This is a stretch or tear of a ligament at the base of the thumb. Ligaments are bands of tissue that connect one bone to another. This sprain involves the ulnar collateral ligament. We call it the "UCL."
How does this sprain happen? You injure the UCL when your thumb is forcefully bent back away from your hand. It can happen if you fall and try to catch yourself with an outstretched hand. It happens during sports. It's also a common injury for skiers who fall while their hand is strapped to a ski pole. In fact, we also call this injury "skier's thumb."
What are the symptoms? You can have bruising, tenderness and swelling. It may be painful, but you could have a sprain without pain. Your thumb may feel loose. It may be hard to punch or grasp things.
How do we treat it? It depends on how badly you've sprained your thumb. A mild sprain may heal with rest and home care. We may need to put your thumb in a splint or a cast. And for a severe sprain, you may need surgery. Your doctor will create a plan that's right for you.