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Objectives and questions: The posterior malleolus (PM) is involved in 46 % of dislocated Weber B or C ankle fractures and its involvement is associated with inferior clinical outcomes. True size and structure of the PM fracture (PMF) as well as intercalary fragments can only be detected by the use of a CT scan. There is no consensus on whether and when a CT scan should be performed in the case of inconspicuous conventional imaging of the PM. How often do we oversee a fracture? The aim of this study is hence to determine the rate of PM involvement in or bimalleolar ankle fractures detected by conventional radiologic imaging and to correlate this to the operative treatment of PMF.

Material and methods: 100 Weber or bimalleolar ankle fractures, 50% with and 50% without PMF visible on the basis of a CT-scan, were analyzed by four observers at two different time points 30 days apart (d1 and d2). The observers had to decide on the basis of plan radiographs whether a PMF was present or not. This was followed by a correlation with the operative treatment of PMF.

Results: In 64% of cases, all examiners made the correct diagnosis of the presence of a PMF. Unanimous recognition from all 4 observers regarding the presence was scarce. In 15% of cases 3 out of 4, in 11% 2 out of 4, in 7% 1 out of 4 examiners were correct. In 3% of cases, nobody made the correct diagnosis. 24 of the 50 PMF were fixed surgically. In 62.5% of these cases, all 4 observers were right with their diagnosis of PMF on the basis of plain radiographs. In 12.5% of cases 3 out of 4, in 12.5% 2 out of 4 and in 12.5% 1 out of 4 observers were right. None of these fractures were misdiagnosed by all observers.

Discussion and conclusions: In our cohort, there are deficits in the detection of PMF in Weber, or bimalleolar ankle fractures only on the basis of plain radiographs, even in PMF that were later fixed surgically. This study aids to the awareness that PMF can be easily overseen in plain radiographs and raises the question if a CT scan might become mandatory in distal fibula fractures that involve the upper ankle joint.