RCOM RADIOLOGICAL CASE OF THE MONTH

A 20-year-old Brazilian man with a history of a right-knee sports injury 2 years prior presented with 2 weeks of right-knee pain and swelling. He denied any history of fever, chills, or any other constitutional symptoms. Physical examination revealed slightly erythematous swelling of the right knee without point of tenderness or limitation of motion.

Diffuse pigmented villonodular synovitis

Radiographs were taken and magnetic resonance (MR) imaging studies were performed (Figures 1 through 3). Radiographs of the right knee reveal widening of the femoral intercondylar fossa, joint effusion, and peripheral erosive changes (Figure 1). MR imaging shows nodular soft-tissue masses of low signal intensity along with pressure erosions and a large amount of joint effusion (Figure 2). Postgadolinium images show extensive enhancement of the thickened synovium (Figure 3).

Pigmented villonodular synovitis (PVNS) was defined by Jaffe et al¹ in 1941. It is a benign, proliferative disorder of synovium resulting in villous and/or nodular changes in synovium-lined joints, tendon sheaths, and bursae. The lesion occurs in both localized and, more commonly, diffuse forms.² Diffuse PVNS occurs most frequently in young adults and usually affects the large joints of the lower limb, most frequently the knee, followed by the hip.³ The lesion is slightly more common in men; the majority are unilateral and monoarticular.⁴ The most common presenting complaint is mild but progressive pain that is insidious at onset. Mechanical derangement with locking and palpable masses may also be present.

Grossly, diffuse PVNS consists of exuberant, frondlike, thickened synovium with a characteristic tan or reddish color caused by deposition of hemosiderin. Microscopically, it is characterized by fibrous stroma containing multinucleated giant cells, lymphocytes, xanthoma cells, and pigmentation due to both intra- and extracelluar hemosiderin, and lipid deposition. The hyperplastic synovium can invade the subchondral bone and produce cysts.⁵ The cause is unknown; however, several theories are accepted, including a chronic inflammatory process, repetitive trauma, benign neoplasia, and disorder of lipid metabolism.^1,6,7 Malignant transformation has been reported, although it is extremely rare and controversial.⁸

Radiographs may be normal or may reveal periarticular soft-tissue swelling with an absence of calcifications. Normal bony mineralization, and preservation of the joint space are characteristic.⁹ Bony changes begin with erosion of the articular cartilage, often near the chondro-osseous junction, with subsequent extension of the process through the cartilage and the underlying cortical bone into the cancellous bone. This gives rise to the juxta-articular cysts evident radiographically.¹⁰ A variety of imaging methods have been used to diagnose PVNS, each having varying degrees of diagnostic value. Arthrography, used more commonly in the pre-MRI period, may show filling defects due to intra-articular nodules, but is not widely used.¹¹ Computed tomography (CT) is especially useful in delineating bony erosions and in revealing high-attenuation tisue due to iron deposit in the synovium. However, CT is limited compared with MRI in its delineation of soft-tissue lesions and its sensitivity to inflammatory tissue.¹²

With its high resolution and excellent soft-tissue contrast, MRI has become the imaging method of choice. On MRI, mass-like synovial proliferations have lobulated margins and the paramagnetic effect of hemosiderin results in the reduction of signal intensity in all pulse sequences, which is enhanced by high-resolution field-strength and gradient-echo sequences.¹³ However, the appearance varies widely, depending on the relative proportions of lipid, hemosiderin, fibrous stroma, septation, pannus formation, fluid, and articular erosions.¹⁴ Pigmented villonodular synovitis characteristically shows intense gadolinium enhancement, reflecting the high vascularity of the synovial proliferation.¹⁵

Definitive diagnosis relies on synovial biopsy. Treatment of choice is synovectomy.¹⁶ Recurrence is presumably due to incomplete excision. Recurrence rates are lower in open procedures than in arthroscopic synovectomies. On the other hand, postoperative morbidity is lower in arthroscopically treated patients than in open surgeries.¹⁷ Nonsurgical therapies, such as local injection of steroids, Y-90 radiosynoviorthesis, or external beam radiation, are controversial.^18-20

Diffuse PVNS is a benign proliferative disorder of the synovium of unknown etiology that is prevalent in lower limbs in young adults. MRI has proved to be the most helpful tool in the investigation, and, in uncertain cases, an arthroscopic biopsy is easily obtainable. Treatment mainly consists of surgical excision of affected tissue.