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Injury to the ulnar collateral ligament of the thumb is fairly common. This strong band of tissue is attached to the middle joint of the thumb, the joint next to the web space of the thumb. This condition is sometimes called gamekeeper's thumb (Scottish gamekeepers commonly injured their thumbs as a result of their job). 

The joint that is affected is called the metacarpophalangeal joint, or MCP joint. Any hard force on the thumb that pulls the thumb away from the hand (valgus force) can cause damage to the ulnar collateral ligaments. When the thumb is straight, the collateral ligaments are tight and stabilise the joint against valgus force. If the force is too strong, the ligaments can tear. They may even tear completely which is called  a rupture.

Today, most cases of ligament damage in the MCP joint are caused from sports injuries. Now surgeons tend to refer to the condition as skier's thumb, since it happens so often in down hill ski accidents.

Sometimes the ligament itself will not tear but instead pulls a small piece of bone off the base of the thumb where it attaches. this is called an avulsion fracture.

Symptoms

When the ulnar collateral ligament of the thumb is injured, the MCP joint becomes painful and swollen, and the thumb feels weak when you pinch or grasp. You may see bruise like discolorations on the skin around the joint. The loose end of the torn ligament may form a bump that can be felt along the edge of the thumb near the palm of the hand. A torn ligament makes it difficult to hold or squeeze things between your thumb and index finger.

Treatment

To check the ligament, your surgeon will perform a valgus stress test. This involves pushing your thumb backwards and out in different positions. Valgus tests can  be painful, so you may be given a local anaesthetic before being tested.  The stress test is done to determine how stable your joint is. If your joint holds steady, the ligaments may only have been strained or partially torn. If your joint is loose and unstable, your ligaments have probably completely ruptured.

If the thumb ligaments are only partially torn, they usually heal without surgery. Your thumb will be immobilised for 4 to 6 weeks in a special cast, called a thumb spica cast.  After that you will begin to do exercises to regain your range of motion and to strengthen your grip. Getting treatment soon after an injury to the ulnar collateral ligament of the thumb may improve your ability to regain strength and range of motion.

If the ligaments are completely torn, you will most likely have surgery to repair them. A torn ligament cannot fully heal itself therefore you will have to undergo a ligament reconstruction.