Foreign body-induced knee synovitis: importance of history by the radiologist

We describe a case of a 7.5-year-old boy with a 7-week history of recurrent right knee swelling despite joint aspiration, persisting limping, and mild motion restriction, following a minor injury. Initial magnetic resonance imaging (MRI), ultrasound, and joint fluid analysis yielded non-specific synovitis. The child was investigated for juvenile idiopathic arthritis (JIA) and referred for a repeat MRI. Questioning by the attending radiologist disclosed falling onto wood sticks 10 weeks previously. Florid enhancing synovitis, prominent popliteal lymph nodes, and a linear foreign body were identified and ultrasonographically confirmed. During the operation, a 3-cm wooden splinter was successfully removed. This case highlights the uncommon challenge of differentiating between JIA and foreign body-induced synovitis and emphasizes the importance of detailed history by the radiologist and meticulous imaging techniques.