I've been trying to read up on a thumb injury I have but I keep getting confused about whether it's a gamekeeper's thumb or a skier's thumb. What's the difference between these two?

There is no difference. Both refer to an injury to the ulnar collateral ligament (UCL) of the thumb. This ligament is a strong band of tissue attached to the middle joint of the thumb, the joint next to the web space of the thumb. The joint that is affected is called the metacarpophalangeal joint, or MCP joint.

Any hard force on the thumb that pulls the thumb away from the palm of the hand (called a valgus force) can cause damage to the ulnar collateral ligament. If the force is too strong, the ligaments can tear. They may even tear completely. A complete tear is also called a rupture.

In the old days it was called a gamekeeper's thumb. It was a common problem in European gamekeepers in the late 1800s and early 1900s. Gamekeepers killed small game by using the thumb and index finger in a way that put enough repetitive force against the UCL to cause an injury.

Today, it's more likely to be from a sports injury. 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 of the thumb. That's when it is referred to as skier's thumb.