“Bag of nuts” in the knee

The patient had primary synovial osteochondromatosis (PSOC), a rare condition characterized by multiple cartilaginous nodules or mineralized loose bodies in the joint and, less commonly, in bursae or tendon sheaths. It is thought to be a benign neoplastic rather than metaplastic disorder, as it was considered previously.¹

PSOC has a slight predominance in men and occurs most frequently in the third to fifth decades of life. The knee is the joint involved most often, followed by the shoulder, elbow, and hip. PSOC is extremely rare; cases with polyarticular involvement have been reported.²

A few conditions may mimic PSOC, although a different shape and no uniformity in size of a few loose bodies distinguishes secondary disease from the primary disorder. In secondary osteochondromatosis, chondromas result from the growth of separated fragments arising from detached osteophytes or articular cartilage in joint diseases, such as osteochondritis dissecans, osteoarthritis, osteochondral fractures, and chondrocalcinosis.³ Conversely, PSOC is characterized by the development of chondroid foci in the synovial membrane, showing numerous nodules, mostly of the same shape and uniform size.

Milgram⁴ suggested that PSOC is self-limited and described 3 overlapping phases: an initial phase (phase 1), with intrasynovial disease and no loose bodies; the intermediate phase (phase 2), with both intrasynovial disease and loose bodies; and a late phase (phase 3), with multiple osteochondral bodies and without demonstrable synovial activity. In the late phase, there are easily recognizable ossified chondromas on radiographs. Modern imaging techniques, such as MRI, are required for the diagnosis in the early stages, when there are no ossified chondromas.

The patient’s condition was in the intermediate stage. Sonographic imaging may be used to identify PSOC when numerous mineralized bodies fill the joint. Hyperechogenic nodules with acoustic shadowing are useful traces for diagnosis. Doppler imaging has shown no evidence of hyperemia in the intermediate stage.⁵

In the early or intermediate stage of PSOC, ultrasonographic diagnosis can be difficult. The role of ultrasonography in this phase has not been defined. However, during the intermediate stage (as in this case), it can show the presence of hypoechoic mobile nodules, which can be missed easily, by obtaining a different image during compression with an ultrasonographic probe—what I call the “spot the difference” sign (above, left and right). Although such findings are not specific, they might be a “red flag” to follow up with invasive or more expensive imaging studies. Power Doppler also might offer additional information in the early and intermediate stages because the synovial membrane shows blood vessels.

There is controversy about the extent of surgical treatment for patients with PSOC. If intrasynovial disease and proliferation are present, however, synovectomy with release of loose bodies is recommended. Removal of loose bodies alone has been reserved for those who do not have synovial hypertrophy, although recurrence has been documented.⁶

In the present case, arthroscopic release of the loose bodies and extended remotion of the hypertrophic synovium were performed; there were no complications. The patient was asymptomatic with no recurrence of synovial proliferation during 22 months of sonographic follow-up.

This case was submitted by Angel Checa, MD, division of rheumatology at Drexel University College of Medicine in Philadelphia.