Knee Arthroplasty

Number: 0660

Table Of Contents

Applicable CPT / HCPCS / ICD-10 Codes


Scope of Policy

This Clinical Policy Bulletin addresses knee arthroplasty.

  1. Medical Necessity

    1. Food and Drug Administration (FDA) approved total knee arthroplasty (TKA) prosthesis is considered medically necessary for adult members when the following criteria are met:

      1. Member has advanced joint disease demonstrated by:

        1. Pain and functional disability that interferes with ADLs from injury due to osteoarthritis, rheumatoid arthritis, avascular necrosis, or post-traumatic arthritis of the knee joint; and
        2. Limited range of motion, crepitus, or effusion or swelling of knee joint on physical examination: and
        3. Member has any of the following:

          1. Radiographic evidence of moderate/severe osteoarthritis of knee joint (i.e., Kellgren-Lawrence Grade 3 or 4) (see Appendix); or
          2. Radiographic evidence of avascular necrosis (osteonecrosis) of tibial or femoral condyle; or
          3. Radiographic evidence or rheumatoid arthritis (joint space narrowing); and
        4. History of unsuccessful conservative therapy (non-surgical medical management; 12 or 24 weeks depending on age/BMI) that is clearly addressed in the medical record (see Footnote1Note*).  Conservative therapy may be inappropriate for severe osteoarthritis with bone-on-bone articulation in the weight-bearing portion of the joint (medial and/or lateral but not patello-femoral) or severe angular deformity, or for avascular necrosis with collapse of tibial or femoral condyle, or progressive flexion contracture. If conservative therapy is not appropriate, the medical record must clearly document why such approach is not reasonable; or

      2. Failure of a previous osteotomy with pain interfering with ADLs; or
      3. Distal femur or proximal tibia malunion by imaging with pain interfering with ADLs;
      4. Distal femur or proximal tibia fracture or nonunion; or
      5. Malignancy of the distal femur, proximal tibia, knee joint or adjacent soft tissues by imaging; or
      6. Failure of previous unicompartmental knee replacement with pain interfering with ADLs.

      Footnote1*Note: Physical therapy needs to be confirmed either by the actual PT notes, or by documentation in the member claims history. Members with osteoarthritis, traumatic arthritis, or avascular necrosis should have at least 12 weeks of non-surgical treatment documented in the medical record (at least 24 weeks for persons with a relative contraindication) - with at least half of the necessary conservative therapy consisting of formal physical therapy (in-person as opposed to home or virtual physical therapy) in the past year, including all of the following, unless contraindicated:

      1. Anti-inflammatory medications or analgesics; and
      2. Flexibility and muscle strengthening exercises; and
      3. Activity modification; and
      4. Supervised physical therapy (in-person as opposed to home or virtual physical therapy; activities of daily living (ADLs) diminished despite completing a plan of care); and
      5. Assistive device use (required for persons with relative contraindicationsFootnote2** to joint replacement, optional for others); and
      6. Therapeutic injections into the knee (required for persons with relative contraindicationsFootnote2** to joint replacement, optional for others).
      7. Total joint replacement is considered not medically necessary in persons with any of the following absolute contraindications:

        1. Active infection of the joint or active systemic bacteremia that has not been totally eradicated; or
        2. Active skin infection (exception recurrent cutaneous staph infections) or open wound within the planned surgical site of the knee; or
        3. Vascular insufficiency, significant muscular atrophy of the leg, or neuromuscular disease severe enough to compromise implant stability or post-operative recovery or quadriplegia; or
        4. Osseous abnormalities that cannot be optimally managed and which would increase the likelihood of a poor surgical outcome (i.e., inadequate bone stock to support the implant); or
        5. Allergy to components of the implant (e.g., cobalt, chromium or alumina).
      8. For members with significant conditions or co-morbidities, the risk/benefit of tota knee arthroplasty should be appropriately addressed in the medical record.

        Footnote2** Relative contraindications to joint replacement include the following: morbid obesity (BMI greater than 40), age less than 50 years). Members with relative contraindications should exhaust all nonsurgical treatment options.

    2. Revision or replacement of total knee arthroplasty is considered medically necessary for the following indications when accompanied by pain and functional disability (interference with ADLs):

      1. Aseptic loosening of one or more prosthetic components confirmed by imaging; or
      2. Fracture of one or more components of the prosthesis or worn or dislocated plastic insert confirmed by imaging; or
      3. Confirmed periprosthetic infection by gram stain and culture; or
      4. Periprosthetic fracture of distal femur, proximal tibia or patella confirmed by imaging; or
      5. Progressive or substantial periprosthetic bone loss confirmed by imaging; or
      6. Bearing surface wear leading to symptomatic synovitis; or
      7. Implant or knee malalignment (valgus/varus or flexion/extension greater than 15 degrees); or
      8. Knee arthrofibrosis; or
      9. Instability of dislocation of the TKA; or
      10. Extensor mechanism instability; or
      11. Upon individual case review, persistent knee pain of unknown etiology not responsive to a period of non-surgical care for 6 months;

      And member does not have any of the following contraindications to revision surgery:

      1. Persistent infectionor
      2. Poor bone qualityor
      3. Highly limited quadriceps or extensor functionor
      4. Poor skin coverageor
      5. Poor vascular status.
    3. Unicompartmental knee arthroplasty using Food and Drug Administration (FDA)-approved devices is considered medically necessary for members with advanced osteoarthritis or posttraumatic arthritis of the knee affecting only a single compartment (medial, lateral or patellofemoral), and who meet the following criteria:

      1. Pain and functional disability that interferes with ADLs due to osteoarthritis or post-traumatic arthritis of the knee joint; and
      2. Limited range of motion, crepitus, or effusion or swelling of knee joint on physical examination: and
      3. Member must have intact, stable ligaments, in particular the anterior cruciate ligament; and
      4. Patient’s knee arc of motion (full extension to full flexion) is not limited to 90 degrees or less; and
      5. Radiographic evidence of moderate/severe osteoarthritis (i.e., Kellgren-Lawrence Grade 3 or 4) (see Appendix) affecting only a single (medial, lateral or patellofemoral) compartment of the knee joint; and
      6. History of of unsuccessful conservative therapy (non-surgical medical management; 12 or 24 weeks depending on age/BMI) that is clearly addressed in the medical record (see NoteFootnote3***) with at least half of the necessary conservative therapy consisting of formal physical therapy (in-person as opposed to home or virtual physical therapy). Physical therapy needs to be confirmed either by the actual PT notes, or by documentation in the member claims history; and

      Footnote3*** Note: Physical therapy needs to be confirmed either by the actual PT notes, or by documentation in the member claims history. Members should have at least 12 weeks of non-surgical treatment documented in the medical record (at least 24 weeks for persons with a relative contraindication; with at least half of the necessary conservative therapy consisting of formal physical therapy (in-person as opposed to home or virtual physical therapy), including all of the following, unless contraindicated:

      1. Anti-inflammatory medications or analgesics; and
      2. Flexibility and muscle strengthening exercises; and
      3. Activity modification; and
      4. Supervised physical therapy (activities of daily living [ADLs] diminished despite completing a plan of care); and
      5. Assistive device use (required for persons with relative contraindicationsFootnote4 to joint replacement, optional for others); and
      6. Therapeutic injections into the knee (required for persons with relative contraindicationsFootnote4 to joint replacement, optional for others).

        Footnote4† Relative contraindications to joint replacement include the following: morbid obesity (BMI greater than 40), age less than 50 years). Members with relative contraindications should exhaust all nonsurgical treatment options.

      7. Member has none of the following contraindications to unicompartmental knee arthroplasty:

        1. Previous proximal tibial osteotomy or distal femoral osteotomy; or
        2. Tibial or femoral shaft deformity; or
        3. Radiographic evidence of medial or lateral subluxation; or
        4. Flexion contracture greater than 15 degrees; or
        5. Varus deformity greater than 15 degrees (medial unicompartmental knee arthroplasty) or a valgus deformity greater than 20 degrees (lateral unicompartmental knee arthroplasty); or
        6. Inflammatory or crystalline arthropathy; or
        7. Subchondral bone loss due to large subchondral cysts or extensive focal osteonecrosis.
      8. Member has none of the following absolute contraindications to joint replacement:

        1. Active infection of the joint or active systemic bacteremia that has not been totally eradicated; or
        2. Active skin infection (exception recurrent cutaneous staph infections) or open wound within the planned surgical site of the knee; or
        3. Vascular insufficiency, significant muscular atrophy of the leg, or neuromuscular disease severe enough to compromise implant stability or post-operative recovery or quadriplegia; or
        4. Osseous abnormalities that cannot be optimally managed and which would increase the likelihood of a poor surgical outcome (i.e., inadequate bone stock to support the implant); or
        5. Allergy to components of the implant (e.g., cobalt, chromium or alumina).
      9. For members with significant conditions or co-morbidities, the risk/benefit of unicompartmental knee arthroplasty should be appropriately addressed in the medical record.

    4. Prophylactic use of tranexamic acid is considered medically necessary in total knee arthroplasty to decrease blood loss.

    Note: Intra-operative use of kinetic balance sensor for implant stability during knee replacement arthroplasty is considered incidental to the primary procedure being performed and is not eligible for separate reimbursement.

  2. Experimental and Investigational

    1. The following are considered experimental and investigational because the effectiveness has not been established:

      1. Bacteriophage therapy for the treatment of knee arthroplasty-related peri-prosthetic joint infection
      2. Bicompartmental, staged bicompartmental, and bi-unicompartmental knee arthroplasty for osteoarthritis of the knee and all other indications
      3. Custom instrumentation for the procedure including cutting blocks
      4. Customized total or partial knee implant
      5. Prophylactic radiation therapy following total knee arthroplasty
      6. UniSpacer interpositional spacer for the treatment of osteoarthritis affecting the medial compartment of the knee.
    2. Computer-assisted musculoskeletal surgical navigation (e.g., MAKOplasty) is considered experimental and investigational for knee arthroplasty because there is a lack of reliable evidence that it improves surgical outcomes. Note: Robotic assistance is considered integral to the primary procedure and not separately reimbursed.
    3. Pre-operative advanced imaging where required for any experimental and investigational procedure (e.g., where required for computer-assisted surgical navigation, robotic-assisted surgical navigation, or for customized patient implants and/or instrumentation).
  3. Related CMS Coverage Guidance

    This Clinical Policy Bulletin (CPB) supplements but does not replace, modify, or supersede existing Medicare Regulations or applicable National Coverage Determinations (NCDs) or Local Coverage Determinations (LCDs). The supplemental medical necessity criteria in this CPB further define those indications for services that are proven safe and effective where those indications are not fully established in applicable NCDs and LCDs. These supplemental medical necessity criteria are based upon evidence-based guidelines and clinical studies in the peer-reviewed published medical literature. The background section of this CPB includes an explanation of the rationale that supports adoption of the medical necessity criteria and a summary of evidence that was considered during the development of the CPB; the reference section includes a list of the sources of such evidence. While there is a possible risk of reduced or delayed care with any coverage criteria, Aetna believes that the benefits of these criteria – ensuring patients receive services that are appropriate, safe, and effective – substantially outweigh any clinical harms.

    Code of Federal Regulations (CFR):

    42 CFR 417; 42 CFR 422; 42 CFR 423.

    Internet-Only Manual (IOM) Citations:

    CMS IOM Publication 100-02, Medicare Benefit Policy Manual; CMS IOM Publication 100-03 Medicare National Coverage Determination Manual.

    Medicare Coverage Determinations:

    Centers for Medicare & Medicaid Services (CMS), Medicare Coverage Database [Internet]. Baltimore, MD: CMS; updated periodically. Available at: Medicare Coverage Center. Accessed November 7, 2023.

  4. Related Policies


Applicable CPT / HCPCS / ICD-10 Codes

Code Code Description

Other CPT codes related to the CPB:

27440 Arthroplasty, knee, tibial plateau
27441     with debridement and partial synovectomy
27442 Arthroplasty, femoral condyles or tibial plateau(s), knee
27443     with debridement and partial synovectomy
27445 Arthroplasty, knee, hinge prosthesis (eg, Walldius type)
73700 - 73702 Computed tomography, lower extremity
73718 - 73723 Magnetic resonance (e.g., proton) imaging, any joint of lower extremity

Total knee arthroplasty (TKA):

CPT codes covered if selection criteria are met:

27447 Arthroplasty, knee, condyle and plateau; medial AND lateral compartments with or without patella resurfacing (total knee arthroplasty)

CPT codes not covered for indications listed in the CPB:

Bacteriophage therapy -no specific code
+ 0396T Intra-operative use of kinetic balance sensor for implant stability during knee replacement arthroplasty (List separately in addition to code for primary procedure)
0054T Computer-assisted musculoskeletal surgical navigation orthopedic procedure, with image-guidance based on fluoroscopic images
0055T Computer-assisted musculoskeletal surgical navigation orthopedic procedure with image-guidance based on CT/MRI images
20985 Computer-assisted surgical navigation procedure for musculoskeletal procedures, image-less
77401 - 77412 Radiation treatment delivery [following total knee arthroplasty]

HCPCS codes covered if selection criteria are met:

C1776 Joint device (implantable) [FDA approved device]

HCPCS codes not covered if selection criteria are met:

Custom instrumentation for the procedure including cutting block- no specific code
S2900 Surgical techniques requiring use of robotic surgical system (list separately in addition to code for primary procedure)

ICD-10 codes covered if selection criteria are met:

M17.0 - M17.9 Osteoarthritis of knee [with radiographic evidence]
M87.061 – M87.066 Idiopathic aseptic necrosis of tibia and fibula

ICD-10 codes not covered if selection criteria are met:

A00.0 - B99.9 Infectious and parasitic diseases
I87.2 Venous insufficiency (chronic) (peripheral)
L08.0, L08.81, L88 Pyoderma
M00.861 - M00.869 Arthritis due to other bacteria, knee
M01.X61 - M01.X69 Direct infection of knee in infectious and parasitic diseases classified elsewhere
M62.551 - M62.559 Muscle wasting and atrophy, not elsewhere classified, thigh [muscle atrophy of the leg]
M62.561 - M62.569 Muscle wasting and atrophy, not elsewhere classified, lower leg [muscle atrophy of the leg]
M89.711 - M89.79 Major osseous defect [osseous abnormalities]
S81.001+ - S81.859 Open wound of knee and lower leg
T78.40x+ Allergy, unspecified, NEC [allergy to components of the implant]
T84.50XA - T84.59XS Infection and inflammatory reaction due to internal joint prosthesis [Knee-arthroplasty related peri-prosthetic joint infection]

Revision or replacement of total knee arthroplasty:

CPT codes covered if selection criteria are met:

27486 - 27487 Revision of total knee arthroplasty, with or without allograft
27488 Removal of prosthesis, including total knee prosthesis, methylmethacrylate with or without insertion of spacer, knee

CPT codes not covered for indications listed in the CPB:

+0396T Intra-operative use of kinetic balance sensor for implant stability during knee replacement arthroplasty (List separately in addition to code for primary procedure)
0054T Computer-assisted musculoskeletal surgical navigation orthopedic procedure, with image-guidance based on fluoroscopic images
0055T Computer-assisted musculoskeletal surgical navigation orthopedic procedure with image-guidance based on CT/MRI images
20985 Computer-assisted surgical navigation procedure for musculoskeletal procedures, image-less
77401 - 77412 Radiation treatment delivery [following total knee arthroplasty]

HCPCS codes covered if selection criteria are met:

C1776 Joint device (implantable) [not covered for customized total knee implant]

ICD-10 codes covered if selection criteria are met:

M89.9 Disorder of bone, unspecified [confirmed by imaging]
M94.9 Disorder of cartilage, unspecified [confirmed by imaging]
M97.11x+ - M97.12x+ Periprosthetic fracture around internal prosthetic, knee joint [confirmed by imaging]
T84.012+ - T84.013+ Broken internal knee prosthesis [confirmed by imaging]
T84.022+ - T84.023+ Instability of internal knee prosthesis
T84.032+ - T84.033+ Mechanical loosening of prosthetic joint [confirmed by imaging]
T84.062+ - T84.063+ Wear of articular bearing surface of internal prosthetic knee joint [confirmed by imaging]
T84.092+ - T84.093+ Other mechanical complication of internal knee prosthesis [confirmed by imaging]
Z96.651 - Z96.659 Presence of artificial knee joint

Unicompartmental knee arthroplasty:

CPT codes covered if selection criteria are met:

27437 Arthroplasty, patella; without prosthesis
27438     with prosthesis
27446 Arthroplasty, knee, condyle and plateau; medial OR lateral compartment [not covered for customized unicompartmental knee arthroplasty (partial)]

CPT codes not covered for indications listed in the CPB:

+0396T Intra-operative use of kinetic balance sensor for implant stability during knee replacement arthroplasty (List separately in addition to code for primary procedure)
0054T Computer-assisted musculoskeletal surgical navigation orthopedic procedure, with image-guidance based on fluoroscopic images
0055T Computer-assisted musculoskeletal surgical navigation orthopedic procedure with image-guidance based on CT/MRI images
20985 Computer-assisted surgical navigation procedure for musculoskeletal procedures, image-less
77401 - 77412 Radiation treatment delivery [following total knee arthroplasty]

HCPCS codes covered if selection criteria are met:

C1776 Joint device (implantable) [not covered for customized total knee implant]

ICD-10 codes covered if selection criteria are met:

M17.0 - M17.9 Osteoarthritis of knee [with radiographic evidence]

ICD-10 codes not covered if selection criteria are met:

I87.2 Venous insufficiency (chronic) (peripheral)
M62.551 - M62.559 Muscle wasting and atrophy, not elsewhere classified, thigh [muscle atrophy of the leg]
M62.561 - M62.569 Muscle wasting and atrophy, not elsewhere classified, lower leg [muscle atrophy of the leg]
M89.711 - M89.79 Major osseous defect [osseous abnormalities]

UniSpacer interpositional spacer:

No specific code

ICD-10 codes not covered if selection criteria are met (not all inclusive):

M17.0 - M17.9 Osteoarthritis of knee

Bicompartmental, bi-unicompartmental knee and staged bicompartmental arthroplasty:

No specific code

ICD-10 codes not covered if selection criteria are met (not all inclusive):

M17.0 - M17.9 Osteoarthritis of knee


Knee joint replacement is indicated for patients with significant loss or erosion of cartilage to bone accompanied by pain and limited range of motion (ROM), in patients who have had an inadequate response to conservative measures.  Guidelines indicate that unicompartmental knee arthroplasty (UKA) is indicated when only 1 compartment is affected, and total knee arthroplasty (TKA) is indicated when 2 or 3 compartments are affected.

According to available literature, UKA is contraindicated in persons with any of the following: active local or systemic infection; loss of musculature, neuromuscular compromise or vascular deficiency in the affected limb, rendering the procedure unjustifiable; poor bone quality; severe instability secondary to advanced loss of osteochondral structure; absence of collateral ligament integrity; or individuals with over 30 degrees of fixed varus or valgus deformity.

The UniSpacer (Sulzer Orthopedics, Austin, TX) is a metallic interpositional spacer for arthritis affecting primarily the medial compartment of the knee.  The device is a U-shaped metallic shim, designed to be implanted in the knee joint following removal of any damaged cartilage.  The UniSpacer has been used for the treatment of isolated, moderate degeneration of the medial compartment (Grade III to IV chondromalacia) with no more than minimal degeneration (Grade I to II chondromalacia, no loss of joint space) in the lateral condyle or patellofemoral compartment.  The UniSpacer is intended to restore the stability and alignment of the knee and relieve pain, thereby delaying or avoiding the need for total knee replacement (TKR).

The manufacturer states that an advantage of the UniSpacer over TKR is that the procedure to implant the UniSpacer involves no cutting of the patient's bone and no cementing of the implant in the knee.  A small incision is required before the implant can be inserted.  The UniSpacer is designed to center itself in the knee, so that no alteration of the surrounding bone or soft tissues is required for implantation.  The manufacturer states that surgery to implant the UniSpacer takes about 1 hour to complete, and the patient usually is only required to stay over-night after the procedure, instead of the 3 to 4 days required by a TKR.

According to the manufacturer's website, approximately 90 patients have been implanted with the UniSpacer.  The manufacturer's website states that outcomes so far have been "excellent", although the follow-up on these patients is relatively short (the longest being approximately 1.5 years).  The manufacturer's website states that there have been no revisions or complications in any of the cases.

The manufacturer's website states that the UniSpacer is targeted for younger patients who have unicompartmental arthritis involving the medial compartment of their knee.  The majority of the patients who have been implanted with the UniSpacer are under 65 and, therefore, are not yet ideal candidates for TKR.

According to the manufacturer's website, the UniSpacer is currently only available through a small group of specially trained surgeons who are participating in an assessment research project of the device.  However, there is insufficient published evidence of the effectiveness and durability of this device.  Because of the lack of adequate prospective studies in the peer-reviewed published medical literature, the clinical value of UniSpacer has yet to be established.

Scott (2003) stated that the eventual role of the UniSpacer in arthroplasty currently is uncertain.  There are no published reports of its effectiveness.  Its indication should be similar to those for McKeever arthroplasty.  A patient with unicompartmental osteoarthritis in whom an osteotomy is contraindicated but is considered too young, heavy, or active for a metal-to-plastic arthroplasty is ideal.  Less than 1 % of patients with osteoarthritis should be appropriate candidates.  Scott (2003) stated that procedure is technically demanding and sensitive, making its widespread success unlikely.

A technology assessment by the California Technology Assessment Forum (Tice, 2003) concluded that the UniSpacer did not meet CTAF’s assessment criteria.  The assessment concluded that “[s]urgical placement of knee joint spacer devices requires evaluation in controlled trials in order to assess the efficacy and safety of the procedure before its widespread adoption can be advocated.”

The Washington State Department of Labor and Industries (2005) has stated that it does not cover the UniSpacer device because of an absence of clinical data and published literature regarding its safety and efficacy.

Guidance from the National Institute for Health and Clinical Excellence (NICE, 2009) concludes: "Current evidence on the safety and efficacy of individually magnetic resonance imaging (MRI)-designed unicompartmental interpositional implant insertion for osteoarthritis of the knee is inadequate in quantity and quality.  Therefore, this procedure should only be used in the context of research studies".

A technology assessment of TKA prepared for the Washington State Health Care Authority (Dettori et al, 2010) identified only 1 randomized controlled trial (RCT) that reported on a comparison between UKA and standard TKA.  Regarding knee function, the report found that, in the 1 RCT comparing UKA with TKA, the mean Bristol Knee Score was similar between the UKA and TKA groups 5 and 15 years following surgery: 91.1 (range of 32 to 100) and 92 (range of 32 to 100) compared with 86.7 (range of 48 to 98) and 88 (range of 48 to 98).  The report observed that a larger percentage of the UKA group reported excellent Bristol scores at 5- and 15-year follow-up (76 % and 71 %, respectively) than in the TKA group (57 % and 53 %, respectively), although this did not reach statistical significance).  Regarding failure rates, the report stated that statistically significant differences in failure rate defined as revision or a Bristol Knee Score less than 60 were not reported; however, at 15-year follow-up, 17 % of the UKA group and 24 % of the TKA group had experienced failure.  The report found no statistically significant differences in revision rates between UKA and TKA at 15-year follow-up.  Thirteen percent of the UKA group and 16 % of the TKA group had experienced revision.  The report also found no statistically significant differences in survival rate at 15-year follow-up: 89.8 % (95 % confidence interval [CI]: 74.3 to 100) for the UKA group and 78.7 % (95 % CI: 56.2 to 100) for the TKA group (p > 0.05).  The report also found knee pain, function and revision rates were comparable between the 2 treatment groups in 14 cohort studies reporting over a variety of follow-up times.  The report identified 2 RCTs providing data on the efficacy of UKA compared with TKA; in these studies, there were no significant differences in knee pain, knee function, failure or revision, or ROM between the groups from 1 year to 10 years of follow-up.  Regarding safety, no deaths and few complications were reported in 1 RCT and 9 cohort studies.  No statistical significance between UKA and TKA was reported in the number of patients experiencing venous thromboembolism, the knees requiring manipulation under anesthesia or the number of knees having delayed wound healing.  Three studies reported complications after treatment with UKA or high tibial osteotomy (HTO); there were no differences between groups.

Bailie and colleagues (2008) reported the findings of a prospective study of 18 patients treated with the Unispacer.  The mean age of the patients was 49 years (40 to 57).  A total of 8 patients (44 %) required revision within 2 years.  In 2 patients, revision to a larger spacer was required, and in 6 conversion to either a UKA or TKR was needed.  At the most recent review 12 patients (66.7 %) had a Unispacer remaining in-situ.  The mean modified visual analog score for these patients at a mean follow-up of 19 months (12 to 26) was 3.0 (0 to 11.5).  The mean pain level was 30 % that of the mean pre-operative level of 10.  The early clinical results using this device have been disappointing.  This study demonstrated that use of the Unispacer in isolated medial compartment osteoarthritis is associated with a high rate of revision surgery and provides unpredictable relief of pain.

Clarius et al (2010) assessed clinical and radiological results of the UniSpacer, whether alignment correction can be achieved by UniSpacer arthroplasty and alignment change in the first 5 post-operative years.  Antero-posterior long leg stance radiographs of 20 legs were digitally analysed to assess alignment change: 2 relevant angles and the deviation of the mechanical axis of the leg were analysed before and after surgery.  Additionally, the change of the post-operative alignment was determined at 1 and 5 years post-operatively.  Analysing the mechanical tibio-femoral angle, a significant leg axis correction was achieved, with a mean valgus change of 4.7 +/- 1.9 degrees ; a varus change occurred in the first post-operative year, while there was no significant further change of alignment seen 5 years after surgery.  The UniSpacer corrects mal-alignment in patients with medial gonarthrosis; however, a likely post-operative change in alignment due to implant adaptation to the joint must be considered before implantation.  The authors concluded that these findings show that good clinical and functional results can be achieved after UniSpacer arthroplasty.  However, 4 of 19 knees had to be revised to a TKA or UKA due to persistent pain, which is an unacceptably high revision rate when looking at the alternative treatment options of medial osteoarthritis of the knee.

Kock et al (2011) examined if an interpositional knee implant based on magnetic resonance imaging (MRI) data can be an alternative treatment option to the established procedures of high tibial osteotomy and UKA.  From June 2004 to May 2008, a total of 33 patients suffering from unicompartmental knee arthritis received a patient-specific interpositional implant (31 medial and 2 lateral) within a single-arm trial.  The mean follow-up time was 26.6 months (range of 1 to 48 months) and the mean age of the patients was 54.5 years (range of 39 to 65 years).  In addition to the clinical results the Western Ontario and McMaster Universities Osteoarthritis index (WOMAC) function scale and the Knee Society scores were measured.  A descriptive data analysis, a variance analysis for repeated measurements and a determination of significance level were carried out.  The 2 to 4 year results showed a significant improvement in the WOMAC function scale as well as the Knee Society scores.  The knee function after 2 years was comparable to the pre-operative situation with an extension to flexion of 0/2/130°.  The dislocation rate was 6 % and the overall revision rate 21 %.  The authors concluded that despite acceptable functional results a significant pain relief, a complete preservation of bone and a lower rate of dislocations compared to the off-the-shelf Unispacer implant there were only limited indications for the customized interpositional knee implant with respect to the given contraindications due to the high 2-year revision rate.

Catier et al (2011) noted that a new concept has been recently developed for use in the treatment of isolated medial tibio-femoral osteoarthritis: the Unispacer implant.  This mobile interpositional, self-centering implant replicates the meniscal shape.  This mini-invasive device does not require bone cuts or component fixation.  The implant trajectory is guided by the medial condyle.  These investigators hypothesized that the Unispacer knee implant enhances knee function in the treatment of isolated tibio-femoral osteoarthritis graded 2 and 3 according to Ahlbäck radiographic evaluation scale.  This prospective study involved 17 Unispacer knee systems implanted in 16 patients between April 2003 and March 2009 within the frame of a clinical research project.  Patients were clinically (IKS score) and radiographically evaluated during a mean follow-up period of 40 months.  A total of 9 patients (10 implants) had a IKS score greater than 160.  The mean overall knee score at re-assessment, including failures, increased from 51 points pre-operatively to 78 points post-operatively.  The mean overall Knee Society Function score increased from 55 pre-operatively to 75/100 post-operatively.  The reported complication rate was 35 % (pain or implant instability); 1/3 of the failures were not technique- or implant-related but rather induced by the use of an inappropriate width in the frontal plane.  The authors concluded that good results regarding pain relief and function are reported when using a mobile implant with no peripheral overhang that could be responsible for medial capsulo-ligamentous impingement.  The Unispacer has 3 theoretical advantages:
  1. no bone resection,
  2. no implant fixation, and
  3. no polyethylene wear debris.
On the basis of its uncertain clinical results and high revision rate (6 cases out of 17), these researchers do not recommend this system despite the expected improvements on this range of implants.

It has been suggested that bicompartmental knee replacement may be indicated for individuals with osteoarthritis limited to the medial and patello-femoral compartments.  Bicompartmental knee replacement replaces only the inside (medial) joint and knee-cap joint (patello-femoral) joint.  It does not re-surface the outside (lateral) part of the knee and allows for the anterior cruciate ligament (ACL) and posterior cruciate ligament to be retained.

A systematic evidence review of TKA prepared for the Washington State Health Care Authority (Dettori et al, 2010) found 2 registry studies providing comparative data between bicompartmental and standard tricompartmental knee arthroplasty.  These 2 registry studies reported low revision rates in both the bi- and tri-compartmental groups: 3.2 % and 2.8 %, respectively, at 2 to 4 years follow-up and 1.5 % and 1.6 %, respectively, at 2 years follow-up.  No significant differences in overall revision rates between the 2 treatment groups were reported by either study.  Complications were not reported for 2 registry studies comparing bi- and tri- compartmental TKA.

In a meta-analysis, Callahan et al (1995) summarized the literature describing patient outcomes following unicompartmental as well as bicompartmental knee arthroplasties.  Original studies were included if they enrolled 10 or more patients at the time of an initial knee arthroplasty and measured patient outcomes using a global knee rating scale.  A total of 46 studies on unicompartmental prostheses and 18 studies on bicompartmental prostheses met these criteria.  For unicompartmental studies, the total number of enrolled patients was 2,391, with a mean enrollment of 47 patients and a mean follow-up period of 4.6 years.  The mean patient age was 66 years; 67 % were women, 75 % had osteoarthritis, and 16 % underwent bilateral knee arthroplasty.  The mean post-operative global rating scale score was 80.9.  The overall complication rate was 18.5 % and the revision rate was 9.2 %.  Studies published after 1987 reported better outcomes, but also tended to enroll older patients and patients with osteoarthritis and higher pre-operative knee rating scores.  For bicompartmental studies, the total number of enrolled patients was 884, with a mean enrollment of 44 patients and a mean follow-up period of 3.6 years.  The mean patient age was 61 years; 79 % were women, 31 % had osteoarthritis, and 29 % underwent a bilateral arthroplasty.  The mean post-operative global rating scale score was 78.3.  The overall complication rate was 30 % and the revision rate was 7.2 %.  Although bicompartmental studies reported lower mean post-operative global rating scale scores, these studies tended to enroll patients with worse pre-operative knee rating scores.  Recent improvements in patient outcomes following UKA appear to be due, at least partially, to changes in patient selection criteria.  Patient outcomes appear to be worse for bicompartmental arthroplasties than for other prosthetic designs; however, patients enrolled in these studies had more poorly functioning knees before surgery and actually had greater absolute improvements in global knee rating scores.

Rolston et al (2007) stated that in the past, treatment of knee osteoarthritis has been limited to UKA or TKA.  Neither option is well-suited for the active patient with mid-stage osteoarthritis of the medial and patello-femoral compartments.  Now an alternative treatment is available that targets the diseased area without sacrifice of normal bone or both the cruciate ligaments.  Minimally invasive surgical techniques are easily used, which reduces tissue trauma and results in a quicker recovery than TKA.  Bicompartmental replacement offers decreased pain, stability through normal ligament structure, and the retention of normal bone for patients with medial and patello-femoral osteoarthritis.

Bi-unicompartmental knee arthroplasty refers to UKA performed in the contralateral compartment of a knee previously treated with a UKA.

