Sorry Jeroen I can't help myself....... Hope I got it right this time!
Injury to the ulnar collateral ligament of the thumb is fairly common. This strong band of tissue is attached to the joint at the bottom of the thumb where the thumb connects to the palm of the hand.
The joint that is affected at the bottom of the thumb is called the metacarpophalangeal, or MCP, joint. 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. When the thumb is straight, the collateral ligaments are tight and stabilize the joint against valgus force. If the force is too strong, the ligaments are damaged. They may even tear completely. A complete tear is also called a rupture.
When the collateral ligaments tear, the MCP joint becomes very unstable. It is especially unstable when the thumb is bent back. If one of the ligaments pulls away from the bone and folds backwards, it won't be able to heal in the correct position. When this happens, surgery is needed to fix the ligament.