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Biomechanical comparison of two total ankle replacement designs : micromotion and kinematic patterns McInnes, Kurtis Anthony


Introduction: Arthritis is a degenerative disease that causes irreversible damage to a joint’s articular cartilage. Despite having high failure rates in early total ankle arthroplasty (TAA) models, recent improvements have increased the success of this procedure, providing end-stage ankle arthritis patients a viable alternative to fusion with better functional outcomes. Currently, the most prevalent cause of failure is aseptic loosening, which is believed to be affected by motion at the bone-implant interface. The objective of this study was to compare micromotion and kinematic patterns of two TAA designs. Methods: A mechanical simulator was designed to apply compressive loads and bending moments to human cadaveric ankles, intact and replaced. It induced a maximal range of motion in the ankle about 3 orthogonal axes: plantarflexion-dorsiflexion (PF-DF), inversion-eversion (INV-EV), and internal-external rotation (IR-ER). Six ankle pairs were tested and compared. The implants analyzed were the Agility™ and the Scandinavian Total Ankle Replacement (S.T.A.R.®). Using an optical motion capture system, tibiocalcaneal kinematics and the relative bone-implant motion for each implant were recorded and analyzed. Results: The Agility exhibited a greater amount of micromotion between the bone and prosthesis than the STAR for the tibial component in INV-EV (p=0.037), and for the talar component in PF-DF (p=0.002) and IR-ER (p=0.038). Micromotion magnitudes were affected by loading direction and compression. Kinematic changes were observed following replacement of the ankle joint. There were decreases in the amount of motion coupling for both implants when loaded in INV-EV and IR-ER. There were increases in joint translation for both implants in the medial/lateral direction under INV-EV loading, and for the STAR in the anterior/posterior and compression/distraction directions under PF-DF loading. No significant ROM differences were found. Discussion: Increased micromotion in the Agility supports the hypothesis that higher aseptic loosening rates are correlated with reduced initial post-op fixation. The effect of loading direction on micromotion magnitude confirms the need to apply a variety of loading conditions to obtain a comprehensive micromotion analysis. Kinematic differences between implanted and intact ankles show that there is still room for improvement towards an ankle replacement design that replicates the performance of a healthy ankle.

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