Gamekeeper's thumb (also known as skier's thumb or UCL tear) is a type of injury to the ulnar collateral ligament (UCL) of the thumb. The UCL is torn at (or in some cases even avulsed from) its insertion site into the proximal phalanx of the thumb in the vast majority (approximately 90%) of cases. This condition is commonly observed among gamekeepers and Scottish fowl hunters, as well as athletes (such as volleyballers). It also occurs among people who sustain a fall onto an outstretched hand.