What causes skier's thumb?

Ligaments connect bones to each other across a joint; injury to a ligament often is called a “sprain.”  The joint between the thumb and the hand in what we call the first web space is a frequently injured joint. When the thumb is “jammed” into a person, object, or the ground, the ulnar collateral ligament can be torn. This is called “gamekeeper’s thumb.”  It also is called “skier’s thumb” after the skier who injures this ligament while holding a pole. The radial collateral ligament (on the other side of this joint) also can be torn.