A systematic evidence review of TKA prepared for the Washington State Health Care Authority (Dettori et al, 2010) reported on studies comparing bi-unicompartmental knee arthroplasty (bi-UKA) and standard TKA.  The report found 1 small retrospective cohort study comparing bi-UKA with TKA.  No difference was found in functional scores at a minimum of 4 years of follow-up, and no revisions were recorded in either group.  No cases of radiological loosening or infection were seen in either the bi-UKA or TKA groups.  Two cases (9 %) of intra-operative fracture of the tibial spine block occurred in the bi-UKA group but did not have any adverse effect on the outcome at last follow-up in either case.

Confalonieri and associates (2009) carried out a matched paired study between 2 groups:
  1. bi-unicompartmental (Bi-UKR) and
  2. TKR for the treatment of isolated bicompartmental tibio-femoral knee arthritis with an asymptomatic patello-femoral joint.
A total of 22 patients with bicompartmental tibio-femoral knee arthritis, who underwent Bi-UKR were included in the study (group A).  In all the knees the arthritic changes were graded according to the classification of Alback.  All patients had an asymptomatic patello-femoral joint.  All patients had a varus deformity lower than 8 degrees, a body-mass index lower than 34, no clinical evidence of ACL laxity or flexion deformity and a pre-operative range of motion of a least 110 degrees.  At a minimum follow-up of 48 months, every single patient in group A was matched with a patient who had undergone a computer-assisted TKR (group B).  In the Bi-UKR group, in 2 cases these researchers registered intra-operatively the avulsion of the treated tibial spines, requiring intra-operative internal fixation and without adverse effects on the final outcome.  Statistical analysis of the results was performed.  At a minimum follow-up of 48 months there were no statistical significant differences in the surgical time while the hospital stay was statistically longer in TKR group.  No statistically significant difference was observed for the Knee Society, Functional and GIUM scores between the 2 groups.  Statistically significant better WOMAC Function and Stiffness indexes were registered for the Bi-UKR group.  Total knee replacement implants were statistically better-aligned with all the implants positioned within 4 degrees of an ideal hip-knee-ankle angle of 180 degrees.  The authors concluded that the findings of this 48-month follow-up study suggested that Bi-UKR is a viable option for bicompartmental tibio-femoral arthritis at least as well as TKR but maintaining a higher level of function.

Available evidence does not provide strong conclusions regarding optimal patient selection criteria as well as improved patient outcomes with bicompartmental knee arthroplasty or bi-UKA.  Currently, there is no clinical practice guideline on either of these procedures.  In this regard, the American Academy of Orthopaedic Surgeons' clinical guideline on osteoarthritis of the knee (2003) did not discuss the use of bicompartmental knee arthroplasty or bi-UKA as methods of treatment for osteoarthritis of the knee.  Furthermore, the Osteoarthritis Research International's recommendations for the management of hip and knee osteoarthritis (Zhang et al, 2008) did not mention the use of bicompartmental or bi-UKA.

Available scientific evidence is insufficient to support the use of bicompartmental knee arthroplasty and bi-UKA as alternatives for TKR.  At present, there is inadequate evidence demonstrating improved patient outcomes from either of these methods.  Well-designed studies are needed to ascertain the safety and effectiveness of these approaches.

Unicompartmental knee arthroplasty is a popular treatment for unicompartmental knee arthritis.  Roche and associates (2009) stated that a recently developed computer-assisted surgery/robotic system has the potential to improve alignment in and results of UKA.  Pearle et al (2009) stated that indications for UKA include mechanical axis of less than 10 degrees varus and less than 5 degrees valgus, intact ACL, and absence of femoro-tibial subluxation.  Appropriately selected patients can expect UKA to last at least 10 years.  Failures in UKA are not common and involve technical errors that are thought to be corrected with use of newly developed robotic technology such as the MAKO robotic arm system (MAKOplasty).  The surgeon using this technology may be able to arrive at a set target, enhance surgical precision, and avoid outliers.  However, whether improved precision will result in improved long-term clinical outcome remains a subject of research.

Sinha (2009) reported that the early outcomes of UKA performed with a robotically assisted navigation system have been favorable.  The surgical technique enhances accuracy of bone preparation and component positioning.  Technical errors of the system have been minimal.  The surgeon's learning curve is not adversely affected.  Early patient outcomes are excellent and complications minimal.  The authors noted that further follow-up studies will help to determine whether these early outcomes are sustained over time.

Lonner (2009) noted that modular bicompartmental arthroplasty is an emerging knee-resurfacing approach that provides a conservative alternative to TKA.  Isolated bicompartmental arthritis involving the medial or lateral and patello-femoral compartments, but with no significant deformity or bone deficiency, preserved motion, and intact cruciate ligaments, can be effectively managed with this treatment method.  For the many young and active patients with isolated bicompartmental arthritis, given the potential durability of the procedure and the prosthesis, it is appropriate to use an approach that is more conservative than TKA.  Robotic arm assistance for modular bicompartmental arthroplasty optimizes component position and alignment, which may improve system performance and long-term durability.  In addition, a percentage of patients who undergo isolated unicompartmental or patello-femoral arthroplasty may later develop progressive arthritis in an unresurfaced compartment.  Their cases may be effectively managed with a staged modular approach to resurfacing the degenerating compartment, but additional study is needed.

In a pilot study, Lonner et al (2010) compared the post-operative radiographical alignment of the tibial component with the pre-operatively planned position in 31 knees in 31 consecutive patients undergoing UKA using robotic arm-assisted bone preparation and in 27 consecutive patients who underwent unilateral UKA using conventional manual instrumentation to determine the error of bone preparation and variance with each technique.  Radiographically, the root mean square error of the posterior tibial slope was 3.1 degrees when using manual techniques compared with 1.9 degrees when using robotic arm assistance for bone preparation.  In addition, the variance using manual instruments was 2.6 times greater than the robotically guided procedures.  In the coronal plane, the average error was 2.7 degrees +/- 2.1 degrees more varus of the tibial component relative to the mechanical axis of the tibia using manual instruments compared with 0.2 degrees +/- 1.8 degrees with robotic technology, and the varus/valgus root mean square error was 3.4 degrees manually compared with 1.8 degrees robotically.  The authors concluded that further study will be necessary to determine whether a reduction in alignment errors of these magnitudes will ultimately influence implant function or survival.

Paratte and associates (2010) stated that recent literature suggests patients achieve substantial short-term functional improvement after combined bicompartmental implants but longer-term durability has not been documented.  These investigators examined if
  1. bicompartmental arthroplasty (either combined medial unicompartmental UKA and femoro-patellar arthroplasty (PFA) or medial UKA/PFA, or combined medial and lateral UKA or bicompartmental UKA) reliably improved Knee Society pain and function scores;
  2. bicompartmental arthroplasty was durable (survivorship, radiographical loosening, or symptomatic disease progression);
  3. durable alignment can be achieved; and
  4. the arthritis would progress in the unresurfaced compartment.
These researchers retrospectively reviewed 84 patients (100 knees) with bicompartmental UKA and 71 patients (77 knees) with medial UKA/PFA.  Clinical and radiographical evaluations were performed at a minimum follow-up of 5 years (mean of 12 years; range of 5 to 23 years).  Bicompartmental arthroplasty reliably alleviated pain and improved function.  Prosthesis survivorship at 17 years was 78 % in the bicompartmental UKA group and 54 % in the medial UKA/PFA group.  The high revision rate, compared with TKA, may be related to several factors such as implant design, patient selection, crude or absent instrumentation, or component mal-alignment, which can all contribute to the relatively high failure rate in this series.

Palumbo et al (2011) evaluated the effectiveness of a novel bicompartmental knee arthroplasty (BKA) prosthesis for the treatment of degenerative disease affecting the medial and patello-femoral compartments.  The study included 36 knees in 32 patients with a mean follow-up of 21 months.  The mean Knee Society functional survey and Western Ontario McMaster Osteoarthritic Index Survey scores were 65.4 and 75.8, respectively.  Thirty-one percent of patients were unsatisfied with the surgery, and 53 % stated that they would not repeat the surgery.  These researchers reported an overall survival rate of 86 % with 1 catastrophically failed tibial baseplate.  The authors concluded that this prosthesis provides inconsistent pain relief and unacceptable functional results for bicompartmental arthritis.  The short-term survival of this prosthesis was unacceptably low, and therefore, these investigators no longer implant it at their institution.

Morrison and colleagues (2011) compared functional outcomes of BKA and TKA in patients with osteoarthritis (OA) of the patello-femoral and medial compartments.  Eligibility criteria included bicompartmental OA with less than grade 2 OA in the lateral compartment and intact cruciate ligaments.  A total fo 56 patients met eligibility criteria (21 BKA, 33 TKA).  Enrolled participants completed Short-Form 12 and Western Ontario and McMaster Universities Osteoarthritis Index assessments at baseline and post-operatively at 3 months, 1 year, and 2 years.  In the early post-operative period, the BKA cohort had significantly less pain (p = 0.020) and better physical function (p = 0.015).  These trends did not continue past 3 months.  When adjusting for age, sex, body mass index, and pre-operative status, only 3-month Western Ontario and McMaster Universities Osteoarthritis Index stiffness scores significantly differed between cohorts (p = 0.048).  Despite less early stiffness in the BKA cohort, a significantly higher BKA complication rate (p = 0.045) has led these investigators to recommend TKA for patients with this pattern of OA.

Lyons et al (2012) examined if TKA would demonstrate
  1. better change in clinical outcome scores from pre-operative to post-operative states and
  2. better survivorship than UKA.
These researchers evaluated 4,087 patients with 5,606 TKAs and 179 patients with 279 UKAs performed between 1978 and 2009.  Patients with TKA were older and heavier than patients with UKA (mean age of 68 versus 66 years; mean BMI of 32 versus 29).  They compared pre-operative, latest post-operative, and change in Knee Society Clinical Rating System (KSCRS), SF-12, and WOMAC scores.  Minimum follow-up was 2 years (UKA: mean of 7 years; range of 2.0 to 23 years; TKA: mean of 6.5 years; range of 2.0 to 33 years).  Pre-operative outcome measure scores (WOMAC, SF-12, KSCRS) were higher in the UKA group.  Patients with UKA had higher post-operative KSCRS and SF-12 mental scores.  Changes in score for all WOMAC domains were similar between groups.  Total KSCRS changes in score were similar between groups, although patients with TKA had higher knee scores (49 versus 43) but lower function scores than UKA (21 versus 26).  Cumulative revision rate was higher for UKA than for TKA (13 % versus 7 %).  Kaplan-Meier survivorship at 5 and 10 years was 95 % and 90 %, respectively, for UKA and 98 % and 95 %, respectively, for TKA.  The authors concluded tht while patients with UKA had higher pre- and post-operative scores than patients with TKA, the changes in scores were similar in both groups and survival appeared higher in patients with TKA.

Tria (2013) stated that replacement of the patella-femoral and medial tibio-femoral joints has been performed since the 1980s.  Bicompartmental replacement was modified.  Two different designs were developed: one custom implant and one with multiple pre-determined sizes.  The surgical technique and instruments are unique and training is helpful.  There are no clinical reports for the custom design as of yet.  The standard implant has several reports in the literature with only fair to good results and has subsequently been withdrawn from the market.  The author concluded that bicompartmental arthroplasty remains a questionable area of knee surgery.

Chung et al (2013) noted that bicompartmental knee arthroplasty features bone and ligament sparing as unicompartmental knee arthroplasty and is presumably better in the recovery of muscle strength and function compared to TKA though not previously reported in the literature.  These researchers compared isokinetic knee muscle strength and physical performance in patients who underwent either bicompartmental knee arthroplasty or TKA.  Each of 24 patients (31 knees) was prospectively examined pre-operatively, at 6 and 12 months after each surgery.  Isokinetic knee extensor and flexor strength as well as position sense were measured using the Biodex system.  Timed up and go test, stair climbing test, and the 6-min walk test were used to assess physical performance.  The results of each group were also compared with those from the corresponding healthy control, respectively.  Demography showed significant difference in the mean age between bicompartment (54.8 ± 5.6 years) and TKA groups (65.7 ± 6.7 years).  Comparing between the 2 groups, knee extensor and flexor torque, hamstring/Quadriceps ratio, position sense, and physical performance were not significantly different pre-operatively, at 6 and 12 months after surgery.  In intra-group analysis, muscle strength and position sense at each time-point were not different in both groups.  In physical performance, both groups resulted in improvement in the 6-min walk test, and only TKA group showed enhancement in stair climbing test.  The authors concluded that although theoretically plausible, bicompartmental knee arthroplasty was not superior in knee muscle strength and physical performance at 1 year compared with TKA.

Thienpont and Price (2013) stated that studies have shown that after TKA neither normal biomechanics nor function is obtained.  Selective resurfacing of diseased compartments could be a solution.  These investigators presented a narrative review of the available literature on bicompartmental arthroplasty.  A literature review of all peer-reviewed published articles on bicompartmental arthroplasty of the knee was performed.  Bicompartmental arthroplasty is by definition the replacement of the tibio-femoral and the patella-femoral joint.  It can be performed with a modular unlinked or a monolithic femoral component.  Bicompartmental arthroplasty performed with modular components obtained good to excellent results at ± 10 years follow-up.  Function and biomechanics are superior to TKA.  Modern monolithic femoral components were reported to give early failure and high revision rates and should be avoided.  The authors concluded that modular bicompartmental arthroplasty is an excellent alternative to treat bicompartmental arthritis of the knee leading to good functional results and superior biomechanics in well-selected patients.  However, they stated that caution is needed since only a few peer-reviewed articles with small series and old implant designs are available on this type of arthritis treatment.  Survivorship in these studies is inferior to TKA.

Furthermore, the Work Loss Data Institute’s guideline on “Knee & leg (acute & chronic)” (2013) listed bicompartmental knee replacement as one of the interventions that were considered, but are not recommended.

Luring et al (2011) stated that isolated OA of the patellofemoral joint occurs in 9 % of patients over 40 years of age and women are more often affected.  Options of treatment are varied and not sufficiently justified by the literature.  These investigators performed a literature research with keywords in the field of femoropatellar osteoarthritis in the relevant databases.  Studies were categorized into different treatment options and analyzed.  There are almost no level I studies comparing the different treatment options.  In the literature there are indications that relief of pain can be achieved by conservative treatment, arthroscopic surgery, cartilage conserving surgery and isolated arthroplasty.  The authors concluded that in view of the fact that there are almost no prospective randomized controlled trials (RCTs), none of the options for treatment can be highly recommended.  There is still no gold standard for the treatment of isolated patellofemoral osteoarthritis. 

An assessment by the Canadian Agency for Drugs and Technologies in Health (CADTH, 2013) summarized the available evidence for patellofemoral knee implants: "Bietzel et al. reported outcomes of patello-femoral knee implants in terms of pain and knee functions. The study compared the scores of patients for these outcomes at baseline and after two years from the implant surgery. The exact scores were not reported; however, the report showed that the scores for pain and knee functions (Lyshlom score and WOMAC scores) were statistically significantly improved from baseline. The results for maximum reflection showed no statistical difference. Starks et al. reported the scores of knee functions after two years from the implantation; however, these scores were not compared to their baseline counterpart values; therefore, their significance could not be interpreted".

Davies (2013) noted that unicompartmental patellofemoral arthroplasties are uncommon however numbers are increasing and there are a variety of new prostheses available.  The Femoro-Patella Vialla (FPV, Wright Medical UK) device was the second most commonly used patellofemoral unicompartmental prosthesis in the 2012 British National Joint Register.  There are however no published outcomes data for this device.  In this study, a total of 52 consecutive cases were studied prospectively using Oxford Knee Score and American Knee Society Scores pre-operatively and at follow-up to a minimum of 2 years.  Overall Oxford Knee Scores improved from 30 points pre-operatively (36.6 %) to 19 points (60 %) at 1-year.  American Knee Society Knee scores improved from 51 points pre-operatively to 81 points at 1-year.  Function scores improved from 42 points pre-operatively to 70 points at 1-year.  Moreover, 13 (25 %) patients had an excellent outcome with pain abolished and near normal knee function; 11 (21 %) patients gained very little improvement and scored their knees similar or worse to their pre-operative state.  There were no infective or thromboembolic complications.  Seven cases have been revised to a total knee replacement for on-going pain in 6 cases and progression of arthritis in the tibio-femoral compartments in 1 case.  The patellar button was found to be very poorly fixed in all cases that were revised.  The authors concluded that early results with the FPV prosthesis demonstrated that successful outcomes can be achieved; however the results were unpredictable and a significant minority of patients had on-going symptoms that they found unacceptable.  They stated that the early revision rate was high in this series. 

Al-Hadithy et al (2014) stated that isolated patellofemoral joint OA affects approximately 10 % of patients aged over 40 years and treatment remains controversial.  The FPV patellofemoral joint replacement has been shown to restore functional kinematics of the knee close to normal.  Despite its increasing popularity in recent years, there are no studies evaluating the mid-term results with an objective scoring assessment.  These investigators reported the clinical and radiological outcomes of FPV patellofemoral joint replacement in patients with isolated patellofemoral arthritis.  Between 2006 and 2012, these researchers performed 53 consecutive FPV patellofemoral arthroplasties in 41 patients with isolated patellofemoral joint osteoarthritis.  The mean follow-up was 3 years.  Mean Oxford Knee Scores improved from 19.7 to 37.7 at latest follow-up.  The progression of tibiofemoral osteoarthritis was seen 12 % of knees.  Two knees required revision to TKR at 7 months post-operatively, which these researchers attributed to poor patient selection.  There were no cases of mal-tracking patellae, and no lateral releases were performed.  The authors concluded that these findings suggested the FPV patellofemoral prosthesis provided good pain relief and survivorship with no significant mal-tracking patellae.  This was a relatively small study (n = 41 patients) with mid-term results.  These findings need to be validated by well-designed studies with larger sample size and long-term follow-up. 

King et al (2015) reported the incidence of patellar fracture after PFA and determined associated factors as well as outcomes of patients with and without this complication.  A total of 77 knees in 59 patients with minimum 2-year follow-up were included; 7 (9.1 %) patients experienced a patellar fracture at a mean of 34 (range of 16 to 64) months post-operatively.  All were treated non-operatively.  Lower BMI (p = 0.03), change in patellar thickness (p < 0.001), amount of bone resected (p = 0.001), and larger trochlear component size (p = 0.01) were associated with a greater incidence of fracture.  Fewer fractures occurred when the post-operative patellar height exceeded the pre-operatively measured height.  No statistically significant differences were found in outcome scores between groups at mean four-year follow-up.  A fair amount of fractures at mid-term; not sure if the incidence would increase at long-term.

Parratte et al (2015) noted that partial knee arthroplasty (PKA), either medial or lateral UKA or PFA are a good option in suitable patients and have the advantages of reduced operative trauma, preservation of both cruciate ligaments and bone stock, and restoration of normal kinematics within the knee joint. However, questions remain concerning long-term survival.  These researchers presented the long-term results of medial and lateral UKA, PFA and combined compartmental arthroplasty for multi-compartmental disease.  Medium- and long-term studies suggested reasonable outcomes at 10 years with survival greater than 95 % in UKA performed for medial OA or osteonecrosis, and similarly for lateral UKA, particularly when fixed-bearing implants were used.  Disappointing long-term outcomes have been observed with the 1st generation of patella-femoral implants, as well as early Bi-Uni (i.e., combined medial and lateral UKA) or bicompartmental (combined UKA and PFA) implants due to design and fixation issues.  The authors concluded that promising short- and med-term results with the newer generations of PFAs and bicompartmental arthroplasties will require long-term confirmation.

Joseph et al (2020) reported on a pragmatic, single-center, double-blind randomized clinical trial that was conducted in a UK National Health Service (NHS) teaching hospital to evaluate whether there is a difference in functional knee scores, quality-of-life outcome assessments, and complications at one-year after intervention between total knee arthroplasty (TKA) and patellofemoral arthroplasty (PFA) in patients with severe isolated patellofemoral arthritis. The parallel, two-arm, superiority trial was powered at 80%, and involved 64 patients with severe isolated patellofemoral arthritis. The primary outcome measure was the functional section of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) score at 12 months. Secondary outcomes were the full 24-item WOMAC, Oxford Knee Score (OKS), American Knee Society Score (AKSS), EuroQol five dimension (EQ-5D) quality-of-life score, the University of California, Los Angeles (UCLA) Physical Activity Rating Scale, and complication rates collected at three, six, and 12 months. For longer-term follow-up, OKS, EQ-5D, and self-reported satisfaction score were collected at 24 and 60 months. Among 64 patients who were randomized, five patients did not receive the allocated intervention, three withdrew, and one declined the intervention. There were no statistically significant differences in the patients' WOMAC function score at 12 months (adjusted mean difference, -1.2 (95% confidence interval -9.19 to 6.80); p = 0.765). There were no clinically significant differences in the secondary outcomes. Complication rates were comparable (superficial surgical site infections, four in the PFA group versus five in the TKA group). There were no statistically significant differences in the patients' OKS score at 24 and 60 months or self-reported satisfaction score or pain-free years. The investigators concluded that, among patients with severe isolated patellofemoral arthritis, this study found similar functional outcome at 12 months and mid-term in the use of PFA compared with TKA.

Odgaard et al (2018) compared the outcome of patellofemoral arthroplasty (PFA) with total knee arthroplasty (TKA) in a blinded randomized controlled trial. Patients were eligible if they had debilitating symptoms and isolated patellofemoral disease. One hundred patients were included from 2007 to 2014 and were randomized to PFA or TKA (blinded for the first year; blinded to patient, therapists, primary care physicians, etc; quasiblinded to assessor). Patients were seen for four clinical followups and completed six sets of questionnaires during the first 2 postoperative years. SF-36 bodily pain was the primary outcome. Other outcomes were range of movement, PROs (SF-36, Oxford Knee Score [OKS], Knee injury and Osteoarthritis Outcome Score [KOOS]) as well as complications and revisions. Four percent (two of 50) of patients died within the first 2 years in the PFA group (none in the TKA group), and 2% (one of 50) became ill and declined further participation after 1 year in the PFA group (none in the TKA group). The mean age at inclusion was 64 years (SD 8.9), and 77% (77 of 100) were women. The area under the curve (AUC) up to 2 years for SF-36 bodily pain of patients undergoing PFA and those undergoing TKA was 9.2 (SD 4.3) and 6.5 (SD 4.5) months, respectively (p = 0.008). The SF-36 physical functioning, KOOS symptoms, and OKS also showed a better AUC up to 2 years for PFA compared with TKA (6.6 [SD 4.8] versus 4.2 [SD 4.3] months, p = 0.028; 5.6 [SD 4.1] versus 2.8 [SD 4.5] months, p = 0.006; 7.5 [SD 2.7] versus 5.0 [SD 3.6] months, p = 0.001; respectively). The SF-36 bodily pain improvement at 6 months for patients undergoing PFA and those undergoing TKA was 38 (SD 24) and 27 (SD 23), respectively (p = 0.041), and at 2 years, the improvement was 39 (SD 24) and 33 (SD 22), respectively (p = 0.199). The KOOS symptoms improvement at 6 months for patients undergoing PFA and those undergoing TKA was 24 (SD 20) and 7 (SD 21), respectively (p < 0.001), and at 2 years, the improvement was 27 (SD 19) and 17 (SD 21), respectively (p = 0.023). Improvements from baseline for KOOS pain, SF-36 physical functioning, and OKS also differed in favor of PFA at 6 months, whereas only KOOS symptoms showed a difference between the groups at 2 years. No patient-reported outcome (PRO) dimension showed a difference in favor of TKA. At 4 months, 1 year, and 2 years, the range of motion (ROM) change from baseline for patients undergoing PFA and those undergoing TKA was (-7° [SD 13°] versus -18° [SD 14°], p < 0.001; -4° [SD 15°] versus -11° [SD 12°], p = 0.011; and -3° [SD 12°] versus -10° [SD 12°], p = 0.010). There was no difference in the number of complications. During the first 2 postoperative years, there were two revisions in patients undergoing PFA (one to a new PFA and one to a TKA). The investigators concluded that patients undergoing PFA obtain a better overall knee-specific quality of life than patients undergoing TKA throughout the first 2 years after operation for isolated patellofemoral osteoarthritis. At 2 years, only KOOS function differs between patients undergoing PFA and those undergoing TKA, whereas other PRO dimensions do not show a difference between groups. The observations can be explained by patients undergoing PFA recovering faster than patients undergoing TKA and the functional outcome being better for patients undergoing PFA up to 9 months. Patients undergoing PFA regain their preoperative ROM, whereas patients undergoing TKA at 2 years have lost 10° of ROM. The investigators stated that they found no differences in complications.

Bunyoz et al (2019) reported on a systematic review to compare outcomes of second-generation PFA and TKA by assessment of patient-reported outcome measures (PROMs). A systematic search was made in PubMed, Medline, Embase, Cinahl, Web of Science, Cochrane Library and MeSH to identify studies using second-generation PFA implants or TKA for treatment of PFOA. Only studies using The American Knee Society (AKSS), The Oxford Knee Score (OKS) or The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) to report on PROMs were included. The postoperative weighted mean AKSS knee scores were 88.6 in the second-generation PFA group and 91.8 in the TKA group. The postoperative weighted mean AKSS function score was 79.5 in the second-generation PFA group and 86.4 in the TKA group. There was no significant difference in the mean AKSS knee or function scores between the second-generation PFA group and the TKA group. The postoperative weighted mean OKS score was 36.7 and the postoperative weighted mean WOMAC score was 24.4. The revision rate was higher in the second-generation PFA group (113 revisions [8.4%]) than in the TKA group (3 revisions [1.3%]). Progression of OA was most commonly noted as the reason for revision of PFA, and it was noted in 60 cases [53.1%]; this was followed by pain in 33 cases [29.2%]. The authors concluded that excellent postoperative weighted mean AKSS knee scores were found in both the second-generation PFA group and in the TKA group, suggesting that both surgical options can result in a satisfying patient-reported outcome. Higher revision rates in the second-generation PFA studies may in part be due to challenges related to patient selection. Based on evaluation of PROMs, the use of second-generation PFA seems to be an equal option to TKA for treatment of isolated PFOA in appropriately selected patients. 

Dudhniwala et al (2016) evaluated the early functional outcome and survivorship of a bicompartmental knee arthroplasty implant (Journey-Deuce) in a cohort of patients with combined medial and patella-femoral degenerative OA. A total of 15 patients with a mean age of 57 years were followed-up prospectively and evaluated with clinical examination, Oxford knee score and radiology imaging.  Poor pain scores, concerns about the tibial fixation, early aseptic loosening of the tibial component and a revision rate of 60 % at a minimum follow-up of 54 months were reported.  Implantation of this prosthesis was stopped at the authors’ institution well before the first revision due to an unfavorable early clinical response.  This was further endorsed by an unacceptable revision rate.  The authors concluded that the outcome of the Journey-Deuce bicompartmental knee replacement was considerably worse than the published outcome of TKR.

Sabatini et al (2016) stated that TKA is the most worldwide practiced surgery for knee OA and its effectiveness is mightily described by literature. Concerns about the invasiveness of TKA let the introduction of segmental resurfacing of the joint for younger patients with localized OA.  Bone stock sparing and ligaments preservation are the essence of both UKA and BKA.  Advantages related to BKA are the respect of knee biomechanics, lower complications rates, shorter hospital stay, faster rehabilitation.  Moreover, in case of failure of the 1st implant the conversion to TKA is undemanding and can be compared to a standard prosthesis.  The authors concluded that their experience suggested that BKA is a reliable technique in selected cases and especially younger people with higher functional requests can favorably profit from it.  They stated that although these results are encouraging, there is still a need for further prospective, randomized, long-term studies to evaluate BKA indications and outcomes.

Staged Bicompartmental Knee Arthroplasty

Pandit and colleagues (2017) noted that lateral progression of arthritis following medial UKA, although infrequent, is still the most common reason for revision surgery.  Treatment options normally include conversion to TKA.  An alternative strategy for some patients may be addition of a lateral UKA.  In an observational study, these investigators reported the first results of staged bi-compartmental UKA (Bi-UKA) strategy.  They retrospectively selected from their UKA database patients who underwent a lateral UKA to treat a symptomatic lateral OA progression after a medial UKA.  The analysis included a clinical and radiological assessment of each patient.  A total of 25 patients for a total of 27 knees of staged Bi-UKA were performed in a single-center.  The mean time interval between primary medial UKA and the subsequent lateral UKA was 8.1 years (SD ± 4.6 years).  The mean age at the time of the Bi-UKA was 77.1 years (SD ± 6.5 years).  The median hospital stay was 3 (range of 2 to 9 days) days, and the mean follow-up after Bi-UKA was 4 years (SD ± 1.9 years).  The functional scores showed a significant improvement as compared to the pre-operative status (paired-t test, p = 0.003).  There were no radiological evidences of failure.  None of the patients needed blood transfusion, and there was no significant complications related to the surgical procedure without further surgeries or revisions at final follow-up.  The authors concluded that these findings suggested that addition of a lateral UKA for arthritis progression following medial UKA is a good option in appropriately selected patients.  Level of Evidence = IV.  The main drawbacks of this study were its small sample size (n = 25), observational design (thus the lack of a control group), and medium term follow-up (mean of 4 years).

Customized Total or Partial Knee Implant

Beal et al (2016) stated that modern total knee arthroplasty (TKA) is effective at treating the pain and disability associated with osteoarthritis.  The number of total knee replacements done in the USA continues to increase.  Despite the great care taken during all of these procedures, some patients remain dissatisfied with their outcome.  While this dissatisfaction is likely multi-factorial, malalignment of the prosthetic components is a major cause of post-operative complications.  A neutral mechanical axis plus or minus 3° is felt to have a positive impact on the survivorship of the prosthesis.  Conventional instrumentation has been shown to have a significant number of total knee replacements (TKRs) that lie well outside a neutral coronal alignment.  With that in mind, significant effort has been placed into the development of technology to improve the overall alignment of the prosthesis . In order to reduce the number of outliers, several companies have developed cost-effective systems to aid the surgeon in achieving a more predictably aligned prosthesis in all 3 planes.  These researchers reviewed the literature that is available regarding several of these tools to examine if navigation or custom guides improve outcomes in TKA.  The authors stated that the review supported that while both navigation and custom implants guides appeared to be a cost effective way to achieve a predictable mechanical alignment of a total knee prosthesis therefore reducing the number of outliers, the cost may be increased operative times with no perceived difference in patient satisfaction with navigation custom guides.  They concluded that while navigation and customized implants have found recent interest in the knee arthroplasty marketplace, in a broad sense and in their current forms, these technologies have yet to reach their full potential in improving outcomes and patient experience.  These researchers stated that while navigation and customized implants have found recent interest in the knee arthroplasty marketplace, in a broad sense and in their current forms, these technologies have yet to reach their full potential in improving outcomes and patient experience.

Culler et al (2017) compared selected hospital outcomes between patients undergoing TKA using either a customized individually made (CIM) implant or a standard off-the-shelf (OTS) implant.  A retrospective review was conducted on 248 consecutive TKA patients treated in a single institution, by the same surgeon.  Patients received either CIM (n = 126) or OTS (n = 122) implants.  Study data were collected from patients' medical record or the hospital's administrative billing record.  Standard statistical methods tested for differences in selected outcome measures between the 2 study arms.  Compared with the OTS implant study arm, the CIM implant study arm showed significantly lower transfusion rates (2.4 % versus 11.6 %; p = 0.005); a lower adverse event (AEs) rate at both discharge (CIM 3.3 % versus OTS 14.1 %; p = 0.003) and 90 days after discharge (CIM 8.1 % versus OTS 18.2 %; p = 0.023); and a smaller percentage of patients were discharged to a rehabilitation or other acute care facility (4.8 % versus 16.4 %; p = 0.003).  Total average real hospital cost for the TKA hospitalization between the 2 groups were nearly identical (CIM $16,192 versus OTS $16,240; p = 0.913).  Finally, the risk-adjusted per patient total cost of care showed a net savings of $913.87 (p = 0.240) per patient for the CIM-TKA group, for bundle of care including the pre-operative computed tomography scan, TKA hospitalization, and discharge disposition.  The authors concluded that patients treated with a CIM implant had significantly lower transfusion rates and lower AEs rates than patients treated with OTS implants.  Patients treated with a CIM implant showed a trend toward a shorter length of stay (LOS) and a better discharge disposition than patients in the OTS arm.  These improved outcomes for the CIM group were achieved without an increase in hospital costs.  They stated that future studies are needed to examine the potential hospital savings associated with lower inventory management and sterilization cost-savings with the single package CIM implant.

