Pigmented villonodular synovitis causing painless chronic knee swelling in an adolescent

It is uncommon for an adolescent to present with a painless chronic knee effusion without a history of trauma. Acute knee effusions are usually caused by internal derangement, patellar dislocations, or fractures. The differential diagnosis of a chronic knee effusion must include the more uncommon diagnoses of juvenile rheumatoid arthritis, septic arthritis, reactive arthritis, synovial osteochondromatosis, synovial hemangioma, synovial sarcoma, or pigmented villonodular synovitis. Radiographic imaging, plain radiographs, and magnetic resonance imaging in addition to a biopsy specimen are most helpful in differentiating these diagnoses. Pigmented villonodular synovitis is an uncommon diagnosis that is rarely seen in an adolescent. Treatment includes synovectomy via arthroscopy or open arthrotomy.