Avulsion fractures of the ulnar collateral ligament of the thumb metacarpophalangeal joint (bony skier's thumb) may result in chronic instability with pain and weakness of pinch if improperly treated. Management requires an understanding of the relevant anatomy and careful clinical examination including stress testing. Undisplaced, or minimally displaced and stable fractures are treated conservatively, whereas displaced, rotated and unstable fractures require surgical treatment.