Triaxial kinematics of ankle instability were studied in vitro by applying an inversion force to seven nonaxially loaded cadaveric ankle-foot specimens. In intact specimens, mean maximal adduction of the tibia with respect to the calcaneus was 38° and mean maximal external rotation was 24°; maximal displacement occurred near full plantar flexion. Increases after release of ligaments were as follows: calcaneofibular, maximal adduction 10%, external rotation 3% near 15° of plantar flexion; anterior talofibular, adduction 30%, external rotation 8% at 30° of plantar flexion; both, adduction 41%, external rotation 65% near 0° of flexion; all three lateral collateral, adduction 42%, external rotation 240% in slight dorsiflexion. Regardless of the status of the lateral collateral ligaments, the talus adducted and externally rotated 18° ± 1° with respect to the calcaneus. Hence, collateral ligament release had no effect on subtalar motion.
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