Localized pigmented villonodular synovitis: arthroscopic diagnosis and management of an "invisible" lesion.

Pigmented villonodular synovitis (PVNS) is a rare disorder that may involve the synovium of joints, bursa, or tendon sheaths. Monoarticular involvement is typical, with the knee most commonly affected. Localized pigmented villonodular synovitis (LPVNS) involves a discrete region of the synovium. Detection and diagnosis of this entity is clinically challenging, and plain radiographs are usually unremarkable. Magnetic resonance imaging (MRI) has been reported to be sensitive for the detection of synovial abnormalities and is the imaging modality of choice in suspected cases of LPVNS. When the diagnosis remains in doubt, arthroscopy may be used for direct visualization of synovial pathology, as well as to obtain tissue for histologic analysis. Definitive treatment may also be performed at the time of arthroscopy. We present a case of LPVNS in which a large (4 cm) lesion was not apparent on preoperative radiographs or MRI and was also missed on initial diagnostic arthroscopy.