Biomechanics of the subtalar joint complex.

The subtalar joint complex is formed by the posterior talocalcaneal joint and the acetabulum pedis lodging the talar head. The basic motion at the joint is that of male ovoid surfaces moving over female ovoid surfaces and vice versa. The motion generated at the joint is that of flexion-supination-adduction or extension-pronation-abduction. The subtalar motion is generated by the contour and orientation of the articular surfaces and is guided by the intrinsic ligaments: the interosseous talocalcaneal ligament of the canalis tarsi and the cervical ligament. Further support is provided by the extrinsic ligaments: the calcaneofibular ligament and the tibiocalcaneal fascicle of the deltoid ligament. Under tibiotalar vertical loading with internal rotation, the subtalar joint complex is in a close-pack position with maximum talar head surface contact in the acetabulum pedis. The component ligaments of the latter are under maximum tension. The posterior talocalcaneal surfaces are interlocked laterally.