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Background
Flatfoot (hyperpronation and flattening-out of the longitudinal arch) (also known as pes planus or pes planovalgus) is a common deformity among children and adults. Another cause of flatfoot can be attributed to posterior tibial tendon dysfunction. Conservative treatments to relieve pain from the foot and leg associated with flatfoot include orthotics, stretching exercises, and medication (e.g., non-steroidal anti-inflammatory drugs). Corticosteroid injections continue to be controversial. These methods may fail to provide relief and do not provide any correction at the point of contact. Various surgical techniques of subtalar joint arthroereisis have been used in the treatment of patients who have failed conservative approaches. Some surgeons use bone blocks and bond grafts placed into the sinus tarsi to limit excessive subtalar joint pronation. Others advocate the use of endoprosthetic devices. Arthroereisis is the limitation of exogenous joint motion without complete arthrodesis. This has been achieved by using a variety of endoprosthetic devices such as the subtalar arthroereisis peg, the Silastic silicone sphere, and the Subtalar Maxwell-Brancheau arthroereisis (MBA) implant (KMI - Kinetikos Medical Incorporated, San Diego, CA).
The Subtalar MBA implant was cleared by the U.S. Food and Drug Administration (FDA) via a 510(k) premarket notification in 1996. It is an "internal orthotic" designed for correction of pediatric pes valgus and adult posterior tibial dysfunction deformity. There are 5 different MBA implant sizes: 6, 8, 9, 10, and 12 mm in diameter. The implant is a soft-threaded titanium device that is inserted into the sinus tars. It aims to restore the arch by blocking the anterior and inferior displacement of the talus and by preventing the foot from pronating; thus allowing normal subtalar joint motion. Tissue grows normally around the implant and aids in holding it in place. In adults, ancillary procedures may be performed simultaneously (e.g., an Achilles tendon lengthening if an equines deformity is present). The patient can ambulate the day after surgery in a Cam Walker for approximately 3 weeks. Thereafter, regular shoes can be worn with an ankle brace for an additional 2 to 3 weeks.
Husain and Fallat (2002) performed biomechanical analysis of MBA implants in fresh-frozen cadaver limbs to quantitate the effects on subtalar joint motion restriction and radiographic angles. This study did not contain any clinical data on the value of MBA implants.
Well-designed studies are needed to ascertain the effectiveness and durability of the Subtalar MBA implant for the treatment of pathologic flatfoot.
Needleman (2006) ascertained the functional outcomes as well as radiographical results of adult patients who had an operation for flexible flatfeet without any hind-foot osteotomies or fusions. A total of 28 feet in 23 patients with problems caused by their flexible flatfoot deformities had reconstructive foot and ankle surgery that included a subtalar arthroereisis with the MBA sinus tarsi implant. The American Orthopedic Foot and Ankle Society (AOFAS) Hind-foot Scale and a patient assessment questionnaire were obtained from all patients before surgery and at final follow-up. Pre-operative and post-operative standing radiographs were analyzed to determine radiographical correction of the deformities. The average follow-up was 44 months. The MBA implant was surgically removed in 11 of 28 feet (39 %) because of sinus tarsi pain. The average pre-operative AOFAS score was 52 and had improved to 87 (p < 0.00001) at final follow-up. The average response to 4 of 5 questions in the patient assessment had significantly improved (p < 0.05). On a 10-point scale, average patient satisfaction was 8.3 points; 78 % said that they would have the surgery again. Correction after surgery was significant (p < 0.0001) in each of the 3 radiographical parameters evaluated for 'correction with MBA' and "final correction". With the numbers available, no significant differences could be detected after the MBA was removed. Complications included sinus tarsi pain in 46 % (13) of the 28 feet in this study; after implant removal, 73 % (8) of 11 feet had less discomfort than before surgery with AOFAS scores 80 or better. The author concluded that reconstructive foot and ankle surgery that included a subtalar arthroereisis with the MBA sinus tarsi implant resulted in favorable clinical outcomes and patient satisfaction in 78 % (18) of 23 patients. In spite of the high incidence of temporary sinus tarsi pain until the implant was removed, this operative approach compares favorably with other operations for flexible flatfoot deformities in adults. The major drawbacks of this study were its small sample size as well as the multiple etiologies of the flexible flatfoot deformity (13 feet had congenital etiologies and 15 feet had acquired etiologies). Other pitfalls of this study included the 42 % occurrence of sinus tarsi discomfort and the associated 8 post-operative months of pain until the implant was removed.
