Contact characteristics of the subtalar joint: the effect of talar neck misalignment.

In this study we determined the effects of misalignment of the talar neck on the contact characteristics of the subtalar joint. Each of seven fresh cadaver lower extremities was mounted in a loading jig and a vertical load was applied, 90% of which was directed through the tibia and 10% through the fibula. The foot was allowed to displace freely in the horizontal plane so that relative rotations, known to occur in the subtalar joint, would not be prevented. Pressure-sensitive film, inserted into the posterior and anterior/middle articulations, was used to quantify changes in contact characteristics. After testing in the normal condition, the talar neck was osteotomized and stabilized with internal and external skeletal fixation. Contact characteristics were then determined in each of the following stages: anatomic realignment, or with 2-mm displacement of the talar neck either dorsally, medially, laterally, or complex (dorsal and varus) with respect to the body of the talus. Measurements showed no significant changes in overall contact area or high pressure area in the posterior facet, although four of the seven specimens demonstrated increased localization of the contact area into two discrete regions. The combined anterior/middle facet, on the other hand, was significantly unloaded by all but medial displacement of the talar neck. An extraarticular load path and/or increased loading directly on the talonavicular joint was presumed to account for the loss of load transfer in the talocalcaneal joint.