Answer

The lateral radiograph of the right knee shows swelling at the posterior joint recess and, to a lesser extent, in the suprapatellar region. No calcification is observed.

Extensive lobulated mass-like synovial proliferation is observed with low-signal intensity on T1 and susceptibility artifact (blooming) on gradient echo sequence. No erosions or joint space narrowing is observed. The diagnosis is pigmented villonodular synovitis (PVNS).

[TAG:2]Discussion[/TAG:2]

PVNS is a rare, benign, proliferative lesion of uncertain etiology that affects synovium, bursae and tendon sheaths. Commonly, PVNS is a monoarticular process and occurs either as a localized or diffuse form.^[1],[2] It affects the knee in 80% of the cases, but can also affect the hip, ankle, shoulder and elbow. Although PVNS can occur at any age, it mostly occurs in adults aged 20–50 years, with no gender predilection.^[2]

Patients with PVNS often have a slow progressive pain along with swelling and a decrease in range of motion. Another commonly observed finding is history of a recurrent, bloody joint effusion.^[2] In the knee, PVNS typically manifests as a nonspecific joint effusion. Bone erosions are observed in 25% of these cases; such a low rate is because of the capacious joint space.^[2]

An MRI, owing to its very specific imaging features, can help differentiate PVNS from other synovial processes, and thus is the most widely used method for diagnosing PVNS. MRI examination would generally reveal a synovial-based mass affecting the entire or most of the joint with low-signal intensity on T1- and T2-weighted pulse sequences. Another signal characteristic is the presence of the magnetic susceptibility artifact in the affected joint space on gradient echo images.^[1],[2] The differential diagnosis of PVNS includes hemophilia-related arthropathy, amyloid arthropathy and synovial chondromatosis.

In terms of management, surgical excision is considered as the optimal treatment. The success of surgical excision depends on complete resection with clear margins. However, in the extensive diffuse form of the disease, complete resection is rarely possible, and thus surgery is carried out primary for debulking before initiating radiation (with external beam and radiosynoviorthesis) or medication therapy (with immunomodulating agents such as infliximab). The recurrence rates for localized and diffuse forms of PVNS have been reported to vary from 0% to 44% and from 8% to 56%, respectively. Notably, higher recurrence rates have been reported in cases where complete synovectomy could not be performed.^[3],[4]

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