A woman in her 60s presented with a one year history of a slowly-growing painful mass at the dorsal right wrist and sudden inability to actively lift her ring and little fingers without any trauma. She had rheumatoid arthritis ten years before and took anti-rheumatic drugs irregularly. On physical exam, a soft tender mass was over the dorsal wrist. The fourth and fifth metacarpophalangeal joint failed to extend actively with normal flexion. The ulnar head protruded dorsally with positive “piano key” sign (Fig.
