Fractures of the posterior malleolus almost always occur in association with a fracture of the lateral malleolus and a medial injury. “Isolated” posterior malleolar fracture should lead to suspicion of a proximal fibular (Maisoneuve) fracture and/or a major soft tissue disruption.
Hamaguchi et al (2006), using CT, found that 2/3 of posterior malleolar fractures were wedge-shaped and related to the posterior tibiofibular ligament, but 20% were transverse, extending in to the medial malleolus, and 15% were small posterior shell fragments.
A number of biomechanical studies suggest that tibiotalar instability occurs with a posterior fracture that separates 30-40% of the joint surface, in the posterolateral position, from the rest of the plafond. It is difficult to measure the proportion of separated joint surface from plain Xrays, as the fracture line is usually oblique. Ebraheim et al (1999) recommended the use of external rotation lateral views, although Hamaguchi et al (2006) found that the angle between the fracture and coronal plane varied unpredictably from -9 to 40 deg. Some studies suggest that adequate reduction of the fibula or an intact DTTL can prevent instability.
Clinical studies, however, have not shown a clear proportion of posterior separation that predicts a poor result. Both Harper (1988) and Jaskulka (1989) found that outcome was determined by the overall severity of the fracture and the adequacy of reduction. There have been no prospective clinical studies or trials.
At the moment we still recommend fixation of a large posterior malleolar fragment (over 25%). If this is unreduced after fibular stabilization we reduce it through a posteromedial approach. However, there is a great need for a RCT comparing fixation with non-fixation.
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