Knee Arthroplasty

Number: 0660


  1. Aetna considers a Food and Drug Administration (FDA) approved total knee arthroplasty (TKA) prosthesis 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 either 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;
        3. Radiographic evidence or rheumatoid arthritis (joint space narrowing); and
      4. History of of unsuccessful conservative therapy (non-surgical medical management) that is clearly addressed in the medical record (see note). Conservative therapy may be inappropriate for severe osteoarthritis with bone-on-bone articulation and severe angular deformity, or for avascular necrosis with collapse of tibial or femoral condyle. 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.

      Note: Members with osteoarthritis, traumatic arthritis, or avascular necrosis should have at least 12 weeks of nonsurgical treatment documented in the medical record (at least 24 weeks for persons with a relative contraindication), 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 contraindicationsFootnotes for Relative contraindicaitons to joint replacement* to joint replacement, optional for others); and
      6. Therapeutic injections into the knee (required for persons with relative contraindicationsFootnotes for Relative contraindicaitons to joint replacement* to joint replacement, optional for others).

        Footnotes for Relative contraindicaitons to joint replacement* Relative contraindicaitons 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. 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 total knee arthroplasty should be appropriately addressed in the medical record.
  2. Aetna considers a revision or replacement of total knee arthroplasty 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 flextion/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 infection,
      2. Poor bone quality,
      3. Highly limited quadriceps or extensor function,
      4. Poor skin coverage, and
      5. Poor vascular status.
  3. Aetna considers unicompartmental knee arthroplasty using Food and Drug Administration (FDA)-approved devices 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) that is clearly addressed in the medical record (see Note); and

      Note: Members should have at least 12 weeks of nonsurgical treatment documented in the medical record (at least 24 weeks for persons with a relative contraindication), 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 contraindicationsFootnotes for Relative contraindicaitons to unicompartmental knee arthroplasty*: to joint replacement, optional for others), and
      6. Therapeutic injections into the knee (required for persons with relative contraindicationsFootnotes for Relative contraindicaitons to unicompartmental knee arthroplasty*: to joint replacement, optional for others).

        Footnotes for Relative contraindicaitons to unicompartmental knee arthroplasty* Relative contraindicaitons to unicompartmental knee arthroplasty 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 15º; or
      5. Varus deformity greater than 15º (medial unicompartmental knee arthroplasty) or a valgus deformity greater than 20º (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. Aetna considers the UniSpacer interpositional spacer for the treatment of osteoarthritis affecting the medial compartment of the knee experimental and investigational because its effectiveness for this indication has not been established.

  5. Aetna considers bicompartmental, staged bicompartmental, and bi-unicompartmental knee arthroplasty experimental and investigational for osteoarthritis of the knee and all other indications because their effectiveness has not been established.

  6. Aetna considers customized total knee implant experimental and investigational because its effectiveness has not been established.

  7. Aetna considers prophylactic radiation therapy following total knee arthroplasty experimental and investigational because its effectiveness has not been established.

  8. Aetna considers computer-assisted musculoskeletal surgical navigation (e.g., MAKOplasty) 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.

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.

See also CPB 0661 - Joint Resurfacing.


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 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 cos- 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 Cutting Guides

Huijbregts et al (2016) 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.

Patient-Specific Implants

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.

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. 

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.


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 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)

Table: CPT Codes / HCPCS Codes / ICD-10 Codes
Code Code Description

Information in the [brackets] below has been added for clarification purposes.   Codes requiring a 7th character are represented by "+":

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]

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)

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)

HCPCS codes covered if selection criteria are met:

C1776 Joint device (implantable) [FDA approved device]

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:

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]

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

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

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

The above policy is based on the following references:

  1. Al-Hadithy N, Patel R, Navadgi B, et al. Mid-term results of the FPV patellofemoral joint replacement. Knee. 2014;21(1):138-141.
  2. American Academy of Orthopaedic Surgeons (AAOS). AAOS clinical guideline on osteoarthritis of the knee (phase II). Rosemount, IL: AAOS; 2003.
  3. 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.
  4. 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.
  5. Argenson JN, Parratte S, Flecher X, Aubaniac JM. Unicompartmental knee arthroplasty: Technique through a mini-incision. Clin Orthop Relat Res. 2007;464:32-36.
  6. 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. 
  7. 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.
  8. 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.
  9. 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.
  10. Borus T, Thornhill T. Unicompartmental knee arthroplasty. J Am Acad Orthop Surg. 2008;16(1):9-18.
  11. 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.
  12. 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.
  13. 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.
  14. 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.
  15. 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.
  16. 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.
  17. Chidel MA, Suh JH, Matejczyk MB. Radiation prophylaxis for heterotopic ossification of the knee. J Arthroplasty. 2001;16(1):1-6.
  18. 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.
  19. Clarius M, Becker JF, Schmitt H, Seeger JB. The UniSpacer: Correcting varus malalignment in medial gonarthrosis. Int Orthop. 2010;34(8):1175-1179.
  20. 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.
  21. 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.
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