Avulsion fracture and ulnar collateral ligament injury of the thumb

Three views of the right hand in posteroanterior (A), oblique (B), and lateral (C) projections demonstrate a non-displaced fracture at the ulnar base of the first proximal phalanx with intra-articular extension to the metacarpophalangeal (MCP) joint (white arrowhead), indicative of an ulnar collateral ligament injury with associated avulsion fracture secondary to forced hyperabduction of the flexed thumb. Associated soft tissue swelling of the thenar eminence is shown (C, white arrow).