Localized pigmented villonodular synovitis presenting as recurrent dislocation of the patella.

A 31-year-old woman presented with a 1-year history of recurrent dislocation of the patella. On examination, she was found to have a mass at the medial patellofemoral joint line. Arthroscopy showed this to be a vascular synovial tumor extending into the medial patellofemoral joint and causing lateral deviation of the patella. The mass was resected with resolution of her symptoms and histologic examination showed this to be pigmented villonodular synovitis. There are an increasing number of case reports of these lesions causing varying mechanical symptoms in the knee and they should be considered as a differential diagnosis.