This condition, also called skier’s thumb, is an acute sprain or tear of the ulnar collateral ligament (UCL) on the ulnar side of the metacarpal-phalangeal (MCP) joint of the thumb. A related condition, called gamekeeper’s thumb, is a chronic injury that develops over time from repeated stretching of the UCL.

Injuries of the UCL occur when the extended thumb is bent radially away from the hand at the MCP joint. Common causes include a fall onto an outstretched hand, gripping the steering wheel during a car accident, and falling while holding a ski pole.

Symptoms typically include pain, swelling, and tenderness on the ulnar side of the thumb MCP joint. Pinching and gripping with the thumb may be difficult, and range of motion may be limited. In some cases, the ends of the torn ligament are held apart by a nearby tendon, forming a bump under the skin called a Stener lesion.

A physician will diagnose a UCL injury by bending the thumb MCP joint away from the hand. Mild or partial tears are painful, but the joint will be stable. If the UCL is completely torn, the joint will be unstable. In severe cases, the joint capsule or volar plate (a thick, ligamentous band at the base of the joint) may also be damaged. X-rays will be required to check for a bone avulsion or fracture. An MRI may also be used to diagnose a displaced ligament.

Partial tears and nondisplaced fractures are commonly treated by placing the thumb in a cast for 4 to 6 weeks. Complete tears and displaced fractures usually require surgery. Stener lesions always require surgery to avoid chronic pain and instability.