Experimental study of the subtalar joint axis: preliminary investigation

An experimental study of the subtalar joint has been conducted with the aim of establishing its axis of movement as well as analysing the associated m ovement. For description of the axis, CT data for five positions of a singl e foot were reconstructed using a 3D programme, the 3D data was processed b y Patran software. Measures of angular displacements were made from three a mputated feet placed in a specially constructed foot frame. Four instantane ous axes of movement could be defined. Calculation of displacements showed an important rolling of the calcaneus (45 degrees). Tacking was evident in inversion, with an opposite displacement between the front and rear part of the calcaneus, whereas during eversion tacking affected only the rear part of the bone: these results were confirmed by 3D reconstructions. Henke's a xis was described as that for the talonavicular joint, but acceptable for t he subtalar joint. Several authors investigating the coordinates of this ax is have reported large differences and described screw-like movements, the latter being incompatible with a fixed axis: instantaneous axes, however ar e compatible with a screw-like movement. The subtalar joint appears to work as a pivot joint during inversion and as a plane joint during eversion. Al though Henke's axis has pedagogical value the subtalar joint has a series o f instantaneous axes.