Sometimes a simple sprain is not so simple. A sprain is a tear (partial or complete) of a ligament, the tough tissue that connects one bone to another, across a joint.

A sprain of the inner side of the thumb, where it connects to the hand, is just such an example. This ligament is called the ulnar collateral ligament (UCL) of the metacarpophalangeal joint (MPJ).

An injury of the UCL typically occurs when a “hyper-abduction” force is applied to the thumb. Such a force is applied when a ski pole gets stuck in the snow and the hand keeps going. The UCL is stretched beyond its breaking point and pulls off its attachment to the thumb. When the joint comes back to normal position, tissue may get stuck between the ligament and its (former) attachment site. This interposed tissue prevents healing.

Poor healing can lead to instability and pain with pinching and grasping activities and ultimately to arthritis at the MPJ.