Can you explain this to me? I had no injury and no accident, but I ruptured a tendon in my thumb. How is this possible?

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.