In a review on acquired adult flatfoot deformity (AAFD), Pinney and Lin (2006) described the key elements of AAFD and outlined therapeutic options based on the peer-reviewed literature. The authors stated that the limited research on subtalar arthroereisis (the use of a sinus tarsi plug or implant to restrict eversion of the subtalar joint) in adult patients with AAFD means that there is insufficient evidence to make a recommendation for or against this treatment option.
The Interventional Procedures Advisory Committee of the National Institute for Clinical Excellence (NICE, 2008) examined sinus tarsi implant insertion for mobile flatfoot. Provisional recommendations from the Committee stated "[c]urrent evidence on the safety and efficacy of sinus tarsi implant for mobile flatfoot is inadequate in quality and quantity". Furthermore, a review of the published literature on this procedure that was commissioned by NICE (2008) identified 8 case series and 4 case reports (643 feet) of sinus tarsi implant insertion for mobile flatfoot; no prospective comparative data were found. The published literature focused mainly on the pediatric population. Only 1 case series (n = 23) was reported in adults. The provisional review found that the procedures described in the studies varied significantly, particularly in relation to the design, size and instrumentation/insertion of the implant(s). The Committee recommended that this procedure should only be used with special arrangements for clinical governance, consent and audit or research. Formal guidance on this procedure (NICE, 2009) concluded: "Current evidence on the safety and efficacy of sinus tarsi implant insertion for mobile flatfoot is inadequate in quality and quantity. Therefore this procedure should only be used with special arrangements for clinical governance, consent and audit or research."
In a retrospective study, Scharer et al (2010) evaluated the outcome of pediatric patients who have undergone MBA subtalar implants for the treatment of painful pediatric flatfoot deformities. A total of 39 patients (68 feet) were evaluated clinically and radiographically. The mean age of the patients was 12 years (range of 6 to16 years). The mean period of follow-up was 24 months (range of 6 to 61 months). Statistical evaluation was performed on all radiographical measurements. Additional surgical procedures (gastrocnemius recession, Achilles tendon lengthening, Kidner posterior tibial tendon advancement) were performed in 22 of 68 feet. There were 10 (15 %) complications, which consisted of 10 re-operations in 10 feet. Implants were exchanged in 9 feet because of implant migration, under-correction, and over-correction. There was 1 re-operation (in 1 foot) for implant removal because of persistent sinus tarsi pain. Radiographical evaluation demonstrated an improvement of all parameters determined. The parameters that were evaluated include talo-navicular joint coverage, as well as lateral and anterior-posterior talo-calcaneal angles. There were significant changes noted in pre- and post-operative measurements (p < 0.001). The authors concluded that the MBA implant is effective for the correction of painful, flexible flatfoot deformity in children in short-term follow-up. However, this is a multi-planar deformity, and additional procedures may be needed in addition to the MBA.
Yu et al (2011) reviewed the application progress of subtalar arthroereisis for the correction of pediatric flatfoot in children and analyzed the problems at present as well as to predict the trend of development in the field. Domestic and abroad literature concerning the methods of subtalar arthroereisis applied in pediatric flatfoot in recent years was reviewed extensively and thoroughly analyzed. Subtalar arthroereisis has proved to yield good results for correction of the flatfoot in children. In addition to the advantages of subtalar arthroereisis for pediatric flatfoot treatment (simple procedure, mature technology, and less complications), it allows further surgery if needed. The authors concluded that subtalar arthroereisis is a simple and effective way to treat flatfoot in children, however, its biomechanics mechanism and managements to complication need to be explored further.
Metcalfe et al (2011) noted that pediatric flexible flatfoot is a common deformity for which a small, but significant number undergo corrective surgery. Arthroereisis is a technique for treating flexible flatfoot by means of inserting a prosthesis into the sinus tarsi. The procedure divides opinion in respect of both its effectiveness and safety. A database search up until 2010 was used to find articles regarding arthroereisis in pediatric patients. These researchers summarized the findings of this study. A total of 76 studies were identified; 8 of the 9 radiographical parameters reported show significant improvement following arthroereisis reflecting both increased static arch height and joint congruency. Calcaneal inclination angle demonstrated the least change with only small increases following arthroereisis. Arthroereisis remains associated with a number of complications including sinus tarsi pain, device extrusion, and under-correction. Complication rates range between 4.8 % and 18.6 % with unplanned removal rates between 7.1 % and 19.3 % across all device types. The authors concluded that current evidence is limited to consecutive case series or ad hoc case reports. Limited evidence exists to suggest that devices may have a more complex mode of action than simple motion blocking or axis altering effects. The interplay between osseous alignment and dynamic stability within the foot may contribute to the effectiveness of this procedure. They stated that although literature suggests patient satisfaction rates of between 79 % to 100 %, qualitative outcome data based on disease specific, validated outcome tools may improve current evidence and permit comparison of future study data.
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