Pigmented villonodular synovitis and rheumatoid arthritis: diagnostic challenges and therapeutic considerations in a case of knee pain.

We present a case of a woman in her 30s who visited the rheumatology clinic due to her persistent knee pain for 5 years, which spread to multiple joints. She was diagnosed with seropositive rheumatoid arthritis (RA). While most joints responded well to methotrexate and subsequently etanercept, persistent unilateral knee pain prompted further investigation. Imaging revealed synovitis and joint effusion in her knee, prompting arthroscopy and synovial biopsy, revealing pigmented villonodular synovitis (PVNS). Despite initial success with a tricompartmental synovectomy, her disease recurred. The decision was made to pursue medical therapy, with pexidartinib initiated by the oncology team. Our case report highlights the importance of considering other underlying conditions in patients with RA who do not achieve full clinical improvement despite standard treatment. Physicians should remain vigilant for atypical presentations and imaging features in patients with RA, for early recognition of PVNS can significantly impact treatment decisions and patient outcomes.