Chondrosarcoma complicating synovial chondromatosis: findings with magnetic resonance imaging.

A 64-year-old man with a tender mass at the ankle was found to have chondrosarcoma complicating synovial chondromatosis. Malignancy was suspected on the basis of the clinical presentation rather than the findings in plain radiographs or magnetic resonance images. Chondrosarcoma arising in synovial chondromatosis is very rare. Misinterpreting the latter condition as malignant and the former as benign can be problematic, and the importance of distinguishing the two entities is discussed.