Trigger thumb, or trigger finger, is caused by an inflamed tendon in the finger. The pain of trigger finger comes from the tendon poking through the sheath that protects and lubricates it, explains WebMD.
The tissue sheath surrounding the tendon is called the synovium, and a normal tendon glides through this sheath when a finger is moved, says WebMD. Inflamed tendons get stuck in this sheath. When this occurs it causes the affected fingers to lock in place as they move or bend. Some people explain this as feeling a popping sensation in the affected finger. The main cause of the tendon irritation that causes trigger finger is prolonged repetitive or forced movements of the fingers and thumbs.