Pigmented Villonodular Synovitis
Pigmented villonodular synovitis (PVNS) is thought to be a neoplastic process related to tenosynovial giant cell tumor. It usually involves the knee joint in young adults and may be focal or diffuse. In most instances, it involves a single joint. The lesion is composed of papillary, villous, and nodular areas covered by synovium. Some foci show large pseudoglandular or cleft-like spaces with a synovial lining.