Inversion injury biomechanics in functional ankle instability: a cadaver study of simulated gait

The purpose of this study was to test pathogenetic models for the "unprovoked" ankle inversion injuries seen in functional ankle unstable subjects. The consequence of spatial mal-alignment of the ankle/foot complex on the risk of producing an ankle inversion torque at heel-strike and during swing-phase follow through was analyzed in cadaver simulations. Heel-strike was simulated using a 5 degrees of freedom rig in a material testing machine. A set-up capable of accelerating lower limb specimens towards a support surface simulated swing-phase follow through. Joint excursions were monitored with flexible wire goniometers. The unloaded ankle/foot complex was placed in increasing positions of talar and subtalar joint excursions. The consequences of these settings on the behavior of the ankle/foot complex at heel-strike and when the lateral part of the foot "caught" the ground during swing-phase follow through were monitored. An inversion torque at heel-strike was first seen when the unloaded foot was set in positions exceeding 30 degrees of inversion combined with full plantar flexion and 10 degrees of internal tibial rotation. A collision between the lateral border of a 20 degrees inverted, but otherwise neutral ankle/foot complex and the ground surface during swing-phase follow through forced the foot into the full limit of inversion, plantar flexion and internal tibial rotation measurable in this set-up. Clinical consequence: The study showed that the foot/ankle complex exhibits a high degree of intrinsic stability at heel-strike. The foot will thus stabilize itself and move into normal eversion at the beginning of the stance-phase even though it is set to the ground in a substantial degree of mal-alignment. In contrast, the swing-phase collision model provides a link that can connect the small deficits in inversion angle awareness measured in chronic functional ankle unstable subjects with an increased risk in this group of sustaining ankle inversion injuries.