Synovial osteochondromatosis: A disorder of a joint that features a change of the normal synovial lining's cellular structure to form bone-cartilage tissue. Synovial osteochondromatosis is uncommon and typically seen in young to middle-aged adults. Synovial osteochondromatosis leads to pain in the affected joint as well as limitation of the range of motion and often locking. The cause of synovial osteochondromatosis is unknown. Synovial osteochondromatosis generally affects only a single joint. The most common joints involved are the knee, hip, or elbow. Synovial osteochondromatosis can be diagnosed with an imaging test of the joint, such as an X-ray, a computerized axial tomography (CAT) scan, or a magnetic resonance imaging (MRI) scan. Treatment of synovial osteochondromatosis typically involves surgical removal of the synovial lining.