ABC of emergency radiology. The ankle.

The os trigonum is a common normal variant of the talus and is due to a separate ossification centre arising from the posterior tubercle. The appearance may resemble an old ununited fracture fragment. However, it is triangular, well corticated, in a classic location, and usually bilateral, which enables it to be distinguished from a fracture. Transverse, sclerotic, linear lines located at the metaphysis of growing long bones are due to short periods of growth arrest and have no clinical importance (fig 5). They may be confused with compression fractures, but again these lines are usually bilateral. Fibrous cortical defects are the most commonly seen benign lesions of long bones and are usually identified incidentally in radiographs taken for another reason. The defect is limited to the cortex, commonly found at the metaphysis, but may be located in the diaphysis as the bone grows. The lesion is well corticated (sclerotic margins) and usually does not produce signs or symptoms.