Influence of Tibial Component Design Features and Interference Fit on Implant-Bone Micromotion in Total Ankle Replacement: A Finite Element Study.

Background: Implant survivorship in uncemented total ankle replacement (TAR) is dependent on achieving initial stability. This is because early micromotion between the implant and bone can disrupt the process of osseointegration, leading to poor long-term outcomes. Tibial implant fixation features are designed to resist micromotion, aided by bony sidewall retention and interference fit. The goal of this study was to investigate design-specific factors influencing implant-bone micromotion in TAR tibial components with interference fit.

Methods: Three implant designs with fixation features representative of current TAR tibial components (ARC, SPIKES, KEEL) were virtually inserted into models of the distal tibias of 2 patients with end-stage ankle arthritis. Tibia models were generated from deidentified patient computed tomography scans, with material properties for modeling bone behavior and compaction during press-fit. Finite element analysis (FEA) was used to simulate 2 fixation configurations: (1) no sidewalls or interference fit, and (2) sidewalls with interference fit. Load profiles representing the stance phase of gait were applied to the models, and implant-bone micromotions were computed from FEA output.

Results: Sidewalls and interference fit substantially influenced implant-bone micromotions across all designs studied. When sidewalls and interference fit were modeled, average micromotions were less than 11 µm, consistent across the stance phase of gait. Without sidewalls or interference fit, micromotions were largest near either heel strike or toe-off. In the absence of sidewalls and interference fit, the amount of micromotion generally aligned inversely with the size of implant fixation features; the ARC design had the largest micromotion (~540 µm average), whereas the KEEL design had the smallest micromotion (~15 µm).

Conclusion: This study presents new insights into the effect of TAR fixation features on implant-bone micromotion. With sidewalls and interference fit, micromotion is predicted to be minimal for implants, whereas with no sidewalls and no interference fit, micromotion depended primarily on the implant design.

Clinical relevance: This study presents new insights into the effect of TAR primary fixation features on implant-bone micromotion. Although design features heavily influenced implant stability in the model, their influence was greatly diminished when interference fit was introduced. The results of this study show the relative importance of design features and interference fit in the predicted initial stability of uncemented TAR, potentially a key factor in implant survivorship.