An injury that happens when one of the bands of connective tissue in the thumb, called a ligament, is stretched too far.

A sprained thumb often happens when the thumb is bent backward, away from the palm of the hand, by a strong force. With a mild sprained thumb, a ligament is stretched but isn't torn. With a moderate sprain, a ligament is partly torn. With a serious sprain, a ligament is completely torn or pulled off the bone it's attached to.

The symptoms of a sprained thumb depend on how serious the injury is. It's possible to have swelling, pain and bruising around the base of the thumb, by the palm. A serious sprain might cause swelling or a lump on the inside of the thumb. This might make it hard to grasp objects between the thumb and index finger.

Treatment for a thumb sprain also depends on how serious the injury is. A mild sprain often is treated with rest, ice and gentle compression. A moderately sprained thumb might be placed in a bandage, cast or splint while it heals. A serious sprain may need to be fixed with surgery.