Sudden, unexplained tears are not uncommon in the extensor pollicis longus (EPL) tendon of the thumb. This tendon extends the thumb when you give a "thumbs up" signal. You can see it under the skin when you make this motion.

Doctors aren't sure why this tendon ruptures so easily. It's a common injury when the lower arm is broken. There's a band of tissue that holds the tendon tightly in place. When a fracture occurs, the tendon can rub against the broken bone and tear.

Overuse can explain some EPL ruptures. Friction when the tendon glides over a bump in the bone can build up during repeated motions. Of course, actual trauma or injury to the thumb from a fall or sports activity can help explain some EPL ruptures.

In the rest of the unknown cases there may be a mixture of vascular and mechanical causes. Vascular refers to a loss of blood supply. Maybe the tendon is okay, but the lining around the tendon is too tight. This can cut off circulation. With reduced blood flow, there's a possibility that friction builds up. Without proper lubrication, high friction scrapes the tendon.