The authors stated that there were several limitations to this analysis that warrant discussion.  First, this analysis used a retrospective study at a single institution with a single surgeon.  Care should be taken when extrapolating clinical outcome to other providers.  However, it should be pointed out that the bias of the retrospective study was diminished due to the consecutive nature of patient enrollment and consistent patient management between both study arms.  In addition, some of the clinical outcomes in the CIM study arm may reflect a learning curve associated with using a new implant device and outcomes, in particular, operation time may reflect the surgeons learning to use the device.  A further limitation was that the study population (248 hospitalizations) limited the ability to reach statistical significance for some outcome measures.  Nevertheless, nearly all the observed trends in outcomes would have reached significance with more study patients and the same observed variance in the study.  A third limitation was that hospital costs were estimated from billed charges.  However, this was a well-established approach to estimate costs, and it was unlikely that the approach used to estimate cost would consistently over-estimate or under-estimate the cost of treating patients in either study group.  Finally, increased focus on discharge planning over the study period may explain some of the observed differences in the proportion of patients discharged to home or home health care in the CIM study arm.  However, this limitation was migrated by the fact that all patients were treated and discharged by the same surgeon.

Li et al (2017) noted that TKR has been performed for patients with end-stage knee joint arthritis to relieve pain and gain functions.  Most knee replacement patients can gain satisfactory knee functions; however, the range of motion of the implanted knee is variable.  There are many designs of TKR implants; it has been suggested by some researchers that customized implants could offer a better option for patients.  Currently, the 3D knee model of a patient can be created from magnetic resonance imaging (MRI) or computed tomography (CT) data using image processing techniques.  The knee models can be used for PSI design, biomechanical analysis, and creating bone cutting guide blocks.  Researchers have developed patient-specific musculoskeletal lower limb model with TKR, and the models can be used to predict muscle forces, joint forces on knee condyles, and wear of tibial polyethylene insert.  These available techniques make it feasible to create customized implants for individual patients.  The authors concluded that customized TKR implant has the potential to greatly improve knee kinematics and patient knee functions compared to off-the-shelf TKR implant; however, further studies are need to be carried out to make the customized TKR implant available for patients.

Customized Unicompartmental Knee Arthroplasty

Fitz (2009) described the surgical technique with a patient-specific resurfacing uni-compartmental knee arthroplasty (UKA). The patient-specific implant is currently designed on the basis of data from pre-operative computed tomography (CT).  The implant is provided with a set of patient-specific, disposable cutting jigs.  Biomechanical and anatomic axes are factored into jigs from a scan obtained through the hip, knee, and ankle, effectively achieving pre-navigation of the cut planes without the need for a navigation system.  The surgical technique is reduced to 5 simple, reproducible steps.  After removing the articular cartilage, the knee is balanced to determine the correct amount of tibial resection; this is followed by femoral preparation, verification of balancing and tibial preparation, and trial and cementing of the implant.  The introduction of personalized three-dimensional (3-D) image-derived resurfacing implants, as well as personalized single-use instrumentation, has the potential to change the common surgical practice of uni-compartmental knee arthroplasty.  Patient-specific resurfacing implants enable a femoral bone-preserving approach and enhance cortical bone support on the tibia, overcoming critical design limitations of commercial off-the-shelf implants.  The author concluded that patient-specific resurfacing implants can restore normal anatomy, the position of the joint line, and normal joint function, with the potential to result in more normal knee kinematics.

Mahoney and Kinsey (2010) stated that recently, much attention has been directed to femoral component overhang in total knee arthroplasty (TKA).  These researchers described the prevalence of femoral component overhang among men and women after TKA, to identify risk factors for overhang, and to examine if overhang was associated with post-operative knee pain or decreased range of motion (ROM).  Femoral component overhang was measured intra-operatively during 437 implantations of the same type of TKA prosthesis.  The overhang of metal beyond the bone cut edge was measured in millimeters at the mid-point of 10 zones after permanent fixation of the implant.  Factors predictive of overhanging fit were identified, and the effect of overhang on post-operative pain and flexion was examined.  Overhang of greater than or equal to 3 mm occurred in at least 1 zone among 40 % (71) of 176 knees in men and 68 % (177) of 261 knees in women, most frequently in lateral zones 2 (anterior-distal) and 3 (distal).  Female sex, shorter height, and larger femoral component size were highly predictive of greater overhang in multivariate models.  Femoral component overhang of greater than or equal to 3 mm in at least 1 zone was associated with an almost 2-fold increased risk of knee pain more severe than occasional or mild at 2 years after surgery (odds ratio [OR], 1.9; 95 % confidence interval [CI]: 1.1 to 3.3).  The authors concluded that in this series, overhang of the femoral component was highly prevalent, occurring more often and with greater severity in women, and the prevalence and magnitude of overhang increased with larger femoral component sizes among both sexes.  Femoral component overhang of greater than or equal to 3 mm approximately doubled the odds of clinically important knee pain 2 years after TKA.

The authors stated that the limitations of this study included its retrospective design, restriction to a single device and surgical technique, and lack of formally validated, more discriminating outcomes instruments.  It was not known how closely the prevalence of overhang that was observed approximated that of the general population of all patients with TKAs; however, the distal aspect ratio of the femoral component used in this study was similar to that of several other widely used designs.  Statistical models and attributable risk calculations were by nature theoretical and have limitations; it is not known how closely these findings and observations represented the status of the general population, and evaluation of clinical importance was ultimately a subjective process.

Koeck et al (2011) noted that implant positioning and knee alignment are 2 primary goals of successful UKA.  This prospective study outlined the radiographic results following 32 patient-specific uni-compartmental medial resurfacing knee arthroplasties.  By means of standardized pre- and post-operative radiographs of the knee in strictly antero-posterior (AP) and lateral view, AP weight bearing long leg images as well as pre-operative CT-based planning drawings an analysis of implant positioning and leg axis correction was performed.  The mean pre-operative coronal femoro-tibial angle was corrected from 7° to 1° (p < 0.001).  The pre-operative medial proximal tibial angle of 87° was corrected to 89° (p < 0.001).  The pre-operative tibial slope of 5° could be maintained.  The extent of the dorsal femoral cut was equivalent to the desired value of 5 mm given by the CT-based planning guide.  The mean accuracy of the tibial component fit was 0 mm in AP and +1 mm in medio-lateral projection.  The authors concluded that patient-specific fixed bearing UKA could restore leg axis reliably, obtain a medial proximal tibial angle of 90°, avoid an implant mal-positioning and ensure maximal tibial coverage.  This was a relatively small study (n = 32); the duration of follow-up was unclear.

Sinha (2012) stated that the efficiency in surgical procedures saves time and money and can decrease medical complications.  Several sources of inefficiency exist in the operating room (OR), including pre-operative and intra-operative.  The instruments used during total knee arthroplasty (TKA) are frequently redundant.  Customized instruments and implants can improve efficiency by reducing steps.  Additional benefits may include improved alignment and kinematics.  The author addressed the various sources of inefficiency, provided suggestions to overcome them, and introduced the concept of customized guides and implants as a method to improve efficiency.  The author concluded that besides the obvious benefit of cost and resource conservation, one added benefit may be improved accuracy and possibly outcomes; further research is needed to confirm these possibilities.

Carpenter et al (2014) noted that poor tibial component fit can lead to issues including pain, loosening and subsidence.  Morphometric data, from 30 patients undergoing UKA were utilized; comparing size, match and fit between patient-specific and off-the-shelf implants; CT images were prospectively obtained and implants modeled in CAD, utilizing sizing templates with off-the-shelf and CAD designs with patient-specific implants.  Virtual surgery was performed, maximizing tibial plateau coverage while minimizing implant overhang.  Each implant evaluated to examine tibial fit.  Patient-specific implants provided significantly greater cortical rim surface area coverage versus off-the-shelf implants: 77 % versus 43 % medially and 60 % versus 37 % laterally.  Significantly less cortical rim over-hang and under-coverage were observed with patient-specific implants.  The authors concluded that patient-specific implants provided superior cortical bone coverage and fit while minimizing over-hang and under-coverage seen in off-the-shelf implants. 

Ivie et al (2014) stated that patient-specific guides can improve limb alignment and implant positioning in TKA, although not all studies have supported this benefit.  These researchers compared the radiographs of 100 consecutively-performed patient-specific total knees to a similar group that was implanted with conventional instruments instead.  The patient-specific group showed more accurate reproduction of the theoretically ideal mechanical axis, with fewer outliers, but implant positioning was comparable between groups.  The odds ratio comparison showed that the patient-specific group was 1.8 times more likely to be within the desired +3° from the neutral mechanical axis when compared to the standard control group.  The authors concluded that these findings suggested that reliable reproduction of the limb mechanical axis may accrue from patient-specific guides in TKA when compared to standard, intra-medullary instrumentation.

In a cohort study, Schwarzkopf et al (2015) examined if there is a significant difference in surgical time, intra-operative blood loss, post-operative range of motion (ROM), and length of stay (LOS) between patient-specific implants (PSIs) and conventional TKA.  A consecutive series of 621 TKA patients, 307 with PSIs and 314 with conventional implants, was reviewed.  Differences in estimated blood loss, LOS, ROM and surgical time/tourniquet time between the 2 cohorts were analyzed.  Linear regression analysis demonstrated that PSI decreased estimated blood loss by 44.72 ml (p < 0.01), decreased LOS by 0.39 days (p < 0.01), decreased post-operative ROM by 3.90° (p < 0.01), and had a negligible difference on surgical and tourniquet time.  The authors concluded that the use of PSI was associated with decreased estimated blood loss, decreased LOS, decreased ROM, and no discernible difference in surgical or tourniquet time, all of which are unlikely to be clinically significant; and future studies are needed to address quality of life (QOL) and patient-reported functional outcome measurements between the 2 cohorts.  Level of evidence = III.

Patil et al (2015) stated that nearly 14 % to 39 % TKA patients reported dissatisfaction causing incomplete return of function.  These researchers proposed that the kinematics of knees implanted with patient-specific prostheses using patient-specific cutting guides would be closer to normal.  A total of 18 matched cadaver lower limbs were randomly assigned to 2 groups: group A was implanted with patient-specific implants using patient-specific cutting guides; group B, the contralateral knee, was implanted with a standard design using intramedullary alignment cutting guides.  Knee kinematics were measured on a dynamic closed-kinetic-chain Oxford knee rig, simulating a deep knee bend and in a passive rig testing varus-valgus laxity.  The difference from normal kinematics was lower for group A compared to group B for active femoral rollback, active tibiofemoral adduction, and for passive varus-valgus laxity.  The authors concluded that these findings supported the hypothesis that knees with patient-specific implants generate kinematics more closely resembling normal knee kinematics than standard knee designs.  They noted that restoring normal kinematics may improve function and patient satisfaction after total knee arthroplasty.

Zeller et al (2017) examined if improving implant design through customized TKA improves kinematic function.  Using state-of-the-art mobile fluoroscopy, tibio-femoral kinematics were analyzed for 24 subjects with a customized individually made (CIM), cruciate-retaining TKA, and 14 subjects having an asymmetric condylar cruciate-retaining TKA.  Subjects performed a weight-bearing deep knee bend and a rise from a seated position.  Each patient was evaluated for weight-bearing ROM, femoro-tibial translation, femoro-tibial axial rotation, and condylar lift-off occurrence.  Subjects having a CIM TKA experienced greater weight-bearing knee flexion compared with the traditional posterior cruciate-retaining (PCR) TKA design.  During flexion, the CIM TKA subjects consistently exhibited more posterior femoral roll-back than the traditional PCR TKA subjects.  The CIM TKA was found to have statistically greater axial rotation compared with the traditional PCR TKA (p = 0.05). Of note, only the CIM TKA patients experienced femoral internal rotation at full extension, as exhibited in a normal knee.  Compared with the traditional PCR TKA, the CIM TKAs demonstrated minimal occurrences of paradoxical sliding and reverse rotation during flexion and extension.  The CIM TKA subjects showed minimal lift-off and hence better stability in early-flexion to mid-flexion compared with the traditional PCR subjects.  The authors concluded that the CIM TKA demonstrated kinematics more similar to a normal knee; thus; using customized implant technology through CIM TKA designs afforded benefits including more normal motion compared with a traditional PCR TKA.

An assessment by the Ludwig Boltzmann Institute for Health Technology Assessment of custom-made or customizable 3D-printed implants and cutting guides (2019) concluded: "3D printed custom-made or customisable implants and cutting guides are currently most frequently applied in knee, maxillofacial, and cranial surgery. Evidence of very low or low quality shows significant differences in precision, both in terms of malalignment and deviation between 3D printed technology and standard instrumentation in knee arthroplasty. Evidence of higher quality is needed to validate these significant results and draw final conclusions. No firm conclusions can be made in mandibular reconstruction and cranioplasty, since no outcomes were significant in favour of either technology. No statements regarding long-term safety outcomes can be made."  

ConforMIS Knee Implant

Wang et al (2018) stated that newer TKR designs have been introduced to the market with the aim of overcoming common sizing problems with older TKR designs.  Furthermore, since a sizable percentage of patients with osteoarthritis (OA) present with disease limited to the medial/lateral knee compartment in addition to the patellofemoral joint, for whom, a customized bi-compartmental knee replacement (BKR) is available as a therapeutic option.  To-date, there is very little information regarding knee strength and mechanics during gait for patients implanted with these modern TKR and BKR designs.  These investigators evaluated knee strength and mechanics during walking for patients with either a modern off-the-shelf TKR or a customized BKR and compared these findings to a cohort of healthy controls.  A total of 12 healthy controls, 8 BKR, and 9 TKR patients participated in the study.  Maximal isometric knee strength was evaluated; 3D kinematic and kinetic analyses were conducted for level walking.  The TKR knee exhibited less peak extensor torque when compared to, both the BKR and control limbs (p < 0.05).  The TKR knee had less extensor moment at stance than both the BKR and control knees (p < 0.05).  Both the BKR and control knees displayed larger internal rotation at stance than that of the TKR knee (p < 0.05).  The authors concluded that the findings of this study suggested that, for patients that exhibit isolated OA of the tibiofemoral joint, using a customized BKR implant is a viable therapeutic option and may contribute to superior mechanical advantages.

The authors stated that there were several drawbacks that need consideration when interpreting these results.  The sample size of participants in each group was smaller than the typical follow-up studies that reported on functional and clinical end-points.  Though sample size played an important role in interpreting results, the authors believed from their experience with conducting such studies, that the sample size chosen was adequate to enable them to make conclusions on their analyses.  Additionally, they were able to maintain a similar sample size in each arm of the study.  This should alleviate any bias due to sample size in any one study arm.  Although participants in the control group were younger with smaller BMI than the other groups, the 2 patient groups were age-, mass-, and height-matched.  These investigators believed that any advantage drawn from this would affect the implant groups equally, thus making comparisons between the implant groups relevant, while still providing context on how they compare to healthy controls.  Ideally, the authors would have liked to test patients pre- and post-operatively and compare results with the patient being their own control.  However, this would mean having to test patients that have end-stage OA, which the authors felt would not provide a clear comparison to healthy controls.  Lastly, in this study, patients’ pre-operative Knee Society scores and gait analysis data were not available due to their cross-sectional study design.  However, they believed their patients’ pre-surgical conditions were similar to patients used in other prospective studies examining functional improvements after knee replacements.  In those studies, patients’ combined Knee Society scores were close to 100 and knee range of motion was around 120° [19, 20, 21, 22].  In general, patients with end-stage knee OA experience joint pain and stiffness, which led to functional limitations of performing daily activities such as walking, going up and down stairs, and rising from a sitting position.  The authors chose the KOS-ADL because it is an effective instrument for measuring functional limitations associated with pathological disorders of the knee.  However, the authors only administered the KOS-ADL during patients’ post-operative laboratory visit.  Ideally, if the KOS-ADL score was obtained prior to surgery, then it would have been possible to quantify how much functional improvement was made at the time of the post-operative laboratory testing.

Tammachote et al (2018) noted that customized cutting block (CCB) was designed to ensure the accurate alignment of knee prostheses during total knee arthroplasty (TKA).  Given the paucity of CCB efficacy data, these researchers compared CCB with conventional cutting guide using a randomized controlled trial (RCT).  A total of 108 osteoarthritic knee patients underwent TKA by 1 experienced surgeon were randomized to receive CCB (n = 54) or conventional cutting instrument (CCI) surgery (n = 54).  The primary outcomes were limb alignment, prostheses position, and operative time.  The secondary outcomes were hemodynamic alteration after surgery, functional outcomes (modified Western Ontario and McMaster University Osteoarthritis Index) and ROM at 2 years after surgery.  Mean hip-knee-ankle angle in the CCB group was 179.4° ± 1.8° versus 179.1° ± 2.4° in the CCI group, Δ = 0 (95 % confidence interval [CI]: -0.6 to 1.1, p = 0.55).  Mean operative time was faster in the CCB arm: 93 ± 12 versus 104 ± 12 mins, Δ = 11 (95 % CI: -16.7 to -7.2, p < 0.0001).  There were no differences in hemodynamic parameters, mean blood loss (446 [CCB] versus 514 ml [CCI], Δ = -68 [95 % CI: -138 to 31 ml, p = 0.21]), post-operative hemoglobin changes, incidence of hypotension (systolic blood pressure less than 90 mmHg), oliguria, and rates of blood transfusion.  Functional outcomes and ROM were also similar.  The authors concluded that there was no improvement in alignment, hemodynamic changes, blood loss, and knee functional outcomes; CCB reduced surgical time by 11 mins in this cohort.  These researchers stated that CCB cost-effectiveness should be further investigated.

Khosravipour et al (2018) noted that contact pressure and stresses on the articulating surface of the tibial component of a TKR are directly related to the joint contact forces and the contact area.  These stresses can result in wear and fatigue damage of the ultra-high-molecular-weight polyethylene.  Thus, conducting stress analysis on a newly designed surface-guided knee implant is needed to evaluate the design with respect to the polyethylene wear.  Finite element modeling is used to analyze the design's performance in level walking, stair ascending and squatting.  Two different constitutive material models have been used for the tibia component to evaluate the effect of material properties on the stress distribution.  The contact pressure results of the finite element analysis were compared with the results of contact pressure using pressure-sensitive film tests.  In both analyses, the average contact pressure remained below the material limits of ultra-high-molecular-weight polyethylene insert.  The peak von Mises stresses in 90° of flexion and 120° of flexion (squatting) are 16.28 and 29.55 MPa, respectively.  All the peak stresses were less than the fatigue failure limit of ultra-high-molecular-weight polyethylene which was 32 MPa.  The average contact pressure during 90° and 120° of flexion in squatting were 5.51 and 5.46 MPa according to finite element analysis and 5.67 and 8.14 MPa according to pressure-sensitive film experiment.  The authors concluded that customized surface-guided knee implants are aimed to resolve the limitations in activities of daily living (ADL) after TKR by providing close to normal kinematics.  The proposed knee implant model provided patterns of motion much closer to the natural target, especially as the knee flexes to higher degrees during squatting.  These laboratory findings need to be validated in the clinical setting.

Levengood and Dupee (2018) determined the accuracy of a customized individually made total knee implant used in conjunction with patient-specific cutting guides in restoring coronal plane mechanical axis alignment using computer-assisted surgery (CAS).  A consecutive series of 63 TKA patients were prospectively measured with intra-operative CAS.  The patient-specific instruments and implants were created utilizing a pre-operative CT scan; CAS system was used for all patients, to determine mechanical alignment.  Bone cuts were made using the patient-specific instruments.  Both bone cuts and final coronal mechanical alignment were recorded utilizing the navigation system for the assessment.  The patient-specific instruments and implants provided perfect neutral coronal mechanical alignment (0°) in 53 patients.  The remaining 10 patients had a post-operative alignment within ± 2° of neutral.  The average pre-operative deformity was 5.57° versus 0.18° post-operatively (p < 0.0001).  The mean correction angle was 5.68°.  No patients had post-operative extension deficits as measured with CAS (7.50° pre-op for 40/63 patients).  Customized, individually made total knee implant with patient-specific cutting jigs showed results that were comparable to those of CAS systems in this study.  The authors concluded that this technology restored the neutral coronal mechanical axis very accurately, while offering unique benefits such as improved implant fit and restoration of the patient's J-curves, which require further investigation.

The authors stated that the use of a CAS as the reference for the measurements of the mechanical axis pre- and post-implantation could be one of the drawbacks of this study.  The measurements arising from CAS were dependent on what was registered and data may be incorrect if the original registration was not accurate.  However, CAS has been commonly used during surgery for aligning implant components.  Also, the lead author was trained in using CAS and has used them in more than 600 surgeries prior to use in this study.  The authors believed that this had a mitigating impact on registration errors.  Additionally, CAS systems had been found to be more accurate than radiographic and CT measurements and prevented the patient from being exposed to additional ionizing radiation.  Another drawback of this study was the fact that this study was conducted on a sample size that was smaller than the average volume of an orthopedic surgeon in the time window analyzed, though it was comparable to similar previously published studies.  This could be seen as a limiting factor in powering the study.  Nevertheless, these were consecutively recruited patients at a sports medicine practice and the results of this study indicated that the results were highly reproducible; thus, these investigators did not anticipate a deviation from the current results by increasing the sample size.  Also, the sample size used for this study was comparable to previously published reports on mechanical alignment using CAS.   As part of the study data collection, sagittal plane alignment of the femoral and tibial bones, pre- and post-implantation was not collected.  Pre-surgery femoral and tibial varus/valgus alignment was not assessed.  The goal of our study was to evaluate the iJig system used in conjunction with the iTotal implant in reproducing overall coronal plane mechanical alignment after implantation and the ability of the system to return the patients to full extension.  These data have been presented in the study.  Future studies that investigate the sagittal alignment in conjunction with the coronal alignment using these jigs will provide a deeper understanding on the ability of the iJig system to restore sagittal and coronal alignment post-surgery.  Finally, the study did not include a control group, which would have provided a direct comparison of outcomes.  There were multiple studies that had examined the use of patient specific instrumentation blocks in conjunction with off-the-shelf implants.  The authors believed comparing their results to the results presented in these manuscripts as an adequate criterion for comparing the outcomes with the customized implant used with the iJigs platform.  Moreover, they stated that it was important to note, however, that many of these studies investigated the use of patient-specific jigs manufactured using MRI imaging.  The patient-specific jigs investigated in this study were manufactured using computed tomography (CT) imaging.  The differences in the imaging modalities were not investigated in this study.

Koh et al (2018) examined post-cam design via finite element analysis to evaluate the most normal-like knee mechanics.  These researchers developed 5 different 3-D computational models of customized posterior-stabilized (PS) TKA involving identical surfaces with the exception of the post-cam geometry.  They included flat-and-flat, curve-and-curve (concave), curve-and-curve (concave and convex), helical, and asymmetrical post-cam designs.  These investigators compared the kinematics, collateral ligament force, and quadriceps force in the customized PS-TKA with 5 different post-cam designs and conventional PS-TKA to those of a normal knee under deep-knee-bend conditions.  The results indicated that femoral rollback in curve-and-curve (concave) post-cam design exhibited the most normal-like knee kinematics, although the internal rotation was the closest to that of a normal knee in the helical post-cam design.  The curve-and-curve (concave) post-cam design showed a femoral rollback of 4.4 mm less than the normal knee, and the helical post-cam design showed an internal rotation of 5.6° less than the normal knee.  Lateral collateral ligament and quadriceps forces in curve-and-curve (concave) post-cam design, and medial collateral ligament forces in helical post-cam design were the closest to that of a normal knee.  The curve-and-curve (concave) post-cam design showed 20 % greater lateral collateral ligament force than normal knee, and helical post-cam design showed medial collateral ligament force 14 % greater than normal knee.  The authors concluded that the results revealed the variation in each design that provided the most normal-like biomechanical effect.  The present biomechanical data were expected to provide useful information to improve post-cam design to restore normal-like knee mechanics in customized PS-TKA.

The authors stated that this study had 4 limitations.  First, the 5 specific post-cam designs used in this study did not represent all the design features of contemporary TKA.  Second, a deep-knee-bend simulation was performed although simulations related to more demanding activities (e.g., chair rising, sitting, stair climbing, and stair descending) were required in the future for a more reliable investigation.  However, the simulation was performed under deep-knee-bend motion because it included both a wide range of flexion-extension and a significant muscular endeavor around the knee joint.  Third, implant kinematics and quadriceps force were evaluated by using computational simulations, and this did not fully represent an in-vivo condition.  Fourth, the anatomy for the customized PS design was based on, and virtually implanted in, only 1 subject.  The use of subjects of various ages would improve the validity of the results because the validity was also dependent on the geometry of the knee joint.  Most significantly, the time and computational cost associated with subject-specific FE model generation were not efficient.  These researchers stated that further design modifications to the customized TKA are needed to achieve normal knee mechanics during deep-knee-bend activity; and future research will increase the number of subjects.  Additionally, it is necessary to consider the design for substituting ACL function.

Kay et al (2018) stated that manipulation under anesthesia (MUA) is a standard treatment for arthrofibrosis after total knee arthroplasty (TKA), with reported rates of 1.5 to 6 %.  Customized TKA may have better outcomes by matching individual patient anatomy.  However, a previous study reported an unacceptably high rate of MUA for customized TKAs.  This study reported the incidence of MUA in a large cohort of 2nd generation customized TKAs.  Data were collected prospectively on 360 2nd generation ConforMIS iTotal cruciate retaining TKAs; MUA was performed for clinically significant arthrofibrosis.  Range of motion (ROM) and New Knee Society Scores (KSS) were evaluated at regular intervals for 2 years; 11/360 (3.05 %) knees underwent MUA; ROM overall improved from 115° to 125°, and from 112° to 122° in patients undergoing MUA; KSS objective and functional scores in MUA patients increased from 57 to 98 and 41 to 90, respectively, and in the entire cohort increased from 65 to 96 and 45 to 86 at 2 years (p < 0.05).  No MUA patients underwent revision surgery.  The authors concluded that customized TKA with 2nd generation ConforMIS iTotal implants resulted in a MUA rate consistent with the literature for all designs.  Additionally, patients exhibited significant increases in ROM and Knee Society Scores.  Moreover, these researchers stated that further follow-up continues at all sites.  Data from longer term follow-up on the entire cohort as well as the patients who experienced MUAs in this study population will provide a deeper understanding of overall survival, patient outcomes and long-term effects of MUA on patients receiving this device.

The authors stated that drawbacks of this study included a lack of standardized indications for undergoing MUA and incomplete follow-up (298 of 360 patients at 1 years, including 8 of 11 patients who underwent MUA).  However, patients who did not complete 1 year follow-up did not report problems that would be indications for MUA at the 6 week or 6 month visit.  Thus, it was unlikely that additional patients in the study will require MUA in the future. 

Arbab et al (2018) noted that incorrect positioning and malalignment of TKA components can result in implant loosening.  Restoration of neutral alignment of the leg is an important factor affecting the long-term results of TKA.  In a retrospective study, these researchers compared mechanical axis in patients with conventional and patient-specific TKAs.  A total of 232 patients who underwent TKA between January 2013 and December 2014 were included to compare post-operative mechanical axis; 125 patients received a patient-specific TKA (iTotal CR®, Conformis) and 107 a conventional TKA (Triathlon®, Stryker).  Standardized pre- and post-operative long-leg standing radiographs were retrospectively evaluated to compare the 2 patient cohorts; 113 (90 %) radiographs of patient-specific TKA and 88 (82 %) of conventional TKA were available for comparison.  The pre-operative deviation from neutral limb axis was 9.0° (0.1 to 27.3°) in the patient-specific TKA cohort and 8.2° (0.2 to 18.2°) in the conventional TKA group.  Post-operatively the patient-specific TKA group showed 3.2° (0.1 to 8.4°) and the conventional TKA cohort 2.3° (0.1 to 12.5°) deviation.  However, the rate of ± 3° outliers from neutral limb axis was 16 % in the patient-specific TKA cohort and 26 % in the conventional TKA group.  The authors concluded that patient-specific TKA demonstrated fewer outliers from neutral leg alignment compared to conventional technique.  Moreover, these researchers stated that potential benefits in the long-term outcome and functional improvement require further investigation.

The authors stated that this study had several drawbacks.  First, it was a retrospective comparative analysis and hence selection bias could not be excluded.  Second, these investigators assessed only 1 patient-specific implant (PSI) design and these findings might not be applicable to other PSIs that currently are commercially available.  They did not perform a power analysis before starting this study.  Third, the findings of this study were limited to the coronal plane and did not take into account lateral or rotational component positioning, which may play a role in long-term survivorship of total knee implants.  Finally, these investigators did not report on clinical outcomes such as pain, stiffness, range of motion, patient satisfaction, or outcome scoring systems, which may limit the clinical relevance of the findings in this study.

Schroeder and Martin (2019) stated that in TKA, surgeons often face the decision of maximizing tibial component fit and achieving correct rotational alignment at the same time.  Customized implants (CIMs) address this difficulty by aiming to replicate the anatomical joint structure, utilizing data from patient-specific knee geometry during the manufacturing.  These investigators intra-operatively compared component fit in 4 tibial zones of a CIM to that of 3 different off-the-shelf (OTS) TKA designs in 44 knees.  Additionally, they evaluated the rotational alignment of the tibia using CT-based computer aided design model analysis.  Overall the CIM device showed significantly better component fit than the OTS TKAs.  While 18 % of OTS designs presented an implant overhang of 3 mm or more, none of the CIM components did (p < 0.05).  There was a larger percentage of CIMs seen with optimal fit (less than or equal to 1 mm implant overhang to less than or equal to 1 mm tibial bone under-coverage) than in OTS TKAs.  Also, OTS implants showed significantly more component under-hang of greater than or equal to 3 mm than the CIM design (37 % versus 18 %).  The rotational analysis revealed that 45 % of the OTS tibial components showed a rotational deviation of more than 5 degrees and 4 % of more than 10 degrees to a tibial rotational axis described by Cobb et al.  No deviation was seen for the CIM, as the device was designed along this axis.  Using the medial 1/3 of the tibial tubercle as the rotational landmark, 95 % of the OTS trays demonstrated a rotational deviation of more than 5 degrees and 73 % of more than 10 degrees compared with 73 % of CIM tibial trays with more than 5 degrees and 27 % with more than 10 degrees.  Based on these findings, the authors believed that the CIM TKA provided both better rotational alignment and tibial fit without causing overhang of the tibial tray than the 3 examined OTS implants.

The authors stated that this study had several drawbacks.  First, all TKAs and intra-operative measurements were done by a single surgeon which may affect the results when measuring tibial bone coverage of the 3 OTS implants from a surgical technique stand-point. However, the surgeon had used all of these implants previously and was especially experienced with the OTS 3 and CIM brands.  A 2nd drawback was that patient-specific jigs manufactured for the iTotal CR were used for the tibial bone resection.  Yet, this tibial cut was similar to any other cut in the resulting shape of the cut tibial bone.  Third, as the CAD analysis of the rotational deviation from an axis described by Cobb et al and an axis to the medial 1/3 of the tibial tubercle was performed manually and for each implant individually.  This may have resulted in intra-observer mistakes.  Varying opinions exist on what landmarks to use when assessing component alignment.  These researchers did not utilize all methods of tibial component rotation, only methods based on the location of the tibial tubercle and by an axis described by Cobb et al which they believed was accurate from what multiple studies have reported.  Aligning the OTS tibial components toward the medial 1/3 of the tubercle has been shown in multiple studies to be the most reproducible clinical landmark in terms of tibial tray rotation and was used by the majority of surgeons in the United States for tibial alignment.  The authors only evaluated 3 OTS implant designs for this study although there were many more different types on the market.  Nonetheless, based on these findings and similarities between the OTS brands, the authors felt these results were likely highly translatable to other OTS brands.  In this study, only symmetrical implant designs were compared with the CIM TKA, despite the fact that implant manufacturers had introduced other asymmetrical designs on the market.  However, as Jin et al emphasized in their  study, although leading to better results in tibial fit, there were still cases with both over- and under-hang on the same tibial trial with the asymmetrical design.  Moreover, these investigators stated that it has to be noted that no precise definitions for absolute tibial component under-hang or overhang can be found in the literature.  However, Mahoney and Kinsey's observations indicated that the presence of an overhang of greater than or equal to 3 mm in at least 1 zone increased the odds of patients reporting knee pain which was why the authors chose this threshold to be of importance.   Jin et al suggested under-hang is more acceptable during surgery than over-hang as the surgeon can remove uncovered bone during the procedure and correct rotation.  To the authors’ knowledge, no studies investigating a possible correlation between tibial under-coverage and implant failure exist.  However, it has been hypothesized that tibial under-coverage may be a causal factor in increased osteolysis, tibial subsidence and implant loosening, and led to pain and early implant failure.  Additionally, studies have shown that blood exudation from exposed bone sections not covered by prostheses were an important source of blood loss and that the control of bleeding was not amenable to methods such as electrocautery, ligature control, or the use of bone wax.  The authors suggested that further research should be made in this field.

