Osteochondromatosis is classified as primary or secondary. Primary synovial osteochondromatosis occurs as a result of cartilage formation through metaplasia of the synovial membrane. It typically presents as a chronic, progressive, mono-articular process most commonly affecting the knee, hip, or elbow. Men are affected twice as commonly as women, usually in the 3rd â€“ 5th decade of life.
Secondary synovial osteochondromatosis is most commonly related to osteoarthritis.
Primary synovial osteochondromatosis may be distinguished from secondary synovial osteochondromatosis caused by osteoarthritis because of the absence of osteoarthritis. In primary osteochondromatosis, the affected joint will not show associated degenerative changes. the loose bodies are numerous, small, and usually of uniform size. In secondary osteoarthritis, the osteochondral bodies are fewer, larger, and of different sizes. Pressure of the osteochondral bodies in the closed space of the joint may cause erosive changes of the adjacent bones. Primary osteochondromatosis and chondrosarcoma are more difficult to distinguish, since the clinical and radiographic features of each overlap significantly. Both have a protracted clinical course, and both demonstrate local recurrence after local resection. Findings that are concerning for chondrosarcoma are frank bone destruction rather than mere erosions, soft tissue mass, and extension outside joint capsule. However, these findings are not specific, and may occur in synovial chondromatosis
The differential diagnosis of synovial chondromatosis especially when the classical classifications are not evident and only a soft tissue mass is present includes pigmented villonodular synovitis, synovial hemangioma, and lipoma arborescens. Each has its own set of imaging characteristics by which it may be distinguished.
Pigmented villonodular synovitis presents as soft tissue masses that may erode the adjacent bone on radiographs. MRI demonstrates high signal intensity masses that are surrounded by decreased intensity on T2-weighted and gradient-recalled echo MR images due to paramagnetic effects of hemosiderin
Synovial hemangioma on the radiographs usually presents as a soft tissue mass sometimes with phleboliths. The hemangioma enhances on post-contrast T1-weighted MR images with fat sat and shows high signal intensity on T2-weighted MR images with fat sat and both fat saturated sequences show areas of low signal intensity areas consistent with fat.
Lipoma arborescens represents a rare villous lipomatous proliferation of synovial membrane, which is of unknown etiology. Clinically, there is increasing but painless synovial thickening and effusion Radiographically, joint effusion and and mass can be seen. This diagnosis is made on the basis of histology. MR imaging shows frond-like mases that show fat signal intensity.