A new spatial mechanism model for the kinematics analysis of the tibiotalar joint

In virtually unloaded conditions, the tibiotalar (ankle) joint behaves as a single degree-of-freedom system, and two fibres within the calcaneal-fibular and tibio-calcaneal liga-ments remain nearly isometric throughout the flexion arc [1]. A relevant theoretical model also showed that three articular surfaces and two ligaments act together as a mechanism to control the passive kinematics [2]. Two equivalent spatial parallel mechanisms were formulated, with ligament fibres assumed isometric and articulating surfaces assumed rigid, either as three sphere-plane contacts, or as a single spherical pair. Predicted and measured motion in three specimens compared fairly well. Important enhancement of this previous work is here presented, with more accurate experimental data, more anatomical model surfaces, and a more robust mathematical model.