Gamekeeper's Thumb involves the hyperextension of the thumb at the metacarpophalangeal (MCP) joint. It is a very common sports injury involving displaced and non-displaced tears of the ulnar collateral ligament. The most common symptom is significant pain in the thumb. Most people may believe that they have broken their thumb. Lateral instability at the metacarpophalangeal joint will also occur. Gamekeeper's Thumb is caused by a traumatic rupture of the ulnar collateral ligament. Usually the result of trying to catch one's self after a backwards fall or jamming the thumb with a basketball or soccer ball. This is a very common sports injury.