The human ankle anatomical complex comprises the ankle and subtalar joints. The ankle represents the tibiotalar joint, connecting the tibia above, the fibula laterally and the talus below. Modelling of human joint passive motion is of great clinical relevance both for ligament reconstruction and for prosthesis design. The use of (equivalent) planar and spatial mechanisms for the kinematic modelling of joint passive motion proved to be a promising approach as it has been successfully utilized for the knee joint recently. Clinical evidence and experimental measurements led to infer that the relative passive motion between the tibia and the talus is a complex but single repeatable path constrained by articular surfaces and ligaments. The relative motion has the feature of a one degree of freedom rigid body guidance motion. Based on these considerations this paper presents two different equivalent spatial parallel mechanisms for this modelling. The mechanism links are taken from bones, ligaments and tendon structures, while kinematic pairs are appropriately chosen according to the corresponding type of anatomical connections. Simulation results are compared with corresponding experiments ones confirming the potential of the proposed approach.

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