Joint kinematics from functional adaptation: An application to the human ankle

The aim of this paper is to exploit the concept of functional adaptation to model the motion of human joints and to present an application to the human tibio-talar articulation. With respect to previous works, a new algorithm is presented here that improves the model outcomes and numerical stability, also reducing the computational cost. Moreover, a refined measure for joint congruence is proposed, which requires only the knowledge of the articular surface shapes. This measure is hypothesized to be proportional to the joint's ability to withstand an applied load. Biological tissues tend to achieve the necessary mechanical resistance with the smallest amount of material (functional adaptation). Conversely, adapted tissues employ their material optimally, maximizing their mechanical resistance. It follows that, as a result of the functional adaptation process, an adapted joint will move along the envelope of maximum resistance and thus maximum congruence configurations. This envelope defines a spatial trajectory along which the functional adaptation requirements are satisfied and it may thus be called functionally adapted trajectory. The functionally adapted trajectory obtained by simulations is compared with in vitro measured one. Preliminary results provided strong support to the theoretical model prediction.