Trigger thumb is the popping, clicking or catching of the thumb when trying to straighten it. In some cases, the thumb locks in a flexed or extended position and must be gently straightened with the other hand.
Trigger thumb occurs when a tendon near the base of the thumb, called the A1 pulley, swells and forms a nodule near the A1. The nodule gets locked at the pulley, and the tendon cannot easily slide inside the tendon sheath. In children, the condition usually happens between the ages of 1 and 3 years old, and it usually occurs in one hand. Triggering can also occur in the fingers, and is considered to be indicative of a more serious problem.