Extra-articular synovial chondromatosis.

Four patients with extra-articular synovial chondromatosis and one patient with combined intra-articular and extra-articular involvement are described. In all five patients the lesion was anomalously located (knee, shoulder, hip, and ankle). The lesion usually occurs in the hands and feet. Accurate diagnosis is important, because if the synovial origin of the cartilaginous proliferations is ignored, the evidence of cellular activity may lead to an erroneous diagnosis of chondrosarcoma. All five patients had a benign clinical course.