Synovial Osteochondromatosis 

Synovial osteochondromatosis (SOC) is a benign condition characterized by synovial membrane proliferation (demonstrated in the images below) and metaplasia. The entity also is termed synovial chondromatosis. The synovial lining of a joint, bursa, or tendon sheath undergoes nodular proliferation, and fragments may break off from the synovial surface into the joint. There, nourished by synovial fluid, the fragments may grow, calcify, or ossify. The intra-articular fragment may vary in size from a few millimeters to a few centimeters.

The images below present a detailed view of SOC of the shoulder.

The degree of calcification varies, and calcification may be seen as a few calcific specks or as foci of frankly ossified bodies. The fragments may be found free within the joint cavity, or they may be embedded within the proliferating synovium, which may extend into the surrounding soft tissues. The natural history of SOC entails gradual progression of disease, joint deterioration, and secondary osteoarthritis. Essentially, the disease is a benign process, and although studies in the literature have reported malignant transformation, this finding is decidedly unusual.

Preferred examination

Radiographic findings are frequently diagnostic. CT scans and CT arthrograms also may be used, especially for demonstrating noncalcified intra-articular bodies. MRI usually helps establish the diagnosis, and the images demonstrate the true extent of the disease. Ultrasonographic examination may be used to investigate accessible joints.

Radiographs should be obtained first.^{[1, 2, 3, 4, 5]} MRI scans should then be obtained preoperatively. When MRI is not readily available, CT arthrography may be performed.

Limitations of techniques

Radiographs may not demonstrate noncalcified bodies. CT scans may not demonstrate the full extent of proliferating synovial disease. SOC may be confused with pigmented villonodular synovitis (PVNS) if only MRI scans are available, and plain radiographs may help in such cases.

Plain radiographs frequently show characteristic features, including multiple (usually >5) calcified or osseous bodies within the joint or bursa, as in the images below. When fragments are not calcified, intrasynovial fragments may not be seen on plain images, and arthrographic studies are required to demonstrate the bodies.

Pressure erosions (saucerization; as demonstrated in the image below) and cyst formation can be seen in adjacent bone, although this is more typical for joints with lax capsules, such as the hip. A similar observation may be found in patients with PVNS.

Degree of confidence

With plain radiography, differential diagnoses include degenerative joint disease in which osteophytes have broken off into the joint; however, SOC tends to have a larger number of bodies in the joint (usually >5). Other differential diagnoses include soft-tissue and intra-articular chondromas. In advanced stages of SOC, secondary degenerative changes are often observed.

CT scanning is rarely necessary to make a diagnosis, and the CT scan's features are similar to findings on plain radiographs. CT scanning may show noncalcified bodies.

See the CT scans depicting synovial osteochondromatosis, below.

In synovial osteochondromatosis (SOC), T1-weighted and proton density–weighted images often demonstrate multiple rounded bodies that are isointense or hypointense relative to muscle. Some bodies demonstrate signal intensity (or lack of it) similar to that of cortical bone.^[6]

See the magnetic resonance images depicting SOC, below.

Some bodies may lie free, and some are completely intra-articular, whereas others may be adherent to the synovial surface. Some bodies may be found deep in the synovial lining, embedded in adjacent soft tissue.

T2-weighted images may show areas of high signal intensity consistent with joint effusion and synovial thickening.

The intravenous administration of a gadolinium-based contrast material usually enables the differentiation of joint effusion from hyperplastic synovium, which shows heterogeneous enhancement. Joint effusion may be hemorrhagic.

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or magnetic resonance angiography scans.

NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.

Degree of confidence

When the presence of calcification is ambiguous, plain radiographs are useful adjuncts to MRI scans for establishing the diagnosis.

False positives/negatives

On MRI scans, SOC may appear somewhat similar to pigmented villonodular synovitis (PVNS), but the 2 conditions are easily differentiated by using plain radiographs.

The distinguishing feature of SOC is calcification, which appears as signal voids in the synovium. This finding is optimally visualized when the intra-articular fragments are calcified.

Radionuclide studies (see the image below) may be obtained when the plain radiographic features are not clear. The affected areas usually show increased radionuclide uptake on technetium-99m bone scans.^[7]