The ulnar collateral ligament of the thumb runs along the ulnar side of the metacarpo-phalangeal joint of the thumb, from the first metacarpal head to the volar aspect of proximal phalanx of the thumb. It is on the radial side of the wrist, but on the ulnar side of the thumb.

Injuries to the ulnar collateral ligament of the thumb cause instability and loss of function of the thumb. Acutely, this injury is known as the skier’s thumb; if it is the result of chronic injury, it is known as gamekeeper’s thumb.

The ulnar collateral ligament of the thumb should not be confused with the ulnar collateral ligament of the wrist joint