The ski thumb is a ligament injury to the base joint of the thumb. In this case, usually by a fall on the spread-out thumb, the inner lateral ligament of the base joint of the thumb, facing the index finger, tears. This lateral ligament is referred to in anatomy as the ulnar lateral ligament, since it lies on the elbow side of the thumb base joint (ulna = Latin: ulna). The ulnar lateral ligament contributes significantly to the stabilization of the thumb in almost all grip shapes. It prevents the thumb from deviating to the outside, e.g. when writing. If the lateral ligament no longer grows together after an injury, chronic instability with loss of strength can occur with many grip forms (e.g. pointed grip). An incorrect load on the joint caused by the misalignment can also lead to osteoarthritis (wear and tear) of the joint over the course of many years. The injury actually occurs very often when skiing, but can also be caused by other causes and accidents, in which the thumb is sheared off suddenly and with a lot of force to the outside.
With the forcible spreading of the thumb in the base joint, a rupture of the elbow-side (ulnar) ligamentous apparatus occurs with sufficient force. This is either the rupture at the attachment of the ligament to the base member, a rupture with a bony fragment of the base of the base member of various sizes, or very rarely, a rupture of the ligament itself. The ulnar collateral ligament is the only stabilizer for the thumb at the base of the thumb when accessing it. As a result of any instability (loosening) of the ligament that may arise in the event of cicatricial malhealing, a painful grasping disorder occurs.
The classic cause is the fall while skiing with the handle of the ski pole fixed to the hand by the ski pole loop, on which the thumb is then forced into a spreading movement in the base joint. This then leads to the ulnar collateral ligament rupture. However, other injury patterns are also possible. In my (Berlin) consultation hours, bicycle or motorcycle crashes are the most common cause.
With the ulnar collateral ligament rupture, there is always a pronounced swelling of the basal joint region of the thumb, which leads to a significant bruising discoloration after a few hours. Sometimes there is an externally recognizable axial deviation of the thumb on the spoke side. There is a compressive pain over the inner aspect of the basal joint of the thumb.