Focal Nodular Synovitis (FNS)! Pigmented Villonodular Synovitis (PVNS)

Focal Nodular Synovitis (FNS)! Pigmented Villonodular Synovitis (PVNS) results from metaplastic proliferation of the synovium, which leads to recurrent bleeding and formation of a pseudomass. PVNS has three presentations: 1. Intra articular diffuse 2. Intra articular localized (FNS) 3. Associated to the tendon sheath (giant cell tumor of the tendon sheath). PVNS is a benign entity with no risk for malignant transformation or metastasis. PVNS is composed of metaplastic synovium and blood products, which gives its characteristic MRI appearance. Monoarticular synovial pseudo masses that show low T1 and low T2 signal intensity related to the Hemosiderin deposition (blood products). It’s important to differentiate FNS from diffuse PVNS as the treatment and prognosis varies. Obviously, surgical excision is “easier” and recurrence lower for FNS. In terms of location, the knee infrapatellar (Hoffa’s) fat pad is a common location for FNS. But the differential diagnosis for an infrapatellar lesion includes Hoffitis, focal Synovitis, gout tophu or a cyclops lesion on those patients with prior arthroscopy. This case was shared by @mgabriel7 - much thanks for your contribution!