A small osseous fragment (lower arrow) is noted on the ulnar side of the 1st MCP joint. A concave defect (upper arrow) is noted in the base of the proximal phalanx of the thumb, consistent with a "donor site". This location is the attachment site of the ulnar collateral ligament, one of the two collateral ligaments of the 1st MCP joint.
The classic gamekeeper's injury is actually a chronic injury, due to chronic repetitive stress to the ulnar collateral ligament. This was allegedly seen in gamekeepers, supposedly from wringing the necks of game animals that were not quite dead yet. This chronic neck wringing is said to have led to avulsion of the ulnar collateral ligament.
Nowadays, this type of injury is seen more commonly in a different type of sport -- skiing. A typical scenario for this injury would be a skier with their wrists up through the loops of a ski pole, who then falls, and has forced radial deviation of their thumb, tearing or avulsing the ulnar collateral ligament. Many injuries of this type will not demonstrate an avulsion fragment as in this case. Clinical examination, and occasionally stress films, may be necessary to make the diagnosis.