Synovial Osteochondroma

Gregory S Hurlock, MD - Case Coordinator
Mitali Bapna, MD - Radiology Discussion
Jason H Smouse, MD - Pathology Discussion
John Anthony Carrino, MD - Attending Radiologist
Pablo R Ros, MD, MPH - Attending Radiologist

July 28, 2003

Presentation

Imaging Findings

A radiograph of the right humerus shows multiple high density opacities in the axilla. It is unclear whether or not calcifications are present, though there is some evidence of mineralization. The lesion does not appear to be contiguous with any bone. In this location, possible soft tissue masses include sarcoma (such as chondrosarcoma and synovial cell sarcoma) and treated lymphoma. An axial CT image shows an irregular, calcified lesion abutting the cortex of the medial aspect of the proximal right humerus. It appears fragmented. There is very little soft tissue component, but the center of the lesion is in the soft tissues. The bone scan shows a highly mineralized focus of uptake in the right axilla.

Differential Diagnosis

Diagnosis

Discussion

The gross specimen is a 7-cm mass with scant attached soft tissue. On cut section, the lesion is diffusely indurated with bone. There is a fibrocartilaginous cap surrounding the mass. The majority of the center of the lesion is composed of cancellous bone with fibrous areas. There is evidence of cartilaginous differentiation, but no atypia. The surrounding edge of the lesion demonstrates a transition from cancellous bone to cartilage, with overlying synovium.

Radiology Discussion:

Synovial osteochondromatosis (a related disorder) is a benign condition in which synovial membrane proliferation and metaplasia result in cartilaginous/ossified synovial fragments in the joint space. The degree of calcification can vary from a few calcific specks to frankly ossified bodies. Free fragments may exist within the joint cavity or may be embedded within the proliferating synovium and can extend to the surrounding soft tissues. This disorder nearly always occurs within joints but occasionally occurs within bursa and tendon sheaths. It is monoarticular and typically affects large joints, such as the knee, hip, elbow, and shoulder. It is 2-4 times more common in men than women and generally affects patients between the ages of 20 and 50 years.

The natural history of synovial osteochondromatosis entails gradual progression of disease, joint deterioration, and secondary osteoarthritis. On radiographs, the multiple lesions (typically more than five) appear as calcified or osseous bodies within the joint or bursa. On MR images, the lesions are typically rounded and often uniform in size and distribution of densities. Signal intensities, however, vary depending on the degree of ossification. The lesions can appear iso- or hypointense on T1-weighted images and range from hypo- to hyperintense on T2-weighted images. The differential diagnosis includes pigmented villonodular synovitis, synovial hemangiomatosis, synovial sarcoma, and secondary chondromatosis (e.g., associated with joint surface disintegration [as may be caused by osteo- or rheumatoid arthritis or tuberculosis] , osteochondral fracture, osteochondritis dessicans). Total open synovectomy is the treatment of choice, but the recurrence rate is over 25%. Although malignant transformation (synovial chondrosarcoma) is rare, it has been reported in cases of repeated recurrence following treatment.

A solitary synovial osteochondroma, as seen in this case, is an extremely uncommon entity; few reports are available in the literature. It is a rare variant of synovial osteochondromatosis. This solitary manifestation is not connected to bone and occurs almost exclusively in the vicinity of the joints in the hands and feet. The differential diagnosis of a discrete soft tissue mass containing mature calcifications includes myositis ossificans, synovial (osteo)chondromatosis, tumoral calcinosis, synovial sarcoma, and extraskeletal osteoarcoma. In addition, consider a diagnosis of extraskeletal osteochondroma when a discrete, ossified mass is localized in the soft tissues.

References

Valmassy R, Ferguson H: Synovial osteochondromatosis. A brief review. J AM Podiatr Med Assoc 1992 Aug;82(8):427-31

McGrory JE, Rock MG: Synovial chondromatosis of the shoulder. AM J Orthop 2000 Oct;29(10):793-5

Erb RE, Nance EP: Skeletal case of the day. Synovial osteochondromatosis. AJR AM J Roentgenal 1992 Jun;158(6):1370-1

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