Patellar Subluxation With Early-Phase Synovial Chondromatosis of the Knee.

Primary synovial chondromatosis is a rare, benign, monoarticular disease process that affects the synovial membrane of the joint, the synovial sheath, or the bursa around the joint. The etiology is unknown, but it has been associated with trauma in some cases. Although it is a benign lesion, if left untreated, it may lead to early secondary osteoarthritis of the joint. The knee joint is affected in 50% to 65% of cases, followed by the elbow and the hip. This article reports a 30-year-old active woman who presented to the author's clinic with a large infrapatellar mass that caused lateral subluxation of the patella, swelling, and episodic pain with crepitations 14 months after direct trauma to the knee. Clinical examination, magnetic resonance imaging, and arthroscopy revealed a large infrapatellar mass causing lateral subluxation of the patella with no loose bodies. Hoffa's disease, para-articular osteochondroma, and early-phase synovial chondromatosis were considered in the differential diagnosis. The histopathologic and clinical features were consistent with early synovial chondromatosis. The patient underwent local excision of the mass through a medial parapatellar arthrotomy. At 5 years of follow-up, she had no recurrence of the lesion or progression of the disease. Early diagnosis of synovial chondromatosis with local excision offers a reliable cure. However, long-term follow-up is advised because of the high recurrence rates as well as the risk of metaplastic transformation.