The pronation capacity of the foot--its consequences for axial deformity after tibial shaft fractures.

In spite of the fact that discomfort from the subtalar area is common after varus-deformed tibial shaft fractures no plausible mechanism is to be found in the literature. A mechanical analysis of the problems shows that a varus deformity is compensated as pronation of the foot. A limited pronation capacity could thus be the cause of the pain. Pronation capacity was accurately measured in ten osteoligamentous preparations. The average pronation capacity was found to be 9.5 degrees +/- 7.0 degrees. There was a marked interindividual variation. In two of the specimens the pronation capacity was less than 1 degree. Capacity decreased by 0.21 degree for every degree increase in plantar flexion of the ankle joint. Thus, a small pronation capacity may be the mechanical basis for ankle complaints after varus-deformed tibial shaft fractures. An anterior angulation, compensated as planar flexion, further decreases the pronation capacity and adds to the risk associated with varus deformities.