In brief: Skiing, hockey, and boxing, as well as occupational hazards and falls in general, are potential causes of skier's thumb-a sprain of the ulnar support structures of the metacarpophalangeal joint. History, physical characteristics, and the findings on plain and special x-rays are necessary for a correct diagnosis. The mechanism of injury is usually excessive extension or radial deviation of the thumb. Minor sprains can be immobilized by taping. More severe injuries require a thumb spica cast. For acute injuries surgery is necessary to reattach the ulnar collateral ligament to the base of the proximal phalanx.