The thumb is sprained when the main ligament (ulnar collateral) that supports pinch and grasp activities is torn. The ulnar collateral ligament helps the hand function properly, acting like a hinge to keep the joint of the thumb (the metacarpophalangeal joint) stable.

With a fall, it is natural to extend the arms to reduce the force of the impact from hitting the ground. This can cause a sprained thumb if the ulnar collateral ligament is stretched or torn. A spill on the ski slopes with the hand strapped to a ski pole might injure the thumb. This is often called a "skier's thumb."

A sprained thumb diminishes the hand's ability to grasp items between the thumb and index finger. There may or may not be pain right away. Other signs include bruising, tenderness, and swelling. To ensure that the injury will not cause long-term weakness, pain, and instability, it is important to have a doctor assess the injury.

The ulnar collateral ligament of the thumb may be partially or completely torn. The physical examination of the thumb will test how stable the thumb joint is by moving the thumb. X-rays may be taken to ensure that the bone is not broken. A stress X-ray is a special type of X-ray that shows the joint as the ligament is being tested. If the test causes pain, a shot of a local anesthetic may help. An X-ray of the uninjured thumb may also be taken to compare it to the injured thumb.

If the ligament is only partially torn, the thumb joint with probably be immobilized with a bandage or cast until it heals. The splint will be worn for about three weeks. After that, it can be removed when performing flexion and extension exercises with the thumb. The splint should be worn at all other times. This should continue for another two or three weeks, until the swelling and tenderness in the thumb are gone. An ice pack can be placed on the injury twice a day for two to three days after the fall.

If the ulnar collateral ligament of the thumb is completely torn, surgery may be needed. Fragments of bone that sometimes are pulled away when the ligament tears may be removed or put back in their correct positions. After surgery, a short arm cast or a splint may be worn to protect the thumb ligament for six to eight weeks while the injury heals.