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Mobility Ankle System

The Mobility Ankle System, developed by Pascal Rippstein, Peter Wood, and Chris Coetzee, is a three-component Buechel-Pappas type prosthesis with a short, conical tibial stem. The talar component of the Mobility implant resurfaces the superior dome of the talus, while the medial and lateral aspects of the talus remain untreated (unlike the Buechel-Pappas prosthesis). The talar component has a central, longitudinal sulcus and two fins, enhancing its intrinsic stability. The nonarticulating surfaces are porous coated with a titanium spray.

Goldberg et al reported results of their questionnaire-based survey sent to all Consultant members of the British Orthopaedic Foot & Ankle Society.1 The Mobility prosthesis was the most commonly used prosthesis among 62% of all surgeons in the United Kingdom.1 The use of this prosthesis has also been documented in the Swedish Ankle Arthroplasty Register 2 and New Zealand Ankle Arthroplasty Register.3 The Swedish Ankle Arthroplasty Register included a total of 531 total ankle replacements (TARs) with 23 Mobility prostheses implanted since 2005.2 No peri- or postoperative complications with the Mobility prosthesis were observed in the Swedish cohort.2 The New Zealand Ankle Arthroplasty Register included a total of 202 TAR with a mean follow-up of 6 years.3 Twenty-nine patients in this registry underwent Mobility prosthesis; no failures were observed in this study.3

Recently, Wood et al published early results from their prospective study that included 100 Mobility implants performed in 96 patients between 2003 and 2005.4 At a minimum follow up of 5 years, a total of five ankles (5%) had to undergo revision surgery -- two ankle fusions and three revision arthroplasties  -- resulting in 3- and 4-year survivorship of 97% (95%CI, 91% -- 99%) and 93.6% (95%CI, 84.7% -- 97.4%), respectively. In 14 ankles a radiolucent line or osteolytic cavity was observed. However, only in five ankles was it more than 10 mm in width. The authors presented encouraging short-term results which are comparable to those obtained using other modern three-component prostheses.4

Naal et al addressed habitual physical activity and sports participation before and after TAR in 137 consecutive patients with 155 implants.5 One hundred one ankles were available for the review at a mean follow up of 44 months, 54 of them were Mobility implants. The percentage of patients who were active in sports did not change after the TAR (62.4% preoperatively and 66.3% postoperatively). The most common sports activities after TAR were swimming, cycling, and fitness/weight training. The authors registered high functional improvement in their patient cohort as assessed by AOFAS scores. No association was found between sports participation, increased physical activity level, and the appearance of periprosthetic radiolucencies.5

Thermann et al reported a single case of a 58-year-old patient who underwent Mobility TAR and tibialis posterior tendon transfer for ankle osteoarthritis (OA) and drop foot deformity.6 Postoperative radiographs showed a medial malleolus fracture despite intraoperative pinning with the Kirschner wires. However, the fracture completely healed after 8 weeks. Three years after the primary procedure, the patient was asymptomatic and had a stable ankle joint with 5° dorsiflexion and 20° plantar flexion.6

Goldberg et al reported two cases of early failure in patients who underwent Mobility TAR due to component malposition.7 In both cases, the talar component was inserted back to front, as was the polyethylene insert, requiring ankle fusion in the first case and revision arthroplasty in the second case. The authors emphasized the need for adequate training of foot and ankle surgeons for performing TARs. They also suggested that designers have a responsibility to improve marketing and education for surgeons using their products to minimize incidents of incorrect use.7

Espinosa et al generated two finite element models of TAR prostheses (Agility and Mobility) to investigate the misalignment of prosthesis components on joint contact pressures.8 The authors have shown that the highly congruent mobile-bearing design of Mobility prosthesis may result in more evenly distributed contact pressures than the less congruent two-component Agility prosthesis. However, in both design the misalignment of prosthesis components may lead to pathologically increased contact stresses.8

Bell and Fisher investigated polyethylene wear in Mobility and Buechel-Pappas prostheses using a modified knee prosthesis simulator.9 The authors showed that wear in the two models was comparable and that the wear rate for both designs significantly increases with the inclusion of an anterior/posterior displacement in the kinematic inputs, simulating malalignment of prosthesis components.9

In conclusion, the Mobility TAR has been in general use since 2003 in Europe, Australia, New Zealand, South Africa, and Canada. In the United States a multicenter FDA trial including this prosthesis type is running.10

References

  1. Goldberg,A.J., Sharp,R.J., and Cooke,P.: Ankle replacement: current practice of foot & ankle surgeons in the United kingdom. Foot Ankle Int, 30:950-954, 2009.
  2. Henricson,A., Skoog,A., and Carlsson,A.: The Swedish Ankle Arthroplasty Register: An analysis of 531 arthroplasties between 1993 and 2005. Acta Orthop, 78:569-574, 2007.
  3. Hosman,A.H., Mason,R.B., Hobbs,T., and Rothwell,A.G.: A New Zealand national joint registry review of 202 total ankle replacements followed for up to 6 years. Acta Orthop, 78:584-591, 2007.
  4. Wood,P.L., Karski,M.T., and Watmough,P.: Total ankle replacement: the results of 100 mobility total ankle replacements. J Bone Joint Surg Br, 92:958-962, 2010.
  5. Naal,F.D., Impellizzeri,F.M., Loibl,M., Huber,M., and Rippstein,P.F.: Habitual physical activity and sports participation after total ankle arthroplasty. Am J Sports Med, 37:95-102, 2009.
  6. Thermann,H., Gavriilidis,I., Longo,U.G., and Maffulli,N.: Total ankle arthroplasty and tibialis posterior tendon transfer for ankle osteoarthritis and drop foot deformity. Foot Ankle Surg,epub ahead of print, 2009.
  7. Goldberg,A.J., Sharp,B., and Cooke,P.: Early failure in total ankle replacements due to component malposition: a report of two cases. Foot Ankle Int, 30:783-787, 2009.
  8. Espinosa,N., Walti,M., Favre,P., and Snedeker,J.G.: Misalignment of total ankle components can induce high joint contact pressures. J Bone Joint Surg Am, 92:1179-1187, 2010.
  9. Bell,C.J. and Fisher,J.: Simulation of polyethylene wear in ankle joint prostheses. J Biomed Mater Res B Appl Biomater, 81:162-167, 2007.
  10. Deorio,J.K. and Easley,M.E.: Total ankle arthroplasty. Instr Course Lect, 57:383-413, 2008.

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