considerations of synovial osteochondromatosis in the knee include pigmented villonodular synovitis, osteoarthritis, chondrosarcoma, osteochondroma, osteochondritis disse- cans, and fracture with an avulsed fragment. 1,2 Synovial osteochondromatosis is a benign intrasynovial process caused by nodular proliferation of the synovial membrane. Fragments can break off of the synovial surface within the joint. These bodies can grow, calcify, and ossify within the periarticular synovial fluid. The condition can occur in a primary or secondary form, but the etiologies are not specifically known. The primary form occurs as ectopic cartilage within synovial fluid without identifiable joint pathology. A secondary form can develop in a setting of preexisting joint pathology, including osteoarthritis, rheumatoid arthritis, osteochondritis dissecans, neuropathic osteoarthropathy, osteonecrosis, or fracture. Etiology of the second form is based on fragment production by the underlying disease process. 2 With the primary etiology, the knee is the joint that is most often affected, comprising 60%-70% of cases. Other sites that can commonly be affected include the shoulder, the elbow, and the hip. The process appears to occur in three phases: initial active intrasynovial disease without loose bodies, a transitional phase with osteochondral nodules on the synovial membrane and free bodies in the cavity, and finally a quiescent disease state with mulitiple free intrasy- novial osteochondral bodies. 3 Transformation tomalignancy is rare. Cases have been reported of coexisting chondrosar- coma, but a causal relationship has not been established. Osteochondromatosis does appear to have a predilection for knees with a history of repeat minor trauma and/or degenerative joint disease. Whether trauma or degenerative disease contribute to the proliferation of synovial nodule or to their fragmentation is not known. 4 Patients with synovial osteochondromatosis often com- plain of chronic, progressive, monoarticular pain, and swell- ing exacerbated by physical activity. Mechanical symptoms can also present, including limited range of motion, grating, and joint locking. Medical therapy with non-steroidal, anti- inflammatory drugs can provide some relief, depending on the degree of nodule proliferation, size and characteristics of the free bodies, and the physical stress placed upon the joint. Surgical excision of free bodies within the joint space is also an appropriate intervention, with partial or total synovectomy depending on the degree of recurrence. 3 The classic radiographic appearance of synovial os- teochondromatosis is multiple, spheroid, calcified masses within the joint cavity. The bodies tend to have a “popcorn ball” appearance characteristic of calcified cartilage. Joint effusion and degenerative changes are frequently noted as well. Serial imaging studies may reveal changes in numbers and sizes of masses, as some of the calcified bodies grow and other are reabsorped into the synovial membrane. Lack of calcification or ossification of free bodies renders them relatively lucent, with opacity similar to water. It is worth noting that the dynamic nature of the fragmented nodules can lead to normal findings in a patient with osteochon-
Figure 5: A coronal proton density MR with fat suppression image. It was obtained more anteriorly than Figure 4. It demonstrates an abormal high signal along the lateral aspect of the proximal tibia (black arrow) and medial femoral condyle (white arrow), again consistent with edema due to bone bruise. RADIOLOGIC DIAGNOSIS: This is a case of osteochon- dromatosis of the proximal tibiofibular joint with associated bone bruise and degenerative osteophytosis. DISCUSSION Synovial osteochondromatosis is an uncommon condi- tion caused by synovial metaplasia that generates multiple calcified periarticular bodies. The lesion commonly presents as monoarticular joint pain and swelling in patients who re- late a history of several years of such symptoms. It presents more commonly inmales than females by a factor of between two and four. While individuals of any age can be affected, it is most often diagnosed in persons aged 20 to 50 years. Current medical therapy is centered around non-steroidal anti-inflammatory drugs, while surgical treatment consists primarily of arthroscopic examination and excision of loose bodies from the joint space with limited synovectomy of the affected synovium. Important differential diagnostic
J La State Med Soc VOL 166 May/June 2014 127
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