Ulnar collateral ligament injuries of the thumb are the most common ligament injuries of the hand. This is because the thumb is positioned in a different plane than the rest of the fingers and becomes vulnerable to falls or participation in ball sports. The stability of the thumb relies on the ulnar collateral ligament situated at the MCP joint, where the thumb meets the hand. The thumb is supported at the MCP joint by a ‘sling’ or two intrinsic muscles, namely the abductor policies brevis and adductor pollicis on the ulnar side. At this level, the ulnar collateral ligament provides the ability for a strong pinch action or tip-to-tip pinching.

UCL injuries of the thumb typically occur when there is a forceful hyperextension or sprain of the thumb, often due to activities like skiing, playing sports, or falling on an outstretched hand. The injury can range from a mild sprain or grade one injury to a complete tear of the UCL and is sometimes even associated with a capsular tear of this joint. Usually, the ligament tears off at its attachment, or it can avulse a small piece of bone (called an ulnar collateral ligament avulsion fracture). Furthermore, the ligament might detach distally from its dorsal capsule with the volar plate, which then assumes a different angle, still creating instability due to rotation instability. This is probably the most common in a grade 2 type injury where the ulnar collateral ligament tears open, but there is some form of soft tissue that restricts the complete opening of the joint.

In the most advanced lesions, the ligament tears completely, as in a grade 3 injury, which is called the Stener lesion. This means that the ligament stump sits between the bone and the adductor pollicis muscle, an intrinsic muscle that prevents the ability of the stump to heal by itself.