[Effects of fibular malunion on contact area and stress distribution at the ankle with six simulated loading conditions].

A shortened, externally rotated malunion in the lateral malleolus following an ankle fracture-dislocation often causes post-traumatic degenerative arthrosis even without talar displacement (occult malunion). The purpose of this study was to examine the effect of malalignment of the fibula on stress distribution in the ankle joint using two grades of pressure sensitive film (super-low and low) as a transducer. The ankles of seven amputated lower legs were mounted in an MTS machine, then loaded in 6 different positions of 3 single leg stance positions at the neutral, abduction and adduction of the foot, and 3 simulated positions in the stance phase of walking. A normal ankle, an ankle with 5 mm shortened fibula only, and an ankle with a shortened fibula plus an additional 20 degrees external rotation of the fibula were tested. Talar shift was examined on X-ray, before and after each test. When the fibula was shortened, and after additional external rotation, there was no talar shift even after loading and the total contact area was not significantly different from normal, however, there was the stress increase on the lateral side of the tibio-talar joint, with the area of higher stress range significantly larger than normal. There was no significant difference between the ankle with a shortened fibula only and that with an additional 20 degrees external rotation. The stress change in the fibular malalignment was concluded to be mainly due to the shortened fibula. It seems that the shortened lateral malleoli affect the ankle joint regardless of the talar shift and may induce degenerative arthrosis.