This work presents a 3D kinematic model for the passive flexion motion of the tibio-talar joint. With only knowledge of the articular surface shapes, the spatial trajectory of passive motion is obtained as the envelop of joint configurations that maximize the surfaces congruence. An increase in joint congruence corresponds to an improved capability of distributing an applied load, allowing the joint to attain better strength with less material. Thus, joint congruence maximization is a simple geometric way to capture the idea of joint energy minimization. The results obtained are compared with in vitro measured trajectories. Preliminary experimental data provide strong support for the predictions of the theoretical model.
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