Analysis of surface-to-surface distance mapping during three-dimensional motion at the ankle and subtalar joints.

Joint surface interaction and ligament constraints determine the kinematic characteristics of the ankle and subtalar joints. Joint surface interaction is characterized by joint contact mechanics and by relative joint surface position potentially characterized by distance mapping. While ankle contact mechanics was investigated, limited information is available on joint distance mapping and its changes during motion. The purpose of this study was to use image-based distance mapping to quantify this interaction at the ankle and subtalar joints during tri-planar rotations of the ankle complex. Five cadaveric legs were scanned using Computed Tomography and the images were processed to produce 3D bone models of the tibia, fibula, talus and calcaneus. Each leg was tested on a special linkage through which the ankle complex was loaded in dorsiflexion/plantarflexion, inversion/eversion, and internal/external rotation and the resulting bone movements were recorded. Fiduciary bone markers data and 3D bone models were combined to generate color-coded distance maps for the ankle and subtalar joints. The results were processed focusing on the changes in surface-to-surface distance maps between the extremes of the range of motion and neutral. The results provided detailed insight into the three-dimensional highly coupled nature of these joints showing significant and unique changes in distance mapping from neutral to extremes of the range of motion. The non-invasive nature of the image-based distance mapping technique could result, after proper modifications, in an effective diagnostic and clinical evaluation technique for application such as ligament injuries and quantifying the effect of arthrodesis or total ankle replacement surgery.