An acute radial stress on the thumb metacarpophalangeal joint may disrupt its ulnar support. Tissue failure is usually rupture of the ulnar collateral ligament from its insertion at the base of the proximal phalanx. Injury can occur in the form of an avulsion fracture, less commonly as a combined fracture and ligament tear, or as a ligament rupture through the central or proximal ligament. The historical eponym refers to the gamekeeper who repeatedly dispatched small animals by using their thumb to push forcefully on the back of the animal's head, breaking its neck. The injury may result in an irreducible displacement of the end of the ligament. For irreducible ligament displacement, the following events must occur: at the time of maximum displacement, the extensor mechanism overlying the ligament tears, allowing the torn ligament end to protrude through a buttonhole, where it becomes trapped in a subcutaneous position (Fig. 16). This specific scenario is referred to as the Stener lesion and is important because spontaneous ligament healing is prevented by interposition of the thumb extensor mechanism, requiring surgery to prevent chronic instability. The Stener lesion occurs in a sizable minority of thumb ulnar collateral ligament injuries, and should be suspected when the metacarpophalangeal joint is grossly unstable, or when there is a persistent firm mass on the ulnar aspect of the thumb metacarpal head. In most cases, ligament reinsertion is possible months or even years after injury, and should be considered to stabilize the thumb and prevent early degenerative changes from persistent subluxation.