Which ligament is injured in Gamekeeper's thumb?

• A. Radial collateral ligament of the MCP joint
• B. Ulnar collateral ligament of the first MCP joint
• C. Volar plate injury
• D. Condyle fracture

Answer: (B)

Gamekeeper’s thumb is an injury to the ulnar collateral ligament of the first metacarpophalangeal joint. This ligament stabilizes the thumb at the MCP joint against valgus stress when the thumb is abducted away from the hand. A force such as a fall onto an outstretched hand or a sudden, forceful pull on a ski pole can cause the thumb to bend away and hyperextend, tearing the UCL or pulling it off its bony attachment. Patients typically experience pain at the inside (ulnar) aspect of the base of the thumb, weakness with pinching or gripping, and a laxity when tested for valgus stability of the MCP joint. If the UCL tears completely or there’s an avulsion fracture, instability can be more pronounced, and a surgery may be required, especially if a Stener lesion—where the torn end is displaced under the adductor aponeurosis—occurs.

The other options describe different problems. A ligament on the radial side would be injured with forces pushing the thumb toward the ulnar side, not the classic mechanism of Gamekeeper’s thumb. A volar plate injury relates to hyperextension injuries and instability at the joint due to soft-tissue stretching rather than a primary UCL tear. A condyle fracture is a bone injury, not a primary ligament injury, and while it can accompany thumb trauma, it is not the injury that defines Gamekeeper’s thumb.