Biomechanics of the first ray part V: The effect of equinus deformity. A 3-dimensional kinematic study on a cadaver model.

The positional change of the medial column of the foot in closed kinetic chain with variable Achilles tendon tension was investigated in seven fresh frozen cadaver specimens using a 3-dimensional radio wave tracking system. The distal tibia and fibula and the intact ankle and foot and were mounted on a non-metallic loading frame. The frame allowed positioning of the foot to simulate midstance phase of gait while the tibia and fibula were axially loaded to 400 N. To record osseous motion, receiving transducers were attached to the first metatarsal, medial cuneiform, navicular, and talus. Movements of these bones in 3-dimensional space were measured as specimens were axially loaded and midstance motor function was simulated using pneumatic actuators. To simulate a progressive equinus influence, force was applied to the Achilles tendon at tensile loads of 0%, 30%, and 60% of predicted maximum strength during each test trial. Osseous positions and orientations were collected and analyzed in all three cardinal planes utilizing data recorded. As Achilles load increased, the position of the first metatarsal became significantly more inverted ( P < .05). Although not statistically significant, remarkable trends of arch flattening motion were detected in the distal segments of the medial column with varied Achilles load. Increased Achilles load reduced the influence of peroneus longus on the medial column.