A new mathematical model for the human ankle joint

The objective of the present study is to develop a preliminary two-dimensional mathematical model that could predict the sagittal kinematics of the human ankle/subtalar joint complex in unloaded conditions. This is a necessary preliminary step to the study of ankle joint stability in response to applied loads. From the results of a preliminary experimental investigation, the calcaneofibular and the tibiocalcaneal ligaments were selected as the ligament links of a four-bar linkage model of the ankle as they showed quasi-isometric patterns of rotation during passive ankle motion. The model predictions of calcaneus movements, ligament orientations, instantaneous centres of rotation, and conjugate talus surface profile compared well with the experimental measurements from one specimen and therefore support the assumptions underlying the formulation of the geometric model. For the first time, many features of rearfoot kinematics were explained by the linkage model. The model is intended to be applied to contact and ligament force predictions during activities, and thus, ultimately to contribute to the improvement of knowledge of these joints, to the enhancement of arthritis treatment and to the development of ankle prosthesis design and techniques of ligament reconstruction.