Geometry and mechanics of the human ankle complex and ankle prosthesis design.

The main objective of the study was to develop a model of the intact human ankle complex. It was also aimed at designing total ankle replacement which would better reproduce the physiological function of the joint. Passive flexion was analysed in seven lower-leg preparations with a stereophotogrammetric system. 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 elucidated the observed kinematics at the intact joint. The experimental evidence and the geometrical model gave the basis for the design of models of replaced ankle in the sagittal plane. A three-component, convex-tibia prosthesis was eventually selected with articular surface shapes compatible with the geometry of the ligaments. It was demonstrated that in 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 pattern of ligament tensioning. Careful reconstruction of the ligaments is recommended in any ankle surgery for maintenance of the normal kinematics and mechanics. A proposed novel design based on ligament/shape compatibility may improve total ankle replacement results.