Synovial (osteo)chondromatosis should be differentiated from the secondary osteochondromatosis caused by osteoarthritis, particularly in the knee and hip joints, and from synovial chondrosarcoma, either primary (arising de novo from the synovial membrane) or secondary (caused by malignant transformation). Distinguishing primary from secondary osteochondromatosis usually presents no problems. In the latter condition, there is invariably radiographic evidence of osteoarthritis with all of its typical features, such as narrowing of the radiographic joint space, subchondral sclerosis, and, occasionally, periarticular cysts or cyst-like lesions (Fig. 23.8). The loose bodies are fewer, larger, and invariably of different sizes. Conversely, in primary synovial (osteo)chondromatosis, the joint is not affected by any degenerative changes. In some cases, however, the bone may show erosions secondary to pressure of the calcified bodies on the outer aspects of the cortex. The intraarticular bodies are numerous, small, and usually of uniform size (see Figs. 23.2, 23.3, 23.4).

It is more difficult to distinguish synovial chondromatosis from synovial chondrosarcoma. The clinical and radiographic features have not been useful in this differentiation and are equally ineffective in distinguishing a secondary malignant lesion arising in synovial (osteo)chondromatosis. In addition, both entities tend to have a protracted clinical course, and local recurrence is common after synovectomy for synovial chondromatosis or local resection of synovial chondrosarcoma. The presence of frank bone destruction rather than merely erosions, and the association of a soft-tissue mass, should always raise a concern for malignancy (see Fig. 23.22). Although extension beyond the joint capsule should heighten the suspicion of malignancy, some cases of synovial chondromatosis have been reported to have extraarticular extension.

The other conditions that can radiologically mimic synovial chondromatosis include pigmented villonodular synovitis (PVNS), synovial hemangioma, and lipoma arborescens. In PVNS (discussed in detail later in this chapter), the filling defects in the joint are more confluent and less distinct. MRI may show foci of decreased intensity of the synovium in all sequences because of the paramagnetic effects of deposition of hemosiderin (see Figs. 23.12 and 23.13). Synovial hemangioma usually presents as a single soft-tissue mass. On MRI, T1-weighted images show that the lesion is either isointense or slightly higher (brighter) in signal intensity than surrounding muscles but much lower in intensity than subcutaneous fat. On T2-weighted images, the mass is invariably much brighter than fat (see Figs. 23.15 and 23.16). Phleboliths and fibrofatty septa in the mass are common findings that show low-signal characteristics. Lipoma arborescens is a villous lipomatous proliferation of the synovial membrane. This rare condition usually affects the knee joint but has occasionally been reported in other joints, including


the wrist and ankle. The disease has been variously reported to have a developmental, traumatic, inflammatory, or neoplastic origin, but its true cause is still unknown. The clinical findings include slowly increasing but painless synovial thickening as well as joint effusion with sporadic exacerbation. Imaging studies reveal a joint effusion occasionally accompanied by various degrees of osteoarthritis (see Fig. 23.17). Histologic examination demonstrates complete replacement of the subsynovial tissue by mature fat cells and the formation of proliferative villous projections (see in the following text).

Treatment of synovial chondromatosis usually consists of removal of the intraarticular bodies and synovectomy, but local recurrence is not uncommon.

The most common diagnostic possibilities include hemophilic arthropathy, synovial chondromatosis, synovial hemangioma, and synovial sarcoma. MRI is very
effective in distinguishing these entities because it can reveal hemosiderin deposition in PVNS. Although this feature may also be present in hemophilic arthropathy, detection of diffuse hemosiderin clumps, synovial irregularity and thickening, and distention of the synovial sac favors the diagnosis of PVNS. In addition, hemophilia, unlike PVNS, commonly affects multiple joints and is associated with growth disturbance at the articular ends of the affected bones. Synovial chondromatosis may manifest with pressure erosions of the bone similar to those of PVNS. However, it can be distinguished by the presence of multiple joint bodies, calcified or uncalcified. Synovial hemangioma is commonly associated with the formation of phleboliths. Synovial sarcoma tends to have a shorter T1 and longer T2 on MRI compared with PVNS, and when calcifications are present, the latter diagnosis can be excluded.