A 5-5 one degree of freedom fully-parallel mechanism for the modelling of passive motion at the human ankle joint

This paper presents a new equivalent spatial mechanism for the passive motion simulation at the human ankle complex joint. The mechanism is based on the geometry of the main anatomical structures of the ankle complex, such as the shape of the talus and tibio/fibula bones at their interface, and the TiCal and CaFil ligament lengths. In particular, three sphere-to-sphere contact points at the interface have been identified and isometric fibers of both TiCal and CaFil ligaments have been considered to devise the equivalent mechanism. The proposed mechanism is a fully-parallel mechanism of type 5-5 with one degree of freedom. A procedure for the optimal synthesis of the mechanism is given. Simulation results compared with experimental data show the efficiency of the proposed mechanism to replicate the ankle passive motion, and also to reflect at the same time the main anatomical structures of the ankle joint. The new mechanism is believed to be a useful tool for both pre-operation planning and prosthesis design.