External rotation-lateral view of the ankle in the assessment of the posterior malleolus

Demonstration of a posterior malleolar fragment on a radiograph of an ankle fracture is important in the diagnosis and evaluation of posterior malleol us fractures. The size and extent of displacement of a posterior malleolar fragment can be evaluated. The diagnosis of nonunion of the posterior malle olus is also important because it can lead to failure of reduction of ankle fractures. The authors present a case in which non-union of the posterior malleolus was diagnosed by an external-rotation lateral view of the ankle. This could not be demonstrated on the AP or the lateral views. Thirteen cad aver feet were then used to study the external-rotation lateral view. A pos terior malleolar fracture was created, and the borders of the fracture line were marked with solder wire. The average external rotation angle required to best demonstrate the posterior malleolar fracture was 50 degrees (range , 43 degrees-55 degrees). The actual size of the posterior malleolus fragme nt was measured and compared to the x-ray measurement. There was a 0.10 cor rection for the determination of the actual size of the fragment. The unmar ked fragment could not be demonstrated on AP and lateral views.