Skier's Thumb involves an injury to the ulnar collateral ligament at the joint where the thumb meets the hand. It is one of the most common injuries to affect skiers. The most common symptom is significant pain in the thumb. Depending on the severity of the injury, the thumb may be able to maintain stability, or the joint may be unstable. Skier's Thumb is caused by an abduction of the metacarpophalangeal (MCP) joint. It is usually a result of a collision, a fall on an icy mountain, or a direct impact on the thumb (stubbed thumb).