Sometimes the ligament itself will not tear but instead pulls a small piece of bone off the base of the thumb where it attaches. When this happens, surgery is needed to fix the ligament. If one of the ligaments pulls away from the bone and folds backwards, it won't be able to heal in the correct position. It is especially unstable when the thumb is bent back. When the collateral ligaments actually tear, the MCP joint becomes very unstable. A complete tear is also called a rupture. If the force is too strong, the ligaments can tear. When the thumb is straight, the collateral ligaments are tight and stabilize the joint against valgus force. Any hard force on the thumb that pulls the thumb away from the hand (called a valgus force) can cause damage to the ulnar collateral ligaments. The joint that is affected is called the metacarpophalangeal joint, or MCP joint.
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