Posterior malleolar ankle fractures: an in vitro biomechanical analysis of stability in the loaded and unloaded states.

The use of internal fixation for posterior malleolar fractures remains controversial. This study assessed the contribution of the posterior malleolus/posterior tibiofibular (PM/PTF) complex to ankle stability in the loaded condition. Total plantarflexion and dorsiflexion, torsion, version, and drawer were measured with 15- and 70-kg loads before and after creation of posterior malleolar fractures that measured 25% of the distal articular surface. After internal fixation, specimens were retested. Fractures caused significant increases with internal rotation and posterior drawer at 15- and 70-kg loads in the neutral position. External rotation, anterior drawer, and dorsiflexion increased at 15-kg load. Fixation increased stability of fractured specimens, but not significantly. Fractures of the PM/PTF ligament complex may lead to excessive internal rotation and posterior instability in the loaded state.