CASE

A 30-year-old white female presented for evaluation of mechanical knee pain and swelling for the past 4 years. Her symptoms were most prominent as she attempted to engage in an exercise program that included walking and core strengthening. Her main goal at the time of the visit was weight loss; her knee symptoms had led to weight gain of 20 lb over the past 4 years.

An orthopedic surgeon had evaluated her 2 years earlier for the knee pain. At that time, MRI of her knee was performed and was reported to show a lateral meniscus tear with an associated meniscal cyst, joint effusion, and a popliteal cyst. Arthroscopic surgery was recommended, but the patient opted for more conservative management. Since the time of that evaluation, the patient had noticed increased knee pain and swelling, especially in the lateral region of her knee. Her symptoms worsened with any type of exercise.

Physical examination demonstrated a mildly obese female approximately 5 ft 7 in tall. Both knees were similar on gross inspection. No effusion was appreciated. Range of motion (ROM) was 0 to 130 degrees of flexion, with no mechanical barrier to motion. Patellar crepitus was noted with ROM testing, and the result of patellar grind testing was positive. Tenderness to palpation was present along the entire lateral joint line and in the lateral popliteal fossa. There was no medial joint line pain with palpation. McMurray's test was negative for medial or lateral meniscus pathology. Lachman's test, along with anterior and posterior drawer tests, showed the anterior cruciate ligament and posterior cruciate ligament to be intact. No appreciable varus or valgus instability was appreciated. Posterior capsular stability was symmetric to the contralateral knee. Pain was elicited with passive extension testing for anterior impingement. No neurovascular deficits were apparent on examination.

Imaging studies included knee radiographs obtained at the initial visit. These demonstrated mild varus deformity of the knee, osteophyte formation in the patellofemoral joint, and calcifications identified in the anterior joint space and popliteal fossa (see Figure 1).

MRI with gadolinium was ordered to further evaluate the calcifications seen on radiographs. Scans demonstrated a joint effusion, mild synovitis, tricompartmental osteoarthritis, and numerous intra-articular bodies (see Figure 2). The two largest of these were noted; one in the anterior recess measured 1.132 cm, and the other in the popliteal space 1.131.4 cm.

Based on these findings, arthroscopic knee surgery was recommended and was performed. The procedure revealed multiple loose bodies and grade 3 chondral erosions of the patellofemoral articulation (see Figure 3). No evidence of a medial or lateral meniscal tear was seen. The loose bodies were removed at arthroscopy and were sent for pathological characterization. They were reported as polypoid fragments of benign cartilage consistent with synovial osteochondromatosis.

DISCUSSION

Knee pain with mechanical symptoms and swelling is a common complaint in the sports medicine setting, and synovial osteochondromatosis is an uncommon diagnosis that will often manifest with these common symptoms. The etiology of this condition remains obscure, and it has no agreed upon definition. Consensus opinion regards the disease as a reactive process caused by either repeated trauma or, possibly, infection.¹ Some reports have suggested a neoplastic etiology, but this has not yet been proven.²

The pathogenesis of synovial osteochondromatosis is founded in metaplasia. Synovial cells may undergo chondroblastic metaplasia and come to resemble chondroblasts. They then can produce deposits of cartilage in the synovial membrane that can eventually undergo calcification. With time, these lesions can increase in size, become detached from the membrane, and become loose bodies within the joint. The loose bodies can continue to grow as the cartilaginous portion continues to be nourished by the synovial fluid.³ Malignant transformation of synovial osteochondromatosis is rare, but transformation into chondrosarcoma has been reported and must be considered in a patient with long-standing disease or in one with known disease and acute worsening symptoms.^4,5

Synovial osteochondromatosis is usually seen in adults aged 30 to 50 years, and men are twice as likely to have the condition as women.^5,6 The knee is the most common site, with other large joints such as the hip, shoulder, and elbow frequently involved. Smaller joints may also be affected, but much less often.^6,7 Cases of synovial osteochondromatosis of the temporomandibular joint have even been reported.⁴

The clinical manifestations of synovial osteochondromatosis are often similar to those of other intra-articular problems, especially in the knee. Pain, swelling, and decreased ROM are commonly associated with synovial osteochondromatosis. Palpable loose bodies may be present in more advanced cases. The differential diagnosis includes the conditions previously mentioned and other less common disorders, such as pigmented villonodular synovitis. Laboratory testing and joint aspiration can help rule out inflammatory and septic arthritides, but no common laboratory findings suggest synovial osteochondromatosis.⁸

Because the history and physical examination findings are often nondescript in patients with synovial osteochondromatosis, imaging is often pursued. Radiographs are generally performed first to evaluate swelling, pain, and decreased ROM. Calcifications are appreciated in 70% to 95% of the cases of synovial osteochondromatosis, and they are often smooth and round and appear throughout the joint space.⁹ CT has a greater capacity than plain films to differentiate synovial osteochondromatosis from other entities but is less often used in imaging of the knee. MRI is more commonly used to help characterize the findings seen on plain films.

The appearance of synovial osteochondromatosis on MRI can be variable, depending on the stage of the disease process. Three patterns of appearance have been described, depending on the degree of calcification and ossification of the synovium and the appearance of loose bodies on plain films.⁹ MRI can also help to differentiate synovial osteochondromatosis from pigmented villonodular synovitis in the knee.⁸

The treatment of synovial osteochondromatosis is almost always surgical. Arthroscopy of the joint is often performed for both diagnostic and therapeutic reasons. Loose body removal is typically accompanied by synovectomy at the time of arthroscopy. Loose body removal with or without synovectomy for synovial osteochondromatosis of the knee has been reported to improve pain, effusion, ROM, and function. Those patients with synovial osteochondromatosis of the knee who undergo both loose body removal and synovectomy have decreased recurrence rates.¹⁰ The overall recurrence rate is reported as 5% to 11%.⁸ The overall prognosis is good, with malignant transformation occurring only rarely.

OUTCOME

The patient underwent loose body removal and synovectomy at the time of knee arthroscopy and did well in the postoperative period. At her last visit, she was no longer experiencing pain, swelling, or catching of the knee and had begun to exercise again. There has been no sign of recurrence of the synovial osteochondromatosis.

REFERENCES

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	3.	Salter RB. Textbook of Disorders and Injuries of the Muskuloskeletal System. 2nd ed. Baltimore, MD: Williams & Wilkins; 1983.
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	6.	Murphy FP, Dahlin DC, Sullivan CR. Articular synovial chondromatosis. J Bone Joint Surg Am. 1962;44A(1):77-86.
	7.	Kumar A, Thomas AP. Recurrent synovial chondromatosis of the index finger—case report and literature review. Hand Surg. 2000;5(2):181-183.
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	10.	Ogilvie-Harris DJ, Saleh K. Generalized synovial chondromatosis of the knee: a comparison of removal of the loose bodies alone with arthroscopic synovectomy. Arthroscopy. 1994;10(2): 166-170.

James Borchers, Rose Backs, and Christopher Kaeding are affiliated with the OSU Sports Medicine Center Division of the Ohio State University Hospitals, Columbus. The authors have indicated no relationships to disclose relating to the content of this article.