Mobility of the subtalar joint in the intact ankle complex

A previous study by these authors showed that the calcaneus follows a uniqu e path of unresisted coupled motion relative to the tibia Juring passive fl exion and that most of this motion occurred at the ankle level. Subtalar mo tion in the intact ankle complex was observed only when perturbations from this path were induced by the application of force to the calcaneus. Relati ve motion of the bones of the ankle complex was tracked by stereophotogramm etry in seven specimens. Anatomical landmarks, reference frames and joint a ngles were defined by standard techniques. Sequential moments were applied to the calcaneus about the long axis of the tibia. Measured movements at su btalar level demonstrated plantarflexion coupled to supination and internal rotation (inversion) and dorsiflexion coupled to pronation and external ro tation (eversion). These movements were resisted and were fully recovered w hen the external load was removed. Subtalar motion diminished as the ankle approached maximal dorsi- and plantarflexion. Two clearly distinguished mea n axes of rotation were observed for inversion and eversion runs. The axes of inversion and eversion of the subtalar complex changed orientation along a preferred and repeatable path. The subtalar joint complex occupied only a single stable position in the unloaded state and with no range of unresis ted motion. It is inferred that mobility was possible only by the stretchin g and lengthening of the ligaments and the indentation of the articular sur faces, requiring the application of loads. The subtalar joint complex behav es like a flexible structure. (C) 2001 Elsevier Science Ltd. All rights res erved.