This is an issue to the ulna collateral ligament of the metacarpo-phalangeal joint. This is a strong ligament that supports the thumb in pinching and gripping tasks. If this is damaged it can cause long term thumb instability which can affect pain and function.
It is most commonly injured when you fall onto an outstretched thumb and even more common if the thumb is also gripping something at the same time (commonly reported when skiers fall holding a pole, hence the name Skier’s thumb). You will usually find swelling and bruising at the thumb.
The treatment for this is usually splinting (either custom-made in clinic or off-the-shelf) for a period of time, usually 6 weeks, followed by a rehabilitation programme to return normal strength and function back to the hand. Sometimes surgery may be required if the ligament has displaced to a position where it cannot heal, this is called a Stener lesion.
In some rarer cases, there is no injury at all and it is a chronic stretching of the ligament that causes the injury- this is what is meant by Gamekeeper’s thumb. You will often find a loss of strength on pinching or gripping with the thumb