| Subtalar instability may be caused by various ligamentous injuries. Combined instability at the ankle and subtalar joint is not adequately diagnosed. Further, isolated subtalar instability is usually misdiagnosed which may lead to long term damage to the joint. Developing a non-invasive and clinically practical tool to diagnose subtalar joint instability would be an important asset. The ability of an ankle brace, a common treatment for hindfoot instability, to promote stability for the subtalar joint was not well established. The purposes of this study were to 1) assess the kinematics of the subtalar, ankle, and hindfoot in the presence of isolated subtalar instability; 2) investigate the effect of bracing in a calcaneofibular ligament (CFL) deficient foot and with a total rupture of the intrinsic ligaments; 3) implement an optimization method to determine the subtalar joint axis in vivo and apply this method in the diagnosis of subtalar joint instability. Kinematics from nine cadaveric feet were collected with the foot placed in neutral, dorsiflexion and plantarflexion. Motion was applied with and without a brace on an intact foot and after sequentially sectioning the CFL and the intrinsic ligaments. A two-hinge joint optimization model was developed to approximate the ankle and subtalar joint axis during inversion based on the kinematics of the calcaneus and the tibia. The optimization determined subject-specific subtalar and ankle joint axis for each condition. Isolated CFL sectioning increased ankle joint inversion while sectioning the CFL and intrinsic ligaments affected subtalar joint stability. Additionally, examining the foot in dorsiflexion significantly reduced ankle and subtalar joint motion. The ankle brace limited inversion at both joints. The inclination and deviation angles of the optimized subtalar joint axis were similar to previous studies. The orientation of the subtalar and ankle joint axes did not change after ligament injury. The optimized subtalar and ankle axes were significantly different than the 'true' subtalar and ankle joint axes determined from inversion-eversion. Future work would improve the optimization to look at the change in the angle of rotation around the optimized subtalar and ankle joint axes to detect subtalar joint instability. |
