Most people don’t think about hand or thumb injuries when they think about skiing or snowboarding, but skier’s thumb is actually quite common: it impacts around 200,000 people each year! What is skier’s thumb? Skier’s thumb is an injury to the ligament at the base of the thumb joint, known as the ulnar collateral ligament. It can range from a mild sprain to a severe ligament tear. How does it happen? What Causes Skier’s Thumb?