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 malleolus fractures. The size and extent of displacement of a posterior malleolar fragment can be evaluated. The diagnosis of non-union of the posterior malleolus is also important because it can lead to failure of reduction of ankle fractures. The authors present a case in which nonunion 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 cadaver feet were then used to study the external-rotation lateral view. A posterior 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 fragment was measured and compared to the x-ray measurement. There was a 0.10 correction for the determination of the actual size of the fragment. The unmarked fragment could not be demonstrated on AP and lateral views.