Secondary synovial osteochondromatosis of the ankle in a child.

We describe a case of a large intra-articular ossifying mass of the ankle joint in a 12-year-old boy who sustained a severe ankle twisting injury at 6 years of age. The mass is presumed to be the result of secondary synovial osteochondromatosis originated from a fractured osteochondral fragment of the medial tubercle of the posterior process of the talus. The mass could be differentiated from os trigonum syndrome, Trevor disease and primary synovial osteochondromatosis based on its location, size and radiologic and histological features.