Musculoskeletal-modelling-based design of a novel ankle prosthesis

Abstract

This work was aimed (a) at geometrical and mechanical modelling of the human ankle complex, and (b) at designing total ankle replacements that would better reproduce the physiological function of the intact joint. Passive flexion was first analysed in intact lower-leg preparations with both optical and roentgen stereophotogrammetry. The articular surfaces and fibres within the calcaneofibular and tibiocalcaneal ligaments prescribed the changing positions of the axis of rotation. Joint motion included rolling as well as sliding. A computer-based model including skeletal bones and ligaments elucidated the observed kinematics in the sagittal plane. The experimental evidence and the modelling predictions gave the basis for model-based designs of ankle replacements. A three-component, convex-tibia prosthesis was devised with articular surface shapes compatible with the geometry of the ligaments. It was demonstrated that in an intact ankle joint, the geometry of the articular surfaces is strictly related to that of the ligaments and that current prosthesis designs do not restore physiological patterns of ligament tension. The overall investigation also demonstrates that a profound knowledge of the changing geometry of the passive joint structures throughout the range of passive flexion (mobility) is mandatory for a successful mechanical analysis of the response to external load (stability).