Pigmented villonodular synovitis developing in a patient with rheumatoid arthritis.

A 56-year-old man developed pigmented villonodular synovitis (PVNS) 3 years after he was diagnosed with rheumatoid arthritis (RA). He had been successfully treated with methotrexate, leflunomide, sulfadiazine, and intra-articular knee injection of etanercept (tumor necrosis factor α inhibitor) in 2010. He stopped all drugs for arthritis 1 year later for disease remission. He was readmitted for right knee pain and swelling in 2013, when the magnetic resonance imaging and arthroscopy of the right knee indicated PVNS. Following surgical resection, the patient was doing well after 1 year. This rare case is the first reported case in English-language literature of PVNS of the knee seen in RA patients and illustrates the importance of differential diagnosis of this condition with synovial cysts, which are commonly found in RA.