Clinics in diagnostic imaging (95). Os trigonum.

A 19-year-old boy presented to the Accident and Emergency Department after sustaining trauma to his left ankle and foot while playing soccer. The radiograph of his left ankle showed a well-corticated triangular fragment of bone posterior to the left talus, typical of an os trigonum. This accessory bone was initially mistaken for a fracture fragment and a plaster cast was applied. The term, do not touch lesion, has been coined to describe this group of benign bony entities which may be classified into three broad categories, namely: normal variants, lesions that are real but obviously benign, and lesions that are related to degenerative disease. The importance of recognising the characteristic radiographical appearances of these entities is emphasised, as the need for further imaging or diagnostic tests can usually be avoided.