Dynamic simulation of the natural and replaced human ankle joint.

Disappointing results for total ankle replacement have been explained by poor knowledge of the mechanics of the intact and replaced joints. Dynamic simulation tools have the capacity to simulate dynamic conditions that occur in human joints. The Working Model 2D tool was used to simulate the mechanics of the intact and replaced ankle joints, based on previously validated mathematical models. Elementary objects were used to model ligaments, articular surfaces, retinacula and muscle-tendon units. The performance of several pairs of prosthetic articular surfaces was also analysed. According to the results of these simulations, rolling as well as sliding motion occurs in the natural ankle, governed by a ligamentous linkage. Elongation of the tibiocalcaneal and calcaneofibular ligaments was found to be 1.5% and 4.8%, respectively. A 13% change in lever arm length occurred for both the tibialis anterior and gastrocnemius muscles during ankle flexion. Unlike the currently available three-component designs, the newly proposed convex-tibial ligament-compatible prosthesis was found to be able to restore the original mobility and physiological function of the ligaments. This prosthesis combines freedom from restraint with congruity of the components throughout the range of flexion.