Effects of isolated Weber B fibular fractures on the tibiotalar contact area.

Fractures of the lateral malleolus can occur without rupture of the deltoid ligament or fracture of the medial malleolus. Controversy exists regarding the necessity of surgery on supination-external rotation stage II ankle fractures. Theoretically, as long as the medial structures are intact, the talus cannot displace enough to cause degenerative arthritis of the ankle joint. The purpose of this study was to measure changes in contact area between the tibial plafond and the talar dome with serial displacement of the distal fibula in both a lateral and a superolateral direction. Twelve cadaver lower extremities were used. Distal fibular fractures were replicated by creating an osteotomy. Displacement was accomplished with a customized apparatus that displaced and held the distal fibula in a malaligned position. Tibiotalar contact area was measured with pressure sensitive film at the following intervals of fibular displacement: 0 mm, laterally 2 mm and 4 mm, and then posteriorly and superiorly 2 mm and 4 mm. A servohydraulic testing apparatus was used to apply the same physiologic load to all limbs while measuring contact area. Key independent variables included the direction and amount of displacement of the distal fibula. Mean tibiotalar contact area decreased from baseline (no displacement) 361.1 mm2 (SD +/- 49.0) to 162.2 mm2 (SD +/- 81.3) and 82.6 mm2 (SD +/- 30.6) for 2 mm and 4 mm lateral displacement of the distal fibula respectively. With posterior/superior displacement of 2 mm and 4 mm mean tibiotalar contact decreased to 219.3 mm2 (SD +/- 56.7) and 109.2 mm2 (SD +/- 39.0), respectively. Statistical significance was found (P <.001) when comparing normal ankle alignment with displaced fractures at all levels of displacement.