How is the ulnar collateral ligament of the thumb usually injured?

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

Any extreme force (valgus force) on the thumb can damage the ligaments. The most common way for this to happen is to fall on your hand with your thumb stretched out. When a skier falls down while holding a ski pole, the thumb may get bent out and back, leading to an injury in the ulnar collateral ligament.