The radiology of skeletal elements in the subtibial region: incidence and significance.

The os subtibiale is a rare, genuine accessory bone and normal variant related to the posterior colliculus of the medial malleolus. Only one example was found in the radiographs of the ankles of 700 patients examined. It can be differentiated from other ossicles in this region by its relatively large size, its rounded and well-defined shape, and its posterior position. Ossicles related to the anterior colliculus, which forms the tip of the malleolus, are smaller and were present in 15 ankles (2.1%). These may represent unfused secondary ossification centres. Post-traumatic ossification is the probable explanation for the small, angular and less well-defined ossified elements encountered. The overall incidence of ossification in the subtibial region in the 700 ankles studied was 4.6%.