Shroeder et al (2019) examined implant survivorship, patient satisfaction, and patient-reported functional outcomes at 2 years for patients implanted with a customized, posterior stabilized (PS) total knee replacement (TKR) system.  A total of 93 patients (100 knees) with the customized PS TKR were enrolled at 2 centers.  Patients’ length of hospitalization and pre-operative pain intensity were assessed.  At a single time-point follow-up, these researchers evaluated patient reported outcomes utilizing the KOOS Jr., satisfaction rates, implant survivorship, patients’ perception of their knee and their overall preference between the 2 knees, if they had their contralateral knee replaced with an off-the-shelf (OTS) implant.  At an average of 1.9 years, implant survivorship was found to be 100 %.  From pre-op until time of follow-up, these investigators observed an average decrease of 5.4 on the numeric pain rating scale.  Satisfaction rate was found to be high with 90 % of patients being satisfied or very satisfied, and  88 % of patients reporting a “natural” perception of their knee some or all the time.  Patients with bilateral implants mostly (12/15) stated that they preferred their customized implant over the standard TKR.  The evaluation of KOOS Jr. showed an average score of 90 at the time of the follow-up.  The authors concluded that based on these findings, they believed that the customized PS implant provided patients with excellent outcomes post-surgery.

Reimann et al (2019) stated that despite recent innovations in TKA, 20 % of the patients are not completely satisfied with the clinical results.  Regarding patient-specific implants (PSI), these investigators compared individual and off-the-shelf implant (OSI) TKA concerning the post-operative outcome like function and global patient satisfaction.  Between 2013 and 2014, a total of 228 patients received a TKA due to primary OA with an indication for a bicondylar, cruciate retaining prosthesis; 125 patients received a PSI and 103 an OSI TKA.  The outcome after surgery was evaluated retrospectively by 2 questionnaires and a clinical follow-up examination.  The Knee Society Score (KSS) was used to evaluate function.  To compare the satisfaction the Knee Injury and Osteoarthrosis Outcome Score (KOOS) and a modified EuroQol (EQ) including 5 additional questions were used.  Finally, 84 patients with PSI and 57 with OSI completed follow-up.  Concerning demographic data, the PSI group showed a significantly younger age, 5 years on average.  The ROM was comparable in both groups.  The KSS and the separate function score achieved significantly better results in the PSI group.  For subjects with PSI TKA, the global satisfaction showed significant better values.  The authors concluded that significantly higher values in KSS and its function score led to a better basic daily function in PSI group.  In addition, the PSI TKA achieved a higher global patient satisfaction.  Nevertheless, both should mainly be assessed in the context of average younger age and the influence of expectations.  The reason why patients with PSI TKA were more satisfied remained unclear because of study design.  These data cannot reveal whether it was because of prosthetic design or of other parameters like expectations and awareness of receiving an individual implant.  They stated that further studies that examine expectations, patient reported outcome measures (PROMs) and kinematics, in particular, are needed.

The authors stated that this study had some limitations.  Besides the retrospective design, there was no randomization and blinding.  In addition, the rate of drop-outs was quite high.  Hence, a selection bias cannot be certainly excluded.  Satisfied patients might be more willing to take part in a study with an examination compared to unsatisfied.  To the authors' knowledge, the study was one of the first to compare PROMs and objective clinical data in subjects with PSI TKA and conventional TKA on a larger scale. 

Buch et al (2019) stated that “Fast-Track” protocols have been introduced in total knee arthroplasty (TKA) with the intention to increase health care savings while maintaining or improving patient outcomes.  The influence of the implant design in a “Fast-Track” setting has not been described yet.  These investigators compared a customized implant with standard off-the-shelf (OTS) devices when utilizing a “Fast-Track” protocol.  A total of 62 patients were prospectively enrolled at a single-center and implanted with either a customized or a standard OTS implant resulting in 30 patients being treated with an OTS design (Columbus Total Knee System) and 32 with the customized design (iTotal®G2, Cruciate Retaining TKA, ConforMIS, Inc.,).  The same institutional fast-track protocol was utilized on all patients and included pre-, intra-, and post-operative medical treatment.  These researchers evaluated total length of stay (LOS), discharge destination and range of motion (ROM) at 6 to 8 weeks post-op and at an average of 16 months post-op follow-up to compare the OTS implant with the customized device.  Implant survivorship was assessed at a minimum of 25 months post-op.  Using the fast track protocol these researchers were able to decrease overall LOS to 2.1 days versus 3.6 days prior to introduction of the protocol.  The use of the customized implant further reduced LOS significantly to 1.6 days.  Significantly higher number of patients who got implanted with the customized device (66 %) were discharged within 24 hours than in the OTS group (30 %).  Patients treated with the customized implant were found to be discharged home more often than patients treated with the OTS implants (97 % versus 80 %) and achieved higher ROM both at 6-8 weeks (114° versus 101°) and at an average of 16 months (122° versus 114°) than patients who got treated with the OTS device.  At an average follow-up of 28 months, there was 1 implant revision in the customized group (due to tibial fracture resulting from patient fall).  For the OTS group there was 1 implant revision (late infection) and 1 poly swap (due to instability).  The authors stated that based on this analysis they observed a positive influence of the customized device on patient outcomes and hospital metrics and concluded that the implant choice is an important factor for TKA in a “fast-track” setting.

The authors believed this was the 1st study to compare the effect of the knee implant design on LOS and hospital metrics in a defined fast-track program.  They stated that this study was not without limitations that have to be taken into consideration when interpreting the results.  This study was performed prospectively with patients selecting the implant design.  Including blind randomization of the patient / component matching may have eliminated potential selection bias between the 2 study groups.  Thus, these researchers had little influence on the composition of the study cohorts that might have led to inequalities between the study groups.  However, since patient demographics and co-morbid conditions were similar and no statistically significant difference was detected between the 2 groups these investigators considered their findings to be valid.  With a total of 62 patients participated in this study the patient cohort was relatively small (n = 32 in the ConforMIS group).  Nevertheless, the differences observed between the groups were large enough to be of significance and the authors believed they would be similar for a larger study population.  These researchers suggested that further research with a larger study population should be carried out in the future.  For this study all TKAs were performed by a single surgeon who is experienced with all devices used.  Experience and a high expertise in performing TKA has been shown to result in better outcomes and additional studies at different sites should be conducted to verify if the implant design does have an impact on a faster discharge.  Lastly, fast track surgery can be implemented in multiple ways with the same guidelines but different protocols.  The authors noted that these findings only reflected the fast-track protocol they utilized in this study.  As there is no single definition of the “fast track protocol” in literature these investigators proposed that their protocol should be used in future research in order to validate their findings.

O'Connor et al (2019) noted that the amount of TKA procedures performed in the United States has been increasing steadily and is projected to reach 3 million procedures annually by 2030 in patients aged greater than or equal to 65 years.  A rise in TKA procedures will increase spending on osteoarthritis (OA) treatments, which is currently the 2nd highest category of spending for Medicare patients.  Because TKA procedures account for a substantial amount of total OA spending, payers and providers are examining methods to reduce spending on the procedure while improving clinical outcomes.  Customized individually made implants have been shown to improve clinical outcomes, such as physical function and limb alignment, compared with OTS implants; however, the economic impact of customized implants has yet to be established.  These investigators analyzed TKA episode expenditures among Medicare fee-for-service (FFS) members who received a customized or an OTS implant.  Members undergoing a TKA procedure using the customized implant technology were identified in the Medicare FFS database and were propensity matched (1:5) to a cohort of members who received OTS implants.  Reimbursement for the initial procedure (i.e., customized and OTS procedure), a pre-operative computed tomography (CT) scan, and 12-month post-operative healthcare utilization were analyzed.  The overall episode expenditures were used to construct a budget impact model to calculate the per-member per-month (PMPM) spending for Medicare FFS beneficiaries.  The average total episode spending was significantly lower among the customized implant cohort ($18,585) compared with the OTS implant cohort ($20,280; a $1,695 difference; p < 0.0001).  This savings resulted in $0.08 PMPM savings for the Medicare FFS program when a portion (10 %) of eligible members received the customized implant technology.  A sensitivity analysis, which varied with the customized implant market penetration and the percent of customized implant-eligible procedures, indicated that the savings could be as great as $0.28 PMPM.  The authors concluded that the findings of this study suggested that compared with OTS implants, customized knee implants can reduce healthcare spending among patients undergoing TKA.  These findings may help to determine the economic impact of customized knee implant technology on specific health plan populations.  In addition, the results may be of benefit for providers who are taking on financial risk for patients undergoing TKA procedures, such as those participating in accountable care organizations or bundled payment programs.  Moreover, these researchers stated that this study did not examine the financial impact of receiving a customized implant in a commercial population with TKA.  Given the positive findings in the Medicare population, a similar review is recommended to be completed in a commercial population among younger patients aged less than 65 years, because the findings may indicate that customized implants could also result in substantial savings for a commercial health plan.  It is also suggested that future studies conduct sub-analyses by sex, race, and co-morbidities to understand the economic impact on these specific populations.  (Funding for this study was provided by Conformis Inc., Billerica, MA; Dr. O'Connor is principal investigator of a clinical trial sponsored by Conformis and her institution receives research support from Conformis for that; Ms. Blau was a consultant to Conformis at the time of this study).

The authors stated that this study had several drawbacks.  Because medical coding does not distinguish between customized and OTS implants in administrative claims data, the customized implant cohort was identified through matching health plan members to multiple demographic and procedural characteristics of customized implant order numbers provided by the manufacturer to ensure that members who were identified as having a customized implant actually received the implant.  The coding methodology used could have limited impact on study findings.  The study selection criteria only allowed for exact matches; thus, there was an extremely low chance that a patient who did not receive a customized implant was included in the customized implant cohort.  However, it was possible for a patient who received a customized implant to be included in the OTS implant cohort if the patient did not receive a pre-operative CT scan in the out-patient setting, which was therefore not listed in the Medicare database.  Because the OTS implant cohort was selected from a large population (i.e., 228,697 procedures), the chance of incorrect categorization was low and was unlikely to have any impact on the study's results.  In addition, as a result of the conservative nature of patient selection used in the study, not all customized implant order numbers were identified in the Medicare FFS database and/or were included in the analysis.  Finally, the driving factor of the index procedure pay amount differences between the 2 cohorts was not identified during the analysis.  These researchers examined multiple factors that could potentially increase or decrease a hospital's diagnosis-relayed group (DRG) pay amount.  The factors examined by the authors included the percent of patients with a short stay (which reduced DRG payments in some instances), the outlier payments (made when hospital costs exceed a certain threshold), and the percent of patients whose index visit was classified with DRG code 469 (reimbursed at a higher rate than DRG code 470).  Although the results of these analyses suggested that each factor may slightly contribute to the index differences observed between the 2 cohorts, no one factor made a meaningful impact that fully explained these differences.

Namin et al (2019) examined the impact of insurance coverage on the adoption of customized individually made (CIM) knee implants and compared patient outcomes and cost-effectiveness of OTS and CIM implants.  A system dynamics simulation model was developed to study adoption dynamics of CIM and meet the research objectives.  The model reproduced the historical data on primary and revision knee replacement implants obtained from the literature and the Nationwide Inpatient Sample.  Then the dynamics of adoption of CIM implants were simulated from 2018 to 2026.  The rate of 90-day re-admission, 3-year revision surgery, recovery period, time savings in operating rooms, and the associated cost within 3 years of primary knee replacement implants were used as performance metrics.  The simulation results indicated that by 2026, an adoption rate of 90 % for CIM implants can reduce the number of re-admissions and revision surgeries by 62 % and 39 %, respectively, and can save hospitals and surgeons 6 % on procedure time and cut down cumulative healthcare costs by approximately $38 billion.  The authors concluded that CIM implants have the potential to deliver high-quality care while decreasing overall healthcare costs, but their adoption requires the expansion of current insurance coverage.  This work presented the 1st systematic study to understand the dynamics of adoption of CIM knee implants and instrumentation.  More broadly, the current modeling approach and systems thinking perspective could be used to consider the adoption of any emerging customized therapies for personalized medicine.

The authors noted that this dynamics simulation model has several limitations.  First, the current simulation model, like most models, cannot portray full reality, but the validated model can potentially help uncover complexities in the healthcare system around TKAs.  The analyses compared the relative potential of different insurance policies rather than predicting precisely the long-term effect of these policies.  Second, the simulation model did not consider indirect costs and delays associated with administrative processes.  Indirect costs may include lost wages due to patients’ disability from the procedures.  Administrative processes may include delays due to the FDA approval process and bureaucratic burdens of ordering system.  All hospital entities have to use FDA-approved medical devices; however, FDA regulations for 3-D printed medical devices are expected to increase in the near future, which could put increased pressure on the adoption of these products.  In this model, these researchers assumed that the FDA would approve new CIM implant manufacturers and their products within a period of 4 months.  Complexity of ordering system may include selection of implant (partial, total, cruciate retaining, etc.) and transferring the CT data to a manufacturer that could cause bureaucratic burdens and limit the adoption.  These investigators considered performance improvements of CIM implants, the design phase and the use phase during surgery, as a “moving target” since the evaluation process takes time and may not reflect the latest effects of product modifications on performance.  OTS implants have been on the market for a long time, and 3-D printed patient-specific surgical guidance for OTS implants and robotically assisted surgery have enhanced their improvements up to the present; CIM implants were introduced only a few years ago.  For this reason, these researchers considered the potentials for improvements of CIM implants in the design phase and the use phase during surgery to be 5 % per year: 2.5 % higher than OTS.  However, to increase the confidence in the model, sensitivity analyses were done on the performance improvement assumptions for each type of implant.  According to the sensitivity analysis results presented, the model is relatively robust to changes in performance improvements.  In addition, the online simulator platform provides decision-makers with the flexibility to incorporate various performance improvement rates for either type of implant (OTS or CIM), initially or midway through the simulation run, and observe the results.  Out-patient total joint arthroplasty has become more popular in recent years because of the economic benefits due to lower costs associated with reduced LOS; CMS removed TKA from inpatient-only list beginning January 2018.  However, according to the American Association of Hip and Knee Surgeons (AAHKS), outpatient TKA should only be utilized for patients who are healthy enough to have a procedure in such settings.  The patient should also have an appropriate home support for being discharged with no hospitalization.  Similar to any other episode of care, there are advantages and disadvantages associated with outpatient TKA, i.e., reduced costs and discharge on the day of surgery that could lead to either patient satisfaction or dissatisfaction if it causes more complications such as implant failure, stiffness, more re-admissions and potentially revision surgeries.  In this generic model, these researchers considered that OTS and CIM implants can be used in either inpatient or outpatient setting uniformly, however, if the dynamic changes and more patients become interested in outpatient procedures, the model can be expanded to distinguish between inpatient and outpatient settings.

Prophylactic Radiation Therapy Following Total Knee Arthroplasty

Chidel and colleagues (2001) stated that heterotopic ossification (HO) occurs in 42 % of patients who have undergone total knee arthroplasty (TKA).  Bone formation usually is found in the quadriceps expansion and causes minimal to no symptoms.  Specific therapy usually is unnecessary, but cases have been reported in which manipulation under anesthesia (MUA) or revision arthroplasty has been required.  These investigators reported a small series of 5 patients (6 knees) who have undergone surgical intervention for HO of the knee with radiotherapy given post-operatively for prophylaxis against future HO.  The authors concluded that although this series was small, it appeared that the use of prophylactic radiation may reduce recurrence after resection of symptomatic HO after TKA.  Moreover, they stated that further investigation is needed to confirm these preliminary findings.

Farid and associates (2013) noted that therapeutic options for arthrofibrosis following TKA include MUA, open or arthroscopic arthrolysis, and revision surgery to correct identifiable problems.  These investigators proposed pre-operative low-dose irradiation and Constrained Condylar or Rotating-hinge revision for severe, idiopathic arthrofibrosis.  Irradiation may decrease fibro-osseous proliferation while constrained implants allow femoral shortening and release of contracted collateral ligaments.  A total of 14 patients underwent 15 procedures for a mean overall motion of 46° and flexion contracture of 30°; 1 patient had worsening range of motion (ROM) while 13 patients had 57° mean gain in ROM (range of 5° to 90°).  Flexion contractures decreased by a mean of 28°.  There were no significant complications at a mean follow-up of 34 months (range of 24 to 74 months).

Furthermore, an UpToDate review on “Total knee arthroplasty” (Martin and Crowley, 2018) does not mention radiation therapy/radiotherapy for post-operative management.

Patient-Specific Implants / Patient-Specific Cutting Guides

In a retrospective study, Nunley et al (2012) examined if the mean coronal alignment following TKA performed with conventional versus patient-specific instrumentation (PSI) would better restore the mechanical and kinematic axes and whether there were more outliers with one of the two methods.  The investigators examined 150 primary TKAs carried out for osteoarthritis (OA): Group 1 (n = 50) conventional instrumentation; Group 2 (n = 50) PSI restoring the mechanical axis; Group 3 (n = 50) PSI restoring the kinematic axis, and measured femorotibial angle, hip-knee-ankle angle, and the zone of the mechanical axis from scout CT images taken 0 to 6 weeks post-operatively.  The mean femorotibial angle differed between the groups: Group 1 had the greatest varus mean alignment and most varus outliers.  The mean hip-knee angle was similar between Groups 1 and 2, with Group 3 having greater valgus mean alignment and the most valgus outliers.  For the zone of the mechanical axis, Groups 1 and 2 had similar percentages of outliers (40 % versus 32 %), whereas Group 3 had a greater number of outliers (64 %) that were valgus.  TKAs with PSI restoring the mechanical axis had a similar number of outliers as conventional instrumentation with both groups having more varus outliers than TKAs with PSI restoring kinematic axis, which had more valgus outliers.  The authors concluded that additional studies are needed to examine if PSI would improve clinical function or patient satisfaction and whether their routine use can be justified in primary TKA.

In a prospective, randomized trial comparing PSI and conventional instrumentation, Parratte (2013) reported the findings of 40 patients (20 in each group) operated in the authors’ institution between September 2012 and January 2013.  Randomization of patients into one of the two groups was carried out by the Hospital Informatics Department with the use of a systematic sampling method.  All patients received the same cemented high-flex mobile bearing TKA.  In the PSI group, implant position was compared to the planed position using previously validated dedicated software.  The position of the implants (frontal and sagittal) was compared in the 2 groups on standard X-rays, and the rotational position was analyzed on post-operative CT-scan; 90 % of the patients added less than 2° or mm of difference between the planned position of the implants and the obtained position, except for the tibial rotation where the variations were much higher.  Mean HKA was 179° (171 to 185) in the PSI group with 4 outliers (2 varus: 171° and 172°:184° and 185°) and 178.3° with 2 outliers (171° and 176°) in the control group.  No difference was observed between the 2 groups concerning the frontal and sagittal position of the implants on the ML and AP X-rays.  No significant difference of femoral rotation was observed between the 2 groups with a mean of 0.4° in the PSI group and 0.2° in the control group (p: non-significant).  Mean tibial rotation was 8° of internal rotation in the PSI group and 15° of internal rotation in the standard group (p: non-significant).  The authors concluded that, based on these results, they were unable to confirm their hypothesis as PSI could not improve rotation in TKA.  These researchers stated that further investigation is needed to define the place of PSI in TKA, to keep on improving the accuracy of the system and to better define the individual targets in TKA in terms of frontal, sagittal and rotational positioning of the implant for each patient.

In a prospective, double-blind, randomized controlled trial (RCT), Boonen et al (2013) addressed the following questions: First, is there a significant difference in outliers in alignment in the frontal and sagittal plane between PSG TKA and conventional TKA.  Second, is there a significant difference in operation time, blood loss and hospital length of stay (LOS) between the 2 techniques.  These researchers hypothesized that there will be fewer outliers with PSG TKA and that operation time, blood loss and hospital LOS could be significantly reduced with PSG.  A total of 180 patients were randomized for PSG TKA (group 1) or conventional TKA (group 2) in 2 centers; patients were stratified per hospital.  Alignment of the mechanical axis of the leg and flexion/extension as well as varus/valgus of the individual prosthesis components were measured on digital, standing, long-leg and standard lateral radiographs by two independent outcome assessors in both centers.  Percentages of outliers (greater than 3°) were determined.  The investigators compared blood loss, operation time and hospital LOS: There was no statistically significant difference in mean mechanical axis or outliers in mechanical axis between groups.  No statistically significant difference was found for the alignment of the individual components in the frontal plane nor for the percentages of outliers.  There was a statistically significant difference in outliers for the femoral component in the sagittal plane, with a higher percentage of outliers in the group 1 (p = 0.017).  No such significant result was found for the tibial component in that plane.  All inter-class correlation coefficients were good.  Blood loss was 100 ml less in group 1 (p < 0.001).  Operation time was 5 mins shorter in group 1 (p < 0.001).  Hospital LOS was identical with a mean of 3.6 days (p = 0.657).  The authors concluded that the results in terms of obtaining a neutral mechanical axis and a correct position of the prosthesis components did not differ between groups.  A small reduction in operation time and blood loss was found with the PSG system.  These investigators stated that future research should focus on cost-effectiveness analysis and functional outcome of PSG TKA.

Russell et al (2014) carried out a meta-analysis of level I and level II studies to examine if PSI would improve the mechanical alignment of the leg compared with conventional instrumentation (CI) in TKA.  A total of 7 studies met inclusion criteria examining 559 patients undergoing TKA.  Mean coronal alignment was within 1 degree of neutral mechanical alignment in both groups (PSI, 0.78 degrees; CI, 0.81 degrees).  There were fewer outliers in the PSI group (21.1 %) than in the CI group (23.2 %); however, this was not statistically significant (p = 0.59).  The authors concluded that based on the data from this analysis, PSI did not significantly improve the post-operative mechanical alignment of the limb after TKA; moreover, PSI did not decrease the number of outliers compared with CI.

Marimuthu et al (2014) observed that patient specific guides (PSGs) are postulated to improve the alignment of components in TKA.  In this study, a total of 300 consecutive TKAs carried out with either conventional (CON) (n = 185) or Visionaire PSG (n = 115) were examined with a CT protocol for coronal limb alignment, coronal and sagittal alignment of individual components and femoral component rotation.  There was no statistically significant difference between the 2 groups in any of the above parameters.  Furthermore, no difference was observed in total operative time.  The authors concluded that PSGs did not offer any benefit over conventional guides in terms improving the coronal alignment of the limb or alignment of individual components.

In a retrospective study, Daniilidis and Tibesku (2014) compared the ability of standard instrumentation (SI) and patient-matched cutting blocks (PMCB) to achieve a hip-knee-ankle angle (HKA) within ± 3° of the ideal alignment of 180°.  Between October 2009 and December 2012, a total of 170 TKAs in 166 patients (4 bilateral) using VISIONAIRE (Smith & Nephew) PMCB technology were carried out.  In addition, 160 TKAs in 160 consecutive patients that had received a TKA using SI during the same time period were used as a control group.  All surgeries were performed by the same surgeon.  Standardized pre- and post-operative long-leg standing X-rays were evaluated to compare the 2 patient cohorts.  X-rays were available for analysis for 156 knees in the SI group and 150 in the PMCB group.  The average post-surgical HKA was 178.7 ± 2.5 in the SI group and 178.4 ± 1.5 in the PMCB group.  However, the rates of ± 3° outliers were 21.2 % in the SI group and 9.3 % in the PMCB group. There were no intra-operative complications with the use of PMCB technology or SI.  The authors concluded that PMCB technology proved superior to conventional instrumentation in achieving a neutral mechanical axis following TKA.  Moreover, these researchers stated that further follow-up are needed to ascertain the long-term impact of these findings.

The authors stated that this analysis had several drawbacks.  First, although a control arm was offered as means of a comparison, there were notable differences in the demographics between the 2 groups (e.g., more male patients in the PMCB group).  It was possible that these factors influenced the overall results.  Second, this trial exclusively dealt with radiographic outcomes.  Expanding the outcomes to other important aspects of TKA such as pain, stiffness, and ROM would have provided important further data with which to compare these 2 separate strategies.  PSI has also been shown to shorten surgical steps and operative times, reduce the burden for surgical instrumentation, and lessen adverse outcomes such as blood loss, infection, and systematic fat emboli.  Thus, the proposed use of this technology should not be limited to radiographic aspects.  Third, it must be noted that the operating surgeon performed approximately 550 knee replacements annually and it was possible that this high level of experience positively influenced the results.  The learning curve for beginner surgeons has been shown to be acceptably low with computer-assisted navigation systems, with the exception of a general increase in operative and tourniquet times during initial cases.  Fourth, these researchers did not perform a power-analysis.  Additional research is needed, however, to examine if less-experienced surgeons would encounter a similar learning curve with this particular PSI technology.

Moubarak and Brilhault (2014) examined the contribution of PSI to post-operative lower limb alignment.  This trial entailed primary TKA cases being carried out with patient-specific cutting guides (PSCG) between October 5, 2010 and May 3, 2013 and then followed prospectively.  The analysis involved the PSCG usage and post-operative measurement of the patient's HKA, medial distal femoral joint angle (MDFA) and medial proximal tibia joint angle (MPTA).  Of the 104 eligible cases, 68 were included; 11 of these cases were not performed completely with the PSCG as initially planned.  Therefore, these investigators compared these 11 cases with the 57 where PSCG were used.  The pre-operative HKA in the included cases was 175.8° ± 7.8; the post-operative angles on average were 179.2° ± 2.9 for the HKA, 89.9° ± 1.6 for the MDFA and 89.0° ± 2.3 for the MPTA.  The average post-operative deviation from the target values was 2.22° ± 2.14 for the HKA angle, 1.07° ± 1.15 for the MDFA and 1.66° ± 1.90 for the MPTA.  There were no significant differences between the 2 groups in any of the measurements.  The lower limb alignment goal was achieved in 50 cases (73 %), with 41 of these achieved with PSCG (82 %).  Of the 18 cases where the target was not achieved, PSCG were used 16 times (88% ).  The authors concluded that lower limb alignment was not significantly closer to an HKA of 180° or achieved more often with the use of PSCG versus standard instrumentation.  Since the results of the 2 groups could be superimposed, the investigators found no evidence that use of PSCG improved post-operative lower limb alignment.

In a randomized, clinical trial, Pfitzner et al (2014) compared the accuracy of MRI- and CT-based PSI with CI and with each other in TKAs.  The 3 approaches also were compared with respect to validated outcomes scores and duration of surgery.  A total of 90 patients were enrolled and divided into 3 groups: CT-based, MRI-based PSI, and CI.  The groups were not different regarding age, male/female sex distribution, and BMI.  In all groups, coronal and sagittal alignments were measured on post-operative standing long-leg and lateral radiographs.  Component rotation was measured on CT scans.  Clinical outcomes (Knee Society and WOMAC scores) were evaluated pre-operatively and at a mean of 3 months post-operatively and the duration of surgery was analyzed for each patient.  MRI- and CT-based PSI groups were first compared with CI, the PSI groups were compared with each other, and all 3 approaches were compared for clinical outcome measures and duration of surgery.  Compared with CI MRI- and CT-based PSI showed higher accuracy regarding the coronal limb axis (MRI versus conventional, 1.0° [range of 0° to 4°] versus 4.5° [range of 0° to 8°], p < 0.001; CT versus conventional, 3.0° [range of 0° to 5°] versus 4.5° [range of 0° to 8°], p = 0.02), femoral rotation (MRI versus conventional, 1.0° [range of 0° to 2°] versus 4.0° [range of 1° to 7°], p < 0.001; CT versus conventional, 1.0° [range of 0° to 2°] versus 4.0° [range of 1° to 7°], p < 0.001), and tibial slope (MRI versus conventional, 1.0° [range of 0° to 2°] versus 3.5° [range of 1° to 7°], p < 0.001; CT versus conventional, 1.0° [range of 0° to 2°] versus 3.5° [range of 1° to 7°], p < 0.001), but the differences were small.  In addition, MRI-based PSI showed a smaller deviation in the post-operative coronal mechanical limb axis compared with CT-based PSI (MRI versus CT, 1.0° [range of 0° to 4°] versus 3.0° [range of 0° to 5°], p = 0.03), while there was no difference in femoral rotation or tibial slope.  Although there was a significant reduction of the duration of surgery in both PSI groups in comparison to CI (MRI versus conventional, 58 mins [range of 53 to 67 mins] versus 76 mins [range of 57 to 83 mins], p < 0.001; CT versus conventional, 63 mins [range of 59 to 69 mins] versus 76 mins [range of 57 to 83 mins], p < 0.001), there were no differences in the post-operative Knee Society pain and function and WOMAC scores among the groups.  The authors concluded that although the findings of this study supported that PSI increased accuracy compared with CI and that MRI-based PSI was more accurate compared with CT-based PSI regarding coronal mechanical limb axis, differences were only subtle and of questionable clinical relevance.  Moreover, these researchers stated that because there are no differences in the long-term clinical outcome or survivorship yet available, the widespread use of this technique could not be recommended.

Abane (2015) randomized 140 patients who were due to undergo primary TKA to have the procedure performed using either PSCG or CI.  The primary outcome measure was the mechanical axis, as measured at 3 months on a standing long-leg radiograph by the HKA angle.  This was undertaken by an independent observer who was blinded to the instrumentation.  Secondary outcome measures were component positioning, operating time, Knee Society and Oxford knee scores, blood loss and hospital LOS.  A total of 126 patients (67 in the CI group and 59 in the PSCG group) had complete clinical and radiological data.  There were 88 women and 52 men with a mean age of 69.3 years (47 to 84) and a mean BMI of 28.6 kg/m(2) (20.2 to 40.8).  The mean HKA angle was 178.9° (172.5 to 183.4) in the CI group and 178.2° (172.4 to 183.4) in the PSCG group (p = 0.34).  Outliers were identified in 22 of 67 knees (32.8 %) in the CI group and 19 of 59 knees (32.2% ) in the PSCG group (p = 0.99).  There was no significant difference in the clinical results (p = 0.95 and 0.59, respectively).  Operating time, blood loss and hospital LOS were not significantly reduced (p = 0.09, 0.58 and 0.50, respectively) when using PSCG.  The authors concluded that the use of PSCG in primary TKA did not reduce the proportion of outliers as measured by post-operative coronal alignment.

Nankivell et al (2015) examined the Visionaire patient-specific system.  The thickness of actual bone resections was compared with the predicted thickness (giving a resection 'error').  Data were also obtained on the number of trays used, skin-to-skin operating time and tourniquet time.  A total of 41 TKRs were performed on 33 females (1 bilateral) and 7 males.  Average resection errors were 0.22 mm medially and 0.05 mm laterally for the distal femur, 0.99 mm medially and 0.74 mm laterally for posterior femoral condyles, and 0.55 mm medially and 0.71 mm laterally for the proximal tibia.  There were no significant differences in tourniquet time, skin-to-skin time or the number of trays used between the patient-specific and historical comparison groups.  The authors concluded that PSCG made accurate resections.  Operative and tourniquet times and the number of trays used were no different to standard TKRs.  These researchers stated that further investigation is needed to examine if PSCG would improve post-operative alignment and patient satisfaction.

Schwarzkopf and colleagues (2015) stated that TKA instrumentation and implant designs have been evolving, with one of the current innovations being patient-specific implants (PSIs).  In a retrospective, cohort study, these researchers examined if there is a significant difference in surgical time, intra-operative blood loss, post-operative ROM, and LOS between PSI and conventional TKA.  A consecutive series of 621 TKA patients, 307 with PSIs and 314 with conventional implants, was reviewed.  Differences in estimated blood loss, LOS, ROM, and surgical time/tourniquet time between the 2 cohorts were analyzed.  Linear regression analysis demonstrated that PSI decreased estimated blood loss by 44.72 ml (p < 0.01), decreased LOS by 0.39 days (p < 0.01), decreased post-operative ROM by 3.90° (p < 0.01), and had a negligible difference on surgical and tourniquet time.  The authors concluded that the use of PSI was associated with decreased estimated blood loss, decreased LOS, decreased ROM, and no discernible difference in surgical or tourniquet time, all of which were unlikely to be clinically significant.  These researchers stated that future studies need to address quality of life (QOL) and patient-reported functional outcome measurements between the 2 cohorts.  Level of Evidence = III.

