Varus malalignment of the talar neck. Its effect on the position of the foot and on subtalar motion.

We performed an in vitro study on twelve specimens of the foot and ankle from cadavera to determine whether varus malalignment of the talar neck alters the position of the foot and subtalar motion. An osteotomy of the talar neck was performed, and the specimens were studied with and without removal of a medially based wedge of bone. Removal of the wedge produced an average varus malalignment of the talar neck of 17.1 +/- 2.4 degrees (range, 12.5 to 21.0 degrees). In the coronal plane, the average arc of motion of the subtalar joint decreased from 17.2 +/- 3.3 degrees before the osteotomy to 11.7 +/- 2.9 degrees after the osteotomy and removal of the wedge. In the transverse plane, it decreased from 17.5 +/- 2.9 degrees to 11.9 +/- 2.4 degrees. In the sagittal plane, it decreased from 8.9 +/- 2.4 degrees to 6.8 +/- 2.3 degrees. The decrease in subtalar motion was characterized by an inability to evert the foot; inversion was not limited, however. The malalignment produced an average of 4.8 +/- 1.2 degrees of varus deformity and 8.7 +/- 2.3 degrees of internal rotation of the hindfoot and an average of 5.5 +/- 2.0 degrees of varus deformity and 11.5 +/- 2.4 degrees of adduction of the forefoot. A linear correlation analysis was used to compare the change in subtalar motion and the position of the foot with the degree of varus malalignment at the talar neck. The correlation coefficient was 0.90 (p < 0.01) for subtalar motion, 0.76 (p < 0.01) for internal rotation of the calcaneus, and 0.81 (p < 0.01) for adduction of the forefoot. This indicated a direct correlation between the degree of varus malalignment at the talar neck and the change in the position of the foot and in subtalar motion.