A simple, anatomically based correction to the conventional ankle joint center.

BACKGROUND Conventional motion analysis studies define the ankle joint center as the midpoint between the most medial and lateral aspects of the malleoli, yet research points toward a more distal joint center location. The purpose of this study was to develop and evaluate an anatomically based correction that would move the conventional ankle joint center to a more accurate location. METHODS Lower extremity radiographs from 30 pediatric patients were analyzed retrospectively. An offset between the conventional and more accurate ankle joint centers was measured and correlated to other common anatomical measures based on conventional skin mounted marker positions. The best correlated measure was used to define a simple correction factor, which was subsequently evaluated by its effect on six degree-of-freedom ankle joint translations during normal gait (n=8). FINDINGS Shank length was found to have the highest bivariate linear correlation (r=0.89) with the offset. Adjusting the ankle joint center using a percentage of shank length (2.7%) was also as accurate as the regression equation in predicting offset (mean error 0.6mm, or 6% offset). Adjusting the ankle joint center using this simple percentage resulted in a 25% reduction in mean ankle joint translations during normal gait. INTERPRETATION The accuracy of the ankle joint center can be increased through a simple, anatomically based correction. This correction may prove beneficial in some kinematic and kinetic applications requiring increased anatomical fidelity.