The authors stated that this study had several drawbacks.  First, the retrospective nature of this study limited their ability to uniform the measurement criteria of the different evaluated variables, thus leading to possible bias.  This study used data for the 2 cohorts from different time periods.  The conventional implants were performed between January 2008 and December 2010, while the PSIs were performed between January 2011 and June 2013.  This could allow for possible bias due to factors such as improved pain management and improved protocols.  This study focused on interpreting the differences in ROM as a measure of difference between the PSI and conventional implants.  Because of the retrospective nature of the study, there was no standardized approach for documenting both pre- and post-surgical ROM measurements.  By not following a specific protocol for collecting ROM values, there was possibly a variability in measurements based on how ROM was measured and by whom.  For future studies, it would be more meaningful to address these issues in a prospective study to lessen the amount of variation.  In the future, it would be important to address factors such as QOL and patient-reported outcome measures.  Ideally, a follow-up study should address these measurements in a prospective, randomized controlled fashion.  These investigators noted that the findings of this trial demonstrated statistically significant differences in estimated blood loss and LOS; however, it appeared that these differences may not represent any clinically significant differences.  This was best explained by the negligible change in estimated blood loss (44.72 ml) and LOS (0.39 days) between cohorts.  Nonetheless, whether these intra-operative and post-operative benefits of individualized implants were truly associated with long-term favorable outcomes for patients remains to be evaluated.  That post-operative increase in ROM was less in the patient-specific group compared with the conventional group suggested that further analysis of other parameters such as post-operative knee score, pain score, and alignment will be useful in determining any long-term advantage of customized instruments and implants.  Of note, conventional instrumentation may be associated with increased operative times and intra-operative blood loss due to its reliance on manual intra-medullary alignment guides.  Furthermore, patient-specific implants provided greater bone coverage, thus eliminating exposed bone, and may contribute to decreased post-operative blood loss compared with conventional implants.

In a RCT, Huijbregts et al (2016a) examined the accuracy of positioning and alignment of the components in TKA, comparing those undertaken using standard intra-medullary cutting jigs and those PSI.  There were 64 TKAs in the standard group and 69 in the PSI group.  The post-operative HKA angle and positioning was examined using CT scans.  Deviation of greater than 3° from the planned position was regarded as an outlier.  The operating time, Oxford Knee Scores (OKS) and Short Form-12 (SF-12) scores were recorded.  There were 14 HKA-angle outliers (22 %) in the standard group and 9 (13 %) in the PSI group (p = 0.251).  The mean HKA-angle was 0.5° varus in the standard group and 0.2° varus in the PSI group (p = 0.492).  The accuracy of alignment in the coronal and axial planes and the proportion of outliers was not different in the 2 groups.  The femoral component was more flexed (p = 0.035) and there were significantly more tibial slope outliers (29 % versus 13 %) in the PSI group (p = 0.032).  Operating time and the median 3-month OKS were similar (p = 0.218 and p = 0.472, respectively).  Physical and mental SF-12 scores were not significantly different at 3 months (p = 0.418 and p = 0.267, respectively) or at 1 year post-operatively (p = 0.114 and p = 0.569).  The median 1-year OKS was 2 points higher in the PSI group (p = 0.049).  the authors concluded that compared with standard intra-medullary jigs, the use of PSI did not significantly reduce the number of outliers or the mean operating time, nor did it clinically improve the accuracy of alignment or the median OKS; these findings did not support the routine use of PSI when undertaking TKA.

Huijbregts et al (2016b) noted that patient-specific instrumentation (PSI) for TKA has been introduced to improve alignment and reduce outliers, increase efficiency, and reduce operation time.  In order to improve the understanding of the outcomes of PSI, these researchers conducted a meta-analysis.  They identified randomized and quasi-randomized controlled trials (RCTs) comparing patient-specific and conventional instrumentation in TKA.  Weighted mean differences (WMDs) and risk ratios (RRs) were determined for radiographic accuracy, operation time, hospital stay, blood loss, number of surgical trays required, and patient-reported outcome measures.  A total of 21 RCTs involving 1,587 TKAs were included.  Patient-specific instrumentation resulted in slightly more accurate hip-knee-ankle axis (0.3°), coronal femoral alignment (0.3°, femoral flexion (0.9°), tibial slope (0.7°), and femoral component rotation (0.5°).  The RR of a coronal plane outlier (greater than 3° deviation of chosen target) for the tibial component was statistically significantly increased in the PSI group (RR =1.64).  No significance was found for other radiographic measures.  Operation time, blood loss, and transfusion rate were similar.  Hospital stay was significantly shortened, by approximately 8 hours, and the number of surgical trays used decreased by 4 in the PSI group.  Knee Society scores and Oxford knee scores were similar.  The authors concluded that PSI did not result in clinically meaningful improvement in alignment, fewer outliers, or better early patient-reported outcome measures.  Efficiency is improved by reducing the number of trays used, but PSI did not reduce operation time.

Nam et al (2016a) noted that custom cutting guides (CCGs) in TKA use pre-operative 3-D imaging to manufacture cutting blocks specific to a patient's anatomy.  In a retrospective, multi-center study, these researchers examined the impact of CCGs versus standard intra-medullary and extra-medullary guides on patient-reported satisfaction and residual symptoms following TKA.  This trial compared a MRI-based CCG system versus standard instrumentation.  All patients received the same, cemented, fixed-bearing, cruciate-retaining component, and had a primary diagnosis of OA.  Data was collected by an independent, 3rd party survey center blinded to surgical technique that administered telephone questionnaires assessing patient satisfaction and symptoms.  Patient age, gender, minority status, education level, income, length of follow-up, and pre-arthritic UCLA scores were considered potential confounders and accounted for using multivariate logistic regression analyses.  A total of 448 patients (107 CCGs, 341 standard) were successfully interviewed.  At a mean follow-up of 3 years, there was no difference in percentage of patients reporting their knee to feel "normal" (74 % CCG versus 78 % standard, p = 0.37).  Residual symptoms including knee stiffness (37 % CCG versus 28 % standard, p = 0.08) and difficulty getting in and out of car (34 % CCG versus 30 % standard, p = 0.40) remained high.  Multi-variate regression analyses demonstrated no differences between the 2 cohorts for both patient-reported satisfaction and residual symptoms (OR 0.72 to 1.48; p = 0.10 to 0.81).  The authors concluded that when interviewed by an independent, blinded 3rd party, the use of CCGs in TKA did not improve patient-reported satisfaction or residual symptoms versus the use of standard alignment guides.

In a retrospective study, Nam et al (2016b) examined if CCGs would improve clinical outcomes as measured by UCLA activity, SF-12, and OKS; and determined coronal mechanical alignment versus standard alignment guides.  This trial included patients undergoing primary TKA using the same cruciate-retaining, cemented TKA system between January 2009 and April 2012.  Patients were included if they were candidates for a unilateral, cruciate-retaining TKA and met other pre-specified criteria; patients were allowed to self-select either an MRI-based CCG procedure or standard TKA; 97 of 120 (80.8 %) patients in the standard and 104 of 124 (83.9 %, p = 0.5) in the CCG cohort with a minimum of 1-year follow-up were available for analysis.  The first 95 patients in the standard (mean follow-up of 3 years; range of 1 to 4 years) and CCG (mean follow-up of 2 years; range of 1 to 4 years) cohorts were compared.  The alignment goal for all TKAs was a HKA angle of 0°. UCLA, SF-12, and OKS were collected pre-operatively and at each patient's most recent follow-up visit.  Post-operatively, rotationally controlled coronal scout CT scans were used to measure HKA alignment.  Independent-sample t-tests and Chi-square tests were used for comparisons with a p value ≤ 0.05 considered significant.  At the most recent follow-up, no differences were present between the 2 cohorts for ROM (114° ± 14° in CCG versus 115° ± 15° in standard, p = 0.7), UCLA (6 ± 2 in CCG versus 6 ± 2 in standard, p = 0.7), SF-12 physical (44 ± 12 in CCG versus 41 ± 12 in standard, p = 0.07), or OKS (39 ± 9 in CCG versus 37 ± 10 in standard, p = 0.1).  No differences were present for the incremental improvement in the UCLA (1 ± 4 in CCG versus 1 ± 3 in standard, p = 0.5), SF-12 physical (12 ± 20 in CCG versus 11 ± 21, p = 0.8), or OKS (16 ± 9 in CCG versus 19 ± 10 in standard, p = 0.1) from preoperatively to postoperatively. There was no difference in the percentage of outliers for alignment (23% in standard versus 31% in CCG with HKA outside of 0° ± 3°; p = 0.2) between the two cohorts.  At a mean follow-up of greater than 2 years, CCGs failed to demonstrate any advantages in validated knee outcome measure scores, or coronal alignment as measured by CT scan versus the use of standard instrumentation in TKA.  The authors concluded that the clinical benefit of CCGs must be proven before continued implementation of this technology.

Pourgiezis et al (2016) compared patient-matched instrumentation (PMI) with CI TKA in terms of limb alignment and component position.  A total of 9 men and 36 women (mean age of 69.5 years) who underwent PMI TKA were compared with 20 men and 25 women (mean age of 69.3 years) who underwent CI TKA by the same team of surgeons with the same prosthesis and protocols in terms of limb alignment and component position using the Perth protocol CT, as well as bone resection measurements, operating time, and the number of trays used.  The PMI and CI TKA groups were comparable in terms of age, BMI, tourniquet time, operating time, and the number of trays used.  For limb alignment and component position, the 2 groups differed significantly in sagittal femoral component position (2.4º versus 0.9º, p = 0.0008) and the percentage of knees with femoral component internally rotated 1° or more with respect to the trans-epicondylar axis (20 % versus 55 %, p = 0.001).  The difference was not significant in terms of limb alignment, coronal and rotational femoral component position, or coronal and sagittal tibial component position.  Intra-operatively, all patient-matched cutting blocks showed acceptable fit and stability.  No instrument-related AEs or complications were encountered; 1 (2.2 %) femur and 6 (13.3 %) tibiae were recut 2 mm for optimal ligament balancing; 2 femoral components were up-sized to the next size, and 2 tibial components were up-sized and 2 down-sized to the next size.  The authors concluded that PMI was as accurate as CI in TKA.  There was no significant difference in limb alignment or femoral and tibial component position in the coronal and sagittal planes between PMI and CI TKA; PMI had a higher tendency to achieve correct femoral component rotation.

Predescu et al (2017) reported on their experience comparing 2 different TKA techniques using PSI with the Visionaire knee and CI from the same system (Genesis II Smith & Nephew).  A total of 80 knees were divided into 2 equal groups, 40 PSI and 40 CI respectively, operated between April 2013 and August 2014.  One female patient had bilateral TKR during this period, at 6 months interval, both with the PSI.  All operated knees had varus deformity, with a mean HKA of 168° (PSI) versus 163° (CI).  These researchers used tranexamic acid (double-dose scheme) and suction drains for 48 hours, with a mean blood drainage in the PSI group of 185 ml and Hb levels of 11.2 g/dL at 3 days post, compared to 260 ml and 10.7 g/dL in the CI.  Mean blood loss was 3.5 g/dL in PSI, and 4.2 g/dL in the CI.  On the long leg standing radiograph at 6 weeks, all knees were aligned in frontal plane,  with similar HKA values (178.9° PSI versus 178.6° CI).  Bone cuts measured intra-operatively proved to be accurate within a 1 mm limit.  The authors concluded that they could not recommend PSI-TKR for a better outcome.  It was an alternative to conventional and computer-assisted TKR; however, further studies are needed to examine if surgical or economic benefits may be achieved by choosing customized instruments.

In a RCT, Young et al (2017) addressed 3 research questions.  First, would 2-year patient-reported outcome scores be enhanced in patients with kinematic alignment (KA) compared with a mechanical alignment (MA) technique?  Second, if post-operative component alignment would differ between the techniques?  Third, is the proportion of patients undergoing re-operation at 2 years different between the techniques?  A total of 99 primary TKAs in 95 patients were randomized to either MA (n = 50) or KA (n = 49) groups.  A pilot study of 20 TKAs was carried out before this trial using the same patient-specific guides positioning in KA.  In the KA group, PSCGs were manufactured using individual pre-operative MRI data.  In the MA group, computer navigation was employed to ensure neutral MA accuracy.  Post-operative alignment was assessed with CT scan, and functional scores (including the PKS, WOMAC, and the Forgotten Joint Score) were assessed pre-operatively and at 6 weeks, 6 months, and 1 and 2 years post-operatively.  No patients were lost to follow-up.  The investigators set sample size at a minimum of 45 patients per treatment-arm based on a 5-point improvement in the mean OKS; the previously reported minimum clinically significant difference for the OKS in TKA, a pooled SD of 8.3, 80 % power, and a 2-sided significance level of 5 %.  These researchers observed no difference in 2-year change scores (post-operative minus pre-operative score) in KA versus MA patients for the OKS (mean of 21, SD 8 versus 20, SD 8, least square means 1.0, 95 % CI: -1.4 to 3.4, p = 0.4), WOMAC score (mean of 38, SD 18 versus 35, SD 8, least square means 3, 95 % CI: -3.2 to 8.9, p = 0.3), or Forgotten Joint score (28 SD 37 versus 28, SD 28, least square means 0.8, 95 % CI: -9.1 to 10.7, p = 0.8).  Post-operative HKA axis was not different between groups (mean KA 0.4° varus SD 3.5 versus MA 0.7° varus SD 2.0).  However, in the KA group, the tibial component was a mean 1.9° more varus than the MA group (95 % CI: 0.8° to 3.0°, p = 0.003) and the femoral component in 1.6° more valgus (95 % CI: -2.5° to -0.7°, p = 0.003).  Complication rates were not different between groups.  The investigators found no difference in 2-year patient-reported outcome scores in TKAs implanted using the KA versus an MA technique.  The theoretical advantages of improved pain and function that formed the basis of the design rationale of KA were not observed in this study.  The authors concluded that it was unclear if the alterations in component alignment observed with KA will compromise long-term survivorship of TKA.  In this study, these researchers were unable to demonstrate an advantage to KA in terms of pain or function that would justify this risk.

In a RCT, Vide et al (2017) compared PSI to standard instrumentation regarding the effectiveness to achieve a good coronal alignment and differences in surgical time, blood loss and LOS.  A total of 95 of 100 randomized patients eligible for TKA were analyzed.  PSI with MRI and long-leg X-ray were carried out in 47 patients, while 48 patients received standard instrumentation.  Primary outcome measure was coronal alignment, evaluated with long-leg X-ray.  Deviation of greater than 3° varus/valgus was considered an outlier.  Surgical time was compared from skin to skin; LOS was a post-hoc analysis.  Blood loss was evaluated comparing the number of blood units spent, fall in hemoglobin (Hb) and hematocrit (Hct)levels.  Standard instrumentation had a higher number of outliers in the coronal alignment with a RR of 3.015, compared to PSI.  Surgical time was reduced by 18 mins (24.8 %) with the PSI, as well as LOS, with a half-day reduction.  Number of blood units spent was significantly less in the PSI group.  Relative risk of transfusion was 7.09 for patients in the standard instrumentation group.  Difference in Hb and Hct levels were not significant.  No patient had to abandon PSI.  Minor changes to pre-operative plan occurred in 14.9 % of the patient: cut review in 4.3 % and insert change in 10.6 %.  The authors concluded that PSI was able to provide important advantages over standard instrumentation in TKA: it lowered the risk of outliers and transfusion, was a faster procedure and allowed a shorter LOS with a low rate of intra-operative adjustments.

In a meta-analysis, Thienpont et al (2017) compared PSI and standard instrumentation in TKA with regard to radiographic and clinical outcomes as well as operative time and blood loss.  This meta-analysis was carried out in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement.  PubMed and Embase were searched from 2011 through 2015.  These investigators included RCTs and cohort studies that reported the effect of PSI on the afore-mentioned outcomes.  The primary endpoint was deviation from the mechanical axis by greater than 3°.  Random and fixed-effect models were used for analysis.  A total of 44 studies, which included 2,866 knees that underwent surgery with PSI and 2,956 knees that underwent surgery with standard instrumentation, were evaluated.  The risk of mechanical axis malalignment was significantly lower for PSI, with a pooled RR of 0.79 (p = 0.013).  The risk of tibial sagittal-plane malalignment was higher for PSI than for standard instrumentation (RR = 1.32, p = 0.001), whereas the risk of femoral coronal-plane malalignment was significantly lower (RR = 0.74, p = 0.043).  The risk of tibial coronal-plane malalignment was significantly higher for PSI only when employing fixed-effect meta-analysis (RR = 1.33, p = 0.042).  Minor reductions in total operative time (-4.4 min, p = 0.002) and blood loss (-37.9 ml, p = 0.015) were noted for PSI.  The authors concluded that PSI improved the accuracy of femoral component alignment and global mechanical alignment, but at the cost of an increased risk of outliers for the tibial component alignment.  The impact of the increased probability of tibial component malalignment on implant longevity remains to be determined.  Meta-analyses indicated significant differences with regard to operative time and blood loss in favor of PSI.  However, these differences were minimal and, by themselves, not a substantial justification for routine use of the technology.

Alvand and colleagues (2018) noted that patient-specific instrumentation (PSI) has been proposed as a means of improving surgical accuracy and ease of implantation during technically challenging procedures such as UKA.  In a prospective RCT, these researchers compared the accuracy of implantation and functional outcome of mobile-bearing medial UKAs implanted with and without PSI by experienced UKA surgeons.  Mobile-bearing medial UKAs were implanted in 43 patients using either PSI guides or conventional instrumentation (CI).  Intra-operative measurements, meniscal bearing size implanted, and post-operative radiographic analyses were carried out to evaluate component positioning.  Functional outcome was determined using the Oxford Knee Score (OKS).  PSI guides could not be used in 3 cases due to concerns regarding accuracy and registration onto native anatomy, especially on the tibial side.  In general, similar component alignment and positioning was achieved using the 2 systems (non-significant for coronal/sagittal alignment and tibial coverage).  The PSI group had greater tibial slope (p = 0.029); while the control group had a higher number of optimum size meniscal bearing inserted (95 % versus 52 %; p = 0.001).  There were no significant differences in OKS improvements.  The authors concluded that component positioning for the 2 groups was similar for the femur but less accurate on the tibial side using PSI, often with some unnecessarily deep resections of the tibial plateau.  These investigators stated that although PSI was comparable to CI based on OKS improvements at 12 months, they continued to use CI for UKA at their institution until further improvements to the PSI guides can be demonstrated.  Level of Evidence = I.

The authors stated that this study had several drawbacks.  First, the surgeons performing the cases were highly experienced Oxford UKA (OUKA) surgeons with extensive previous experience of the CI.  These experts were therefore likely to have a very low number (if any) of surgical outliers when using the CI.  This potential bias was further amplified by the surgical learning curve associated with the use of the PSI.  A concerted effort was made by the surgeons to address this issue by familiarizing themselves with the PSI in a previous pilot study.  Nevertheless, the previous experience of the surgeons with the CI was very likely to have favored the CI group; and hence it would have been difficult to demonstrate more superior surgical accuracy using PSI.  Second, the radiological assessment was based on coronal and sagittal alignment.  Further evaluation of component rotation using CT scans would have been desirable but was limited by resources.  Nevertheless, the radiographic parameters were those recommended by the designers of the OUKA and previous studies have validated their use.  This study was powered to detect important differences in component alignment and positioning.  It was likely to have been under-powered for detecting clinically important changes in OKS; however, the rationale for the present study could be justified based on patient safety factors.  Finally, this study did not examine the long-term implant survival and risk of revision surgery -- a factor that was key in determining the efficacy of PSI technology.  These researchers stated that PSI technology is an exciting development that has received significant attention over the last 10 years.  However, further evaluation and improvement of the PSI guides used in this study are needed before they can be used on a regular basis in day-to-day clinical work.

In a RCT, Tammachote et al (2018) compared customized cutting blocks (CCB) with conventional cutting guide.  A total of 108 patients with knee OA underwent TKA by 1 experienced surgeon were randomized to receive CCB (n = 54) or conventional cutting instrument (CCI) surgery (n = 54).  The primary outcomes were limb alignment, prostheses position, and operative time.  The secondary outcomes were hemodynamic alteration after surgery, functional outcomes (modified WOMAC) and ROM at 2 years after surgery.  Mean HKA angle in the CCB group was 179.4° ± 1.8° versus 179.1° ± 2.4° in the CCI group, Δ = 0 (95 % CI: -0.6 to 1.1, p = 0.55).  Mean operative time was faster in the CCB arm: 93 ± 12 mins versus 104 ± 12 mins, Δ = 11 (95 % CI: -16.7 to -7.2, p < 0.0001).  There were no differences in hemodynamic parameters, mean blood loss (446 [CCB] versus 514 ml [CCI], Δ = -68 [95 % CI: -138 to 31 ml, p = 0.21]), post-operative Hb changes, incidence of hypotension (systolic less than 90 mm Hg), oliguria, and rates of blood transfusion.  Functional outcomes and ROM were also similar.  The authors concluded that there was no improvement in alignment, hemodynamic changes, blood loss, and knee functional outcomes; CCB reduced surgical time by 11 mins in this cohort.  The investigators stated that CCB cost-effectiveness should be further examined.

In a retrospective study, Stone et al (2018) examined alignment accuracy and functional outcomes of PSI as compared with standard instruments (SIs).  The investigators examined a consecutive series of TKA procedures using PSI.  A total of 85 PSI procedures were identified, and these were compared with a matched cohort of 85 TKAs using SI.  Intra-operative decision-making, EBL, efficiency, Knee Society Scores, and post-operative radiographs were evaluated.  A total of 170 patients with comparable patient demographics were reviewed; 81 % of the PSI procedures were within target (180 ± 3°) MA, while the SI group had 70 % of cases within the target plane (p = 0.132).  Mean target alignment (2.0° PSI versus 2.2° SI, p = 0.477) was similar between groups; 27 % of patients in the PSI group had surgeon-directed intra-operative recuts to improve the perceived coronal alignment.  The change in Hct was reduced in the PSI group (8.89 versus 7.21, p = 0.000).  Procedure time and total operating room time were equivalent.  Knee Society Scores did not differ between groups at 6 months or at 1 year.  The authors concluded that PSI decreased change in Hct,  though coronal alignment and efficiency were equivalent between groups.  These investigators stated that surgeons must evaluate cuts intra-operatively to confirm alignment; functional outcomes were equivalent for PSI and SI groups.

In a prospective RCT, Kosse et al (2018) examined stability and alignment after TKA using PSI and CI.  These researchers hypothesized that stability and alignment would be better using PSI than CI, 12 months post-operatively.  The secondary objective included the evaluation of clinical outcomes after TKA.  This study included 42 patients with knee OA who received a Genesis II PS prosthesis with either PSI or CI.  Patients visited the hospital pre-operatively and post-operatively after 6 weeks and 3 and 12 months.  To evaluate stability, varus-valgus laxity was determined in extension and flexion using stress radiographs 12 months post-operatively.  Three months post-operatively, a long-leg X-ray and CT scan were obtained to measure HKA alignment and component rotation.  In addition, frontal and sagittal alignment of the components, the Knee Society Score, VAS Pain, VAS Satisfaction, Knee injury and Osteoarthritis Outcome score, Patella score (Kujala), UCLA activity score, anterior-posterior laxity, (serious) adverse device-related events, and intra-operative complications were reported.  The clinical outcomes were compared using independent t tests or non-parametric alternatives, and repeated measurements ANOVA with a significance level of p < 0.05.  No significant differences were found between the 2 groups regarding stability, HKA angle, and rotational alignment.  In 4 patients, the PSI did not fit correctly on the tibia and/or femur requiring intra-operative modifications.  Both groups improved significantly over time on all clinical outcomes, with no significant differences between the groups 12 months post-operatively.  The PSI group showed less tibial slope than the patients in the CI group [PSI 2.6° versus CI 4.8° (p = 0.02)].  Finally, the PSI group more frequently received a thinner insert size than the CI group (p = 0.03).  Patients operated with PSI did not differ from CI in terms of stability and alignment.  However, in the PSI group ligament releases were more often required intra-operatively.  Furthermore, the 2 methods did not show different clinical results.  It appeared that the pre-operative planning for the PSI facilitated more conservative bone cuts than CI; however, whether this was clinically relevant should be further examined.  These investigators stated that since PSI was more expensive and time-consuming than CI; and did not outperform CI with regard to clinical results, they recommended the use of CI in TKA.

Stolarczyk et al (2018) compared the rotational alignment of the femoral and tibial components in TKA patients when performed with either CI or PSI.  A total of 60 patients with primary knee OA were randomly divided into 2 groups treated surgically with TKA: one with CI and the other with the Visionaire PSI system.  Computerized tomography and X-ray imaging were performed pre-operatively and 12 weeks after surgery.  The rotational alignment of the femoral and tibial component in all patients was assessed post-surgically using CT imaging according to the Berger protocol.  Both groups were clinically examined in a blinded fashion using the Knee Society Score (KSS) and a VAS.  A total of 58 patients were prospectively assessed.  The mean post-surgical follow-up was 3.0 ± 0.4 months.  CT images did not reveal any significant improvement in the rotational alignment of the implant components between the groups.  X-rays revealed a significant improvement in the deviation from the optimal alignment range of the femoral component in the coronal plane in both groups.  Patients operated with Visionaire PSI assistance had poorer functional outcomes.  The authors concluded that there were no improvements in clinical outcomes or knee component alignment in patients treated with PSI compared with those treated with CI.  Furthermore, clinical and functional assessment showed inferior results in terms of KSS and VAS scores at the mid-term follow-up in patients treated with PSI.

Woon et al (2018) combined raw data from RCTs, aiming to compare functional outcomes between KA using PSI and MA, and whether any patient subgroups may benefit more from KA technique.  These investigators carried out a literature search in PubMed, Embase and Cochrane databases; they identified 4 RCTs comparing patients undergoing TKA using PSI-KA and MA.  Unpublished data including WOMAC and KSS were obtained from study authors.  Meta-analysis compared MA to KA change (post-op minus pre-op) scores.  Subgroup-analysis on KA patients looked for subgroups more likely to benefit from KA and the impact of PSI accuracy.  Meta-analyses of change scores in 229 KA patients versus 229 MA patients were no different from WOMAC (MD of 3.4; 95 % CI: - 0.5 to 7.3), KSS function (1.3, - 3.9 to 6.4) or KSS combined (7.2, - 0.8 to 15.2).  A small advantage was observed for KSS pain in the KA group (3.6, 95 % CI: 0.2 to 7.1).  Subgroup-analysis showed no difference between varus, valgus and neutral pre-operative alignment groups, and those who did and did not achieve KA plans.  Pain-free patients at 1-year were more likely to achieve KA plans.  The authors concluded that patient-reported outcome scores following TKA using PSI-KA were similar to MA.  No identifiable subgroups benefited more from KA, and long-term results remain unknown.  Inaccuracy of the PSI system used in KA patients could potentially affect outcome.

In a prospective RCT, Randelli et al (2019) examined if PSI would improve the rotational alignment of the femoral component in comparison to CI TKA.  These investigators evaluated 133 consecutive patients for eligibility.  Block randomization was carried out to allocated patients in the treatment (PSI) or control group.  During hospital stay, surgical times were recorded, and total blood volume loss and estimated red blood cell were calculated.  Two months after surgery, a CT of the knee was obtained to measure femoral component rotation to the trans-epicondylar axis and tibial component slope.  The investigators enrolled 69 patients.  PSI did neither result in a significant improvement in femoral component rotation nor result in a reduction of outliers, as compared with CI.  No significant improvement in terms of tibial slope, blood loss, total surgical time, and ischemia time could be identified.  The number of tibial recuts required in the PSI group was significantly higher than in the control group (p = 0.0003).  The authors concluded that PSI did not improve the accuracy of femoral component rotation in TKA in comparison to CI.  Moreover, PSI did not appear to influence any of the other variables examined as secondary goals by this study.  The results of this study did not support the routine use of PSI during standard TKA.

In a prospective RCT, Teeter et al (2019) compared PSI and CI for TKA in terms of early implant migration, alignment, surgical resources, patient outcomes, and costs.  This study included 50 patients undergoing TKA.  There were 25 patients in each of the PSI and CSI groups.  There were 12 male patients in the PSI group and 7 male patients in the CSI group.  The patients had a mean age of 69.0 years (SD of 8.4) in the PSI group and 69.4 years (SD of 8.4) in the CSI group.  All patients received the same TKA implant.  Intra-operative surgical resources and any surgical waste generated were recorded.  Patients underwent radio-stereometric analysis (RSA) studies to measure femoral and tibial component migration over 2 years.  Outcome measures were recorded pre- and post-operatively.  Overall costs were calculated for each group.  There were no differences (p > 0.05) in any measurement of migration at 2 years for either the tibial or femoral components.  Movement between 1 and 2 years was less than 0.2 mm, indicating stable fixation.  There were no differences in coronal or sagittal alignment between the 2 groups.  The PSI group took a mean 6.1 mins longer (p = 0.04) and used a mean 3.4 less trays (p < 0.0001).  Total waste generated was similar (10 kg) between the 2 groups.  The PSI group costed a mean CAD$1,787 more per case (p < 0.01).  The authors concluded that RSA criteria suggested that both groups would have revision rates of approximately 3 % at 5 years.  The advantages of PSI were minimal or absent for surgical resources used and waste eliminated, and for meeting target alignment, yet had significantly greater costs; thus, these researchers stated that PSI may not offer any advantage over CI for routine primary TKA cases.

In a double-blinded RCT, Turgeon et al (2019) compared the effectiveness of PSI and CI in achieving neutral alignment and accurate component positioning in TKA.  Patients were randomly assigned to treatment with either PSI or CI.  A total of 54 patients were included in the study.  No relevant improvement in coronal alignment was found between the PSI and CI groups with post-hoc power of 0.91.  Tibial slope was found to be more accurately reproduced to the pre-operative target of 3° with PSI than with CI (3.8°± 3.1° versus 7.7°± 3.6°, respectively, p < 0.001).  There were no differences found in patient-reported outcome measures, surgical time or hospital LOS.  The authors concluded that given the added cost of the PSI technique, its use was difficult to justify given the small improvement in only a single alignment parameter.

Meier and colleagues (2019) noted that previous studies analyzing femoral components of TKAs have demonstrated the limited ability of these components to accommodate size variations observed in the patient population, especially width and femoral offset.  These investigators used a large data set of knee CT scans to determine the variations in the distal and posterior femoral geometries and to examine if there is a correlation between distal condylar offset and posterior femoral offset as a potential parameter for symmetry/asymmetry; and to evaluate what proportion of knees would have a substantial mismatch between the implant's size or shape and the patient's anatomy if a femoral component of a modern standard TKA of symmetric (sTKA) or asymmetric (asTKA) designs were to be used.  These researchers carried out a retrospective study on 24,042 data sets that were generated during the design phase for a customized TKA implant.  These data set were drawn from European and US-American patients.  Measurements recorded for the femur included the overall AP and medio-lateral (ML) widths, widths of the lateral condyle and the medial condyle, the distal condylar offset (DCO) between the lateral and medial condyles in the supero-inferior direction, and the posterior femoral offset (PFO) as the difference between the medial and lateral posterior condylar offset (PCO) measured in the AP direction.  A consecutively collected subset of 2,367 data sets was further evaluated to determine the difference between the individual AP and ML dimensions of the femur with that of modern TKA designs using two commercially available implants from different vendors.  These investigators observed a high degree of variability in AP and ML widths as well as in DCO and PFO.  Furthermore, they found no correlation between DCO and PCO of the knees studied.  Instances of a patient having a small DCO and higher PCO were commonly seen.  Analysis of the DFOs revealed that overall, 62 % (14,906 of 24,042) of knees exhibited DCO of greater than 1 mm and 83 % (19,955 of 24,042) of femurs exhibited a greater than 2-mm difference between the lateral and medial PCO.  Concerning AP and ML measurements, 23 % (544 of 2,367) and 25 % (592 of 2,367) would have a mismatch between the patient's bony anatomy and the dimensions of the femoral component of ± 3 mm if they would have undergone a modern standard sTKA or asTKA design, respectively.  The authors concluded that analysis of a large number of CT scans of the knee showed that a high degree of variability exists in AP and ML widths as well as in DCO and PFO.  The investigators stated that these findings suggested that it is possible that a greater degree of customization could result in surgeons performing fewer soft tissue releases and medial resections than now are being done to fit a fixed-geometry implant into a highly variable patient population.  However, as an imaging study, it could not support one approach to TKA over another; comparative studies that assess patient-reported outcomes and survivorship are needed to help surgeons decide among sTKA, asTKA, and customized TKA (cTKA).

The authors stated that this study had several drawbacks.  First and most important was the virtual nature of measurements, which could not be directly transferred to the intra-operative situation and may not be associated with differences in pain or function after TKAs performed in clinical practice.  Another drawback of the present study was that CT did not display cartilage thickness, which varied between 0 and 5 mm; Clarke reported a mean of 2 mm for the posterior condyle, therefore making pre-operative measurement of the PCO inaccurate.  Furthermore, when the posterior condyles in knees with varus alignment are considered, the cartilage thickness of the medial condyle is usually found to be less than the cartilage thickness of the lateral condyle.  As a consequence, over-resection of the medial posterior condyle and under-resection of the lateral posterior condyle may occur.  A further consequence may be additional rotational requirements and balancing.  However, no standard TKA instrumentation allowed for cartilage estimation but focused on bony landmarks, cuts, and ligament balance.  That being so, the authors believed that although their measurement approach may have shortcomings, those shortcomings directly parallel those that are in common use in clinical practice in that their measurement approaches based on cartilage were similar to the alignment guides used during TKA.  Even so, this issue should be considered -- and the authors hope remedied -- by future studies and perhaps future instrument systems.  Furthermore, cartilage and bone loss could influence ligament balance and laxity, and these factors differ between patients; likewise, surgeons may differ in terms of how they achieve ligament balance, making this even more complicated.  To try to mitigate this, given that these differences were likely to be more severe in knees with large deformities, the authors excluded knees with varus or valgus deformities of greater than 15°.  The authors also noted that these data set included implant dimensions that were generated from the design process of a cTKA but did not include patient demographic information; that being so, they could not assume that these findings applied equally to men and women or different ethnicities.  Furthermore, because mapping the entire database of implant dimensions was prohibitive when comparing sTKA and asTKA, a large consecutive series was selected to limit the effect of selection bias.  Because the conclusions drawn were limited to cases that fell into the range of sizes supported by the collected data, the conclusions should apply to patients having knees with dimensions falling into the FDA clearance range of cTKA.  Thus, these conclusions did not apply to small knees with dimensions that did not fall in the clearance range, thereby probably excluding parts of the Asian population.  However, to the authors’ best knowledge, this was the largest data set evaluated so far depicting a large cross-section of European and US-American patients and highlighting that surgeons intra-operatively had to deal with individual anatomic geometries.  Finally, the comparisons were done using 3 modern TKA designs, including symmetric and asymmetric designs; therefore, these findings may not apply to every available commercial implant.  However, said modern standard TKA designs are of particular interest because they are commonly used worldwide.

Namin et al (2019) stated that more than 6 million people were living with knee replacement implants in the U.S. as of 2017.  This number is expected to increase to more than 3.5 million/year by 2030.  The cost-effectiveness of total joint replacement procedures has been broadly studied; however, there is a compelling need to improve beyond the value afforded by off-the-shelf knee implants.  These investigators examined the impact of insurance coverage on the adoption of customized individually made (CIM) knee implants, and compared patient outcomes and cost-effectiveness of off-the-shelf and CIM implants.  The drawbacks of CIM implants include (typically) expensive than OTS implant, lack of long-term evidence for clinical outcomes, need for customized instrumentation, higher exposure to radiation in the process of axial imaging such as computed tomography (CT)  scanning, and increased complexity of the implant ordering system.  These researchers developed a system dynamics model to reproduce the historical data on primary and revision knee replacement implants obtained from the literature and the Nationwide Inpatient Sample.  In simulation analyses, rate of 90-day re-admission, 3-year revision surgery, hospitalization and recovery period, time savings in operating rooms (ORs), and the associated cost within 3 years of primary knee replacement implants were used as comparison indicators.  The results compared the adoption of CIM and its economic and patient outcome impacts to off-the-shelf implants under different insurance coverage for CIM implants.  The simulation results indicated that, by 2025, an adoption rate of 90 % for CIM implants will reduce the number of re-admissions and revision surgeries by 62 % and 39 %, respectively, and save hospitals and surgeons 6 % on procedure time, resulting in cumulative savings of approximately $40 billion in healthcare costs.  The authors concluded that CIM implants have the potential to deliver high-quality care while decreasing total costs, but their adoption requires the expansion of current insurance coverage.

The authors stated that the objective of the present study was to take a systematic look at the adoption of CIM knee implants.  The goal was not to explore how to improve treatment, but rather to perform what-if analyses.  The flexible nature of the model lends itself to extending it to study innovative policies and interventions focused on economic burden and patient outcomes when new information becomes available.  The model allows decision and policy makers to test different coverage policies on the basis of their preference.  For instance, they can consider a dynamic scenario for their coverage rate for CIM procedures on the basis of their initial investment and savings throughout the simulation time.  They can also test the effect of time delays on the preparation of the infrastructure.  These investigators stated that these findings may help policy makers consider CIM implants as an attractive option for improving patient outcomes while reducing the total costs of healthcare associated with TKA.  The result could inform decision-making among the Centers for Medicare & Medicaid Services, private insurance providers, and hospitals, spurring them to consider adoption of CIM implants and to offer alternative payment methodologies that would encourage widespread use of CIM knee implants.

Wheatley et al (2019) noted that patient-specific implants have been linked to stiffness.  These researchers evaluated outcomes in patient-specific implants.  They performed a retrospective review with a primary outcome of manipulation under anesthesia (MUA); secondary outcomes included Knee Society Scores (KSS), Knee Society Functional Scores (KSFS), range of motion (ROM), and Forgotten Joint Scores (FJS).  Pre-operative measurements were similar in both groups.  There was 1 MUA in the custom patient specific (CPS) and 2 in the off-the-shelf (OTS) groups.  There was no difference in post-operative scores.  The authors concluded that the findings of this study suggested that patient-specific implants had comparable rates of MUA and functional outcomes as conventional implants.  These researchers stated that future studies should take into account the inclusion of additional outcome measures and longer term follow-up.

The authors stated that this study had several drawbacks.  First, its retrospective nature has inherent potential for selection bias with potentially more motivated patients requesting the CPS TKA.  Furthermore, as the primary surgeon also noticed an increase in the manipulation rate of his custom CR implants, a decrease in stiffness may represent a learning curve in the implantation of the CPS knee arthroplasty.  However, the CPS group included his first CPS implantation, so the authors believed the lack of difference found in manipulation rates was likely a true finding.  Another drawback was that due to the relatively high Knee Society Scores in the off-the-shelf group, it would be difficult to demonstrate a significant increase in the functional scores of the CPS group.  Additionally, less than 50 % of the patients included in the original cohort completed the FJS survey once contacted.  The relatively low response rate, 43 %, could be considered a potential source of bias.  With the low response and relatively good functional scores in both groups, the study was under-powered to detect a small, but potentially significant difference in FJS scores.  Furthermore, 3 months was a short time period for follow-up to examine outcomes after knee arthroplasty.  However, the authors felt that this was sufficient length of follow-up to address the primary outcome of manipulation rates as the majority of manipulations were typically performed within the first 6 to 12 weeks following surgery.  However, since the principal objective of this study was to compare manipulation rates, they believed the results were still valuable. 

Freigang et al (2020) noted that unicondylar knee arthroplasty was introduced in the late 1960s and remains a topic of controversial discussion; PSI and patient-specific implants are not yet the standard of care.  The question remains whether this time-consuming and costly technique can be beneficial for the patient.  In a retrospective study, these researchers examined if a custom-made unicondylar knee arthroplasty would lead to improved patient-reported outcome.  This trial examined the patient-reported outcome after custom-made unicondylar knee arthroplasty (CM-UKA; ConforMIS™ iUni® G2).  These investigators evaluated 29 patients (31 knees) at an average of 2.4 years (range of 1.2 to 3.6 years) after operation for unicondylar osteoarthritis of the knee.  The target zone for the post-operative leg axis was a slight under-correction of 0 to 2° varus.  Follow-up evaluation included the Forgotten Joint Score (FJS), the KSS, a visual analog scale (VAS) and a radiographic evaluation including a long-leg radiograph.  Primary outcome measure was patient satisfaction based on the FJS.  These researchers found an excellent post-operative health-related QOL with a mean FJS of 76.8 (SD 17.9) indicating a low level of joint awareness after CM-UKA.  The mean pre-operative KSS was 66.0 (SD 13.71) and 59.4 (17.9) for the KSS function score.  The increase was 22.8 points for the KSS knee score (p < 0.0001) and 34.8 points for the KSS function score (p < 0.0001).  The VAS for pain decreased from a mean of 5.4 (SD 1.8) to 1.1 (SD 1.2) (p < 0.0001).  The mal-alignment rate with a post-operative deviation of more than 2° in the leg axis was 29 %.  There was no evidence of component loosening after a mean follow-up of 2.4 years.  The authors concluded that CM-UKA could provide improved clinical and functional outcomes for patients with isolated knee OA of the medial compartment.  They found excellent results regarding patient satisfaction and a low mal-alignment rate for CM-UKA.  Moreover, these researchers stated that further studies are needed to examine long-term survivorship of the implant.  Level of Evidence = IV.

The authors stated that this study had several drawbacks.  The principal drawback was the non-controlled design of the study.  Since UKA is a well-established procedure, there are a number of studies on FJS after UKA that were published since these researchers compared their data to literature.  From a scientific point of view, a post-operative evaluation by CT would have provided greater accuracy than radiographic measurements.  However, since this was a clinical study involving patients, this would not have been compliant with the Ethics Committee’s requirements for patient safety.  Furthermore, follow-up spanned a long period of time for logistic reasons.  The procedure of custom-made implants and instruments is time-consuming and costly; thus, the recruitment period was 2 years, affecting the range of follow-up as well.  Lastly, the follow-up period did not allow a conclusion on the long-term results.

Kalache et al (2020) stated that PSI may potentially improve UKA implant positioning and alignment.  In a retrospective study, these researchers compared early radiographic coronal alignment of medial UKA performed using PSI versus CI for tibial resections.  A consecutive series of 47 knees (47 patients) received medial UKA, with the tibial resections performed using CI (first 22 knees) or PSI (next 25 knees), while femoral resections were performed with CI in both groups.  The target mechanical medial proximal tibial angle (mMPTA) was 87° ± 3°, and the target hip-knee-ankle (HKA) angle was 177° ± 2°.  The post-operative mMPTA and HKA were evaluated from post-operative radiographs at a follow-up of 2 months.  Differences in post-operative mMPTA (p = 0.509) and HKA (p = 0.298) between the 2 groups were not statistically significant.  For the mMPTA target, 24 % of knees in the PSI group (85.6° ± 2.1°) and 32 % of the CI group (85.0° ± 3.6°) were outliers.  For the HKA target, 44 % of knees in the PSI group (176.3° ± 2.8°) and 18 % of the CI group (177.1° ± 2.3°) were outliers.  Considering the 2 criteria simultaneously, 60 % of knees in the PSI group and 45 % of knees in the CI group were outside the target zone (p = 0.324), whereas 28 % of knees in the PSI group and 41 % of knees in the CI group were outside the target zone by more than 1° (p = 0.357).  The authors concluded that the findings of this study revealed no statistically significant difference in radiographic coronal alignment of UKA performed using PSI versus CI for tibial resections.

The authors stated that this study had several limitations.  First, this was a retrospective study without any randomization.  However, the radiographs were systematically obtained as part of normal follow-up for UKA; and blinded before measurement of the radiographic outcomes.  Second, pre-operative mMPTA was statistically different between the 2 groups, but it was by random chance since CI was used on the first 22 knees and PSI on the next 25 knees.  Moreover, although statistically significant, a mean difference of 1.6° was not clinically relevant.  Third, no clinical scores were recorded, and it was unclear if the radiographic outcomes were related to clinical outcomes.  Fourth, the study might be under-powered due to the relatively small number of knees in the 2 groups (n = 25 for the PSI group; and n = 2 2for the CI group).  Fifth, radiographic measurements were obtained at 2 months follow-up, and some knees could still have residual stiffness with some amount of flexion contracture.  This might affect the radiographic analysis and alter the measurements, although all patients followed the same rehabilitation protocol.

In a prospective study, Zahn et al (2020) allocated 300 patients to 4 different groups using a randomization process (2 innovative and 2 conventional) techniques of tibial instrumentation (conventional: extra-medullary, intra-medullary; innovative: navigation and PSI; n = 75 for each group).  The objectives were to reconstruct the medial proximal tibial angle (MPTA) to 90° and the mechanical tibio-femoral axis (mTFA) to 0°.  Both angles were evaluated and compared between all groups 3 months after the surgery.  Patients who presented with a post-operative mTFA of greater than 3° were classified as outliers.  The navigation and intra-medullary technique both demonstrated that they were significantly more precise in reconstructing a neutral mTFA and MPTA compared to the other 2 techniques.  The OR for producing outliers was highest for the PSI method (PSI OR = 5.5, p < 0.05; extra-medullary positioning OR = 3.7, p > 0.05; intra-medullary positioning OR = 1.7, p > 0.05; navigation OR = 0.04, p < 0.05).  These researchers stated that they could only observe significant differences between pre- and post-operative MPTA in the navigation and intra-medullary group.  The MPTA showed a significant negative correlation with the mTFA in all groups pre-operatively and in the extra-medullary, intra-medullary and PSI post-operatively.  The authors concluded that navigation and intra-medullary instrumentation provided the precise positioning of the tibial component.  Outliers were most common within the PSI and extra-medullary technique.  Optimal alignment was dependent on the technique of tibial instrumentation and tibial component positioning determined the accuracy in TKA since mTFA correlated with MPTA pre- and post-operatively.

In a network meta-analysis, Bouche et al (2020) examined if novel approaches to achieving implant alignment, such as PSI, navigation, accelerometer-based navigation, and robotic guidance, would provide any advantage over standard cutting guides in terms of HKA alignment outliers greater than ± 3°; outcome scores (1989 - Knee Society Score and WOMAC score) measured 6 months after surgery; or femoral and tibial implant malalignment (greater than ± 3°), taken separately, in the frontal and sagittal plane, as well as other secondary outcomes including validated outcome scores 1 and 2 years after surgery.  These investigators included RCTs comparing the different cutting guides by using at least 1 of the previously specified criteria, without limitation on language or date of publication.  They searched electronic databases, major orthopedic journals, proceedings of major orthopedic meetings,, and the World Health Organization's International Clinical Trials Registry Platform until October 1, 2018.  They identified 90 RCTs involving 9,389 patients (mean age of 68.8 years) with 10,336 TKAs.  Two reviewers independently selected trials and extracted data.  The primary outcomes were the proportion patients with malalignment of the HKA angle (defined as HKA greater than 3° from neutral) and the KSS and WOMAC scores at 6 months post-operatively.  They combined direct and indirect comparisons using a Bayesian network meta-analysis framework to assess and compare the effect of different cutting guides on outcomes.  Bayesian estimates were based on the posterior distribution of an endpoint and were called CIs.  Usually the 95 % CI, corresponding to a posterior probability of 0.95 that the endpoint lies in the interval, was computed.  Unlike the frequentist approach, the Bayesian approach does not allow the calculation of the p value.  The proportion of HKA outliers was lower with navigation than with PSI (RR 0.46; 95 % CI: 0.34 to 0.63) and standard cutting guides (RR 0.45; 95 % CI: 0.37 to 0.53); however, this corresponded to an actual difference of only 12 % of patients for navigation versus 21 % of patients for PSI, and 12 % of patients for navigation versus 25 % for standard cutting guides.  They found no differences for other comparisons between different cutting guides, including robotics and the accelerometer.  They found no differences in the KSS or WOMAC score between the different cutting guides at 6 months.  Regarding secondary outcomes, navigation reduced the risk of frontal and sagittal malalignments for femoral and tibial components compared with the standard cutting guides, but none of the other cutting guides showed superiority for the other secondary outcomes.  Navigation resulted in approximately 10 % fewer patients having HKA outliers of more than 3°, without any corresponding improvement in validated outcomes scores.  It was unclear if this incremental reduction in the proportion of patients who had alignment outside a window that itself has been called into question would justify the increased costs and surgical time associated with the approach.  The authors concluded that that they believed that until or unless these new approaches either convincingly demonstrate superior survivorship, or convincingly demonstrate superior outcomes, surgeons and hospitals should not use these approaches since they add cost, have a learning curve (during which some patients may be harmed), and have the risks associated with uncertainty of novel surgical approaches.

Wang et al (2021) systematically reviewed the literature of PSI for total ankle arthroplasty (TAA).  PubMed, Embase, Web of Science, and Cochrane Library databases were systematically reviewed according to the PRISMA guidelines for PSI TAA.  The quality of the included studies was evaluated according to Methodological Index for Non-Randomized Studies (MINORS).  A total of 9 studies were included in the systematic review.  The implant position and function outcome of TAA was similar between PSI and SI.  Prediction accuracy of implant size remained great difference.  PSI could shorten the operative time and fluoroscopy time.  The authors concluded that the quality of current studies on PSI TAA was insufficient to produce high-level evidence.  PSI could get similar implant position and clinical outcome in TAA compared to SI; however, current evidence is not strong enough to evaluate PSI TAA.

ALShammari et al (2021) examined the advantage of using PSI over conventional inter-medullary (IM) guides for primary TKA with bilateral severe femoral bowing (greater than 5°).  A parallel trial design was used with 1:1 allocation.  These researchers hypothesized that PSI would support more accurate alignment of components and the lower-limb axis during TKA with severe femoral bowing in comparison with conventional IM guides.  Among 336 patients undergoing bilateral TKAs due to knee OA, 29 patients with bilateral lateral femoral bowing of more than 5° were included in this study.  Every patient was assigned randomly to PSI on 1 side and to CI lateralization of the entry point of the femoral IM guide was applied on the other side with the objective of neutral mechanical alignment.  The assessment of coronal alignment was completed by measuring the HKA angle on pre-operative and post-operative long film standing X-rays.  Coronal and sagittal orientations of femoral and tibial components were assessed on weight-bearing radiographs.  The rotational alignment of the femoral component was evaluated using CT.  The post-operative mean ± (SD) HKA angle was varus 4.0° (± 2.7°) for conventional technique and varus 4.1° (± 3.1°) for PSI, with no differences between the 2 groups (p = 0.459).  The component orientation showed no significant differences except with respect to the sagittal alignment of the femoral component (p = 0.001), with a PSI mean ± SD flexion of 5.8° (± 3.7°) and a conventional method mean ± SD flexion of 3.2° (± 2.5°), due to the intentional 3° flexion incorporated in the sagittal plane to prevent femoral notching in PSI planning.  Computed tomography assessment for rotational alignment of the femoral components showed no difference between the 2 groups concerning the trans-epicondylar axis (p = 0.485) with a PSI mean ± SD external rotation of 1.5° (± 1.3°) and conventional mean ± SD external rotation of 1.5° (± 1.6°).  The authors concluded that PSI showed no advantage over lateralization of the femoral entry for IM guidance.

Meier and associates (2021) stated that unicondylar knee arthroplasty offers the advantage that partial degenerative changes can be addressed with partial prosthetic solutions; thus, preserving as much of the native joint as possible, including the cruciate ligaments.  On the other hand, the number of revisions is still higher than for total knee endoprosthetics; the causes are insufficient fit of the components as well as surgical errors.  Thus, the use of new technologies to achieve a better fit and higher surgical precision and reproducibility represents a promising approach.  Individual endoprosthetics offers the advantage that the prosthesis is adapted to the individual anatomy of each patient and not the patient's anatomy to the prosthesis, as is the case with standard prostheses.  This allows for an optimal fit of the prosthesis while avoiding excessive bone resections and soft tissue releases.  The authors stated that the use of robotics in endoprosthetics makes it easier to correctly perform bone resections and align components; thus, ensuring high and reproducible precision even for less experienced surgeons.  Moreover, these researchers stated that studies on individual unicondylar endoprosthetics and robotics are reporting promising results; however, long-term results of high-quality randomized studies are awaited in order to make a scientifically sound statement.

Merle and co-workers (2021) stated that many long-term results for both medial and lateral UKA demonstrated that UKA is a reliable and successful treatment for isolated antero-medial or lateral OA of the knee when the correct indications are used.  The relationship between operation volume and implant performance has clearly been established from recent studies and registry data.  The use of novel technologies allows for an improvement in the accuracy of implant positioning with fewer outliers; however, evidence-based target zones for the positioning of available implants have not been sufficiently established.  Current data does not support the routine use of PSI or custom-made implants.  The authors concluded that robot-assisted procedures must be interpreted as a very promising approach for the future.  So far, there is insufficient evidence that robotically assisted surgical techniques improve implant performance or lead to better functional results from the patient's point of view.

Beyer et al (2022) compared TKA with PSI and CI with regard to the use of resources in the operating room (OR), alignment and patient-reported outcome.  A total of 139 TKA with PSI or CI were included in 3 centers.  Economic variables of the surgery (number of instrument trays, set-up and cut-sew-time), radiological alignment and patient reported outcomes (VAS Pain Scale, OKS, EQ-5D) were assessed after 6 weeks, 6 and 12 months.  There was a significant reduction of instrument trays and of time in the OR in the PSI group.  The reduction varied between the centers.  With strict reorganization, more than 50 % of the instrument trays could be reduced while using PSI.  There were no significant differences in cut-sew-time, implant position, leg axis, pain and function.  The authors concluded that the use of PSI was associated with significantly less OR resources; however, the savings did not compensate the costs for this technology.

Rudran et al (2022) carried out a literature search of electronic databases Embase, Medline and registry platform portals on the May 16, 2021.  The search was conducted using a pre-designed search strategy.  Eligible studies were critically appraised for methodological quality.  The primary outcome measure was Knee Society Function Score.  Functional scores were also collected for the secondary outcome measures: OKS, WOMAC, KOOS and VAS for pain.  Review Manager 5.3 was used for all data synthesis and analysis.  A total of 23 studies were identified for inclusion in this study; 22 studies (18 RCTs and 4 prospective studies) were included in the meta-analysis, with a total of 2,277 TKAs.  There were 1,154 PSI TKA and 1,123 CI TKA.  The majority of outcomes at 3-month, 6-month and 12-month show no statistical difference.  There was statistical significance at 24 months in favor of PSI group for KSS function (mean difference 4.36, 95 % CI: 1.83 to 6.89).  The MD did not exceed the MCID of 6.4.  KSS knee scores demonstrated statistical significance at 24 months (MD 2.37, 95 % CI: 0.42 to 4.31), with a MCID of 5.9. WOMAC scores were found to be statistically significant favoring PSI group at 12 months (MD -3.47, 95 % CI: -6.57 to -0.36) and 24 months (MD -0.65, 95 % CI: -1.28 to -0.03), with high level of bias noted in the studies and a MCID of 10.  The authors concluded that this meta-analysis of level 1 and level 2 evidence showed there was no clinical difference when comparing PSI and CI KSS function scores for TKA at definitive post-operative time-points (3 months, 6 months, 12 months and 24 months).  Within the secondary outcomes for this study, there was no clinical difference between PSI and CI for TKA.  Although there was no clinical difference between PSI and CI for TKA, there was statistical significance noted at 24 months in favor of PSI compared to CI for TKA when considering KSS function, KSS knee scores and WOMAC scores.  Moreover, these investigators stated that Studies included in this meta-analysis were of limited cohort size and prospective studies were prone to methodological bias.  The current literature is limited and insufficiently robust to make explicit conclusions and therefore further high-powered robust RCTs are needed at specific time-points.

Hinloopen et al (2023) noted that PSI for primary TKA surgery has been shown to increase accuracy of component positioning; however, it is unclear if this would translate to actual benefits for patients in terms of better outcomes (effectiveness) or less complications such as revisions (safety).  In a systematic review, these investigators examined available evidence to determine the safety and effectiveness of PSI in primary TKA; RCTs comparing PSI to non-PSI in primary TKA were included.  A random effects model was used with meta-regression in case of heterogeneity.  A total of 43 studies were included with a total of 1,816 TKA in the PSI group and 1,887 TKA in the control group.  There were no clinically relevant differences between the PSI-group and non-PSI group regarding all outcomes.  There was considerable heterogeneity: meta-regression analyses showed that the year the study was published was an important effect modifier.  Early publications tended to show a positive effect for PSI compared to non-PSI TKA, whereas later studies found the opposite.  The authors concluded that based on evidence of moderate certainty, this study suggested that there were no clinically relevant differences in the safety and effectiveness between patients treated with PSI TKA and patients treated with non-PSI TKA.

Unicompartmental Versus Total Knee Replacement

In a systematic review and meta-analysis, Wilson and colleagues (2019) presented a comprehensive summary of the published data on UKA or TKA, comparing domains of outcome that have been shown to be important to patients and clinicians to allow informed decision-making.  These investigators carried out the c review using data from RCTs, nationwide databases or joint registries, and large cohort studies.  Medline, Embase, Cochrane Controlled Register of Trials (CENTRAL), and Clinical, were searched between January 1, 1997 and December 31, 2018.  Studies published in the past 20 years, comparing outcomes of primary UKA with TKA in adult patients.  Studies were excluded if they involved fewer than 50 subjects, or if translation into English was not available.  A total of 60 eligible studies were separated into 3 methodological groups: 7 publications from 6 RCTs, 17 national joint registries and national database studies, and 36 cohort studies.  Results for each domain of outcome varied depending on the level of data, and findings were not always significant.  Analysis of the 3 groups of studies showed significantly shorter hospital stays after UKA than after TKA (-1.20 days (95 % CI: -1.67 to -0.73), -1.43 (-1.53 to -1.33), and -1.73 (-2.30 to -1.16), respectively).  There was no significant difference in pain, based on patient reported outcome measures (PROMs), but significantly better functional PROM scores for UKA than for TKA in both non-trial groups (MD -0.58 (-0.88 to -0.27) and -0.32 (-0.48 to -0.15), respectively).  Regarding major complications, trials and cohort studies had non-significant results, but mortality after TKA was significantly higher in registry and large database studies (RR 0.27 (0.16 to 0.45)), as were venous thromboembolic events (VTE; 0.39 (0.27 to 0.57)) and major cardiac events (0.22 (0.06 to 0.86)).  Early re-operation for any reason was higher after TKA than after UKA, but revision rates at 5 years remained higher for UKA in all 3 study groups (RR 5.95 (1.29 to 27.59), 2.50 (1.77 to 3.54), and 3.13 (1.89 to 5.17), respectively).  The authors concluded that TKA and UKA are both viable options for the treatment of isolated unicompartmental osteoarthritis.  By directly comparing the 2 treatments, this study demonstrated better results for UKA in several outcome domains.  However, the risk of revision surgery was lower for TKA.  This information should be available to patients as part of the shared decision-making process in choosing treatment options.

Ceramic Femoral Prosthesis in Total Knee Arthroplasty

In a prospective, comparative study, Bergschmidt and co-workers (2016) examined the clinical and radiological outcomes of a TKA system, comparing a ceramic (BIOLOX delta) and metallic (Co28Cr6Mo) femoral component over a 5-year follow-up period.  A total of 43 TKA patients (17 metallic and 26 ceramic femoral components) were enrolled in the study.  Clinical and radiological evaluations were performed pre-operatively and at 3, 12, 24 and 60 months post-operatively, using the Hospital for Special Surgery (HSS) Knee Score, WOMAC function scale and Short Form-36 (SF-36) score, in addition to standardized X-rays.  The HSS-Score improved significantly from 58.7 ± 12.7 points pre-operatively to 88.5 ± 12.3 points at 5-years post-operative in the ceramic group, and 60.8 ± 7.7 to 86.2 ± 9.4 points in the metallic group.  WOMAC- and SF-36-Scores showed significant improvement over time in both groups.  There were no significant differences between groups for HSS-, WOMAC- and SF-36-Scores, nor for ROM (p ≤ 0.897) at any follow-up evaluation.  Furthermore, radiological evaluation showed no implant loosening or migration in either group.  The authors concluded that mid-term outcomes for the ceramic femoral components demonstrated good clinical and radiological results, as well as comparable survivorship to the metallic femoral component of the same total knee system, and to other commonly used metallic total knee systems.  Thus, ceramic knee implants may be a promising solution for the population of patients with osteoarthritis and metal sensitivity.  These researchers stated that long-term studies are needed to confirm the positive mid-term results, and to follow the implant survival rate in regard to the enhanced wear resistance of ceramic implants.

Nakamura and associates (2017) stated that ceramic bearings are not commonly used in TKA.  Currently, little information is available regarding if long-term survivorship and good clinical outcomes can be ensured with ceramic knee implants.  These researchers examined the clinical and radiological outcomes, and evaluated the long-term durability of a ceramic tri-condylar implant.  A total of 507 consecutive TKAs were performed using a ceramic tri-condylar femoral implant.  The posterior cruciate ligament was sacrificed, and all components were fixed with bone cement.  Clinical outcomes were assessed retrospectively with the Knee Society scoring system.  Kaplan-Meier survivorship was calculated to determine the cumulative survival rate.  A total of 167 knees (114 patients) were available for clinical outcomes.  The average range of flexion improved from 118.1° pre-operatively to 123.7° at a minimum 15-year follow-up (p < 0.001).  The average Knee Society knee score improved from 39.1 to 92.8 (p < 0.001).  The functional score also improved from 36.0 to 47.0 (p < 0.001).  With revision for any surgery or radiographic failure as the end-point, Kaplan-Meier survivorship at 15 years was 94.0 %.  With revision of any component as the end-point, the corresponding survivorship was 96.2 %.  The authors concluded that clinically, the post-operative knee flexion range and Knee Society scores were good after long-term follow-up.  The survivorship of the ceramic knee implant was excellent over the 15-year follow-up, and long-term durability was achieved, making ceramic a promising alternative material for the femoral component in TKA.

Xiang and colleagues (2019) noted that ceramic bearings have been widely used in total hip arthroplasty (THA), which resulted in satisfactory clinical outcomes due to the excellent tribological characteristics of the implants.  However, ceramic components are not commonly used in TKA because of brittleness.  These researchers analyzed information regarding the clinical outcomes (including survival without revision, causes of revision, functional outcome, and incidence of loosening) and reached a definitive conclusion about the use of ceramic femoral components in TKA.  Medline, Embase, Cochrane, and databases were searched for studies that reported the clinical and/or radiological outcomes with or without survival data of ceramic TKA implants and that included more than 10 patients with a minimum of 1 year follow-up.  From an initial sample of 147, there were 14 studies that met the inclusion criteria.  Overall, there was a notable enhancement of joint function after the procedure, with a satisfactory mid- and long-term survival of the ceramic components, which is comparable to that of the conventional alloy components reported previously.  In addition, the revision rate was reported to be between 0 % and 14.37 % according to the included studies.  However, revision due to aseptic loosening, wear, and component fracture appeared to be rare, demonstrating the safety of in-vivo use of ceramic bearings in the TKA procedure.  The authors concluded that ceramic TKA implants showed similar post-operative clinical results and survival rate compared to their conventional metallic counterparts.  These findings confirmed the safety of in-vivo use of ceramic bearings in TKA, with rare implant breakage and aseptic loosening.  They stated that considering the excellent characteristics of the tribology of ceramics, the clinical use of ceramic prostheses in TKA could be promising.

The authors stated that by systematically reviewing these single-armed studies, they found that ceramic components could be used in the TKA procedure, with excellent long-term joint function and survival.  However, because of the limited use of ceramic TKA components worldwide, RCTs and cohort studies comparing the long-term clinical results and survival between ceramic TKA components and conventional cobalt-chromium prostheses were not available.  This may jeopardize the strength of this conclusion.  These researchers stated that more research on ceramic TKA components, especially comparative studies with a higher level of evidence, are needed to support the use of ceramic components in the TKA procedure.

MAKOplasty of the Knee

Kouk et al (2018) stated that total patellectomies are uncommon procedures that are reserved as salvage treatment for severely comminuted fractures of the patella.  Due to the alteration of normal joint mechanics, these patients presented later on in life with degenerative cartilage damage to the femoro-tibial joint and altered extensor mechanism.  There are very few reports of unicondylar knee arthroplasties following previous patellectomy and none that specifically address robot-assisted unicompartmental knee arthroplasty (UKA).  A recent case report by Pang et al described the use of minimally invasive fixed-bearing unicondylar knee arthroplasty in a patellectomized patient with moderate medial compartment osteoarthritis (OA).  This report detailed a case with more significant chondral loss along with patellar tendon subluxation.  This was a case report of a patient with severe medial compartment OA after a patellectomy following a motor vehicle collision.  After failing conservative treatment, the patient underwent a medial MAKOplasty with complete resolution of arthritic pain.  The authors concluded that significant pain relief and improved knee function could be achieved with MAKOPlasty partial knee resurfacing system in a previously patellectomized patient with severe medial compartment OA.  Moreover, this case report was limited by its length of follow-up (5 weeks) and its subsequent evaluation on the efficacy of the treatment.  To fully evaluate the treatment, the patient should be followed longitudinally and additional patients should be added.  These researchers stated that robot-assisted unicondylar arthroplasty may be a potential therapeutic option in a patient with severe, isolated medial compartment OA status after remote patellectomy.

Deese et al (2018) stated that UKA originated in the 1950's.  There have been many enhancements to the implants and the technique, improving the precision and accuracy of this challenging operation.  Specifically for Robotic Arm Interactive Orthopedic System (Rio; Mako Stryker, Fort Lauderdale, FL), there are many studies reporting clinical outcomes, but this search offered nothing regarding patient reported outcomes using validated surveys.  Patients with onlay tibial components presenting for routine follow-up of robotic-arm assisted UKA performed between May 2009 and September 2013 were invited to participate; 4 joints had simultaneous patella femoral resurfacing.  Knee Injury and Osteoarthritis Outcomes Score (KOOS) and the 2011 Knee Society Scores were collected.  Radiographic evidence of OA in the non-operative knee compartments was documented.  A total of 81 patients presented for follow-up and consented to participate.  Mean follow-up was 54 months; mean patient reported KOOS activities of daily living (ADL) and pain scores were each 90.  Knee Society 2011 mean objective score was 96 and mean function score 81.  There was 1 revision to total knee at 40 months post-op for pain after injury; 77 % reported their knee always felt "normal", 20 % sometimes, and only 3 % reported that it never felt normal.  The authors concluded that the literature on UKA failure rates suggested that UKA may be a less forgiving procedure than total knee arthroplasty (TKA).  Robotic-arm assisted surgery has been reported to improve the accuracy of implant placement.  Based on the authors’ prospectively collected positive patient outcomes, they have achieved good results from performing robotic-arm assisted UKA on select patients.

The authors stated that this study had several drawbacks.  One was the number of subjects – only 54 % of the possible participants presented for follow-up consented to participate.  Patients were consented and enrolled at the time of routine annual follow-up and therefore the authors could not enroll any patients that did not show up and did not reschedule their appointment; 46 % were of working age 65 and under, which these researchers agreed contributed to poor follow-up compliance.  Close proximity to military base may explain the high number lost to follow-up as this population tended to move frequently making it difficult to keep up with contact information.  The authors’ institution was the first facility in the region to offer this technique and many patients came from out-of-state, hence making it difficult for them to return to clinic.  Unfortunately, these investigators did not have a consecutive series and therefore introduced selection bias.  There were no outcome scores collected or consistency in the documented pain or function levels pre-operatively; thus, these researchers had no baseline for comparison to the follow-up scores.

Kim et al (2020) stated that robotic-assisted total knee arthroplasty (TKA) was introduced to enhance the precision of bone preparation and component alignment with the goal of improving the clinical results and survivorship of TKA.  Although numerous reports suggested that bone preparation and knee component alignment may be improved using robotic assistance, no long-term randomized trials of robotic-assisted TKA have shown whether this results in improved clinical function or survivorship of the TKA.  In a prospective, randomized, controlled trial (RCT), these researchers compared robotic-assisted TKA to manual-alignment techniques at long-term follow-up in terms of (i) functional results based on Knee Society, WOMAC, and UCLA Activity scores; (ii) numerous radiographic parameters, including component and limb alignment; (iii) Kaplan-Meier survivorship; and (iv) complications specific to robotic-assistance, including pin-tract infection, peroneal nerve palsy, pin-site fracture, or patellar complications.  From January 2002 to February 2008, 1 surgeon performed 975 robotic-assisted TKAs in 850 patients and 990 conventional TKAs in 849 patients.  Among these patients 1,406 patients were eligible for participation in this study based on pre-specified inclusion criteria . Of those, 100 % (1,406) patients agreed to participate and were randomized, with 700 patients (750 knees) receiving robotic-assisted TKA and 706 patients (766 knees) receiving conventional TKA.  Of those, 96 % (674 patients) in the robotic-assisted TKA group and 95 % (674 patients) in the conventional TKA group were available for follow-up at a mean of 13 (± 5) years.  In both groups, no patient older than 65 years was randomized because the authors anticipated long-term follow-up.  These researchers examined 674 patients (724 knees) in each group for clinical and radiographic outcomes, and they examined Kaplan-Meier survivorship for the endpoint of aseptic loosening or revision.  Clinical evaluation was carried out using the original Knee Society knee score, the WOMAC score, and the UCLA activity score preoperatively and at latest follow-up visit.  They also evaluated loosening (defined as change in the position of the components) using plain radiographs, osteolysis using CT scans at the latest follow-up visit, and component, and limb alignment on mechanical axis radiographs.  To minimize the chance of type-2 error and increase the power of this study, these researchers assumed the difference in the Knee Society score to be 5 points to match the MCID of the Knee Society with power of 0.99, which revealed that a total of 628 patients would be needed in each group.

Clinical parameters at the latest follow-up including the Knee Society knee scores (93 ± 5 points in the robotic-assisted TKA group versus 92 ± 6 points in the conventional TKA group (95 % confidence interval [CI]: 90 to 98); p = 0.321) and Knee Society knee function scores (83 ± 7 points in the robotic-assisted TKA group versus 85 ± 6 points in the conventional TKA group (95 % CI: 75 to 88); p = 0.992), WOMAC scores (18 ± 14 points in the robotic-assisted TKA group versus 19 ± 15 points in the conventional TKA group (95 % CI: 16 to 22); p = 0.981), range of knee motion (125 ± 6° in the robotic-assisted TKA group versus 128 ± 7° in the conventional TKA group (95 % CI: 121 to 135); p = 0.321), and UCLA patient activity scores (7 points versus 7 points in each group (95 % CI: 5 to 10); p = 1.000) were not different between the 2 groups at a mean of 13 years’ follow-up.  Radiographic parameters such as the femoro-tibial angle (mean 2° ± 2° valgus in the robotic-assisted TKA group versus 3° ± 3° valgus in the conventional TKA group (95 % CI: 1 to 5); p = 0.897), femoral component position (coronal plane: mean 98° in the robotic-assisted TKA group versus 97° in the conventional TKA group (95 % CI: 96 to 99); p = 0.953; sagittal plane: mean 3° in the robotic-assisted TKA group versus 2° in the conventional TKA group (95 % CI: 1 to 4); p = 0.612) and tibial component position (coronal plane: mean 90° in the robotic-assisted TKA group versus 89° in the conventional TKA group (95 % CI: 87 to 92); p = 0.721; sagittal plane: 87° in the robotic-assisted TKA group versus 86° in the conventional TKA group (95 % CI: 84 to 89); p = 0.792), joint line (16 mm in the robotic-assisted TKA group versus 16 mm in the conventional TKA group (95 % CI: 14 to 18); p = 0.512), and posterior femoral condylar offset (24 mm in the robotic-assisted TKA group versus 24 mm in the conventional TKA group (95 % CI: 21 to 27 ); p = 0.817) also were not different between the 2  groups (p > 0.05).  The aseptic loosening rate was 2 % in each group, and this was not different between the 2 groups.  With the endpoint of revision or aseptic loosening of the components, Kaplan-Meier survivorship of the TKA components was 98 % in both groups (95 % CI: 94 to 100) at 15 years (p = 0.972).  There were no between-group differences in terms of the frequency with which complications occurred.  In each group, 2 % of knees (n = 15) had a superficial infection treated with intravenous antibiotics for 2 weeks.  No deep infection occurred in these knees.  In the conventional TKA group, 0.8 % of knees (n = 6) had a motion limitation (less than 60°).

The authors concluded that at a minimum follow-up of 10 years, these researchers found no differences between robotic-assisted TKA and conventional TKA in terms of functional outcome scores, aseptic loosening, overall survivorship, and complications.  Considering the additional time and expense associated with robotic-assisted TKA, these investigators cannot recommend its widespread use.  Level of Evidence = I.

Prophylactic Use of Tranexamic Acid in Total Knee Arthroplasty

Alshryda et al (2011) carried out a systematic review and meta-analysis of randomized controlled trials (RCTs) evaluating the effect of tranexamic acid (TXA) upon blood loss and transfusion in primary total knee replacement (TKR).  The review used the generic evaluation tool designed by the Cochrane Bone, Joint and Muscle Trauma Group.  A total of 19 trials were eligible: 18 used intravenous administration, 1 also evaluated oral dosing and 1 trial evaluated topical use.  TXA led to a significant reduction in the proportion of patients requiring blood transfusion (risk ratio (RR) 2.56, 95 % confidence interval (CI): 2.1 to 3.1, p < 0.001; heterogeneity I(2) = 75 %; 14 trials, 824 patients).  Using TXA also reduced total blood loss by a mean of 591 ml (95 % CI: 536 to 647, p < 0.001; I(2) = 78 %; 9 trials, 763 patients).  The clinical interpretation of these findings was limited by substantial heterogeneity.  However, subgroup analysis of high-dose (greater than 4 g) TXA showed a plausible consistent reduction in blood transfusion requirements (RR 5.33; 95 % CI: 2.44 to 11.65, p < 0.001; I(2) = 0 %), a finding that should be confirmed by a further well-designed trial.  The authors concluded that the current evidence from trials did not support an increased risk of deep-vein thrombosis (DVT) (13 trials, 801 patients) or pulmonary embolism (PE) (18 trials, 971 patients) due to TXA administration.

In a systematic review and meta-analysis, Panteli et al (2013) examined the safety and efficacy of topical use of TXA in total knee arthroplasty (TKA).  These researchers carried out an electronic literature search of PubMed Medline; Ovid Medline; Embase; and the Cochrane Library, identifying studies published in any language from 1966 to February 2013.  The studies enrolled adults undergoing a primary TKA, where topical TXA was used.  Inverse variance statistical method and either a fixed or random effect model, depending on the absence or presence of statistical heterogeneity were used; subgroup analysis was performed when possible.  They identified a total of 7 eligible reports for analysis.  The meta-analysis indicated that when compared with the control group, topical application of TA limited significantly post-operative drain output (mean difference [MD]: -268.36 ml), total blood loss (MD = -220.08 ml), Hb drop (MD = -0.94g/dL) and lowered the risk of transfusion requirements (RR = 0.47, 95 % CI: 0.26 to 0.84), without increased risk of thromboembolic events.  Sub-group analysis indicated that a higher dose of topical TXA (greater than 2g) significantly reduced transfusion requirements.  The authors concluded that although the present meta-analysis proved a statistically significant reduction of post-operative blood loss and transfusion requirements with topical use of TXA in TKA, the clinical importance of the respective estimates of effect size should be interpreted with caution.  Level of Evidence = I, II.

Wang et al (2015) noted that there has been much debate and controversy about the safety and efficacy of the topical use of TXA in primary TKA.  These researchers performed a meta-analysis to examine if there is less blood loss and lower rates of transfusion after topical TXA administration in primary TKA.  They carried out a systematic review of the electronic databases PubMed, CENTRAL, Web of Science, and Embase.  All RCTs and prospective cohort studies evaluating the effectiveness of topical TXA during primary TKA were included.  The focus of the analysis was on the outcomes of blood loss results, transfusion rate, and thromboembolic complications.  Subgroup analysis was performed when possible.  Of 387 studies identified, 16 comprising 1,421 patients (1,481 knees) were eligible for data extraction and meta-analysis.  This study indicated that when compared with the control group, topical application of TXA significantly reduced total drain output (MD, -227.20; 95 % CI: -347.11 to -107.30; p < 0.00001), total blood loss (MD, -311.28; 95 % CI: -404.94 to -217.62;  p < 0.00001), maximum post-operative Hb decrease (MD, -0.73; 95 % CI: -0.96 to -0.50; p < 0.00001), and blood transfusion requirements (RRs, 0.33; 95 % CI: 0.24 to 0.43; p = 0.14).  The authors found a statistically significant reduction in blood loss and transfusion rates when using topical TXA in primary TKA.  Furthermore, the currently available evidence does not support an increased risk of DVT or PE due to TXA administration.  These researchers stated that topical TXA was effective for reducing post-operative blood loss and transfusion requirements without increasing the prevalence of thromboembolic complications.

In a meta-analysis, Meena et al (2017) examined the safety and efficacy of intra-articular TXA when compared to intravenous (IV) route.  These investigators performed a literature search using PubMed, Cochrane Library, Medline, Embase and China National Knowledge Infrastructure (CNKI).  All RCTs evaluating the effectiveness of topical route and IV route of TXA administration were included.  A total of 8 RCTs comprising of 857 patients were included in this analysis.  These researchers found no statistically significant difference in terms of total blood loss, drain output, transfusion requirement, thromboembolic complication, tourniquet time and surgical duration.  The authors concluded that topical TXA had a similar efficacy to IV-TXA in reducing total blood loss, drain output, transfusion rate and Hb drop without any increase in thromboembolic complications.

In a systematic review and meta-analysis, Guo et al (2018) evaluated the safety and efficacy of oral TXA versus control for blood loss after TKA.  These investigators searched PubMed, Embase, Medline, Web of Science, and Cochrane Library databases for relevant studies through August 2017.  The MD of total blood loss, Hb drop, hematocrit (Hct), drain output, and risk difference (RD) of transfusion rate and thromboembolic complications in the TXA and control groups were pooled throughout the study.  The outcomes were pooled by Stata 12.0.  A total of 5 RCTs (608 patients) were included in this study.  All the included studies were randomized; and the quality of included studies was relatively high.  The pooled results indicated that the oral TXA group had significantly less Hb drop (standardized MD [SMD], -0.936; 95 % CI: -1.118 to -0.754), Hct drop (SMD, -0.693; 95 % CI: -1.113 to -0.274), and drain output (SMD, -0.793; 95 % CI: -0.959 to -0.628) than the control group.  No statistically significant differences were found in transfusion rate and the incidence of thromboembolic complications between the 2 groups.  Total blood loss could not be evaluated for the insufficient date.  The authors concluded that this meta-analysis suggested that the administration of oral TXA provided significantly better results with respect to Hb drop, Hct drop, and drain output without increasing the transfusion rate and the risk of thromboembolic complications after TKA.  Nevertheless, the current study with some limitations such as the small sample size only provided limited quality of evidence, confirmation from further meta-analysis with large-scale, well-designed RCTs is needed.

In a systematic review and meta-analysis, Liu et al (2018) compared the safety and efficacy of TXA and epsilon-aminocaproic acid (EACA) for reducing blood loss and transfusion requirements after TKA and total hip arthroplasty (THA).  These researchers conducted electronic searches of Medline (1966 to November 2017), PubMed (1966 to November 2017), Embase (1980 to November 2017), ScienceDirect (1985 to November 2017) and the Cochrane Library (1900 to November 2017).  The primary outcomes included total blood loss, Hb decline and transfusion requirements; secondary outcomes included length of hospital stay (LOS) and post-operative complications such as the incidence of DVT and PE.  Each outcome was combined and calculated using the statistical software STATA 12.0.  Fixed/random effect model was adopted based on the heterogeneity tested by I2 statistic.  A total of 1,714 patients were analyzed across 3 RCTs and 1 non-RCT.  The present meta-analysis revealed that TXA was associated with a significantly reduction of total blood loss and post-operative Hb drop compared with EACA.  No significant differences were identified in terms of transfusion rates, LOS, and the incidence of post-operative complications.  The authors concluded that although total blood loss and post-operative Hb drop were significant greater in EACA groups, there was no significant difference between TXA and EACA groups in terms of transfusion rates.  These researchers stated that based on the current evidence available, higher quality RCTs are still needed for further research.

In a systematic review and meta-analysis, Xu et al (2019) examined the safety and efficacy of different routes of TXA administration in reducing blood transfusion after THA and TKA.  The secondary aim was to find the safest and most effective route and dose of TXA.  PubMed, Embase, Cochrane library, China National Knowledge Infrastructure, and OpenGrey were systemically searched for RCTs investigating the safety and/or efficacy of TXA for THA and/or TKA.  Network meta-analysis, comparing the number of transfusion and DVT among different interventions, was performed using a multi-variate meta-regression model with random effects, adopting a frequentist approach.  A total of 211 publications (20,639 individuals) were included.  For outcome of transfusion, all interventions showed significantly lower transfusion rates compared to placebo.  When compared to placebo, TXA via IV and topical showed statistically significant lowest risk ratio (RR = 0.11, 95 % CI: 0.03 to 0.41).  For safety, TXA via topical showed relatively lowest risk ratio (RR = 0.75, 95 % CI: 0.44 to 1.30).  TXA via topical and intra-articular had the highest but statistically insignificant RR (RR = 1.10, 95 % CI: 0.51 to 2.38).  Thus, current studies did not reveal any significant safety issue in using TXA.  The authors concluded that all forms of TXA administration showed significantly lower transfusion rate compared to control.  There is a trend towards better efficacy with IV and topical.  In patients with higher risk of thrombosis, physicians may consider topical alone for its best safety profile.

In a systematic review and meta-analysis, Chen et al (2019) compared the safety and efficacy of oral TXA with IV TXA in reducing peri-operative blood loss in TKA and THA.  PubMed, Web of Science, Embase, and Cochrane Library were fully searched for relevant studies.  Studies comparing the safety and efficacy of oral TXA with IV TXA in TKA and THA were included in this research.  Odds ratio (OR) or RD was applied to compare dichotomous variables, while MD was used to compare continues variables.  A total of 7 studies (5 RCTs and 2 retrospective studies) were included into this study.  As for patients undergoing TKA or THA, there were no obvious differences between oral TXA group and IV TXA group in Hb drop (MD = 0.06, 95 % CI: -0.01 to 0.13, p = 0.09), transfusion rate (OR = 0.78, 95 % CI: 0.54 to 1.13, p = 0.19), total blood loss (MD = 16.31, 95 % CI: -69.85 to 102.46, p = 0.71), total Hb loss (MD = 5.18, 95 % CI: -12.65 to 23.02, p = 0.57), LOS (MD = -0.06, 95 % CI: -0.30 to 0.18, p = 0.63), drain out (MD = 21.04, 95 % CI: -15.81 to 57.88, p = 0.26), incidence of DVT (RD = 0.00, 95 % CI: -0.01 to 0.01, p = 0.82) or PE (RD = 0.00, 95 % CI: -0.01 to 0.01, p = 0.91).  The sample size of this study was small; and several included studies were with relatively low quality.  The authors concluded that oral TXA was equivalent to IV TXA in reducing peri-operative blood loss and should be recommended in TKA and THA; however, these researchers stated that more high-quality studies are needed to elucidate this issue.

In a randomized, double-blinded, placebo-controlled study, Wang et al (2019) examined the safety and efficacy of IV and subsequent long-term oral TXA following TKA without a tourniquet.  A total of 118 patients undergoing primary TKA were randomized into 2 groups: the patients in group A received IV TXA at 20-mg/kg 10 mins before the surgery and 3 hours post-operatively, and then oral 1 g TXA from post-operative day (POD) 1 to POD 14, and the patients in group B received IV TXA at 20-mg/kg 10 mins before surgery and 3 hours post-operatively, and then oral 1 g placebo from POD 1 to POD 14.  The primary outcome was total blood loss.  Secondary outcomes included ecchymosis area and morbidity, post-operative transfusion, post-operative laboratory values, post-operative knee function and LOS.  Complications, and patient satisfaction were also recorded.  The mean total blood loss was lower in Group A than in Group B (671.7 ml versus 915.8 ml, p = 0.001).  There was no significant difference in the transfusion rate between the 2 groups.  Group A had a higher Hb than Group B on POD 3 (106.0 g/L versus 99.7 g/L, p = 0.001).  However, no significant difference was found for Hb or Hct on POD 1 or POD 14 between the 2 groups.  Patients in Group A had less ecchymosis morbidity (7 versus 38, p = 0.001), smaller ecchymosis area (1.6 versus 3.0, p = 0.001) than Group B.  The blood coagulation level as measured by fibrinolysis (D-Dimer) was lower in Group A than in Group B on POD 1 and POD 3 (4.6 mg/L versus 8.4 mg/L, respectively, p = 0.001; 1.5 mg/L versus 3.3 mg/L, respectively, p = 0.001).  However, there was no significant difference on POD 14, and the fibrin degradation products showed the same trend.  Patients in Group A had less swelling than those in Group B on POD 3 and POD 14.  The circumference of the knee was 43.1 cm versus 46.1 cm (POD 3, p = 0.001) and 41.4 cm versus 44.9 cm (POD 14, p = 0.001) in Group A versus Group B, respectively.  Nevertheless, the circumference of the knee in the 2 groups was similar on POD 1 and POD 3 M.  No significant differences were identified in knee function, pain score, or LOS.  No significant differences were identified in thromboembolic complications, infection, hematoma, wound healing and patient satisfaction between the 2 groups.  The authors concluded that IV and subsequent long-term oral TXA produced less blood loss and less swelling and ecchymosis compared with short-term TXA without increasing the risk of complications.

Balachandar and Abuzakuk (2019) stated that there is no consensus on the optimum timing of administration of TXA)in bilateral TKA.   These researchers examined if the timing of administration of single-dose IV TXA (either given pre-operatively or intra-operatively) has a significant effect on blood loss reduction.  They compared 2 cohorts of patients with end-stage arthritis of knees who underwent bilateral TKA and were given single-dose IV TXA (1 g or 15 mg/kg) at different times during surgery.  The retrospective cohort group consisting of 40 patients (pre-operative (PO) group) received TXA before the skin incision.  The prospective cohort consisting of 40 patients (intra-operative (IO) group) received TXA 10 mins before deflating the tourniquet on the first knee.  Primary outcome measures were mean Hb difference, A (between PO and day 1 post-operative Hb), mean Hb difference, B (between PO and lowest post-operative Hb), and rate of allogeneic blood transfusion.  Secondary measure was drain blood loss.  Both cohorts were well-matched with respect to age, gender, duration of surgery, and LOS.  The Hb drop in the IO group was significantly lesser than the PO group on the 1st POD (2 versus 2.9 g/dL, p < 0.001).  Although statistically insignificant, the patients in the IO group received less allogenic transfusion of packed cell units than in the PO group (11/40, 27.5 % versus 14/40, 35 % ).  Mean Hb difference, B, and secondary drain loss were comparable in both groups.  The authors concluded that single-dose IV TXA given before the start of surgery was as effective as a dose given during arthroplasty of the first knee in reducing blood loss in bilateral TKA.

In a randomized, double-blinded controlled study, Jules-Elysee et al (2019) compared local and systemic levels of thrombogenic markers, interleukin (IL)-6, and TXA between patients who received IV TXA and those who received topical TXA.  A total of 76 patients scheduled for TKA were enrolled in this study.  The IV group received 1.0 g of IV TXA before tourniquet inflation and again 3 hours later; a topical placebo was then administered 5 mins before final tourniquet release.  The topical group received an IV placebo before tourniquet inflation and again 3 hours later; 3.0 g of TXA was administered topically 5 mins before final tourniquet release.  Peripheral and wound blood samples were collected to measure levels of plasmin-anti-plasmin (PAP, a measure of fibrinolysis), prothrombin fragment 1.2 (PF1.2, a marker of thrombin generation), IL-6, and TXA.  At 1 hour after tourniquet release, systemic PAP levels were comparable between the IV group (after a single dose of IV TXA) and the topical group.  At 4 hours after tourniquet release, the IV group had lower systemic PAP levels than the topical group (mean and standard deviation, 1,117.8 ± 478.9 µg/L versus 1,280.7 ± 646.5 µg/L; p = 0.049), indicative of higher anti-fibrinolytic activity after the 2nd dose.  There was no difference in PF1.2 levels between groups, indicating that there was no increase in thrombin generation.  The IV group had higher TXA levels at all time-points (p < 0.001); 4 hours after tourniquet release, wound blood IL-6 and TXA levels were higher than systemic levels in both groups (p < 0.001).  Therapeutic systemic TXA levels (mean, 7.2 ± 7.4 mg/L) were noted in the topical group.  Calculated blood loss and the LOS were lower in the IV group (p = 0.026 and p = 0.025).  The authors concluded that given that therapeutic levels were reached with topical TXA and the lack of a major difference in the mechanism of action, coagulation, and fibrinolytic profile between topical TXA and a single dose of IV TXA, it may be a simpler protocol for institutions to adopt the use of a single dose of IV TXA when safety is a concern.  Level of Evidence = I.

Bacteriophage Therapy for the Treatment of Knee Arthroplasty-Related Peri-Prosthetic Joint Infection

Khalifa and, Hussien (2023) stated that total hip and knee arthroplasty peri-prosthetic joint infection (PJI) poses a management dilemma owing to the emergence of resistant organisms.  A promising option is bacteriophage therapy (BT) that was used as an adjuvant for PJI management, aiming at treating resistant infections, decreasing morbidity, and mortality.  In a systematic review, these investigators examined the role and safety of using BT as an adjuvant for the treatment of PJIs.  They carried out a systematic search via 4 databases (Embase, PubMed, Web of Science, and Scopus) up to March 2022, according to the pre-determined inclusion and exclusion criteria.  This systematic review included 11 case reports of 13 patients in which 14 joints (11 TKAs and 3 THAs) were treated.  The patients' average age was 73.7 years; they underwent an average of 4.5 previous surgeries.  The most common organism was the Staphylococcus aureus species.  All patients underwent surgical debridement; for the 13 patients, 8 received a cocktail, and 5 received monophage therapy.  All patients received post-operative suppressive antibiotic therapy.  After an average follow-up of 14.5 months, all patients had satisfactory outcomes.  No recurrence of infection in any patient.  Transaminitis complicating BT was developed in 3 patients, needed stoppage in only 1, and the condition was reversible and non-life-threatening.  The authors concluded that BT was a safe and potentially effective adjuvant therapy for the treatment of resistant and relapsing PJIs; however, further studies are needed to clarify some issues related to BT’s best route and duration.  Furthermore, new ethical regulations should be implemented to facilitate its widespread use.

The authors stated that this review had several drawbacks.  First, the exclusive inclusion of English literature.  While BT is a common practice in Western Europe; this might have led to depriving the review of studies published in languages other than English.  Second, the inclusion of only case reports; however, this was related to the search results based on the search terms and search engines we used.  Third, these researchers could not report on BT’s exact availability and cost as these data were lacking in the included reports.

Robotic-Assisted Versus Conventional Total Knee Arthroplasty

Ruangsomboo et al (2023) noted that robotic-assisted TKA (RATKA) is an alternative surgical treatment method to conventional TKA (COTKA) that may deliver better surgical accuracy.  However, its impact on patient outcomes is uncertain.  In a systematic review and meta-analysis, these investigators examined if RATKA could improve functional and radiological outcomes compared with COTKA in adult patients with primary OA of the knee.  They searched Ovid Medline, Embase, Scopus, and the Cochrane Library to identify published RCTs comparing RATKA with COTKA; 2 reviewers independently screened eligible studies, reviewed the full texts, assessed risk of bias using the Risk of Bias 2.0 tool, and extracted data.  Outcomes were patient-reported outcomes, ROM, and mechanical alignment (MA) deviation and outliers, and complications.  These researchers included 12 RCTs involving 2,200 patients.  RATKA probably resulted in little to no effect on patient-reported outcomes (mean difference (MD) in the WOMAC score of -0.35 (95 % CI: -0.78 to 0.07) and ROM (MD -0.73°; CI: -7.5° to 6.0°) compared with COTKA.  However, RATKA likely resulted in a lower degree of MA outliers (RR 0.43; CI: 0.27 to 0.67) and less deviation from neutral MA (MD -0.94°; CI: -1.1° to -0.73°) . There were no differences in revision rate or major adverse effects associated with RATKA.  The authors concluded that although RATKA likely resulted in higher radiologic accuracy than COTKA, this may not be clinically meaningful.  Furthermore, there was probably no clinically important difference in clinical outcomes between RATKA and COTKA, while it is as yet inconclusive regarding the revision and complication rates due to insufficient evidence.


Kellgren-Lawrence Classification System

The Kellgren-Lawrence classification system uses a 0 to 4 grading method for classifying the severity of osteoarthritis (OA) based on radiographs.

Table: Kellgren-Lawrence Classification System for Osteoarthritis
Grade Description
Grade 0 (none) Definite absence of x-ray changes of osteoarthritis.
Grade 1 (doubtful) Doubtful narrowing of the joint space with possible osteophytic lipping.
Grade 2 (minimal) Definite osteophyte formation and possible joint space narrowing.
Grade 3 (moderate) Moderate multiple osteophytes formation, definite narrowing of joint space, some sclerosis, and possible deformity of bone ends.
Grade 4 (severe) Large osteophytes formation, severe narrowing of joint space with marked sclerosis and definite deformity of bone ends.

Source: Kohn, Sassoon, Fernando (2016) and Knipe et al. (2020)


The above policy is based on the following references:

  1. Abane L, Anract P, Boisgard S, et al. A comparison of patient-specific and conventional instrumentation for total knee arthroplasty: A multicentre randomised controlled trial. Bone Joint J. 2015;97-B(1):56-63.
  2. Al-Hadithy N, Patel R, Navadgi B, et al. Mid-term results of the FPV patellofemoral joint replacement. Knee. 2014;21(1):138-141.
  3. ALShammari SA, Choi KY, Koh IJ, et al. Total knee arthroplasty in femoral bowing: Does patient specific instrumentation have something to add? A randomized controlled trial. BMC Musculoskelet Disord. 2021;22(1):321.
  4. Alshryda S, Sarda P, Sukeik M, et al. Tranexamic acid in total knee replacement: A systematic review and meta-analysis. J Bone Joint Surg Br. 2011;93(12):1577-1585.
  5. Alvand A, Khan T, Jenkins C, et al. The impact of patient-specific instrumentation on unicompartmental knee arthroplasty: A prospective randomised controlled study. Knee Surg Sports Traumatol Arthrosc. 2018;26(6):1662-1670.
  6. American Academy of Orthopaedic Surgeons (AAOS). AAOS clinical guideline on osteoarthritis of the knee (phase II). Rosemount, IL: AAOS; 2003.
  7. American College of Rheumatology Subcommittee on Osteoarthritis Guidelines. Recommendations for the medical management of osteoarthritis of the hip and knee: 2000 update. Arthritis Rheum. 2000;43(9):1905-1915.
  8. Arbab D, Reimann P, Brucker M, et al. Alignment in total knee arthroplasty - A comparison of patient-specific implants with the conventional technique. Knee. 2018;25(5):882-887.
  9. Argenson JN, Parratte S, Flecher X, Aubaniac JM. Unicompartmental knee arthroplasty: Technique through a mini-incision. Clin Orthop Relat Res. 2007;464:32-36.
  10. Asakawa K, Spry C. Unicompartmental knee arthroplasty (UKA): A review of the clinical and cost- effectiveness and guidelines for use. Health Technology Inquiry Service (HTIS). Ottawa, ON: Canadian Agency for Drugs and Technologies in Health (CADTH); August 8, 2008. 
  11. Bailie AG, Lewis PL, Brumby SA, et al. The Unispacer knee implant: Early clinical results. J Bone Joint Surg Br. 2008;90(4):446-450.
  12. Balachandar G, Abuzakuk T. Is there an optimal timing of administration of single-dose intravenous tranexamic acid in bilateral total knee arthroplasty? A comparison between preoperative and intraoperative dose. J Orthop Surg (Hong Kong). 2019;27(3):2309499019880915.
  13. Beal MD, Delagramaticas D, Fitz D. Improving outcomes in total knee arthroplasty-do navigation or customized implants have a role? J Orthop Surg Res. 2016;11(1):60.
  14. Bergschmidt P, Ellenrieder M, Bader R, et al. Prospective comparative clinical study of ceramic and metallic femoral components for total knee arthroplasty over a five-year follow-up period. Knee. 2016;23(5):871-876.
  15. Beyer F, Lutzner C, Stalp M, et al. Does the use of patient-specific instrumentation improve resource use in the operating room and outcome after total knee arthroplasty? -- A multicenter study. PLoS One. 2022;17(11):e0277464.
  16. Boonen B, Schotanus MG, Kerens B, et al. Intra-operative results and radiological outcome of conventional and patient-specific surgery in total knee arthroplasty: A multicentre, randomized controlled trial. Knee Surg Sports Traumatol Arthrosc. 2013;21(10):2206-2212.
  17. Borus T, Thornhill T. Unicompartmental knee arthroplasty. J Am Acad Orthop Surg. 2008;16(1):9-18.
  18. Bouche PA, Corsia S, Dechartres A, et al. Are there differences in accuracy or outcomes scores among navigated, robotic, patient-specific instruments or standard cutting guides in TKA? A network meta-analysis. Clin Orthop Relat Res. 2020;478(9):2105-2116.
  19. Brown A. The Oxford unicompartmental knee replacement for osteoarthritis. Issues in Emerging Health Technologies Issue 23. Ottawa, ON: Canadian Coordinating Office for Health Technology Assessment (CCOHTA); 2001.
  20. Buch RG, Schroeder L, Buch R, Eberle R. Does implant design affect hospital metrics and patient outcomes? TKA utilizing a "fast-track" protocol. Reconstructive Review. 2019;9(1):11-16.
  21. Bunyoz KI, Lustig S, Troelsen A. Similar postoperative patient-reported outcome in both second generation patellofemoral arthroplasty and total knee arthroplasty for treatment of isolated patellofemoral osteoarthritis: A systematic review. Knee Surg Sports Traumatol Arthrosc. 2019;27(7):2226-2237.
  22. Callahan CM, Drake BG, Heck DA, et al. Patient outcomes following unicompartmental or bicompartmental knee arthroplasty: A meta-analysis. J Arthroplasty. 1995;10(2):141-150.
  23. Carpenter DP, Holmberg RR, Quartulli MJ, Barnes CL. et al. Tibial plateau coverage in UKA: A comparison of patient specific and off-the-shelf implants. J Arthroplasty. 2014;29(9):1694-1698.
  24. Catier C, Turcat M, Jacquel A, Baulot E. The Unispacer™ unicompartmental knee implant: Its outcomes in medial compartment knee osteoarthritis. Orthop Traumatol Surg Res. 2011;97(4):410-417.
  25. Chen X, Zheng F, Zheng Z, et al. Oral vs intravenous tranexamic acid in total-knee arthroplasty and total hip arthroplasty: A systematic review and meta-analysis. Medicine (Baltimore). 2019;98(20):e15248.
  26. Chidel MA, Suh JH, Matejczyk MB. Radiation prophylaxis for heterotopic ossification of the knee. J Arthroplasty. 2001;16(1):1-6.
  27. Chung JY, Min BH. Is bicompartmental knee arthroplasty more favourable to knee muscle strength and physical performance compared to total knee arthroplasty? Knee Surg Sports Traumatol Arthrosc. 2013;21(11):2532-2541.
  28. Clarius M, Becker JF, Schmitt H, Seeger JB. The UniSpacer: Correcting varus malalignment in medial gonarthrosis. Int Orthop. 2010;34(8):1175-1179.
  29. Confalonieri N, Manzotti A, Cerveri P, De Momi E. Bi-unicompartmental versus total knee arthroplasty: A matched paired study with early clinical results. Arch Orthop Trauma Surg. 2009;129(9):1157-1163.
  30. Culler SD, Martin GM, Swearingen A. Comparison of adverse events rates and hospital cost between customized individually made implants and standard off-the-shelf implants for total knee arthroplasty. Arthroplast Today. 2017;3(4):257-263.
  31. Daniilidis K, Tibesku CO. A comparison of conventional and patient-specific instruments in total knee arthroplasty. Int Orthop. 2014;38(3):503-508.
  32. Davies AP. High early revision rate with the FPV patello-femoral unicompartmental arthroplasty. Knee. 2013;20(6):482-484.
  33. Deese JM, Gratto-Cox G, Carter DA , et al. Patient reported and clinical outcomes of robotic-arm assisted unicondylar knee arthroplasty: Minimum two year follow-up. J Orthop. 2018;15(3):847-853.
  34. Dettori JR, Ecker E, Norvell D, et al. Total knee arthroplasty. Health Technology Asessment. Prepared for the Washington State Health Care Authority by Spectrum Research, Inc. Olympia, WA: Washington State Health Care Authority; August 20, 2010.
  35. Dudhniwala AG, Rath NK, Joshy S, et al. Early failure with the Journey-Deuce bicompartmental knee arthroplasty. Eur J Orthop Surg Traumatol. 2016;26(5):517-521.
  36. Emerson RH Jr, Potter T. The use of the McKeever metallic hemiarthroplasty for unicompartmental arthritis. J Bone Joint Surg Am. 1985;67(2):208-212.
  37. Farid YR, Thakral R, Finn HA. Low-dose irradiation and constrained revision for severe, idiopathic, arthrofibrosis following total knee arthroplasty. J Arthroplasty. 2013;28(8):1314-1320.
  38. Fitz W. Unicompartmental knee arthroplasty with use of novel patient-specific resurfacing implants and personalized jigs. J Bone Joint Surg Am. 2009;91 Suppl 1:69-76.
  39. Freigang V, Rupp M, Pfeifer C, et al. Patient-reported outcome after patient-specific unicondylar knee arthroplasty for unicompartmental knee osteoarthritis. BMC Musculoskelet Disord. 2020;21(1):773.
  40. Fu D, Li G, Chen K, et al. Comparison of high tibial osteotomy and unicompartmental knee arthroplasty in the treatment of unicompartmental osteoarthritis: A meta-analysis. J Arthroplasty. 2013;28(5):759-765.
  41. Fuchs S, Rolauffs B, Plaumann T, et al. Clinical and functional results after the rehabilitation period in minimally-invasive unicondylar knee arthroplasty patients. Knee Surg Sports Traumatol Arthrosc. 2005;13(3):179-186.
  42. Geller JA, Yoon RS, Macaulay W. Unicompartmental knee arthroplasty: A controversial history and a rationale for contemporary resurgence. J Knee Surg. 2008;21(1):7-14.
  43. Gesell MW, Tria AJ Jr. MIS unicondylar knee arthroplasty: Surgical approach and early results. Clin Orthop Relat Res. 2004;(428):53-60.
  44. Griffen T, Maddern G, Rowden N, et al. Unicompartmental knee arthroplasty for unicompartmental osteoarthritis: A systematic review. ASERNIP-S Report; 44. North Adelaide, SA: Royal Australasian College of Surgeons, Australian Safety and Efficacy Register of New Interventional Procedures (ASERNIP) - Surgical; 2005.
  45. Griffin T, Rowden N, Morgan D, et al. Unicompartmental knee arthroplasty for the treatment of unicompartmental osteoarthritis: A systematic study. ANZ J Surg. 2007;77(4):214-221.
  46. Guo P, He Z, Wang Y, et al. Efficacy and safety of oral tranexamic acid in total knee arthroplasty: A systematic review and meta-analysis. Medicine (Baltimore). 2018;97(18):e0587.
  47. Hinloopen JH, Puijk R, Nolte PA, et al. The efficacy and safety of patient-specific instrumentation in primary total knee replacement: A systematic review and meta-analysis. Expert Rev Med Devices. 2023;20(3):245-252.
  48. Huijbregts HJ, Khan RJ, Fick DP, et al. Component alignment and clinical outcome following total knee arthroplasty: A randomised controlled trial comparing an intramedullary alignment system with patient-specific instrumentation. Bone Joint J. 2016a;98-B(8):1043-1049.
  49. Huijbregts HJ, Khan RJ, Sorensen E, et al. Patient-specific instrumentation does not improve radiographic alignment or clinical outcomes after total knee arthroplasty. Acta Orthop. 2016b;87(4):386-394.
  50. Institute for Clinical Systems Improvement (ICSI). Diagnosis and treatment of adult degenerative joint disease (DJD) of the knee. ICSI Healthcare Guidelines. Bloomington, MN: ICSI; May 2002. 
  51. Ivie CB, Probst PJ, Bal AK, et al. Improved radiographic outcomes with patient-specific total knee arthroplasty. J Arthroplasty. 2014;29(11):2100-2103.
  52. Jensen LG, Løvschall C, Ladehoff Thomsen AM, et al. Custom-made or customisable 3D printed implants and cutting guides versus non- 3D printed standard implants and cutting guides for improving outcome in patients undergoing knee, maxillofacial, or cranial surgery. HTA-Projektbericht 117. Vienna, Australia: Ludwig Boltzmann Institute for Health Technology Assessment; 2019.
  53. Johnson TC, Tatman PJ, Mehle S, Gioe TJ. Revision surgery for patellofemoral problems: Should we always resurface? Clin Orthop Relat Res. 2012;470(1):211-219.
  54. Joseph MN, Achten J, Parsons NR, Costa ML; PAT Trial Collaborators. The PAT randomized clinical trial. Bone Joint J. 2020;102-B(3):310-318. 
  55. Jules-Elysee KM, Tseng A, Sculco TP, et al. Comparison of topical and intravenous tranexamic acid for total knee replacement: A randomized double-blinded controlled study of effects on tranexamic acid levels and thrombogenic and inflammatory marker levels. J Bone Joint Surg Am. 2019;101(23):2120-2128.
  56. Kalache H, Muller JH, Saffarini M, Gancel E. Patient-specific instrumentation does not improve tibial component coronal alignment for medial UKA compared to conventional instrumentation. J Exp Orthop. 2020;7(1):42.
  57. Kay A, Kurtz W, Martin G, et al. Manipulation rate is not increased after customized total knee arthroplasty.  Reconstructive Review. 2018;8(1):37-42.
  58. Khalifa AA, Hussien SM. The promising role of bacteriophage therapy in managing total hip and knee arthroplasty related periprosthetic joint infection, a systematic review. J Exp Orthop. 2023;10(1):18.
  59. Khanna G, Levy BA. Oxford unicompartmental knee replacement: Literature review. Orthopedics. 2007;30(5 Suppl):11-14.
  60. Khosravipour I, Pejhan S, Luo Y, Wyss UP. Customized surface-guided knee implant: Contact analysis and experimental test. Proc Inst Mech Eng H. 2018;232(1):90-100.
  61. Kim Y-H, Yoon S-H, Park J-W. Does robotic-assisted TKA result in better outcome scores or long-term survivorship than conventional TKA? A randomized, controlled trial. Clin Orthop Relat Res. 2020;478(2):266-275.
  62. King AH, Engasser WM, Sousa PL, et al. Patellar fracture following patellofemoral arthroplasty. J Arthroplasty. 2015;30(7):1203-1206.
  63. Knipe H, Pai V, et al. Kellgren and Lawrence system for classification of osteoarthritis. Radiopaedia [online], 2020. Available at: Accessed June 16, 2020.
  64. Kock FX, Beckmann J, Lechler P, et al. The 2-year follow-up results of a patient-specific interpositional knee implant. Orthopade. 2011;40(12):1103-1110.
  65. Kock FX, Weingartner D, Beckmann J, et al. Operative treatment of the unicompartmental knee arthritis - results of a nationwide survey in 2008. Z Orthop Unfall. 2011;149(2):153-159.
  66. Koeck FX, Beckmann J, Luring C, et al. Evaluation of implant position and knee alignment after patient-specific unicompartmental knee arthroplasty. Knee. 2011;18(5):294-299.
  67. Koh YG, Son J, Kwon OR, et al. Effect of post-cam design for normal knee joint kinematic, ligament, and quadriceps force in patient-specific posterior-stabilized total knee arthroplasty by using finite element analysis. Biomed Res Int. 2018;2018:2438980.
  68. Kohn MD, Sassoon AA, Fernando ND. Classifications in brief: Kellgren-Lawrence classification of osteoarthritis. Clin Orthop Relat Res. 2016;474:1186-1893.
  69. Kosse NM, Heesterbeek PJC, Schimmel JJP, et al. Stability and alignment do not improve by using patient-specific instrumentation in total knee arthroplasty: A randomized controlled trial. Knee Surg Sports Traumatol Arthrosc. 2018;26(6):1792-1799.
  70. Kouk S, Kalbian I, Wolfe E, Strickland SM. Robot-assisted medial compartment arthroplasty following remote patellectomy: A case report. J Orthop Case Rep. 2018;8(1):11-14.
  71. Levengood GA, Dupee J. Accuracy of coronal plane mechanical alignment in a customized, individually made total knee replacement with patient-specific instrumentation. J Knee Surg. 2018;31(8):792-796.
  72. Li X, Wang C, Guo Y, Chen W. An approach to developing customized total knee replacement implants. J Healthc Eng. 2017;2017:9298061.
  73. Liu Q, Geng P, Shi L, et al. Tranexamic acid versus aminocaproic acid for blood management after total knee and total hip arthroplasty: A systematic review and meta-analysis. Int J Surg. 2018;54(Pt A):105-112.
  74. Lonner JH, John TK, Conditt MA. Robotic arm-assisted UKA improves tibial component alignment: A pilot study. Clin Orthop Relat Res. 2010;468(1):141-146.
  75. Lonner JH. Modular bicompartmental knee arthroplasty with robotic arm assistance. Am J Orthop. 2009;38(2 Suppl):28-31.
  76. Luring C, Tingart M, Drescher W, et al. Therapy of isolated arthritis in the patellofemoral joint: Are there evidence-based options? Orthopade. 2011;40(10):902-906.
  77. Lyons MC, MacDonald SJ, Somerville LE, et al. Unicompartmental versus total knee arthroplasty database analysis: Is there a winner? Clin Orthop Relat Res. 2012;470(1):84-90.
  78. Mahoney OM, Kinsey T. Overhang of the femoral component in total knee arthroplasty: Risk factors and clinical consequences. J Bone Joint Surg Am. 2010;92(5):1115-1121.
  79. Marimuthu K, Chen DB, Harris IA, et al. A multi-planar CT-based comparative analysis of patient-specific cutting guides with conventional instrumentation in total knee arthroplasty. J Arthroplasty. 2014;29(6):1138-1142.
  80. Martin GM, Crowley M. Total knee arthroplasty. UpToDate [online serial]. Waltham, MA: UpToDate; reviewed April 2018.
  81. Meena S, Benazzo F, Dwivedi S, Ghiara M. Topical versus intravenous tranexamic acid in total knee arthroplasty. J Orthop Surg (Hong Kong). 2017;25(1):2309499016684300.
  82. Meier M, Calliess T, Tibesku C, Beckmann J. New technologies (robotics, custom-made) in unicondylar knee arthroplasty-pro. Orthopade. 2021;50(2):130-135.
  83. Meier M, Zingde S, Steinert A, et al. What Is the possible impact of high variability of distal femoral geometry on TKA? A CT data analysis of 24,042 knees. Clin Orthop Relat Res. 2019;477(3):561-570.
  84. Merle C, Aldinger PR. New technologies (robotics, "custom-made") for unicondylar knee arthroplasty-contra. Orthopade. 2021 Feb;50(2):124-129.
  85. Mont MA, Stuchin SA, Paley D, et al. Different surgical options for monocompartmental osteoarthritis of the knee: High tibial osteotomy versus unicompartmental knee arthroplasty versus total knee arthroplasty: Indications, techniques, results, and controversies. Instr Course Lect. 2004;53:265-283.
  86. Morrison TA, Nyce JD, Macaulay WB, Geller JA. Early adverse results with bicompartmental knee arthroplasty. A prospective cohort comparison to total knee arthroplasty. J Arthroplasty. 2011;26(6 Suppl):35-39.
  87. Moubarak H, Brilhault J. Contribution of patient-specific cutting guides to lower limb alignment for total knee arthroplasty. Orthop Traumatol Surg Res. 2014;100(4 Suppl):S239-S242.
  88. Nakamura S, Ito H, Nakamura K, et al. Long-term durability of ceramic tri-condylar knee implants: A minimum 15-year follow-up. J Arthroplasty. 2017;32(6):1874-1879.
  89. Nam D, Nunley RM, Berend KR, et al. The impact of custom cutting guides on patient satisfaction and residual symptoms following total knee arthroplasty. Knee. 2016a;23(1):144-148.
  90. Nam D, Park A, Stambough JB, et al. The Mark Coventry Award: Custom cutting guides do not improve total knee arthroplasty: Clinical outcomes at 2 years followup. Clin Orthop Relat Res. 2016b;474(1):40-46.
  91. Namin AT, Jalali MS, Vahdat V, et al. Adoption of new medical technologies: The case of customized individually made knee implants. Value Health. 2019;22(4):423-430.
  92. Nankivell M, West G, Pourgiezis N. Operative efficiency and accuracy of patient-specific cutting guides in total knee replacement. ANZ J Surg. 2015;85(6):452-455.
  93. National Institute for Health and Clinical Excellence (NICE). Individually magnetic resonance imaging-designed unicompartmental interpositional implant insertion for osteoarthritis of the knee. Interventional Procedure Guidance 317. London, UK: NICE; September 2009.
  94. Newman J, Pydisetty RV, Ackroyd C. Unicompartmental or total knee replacement: The 15-year results of a prospective randomised controlled trial. J Bone Joint Surg Br. 2009;91(1):52-57.
  95. Nunley RM, Ellison BS, Zhu J, et al. Do patient-specific guides improve coronal alignment in total knee arthroplasty? Clin Orthop Relat Res. 2012;470(3):895-902.
  96. O'Connor MI, Blau BE. The economic value of customized versus off-the-shelf knee implants in Medicare fee-for-service beneficiaries. Am Health Drug Benefits. 2019;12(2):66-73.
  97. Odgaard A, Eldridge J, Madsen F. Patellofemoral arthroplasty. JBJS Essent Surg Tech. 2019;9(2):e15. 
  98. Odgaard A, Madsen F, Kristensen PW, et al. The Mark Coventry Award: Patellofemoral arthroplasty results in better range of movement and early patient-reported outcomes than TKA. Clin Orthop Relat Res. 2018;476(1):87-100.
  99. Odumenya M, McGuinness K, Achten J, et al. The Warwick patellofemoral arthroplasty trial: A randomised clinical trial of total knee arthroplasty versus patellofemoral arthroplasty in patients with severe arthritis of the patellofemoral joint. BMC Musculoskelet Disord. 2011;12:265. 
  100. Ontario Ministry of Health and Long-Term Care, Medical Advisory Secretariat (MAS). Total knee replacement. Health Technology Literature Review. Toronto, ON: MAS; June 2005.
  101. Palumbo BT, Henderson ER, Edwards PK, et al. Initial experience of the Journey-Deuce bicompartmental knee prosthesis. A review of 36 Cases. J Arthroplasty. 2011;26(6 Suppl):40-45.
  102. Pandit H, Beard DJ, Jenkins C, et al. Combined anterior cruciate reconstruction and Oxford unicompartmental knee arthroplasty. J Bone Joint Surg Br. 2006;88(7):887-892.
  103. Pandit H, Mancuso F, Jenkins C, et al. Lateral unicompartmental knee replacement for the treatment of arthritis progression after medial unicompartmental replacement. Knee Surg Sports Traumatol Arthrosc. 2017;25(3):669-674.
  104. Panteli M, Papakostidis C, Dahabreh Z, Giannoudis PV. Topical tranexamic acid in total knee replacement: A systematic review and meta-analysis. Knee. 2013;20(5):300-309.
  105. Parratte S, Blanc G, Boussemart T, et al. Rotation in total knee arthroplasty: No difference between patient-specific and conventional instrumentation. Knee Surg Sports Traumatol Arthrosc. 2013;21(10):2213-2219.
  106. Parratte S, Ollivier M, Lunebourg A, et al. Long-term results of compartmental arthroplasties of the knee: Long term results of partial knee arthroplasty. Bone Joint J. 2015;97-B(10 Suppl A):9-15.
  107. Parratte S, Pauly V, Aubaniac JM, Argenson JN. Survival of bicompartmental knee arthroplasty at 5 to 23 years. Clin Orthop Relat Res. 2010;468(1):64-72.
  108. Patil S, Bunn A, Bugbee WD, et al. Patient-specific implants with custom cutting blocks better approximate natural knee kinematics than standard TKA without custom cutting blocks. Knee. 2015;22(6):624-629.
  109. Pearle AD, Kendoff D, Stueber V, et al. Perioperative management of unicompartmental knee arthroplasty using the MAKO robotic arm system (MAKOplasty). Am J Orthop. 2009;38(2 Suppl):16-19.
  110. Pennington DW, Swienckowski JJ, Lutes WB, Drake GN. Lateral unicompartmental knee arthroplasty: Survivorship and technical considerations at an average follow-up of 12.4 years. J Arthroplasty. 2006;21(1):13-17.
  111. Pfitzner T, Abdel MP, von Roth P, et al. Small improvements in mechanical axis alignment achieved with MRI versus CT-based patient-specific instruments in TKA: A randomized clinical trial. Clin Orthop Relat Res. 2014;472(10):2913-22.
  112. Pourgiezis N, Reddy SP, Nankivell M, et al. Alignment and component position after patient-matched instrumentation versus conventional total knee arthroplasty. J Orthop Surg (Hong Kong). 2016;24(2):170-174.
  113. Predescu V, Prescura C, Olaru R, et al. Patient specific instrumentation versus conventional knee arthroplasty: Comparative study. Int Orthop. 2017;41(7):1361-1367.
  114. Randelli PS, Menon A, Pasqualotto S, et al. Patient-specific instrumentation does not affect rotational alignment of the femoral component and perioperative blood loss in total knee arthroplasty: A prospective, randomized, controlled trial. J Arthroplasty. 2019;34(7):1374-1381.
  115. Rees JL, Price AJ, Beard DJ, et al. Minimally invasive Oxford unicompartmental knee arthroplasty: Functional results at 1 year and the effect of surgical inexperience. Knee. 2004;11(5):363-367.
  116. Reimann P, Brucker M, Arbab D, Lüring C. Patient satisfaction - A comparison between patient-specific implants and conventional total knee arthroplasty. J Orthop. 2019;16(3):273-277. 
  117. Roche M, O'Loughlin PF, Kendoff D, Robotic arm-assisted unicompartmental knee arthroplasty: Preoperative planning and surgical technique. Am J Orthop. 2009;38(2 Suppl):10-15.
  118. Rolston L, Bresch J, Engh G, et al. Bicompartmental knee arthroplasty: A bone-sparing, ligament-sparing, and minimally invasive alternative for active patients. Orthopedics. 2007;30(8 Suppl):70-73.
  119. Ruangsomboon P, Ruangsomboon O, Pornrattanamaneewong C, et al. Clinical and radiological outcomes of robotic-assisted versus conventional total knee arthroplasty: A systematic review and meta-analysis of randomized controlled trials. Acta Orthop. 2023;94:60-79.
  120. Rudran B, Magill H, Ponugoti N, et al. Functional outcomes in patient specific instrumentation vs. conventional instrumentation for total knee arthroplasty; a systematic review and meta-analysis of prospective studies. BMC Musculoskelet Disord. 2022;23(1):702.
  121. Russell R, Brown T, Huo M, Jones R. Patient-specific instrumentation does not improve alignment in total knee arthroplasty. J Knee Surg. 2014;;27(6):501-504.
  122. Sabatini L, Giachino M, Risitano S, Atzori F. Bicompartmental knee arthroplasty. Ann Transl Med. 2016;4(1):5.
  123. Saldanha KA, Keys GW, Svard UC, et al. Revision of Oxford medial unicompartmental knee arthroplasty to total knee arthroplasty - results of a multicentre study. Knee. 2007;14(4):275-279.
  124. Schroeder L, Martin G. In vivo tibial fit and rotational analysis of a customized, patient-specific TKA versus off-the-shelf TKA. J Knee Surg. 2019;32(6):499-505.
  125. Schroeder L, Neginhal V, Kurtz WB. Patient satisfaction, functional outcomes and survivorship in patients with a customized posterior stabilized total knee arthroplasty [abstract]. Orthop Proc. 2019;101-B (Suppl 4).
  126. Schwarzkopf R, Brodsky M, Garcia GA, Gomoll AH. Surgical and functional outcomes in patients undergoing total knee replacement with patient-specific implants compared with "off-the-shelf" implants. Orthop J Sports Med. 2015;3(7):2325967115590379.
  127. Scott RD, Joyce MJ, Ewald FC, Thomas WH. McKeever metallic hemiarthroplasty of the knee in unicompartmental degenerative arthritis. Long-term clinical follow-up and current indications. J Bone Joint Surg Am. 1985;67(2):203-207.
  128. Scott RD. UniSpacer: Insufficient data to support its widespread use. Clin Orthop. 2003;(416):164-166.
  129. Seeger JB, Cardenas-Montemayor E, Becker JF, et al. The UniSpacer: Correcting varus malalignment in medial gonarthrosis. Preliminary results. Rev Esp Cir Ortop Traumatol. 2013;57(1):15-20.
  130. Sinha RK. Outcomes of robotic arm-assisted unicompartmental knee arthroplasty. Am J Orthop. 2009;38(2 Suppl):20-22.
  131. Sinha RK. The use of customized TKA implants for increased efficiency in the OR. Curr Rev Musculoskelet Med. 2012;5(4):296-302.
  132. Sisto DJ, Mitchell IL. UniSpacer arthroplasty of the knee. J Bone Joint Surg Am. 2005;87(8):1706-1711.
  133. Steinert AF, Beckmann J, Holzapfel BM, et al. Bicompartmental individualized knee replacement: Use of patient-specific implants and instruments (iDuo™). Oper Orthop Traumatol. 2017;29(1):51-58.
  134. Stolarczyk A, Nagraba L, Mitek T, et al. Does patient-specific instrumentation improve femoral and tibial component alignment in total knee arthroplasty? A prospective randomized study. Adv Exp Med Biol. 2018;1096:11-17.
  135. Stone AH, Sibia US, MacDonald JH. Functional outcomes and accuracy of patient-specific instruments for total knee arthroplasty. Surg Innov. 2018;25(5):470-475.
  136. Sulzer Orthopedics Inc. UniSpacer Knee System. Austin, TX: Sulzer Orthopedics; September 30, 2002. Available at: Accessed September 30, 2002.
  137. SulzerMedica. New minimally invasive surgical procedure for arthritis may delay knee replacement surgery. Press Release. Zurich, Switzerland: SulzerMedica; April 4, 2002.  Available at:
    UniSpacer/press_area/UniSpacer_press_release.pdf. Accessed September 30, 2002.
  138. Tammachote N, Panichkul P, Kanitnate S. Comparison of customized cutting block and conventional cutting instrument in total knee arthroplasty: A randomized controlled trial. J Arthroplasty. 2018;33(3):746-751.
  139. Teeter MG, Marsh JD, Howard JL, et al. A randomized controlled trial investigating the value of patient-specific instrumentation for total knee arthroplasty in the Canadian healthcare system. Bone Joint J. 2019;101-B(5):565-572.
  140. Thienpont E, Price A. Bicompartmental knee arthroplasty of the patellofemoral and medial compartments. Knee Surg Sports Traumatol Arthrosc. 2013;21(11):2523-2531.
  141. Thienpont E, Schwab PE, Fennema P. Efficacy of patient-specific instruments in total knee arthroplasty: A systematic review and meta-Analysis. J Bone Joint Surg Am. 2017;99(6):521-530.
  142. Tibesku CO, Haas SB, Saunders C, Harwood DA. Comparison of clinical outcomes of VISIONAIRE patient-specific instrumentation with conventional instrumentation in total knee arthroplasty: A systematic literature review and meta-analysis. Arch Orthop Trauma Surg. 2022 Nov 30 [Epub ahead of print].
  143. Tice JA. Knee joint spacer (UniSpacer) system for osteoarthritis of the knee. Technology Assessment. San Francisco, CA: California Technology Assessment Forum; February 13, 2003. 
  144. Tria AJ Jr. Bicompartmental knee arthroplasty: The clinical outcomes. Orthop Clin North Am. 2013;44(3):281-286.
  145. Turgeon TR, Cameron B, Burnell CD, et al. A double-blind randomized controlled trial of total knee replacement using patient-specific cutting block instrumentation versus standard instrumentation. Can J Surg. 2019;62(6):460-467.
  146. van der List JP, Chawla H, Zuiderbaan HA, Pearle AD. Survivorship and functional outcomes of patellofemoral arthroplasty: A systematic review. Knee Surg Sports Traumatol Arthrosc. 2017;25(8):2622-2631.
  147. Vide J, Freitas TP, Ramos A, et al. Patient-specific instrumentation in total knee arthroplasty: Simpler, faster and more accurate than standard instrumentation -- a randomized controlled trial. Knee Surg Sports Traumatol Arthrosc. 2017;25(8):2616-2621.
  148. Walton NP, Jahromi I, Lewis PL, et al. Patient-perceived outcomes and return to sport and work: TKA versus mini-incision unicompartmental knee arthroplasty. J Knee Surg. 2006;19(2):112-116.
  149. Wang H, Foster J, Franksen N, et al. Gait analysis of patients with an off-the-shelf total knee replacement versus customized bi-compartmental knee replacement. Int Orthop. 2018;42(4):805-810.
  150. Wang H, Shen B, Zeng Y. Blood loss and transfusion after topical tranexamic acid administration in primary total knee arthroplasty. Orthopedics. 2015;38(11):e1007-e1016.
  151. Wang HY, Wang L, Luo ZY, et al. Intravenous and subsequent long-term oral tranexamic acid in enhanced-recovery primary total knee arthroplasty without the application of a tourniquet: A randomized placebo-controlled trial. BMC Musculoskelet Disord. 2019;20(1):478.
  152. Wang Q, Zhang N, Guo W, et al. Patient-specific instrumentation (PSI) in total ankle arthroplasty: A systematic review. Int Orthop. 2021;45(9):2445-2452.
  153. Washington State Department of Labor and Industries. UniSpacer Coverage Decision. Coverage Decisions for Medical Technologies and Procedures. Olympia, WA: Washington State Department of Labor and Industries; 2005. 
  154. Washington State Department of Labor and Industries. Criteria for knee surgery. Medical Treatment Guidelines. Olympia, WA: Washington State Department of Labor and Industries; June 1999.
  155. Wheatley B, Nappo K, Fisch J, et al. Early outcomes of patient-specific posterior stabilized total knee arthroplasty implants. J Orthop. 2019;16(1):14-18.
  156. Wilson HA, Middleton R, Abram SGF, et al. Patient relevant outcomes of unicompartmental versus total knee replacement: Systematic review and meta-analysis. BMJ. 2019;364:l352.
  157. Woon JTK, Zeng ISL, Calliess T, et al. Outcome of kinematic alignment using patient-specific instrumentation versus mechanical alignment in TKA: A meta-analysis and subgroup analysis of randomised trials. Arch Orthop Trauma Surg. 2018;138(9):1293-1303.
  158. Work Loss Data Institute. Knee & leg (acute & chronic). Encinitas, CA: Work Loss Data Institute; November 29, 2013.
  159. Xiang S, Zhao Y, Li Z, et al. Clinical outcomes of ceramic femoral prosthesis in total knee arthroplasty: A systematic review. J Orthop Surg Res. 2019;14(1):57.
  160. Xu S, Chen JY, Zheng Q, et al. The safest and most efficacious route of tranexamic acid administration in total joint arthroplasty: A systematic review and network meta-analysis. Thromb Res. 2019;176:61-66.
  161. Yang KY, Wang MC, Yeo SJ, Lo NN. Minimally invasive unicondylar versus total condylar knee arthroplasty -- early results of a matched-pair comparison. Singapore Med J. 2003;44(11):559-562.
  162. Young SW, Walker ML, Bayan A, et al. The Chitranjan S. Ranawat Award: No difference in 2-year functional outcomes using kinematic versus mechanical alignment in TKA: A randomized controlled clinical trial. Clin Orthop Relat Res. 2017;475(1):9-20.
  163. Zahn RK, Graef F, Conrad JL, et al. Accuracy of tibial positioning in the frontal plane: A prospective study comparing conventional and innovative techniques in total knee arthroplasty. Arch Orthop Trauma Surg. 2020;140(6):793-800.
  164. Zeller IM, Sharma A, Kurtz WB, et al.  Customized versus patient-sized cruciate-retaining total knee arthroplasty: An in vivo kinematics study using mobile fluoroscopy. J Arthroplasty. 2017;32(4):1344-1350.
  165. Zhang W, Moskowitz RW, Nuki G, et al. OARSI recommendations for the management of hip and knee osteoarthritis, Part II: OARSI evidence-based, expert consensus guidelines. Osteoarthritis Cartilage. 2008;16(2):137-162.