I fell and broke my left wrist about seven weeks ago. Doctor slapped a cast on that baby and I thought I was good to go. Then I discovered last week that a tendon to my thumb ruptured. How can that happen with a cast on? I don’t get it.

Wrist fractures involving the radius bone of the forearm has a well-known complication associated with it. And you have just experienced that — rupture of the extensor pollicis longus (EPL) tendon of the thumb. This tendon helps move both the tip of the thumb and the wrist, so damage to it can impair function of the hand.

There are reports of how often this tendon rupture occurs after radial wrist fractures. The incidence ranges between 0.3 and two per cent. In a recent study, surgeons from Brigham and Women’s Hospital in Boston reported a five per cent incidence. This is much higher than previously thought and bears mention as well as closer study.

EPL tendon ruptures are more common with nondisplaced radial fractures. Nondisplaced means the bones did not separate or shift after the break occurred. What’s the connection between these two events (bone fracture and tendon rupture)?

When the force that breaks a bone is not enough to rip or tear the soft tissues around the bone, complications like the tendon rupture can occur later. This is because the tendon is held tightly against the bone. The fracture results in swelling, bleeding into the area, and the formation of a bone callus as healing takes place.

All of these events decrease the space around the tendon and put pressure on the tendon. The EPL tendon in particular doesn’t have a very good blood supply to its own tendon sheath (outer protective covering). Anything that disrupts this area can reduce blood flow and nutrition causing avascular necrosis. Avascular necrosis means death of the tissue due to loss of blood. The end result is rupture of the tendon.

Some surgeons have suggested other possible mechanisms for the EPL tendon rupture. There may be a bone spike that forms as a part of the healing process that cuts into the tendon. Or the tendon might have been injured if the fracture was displaced (separated) requiring a procedure called reduction to reset the bone alignment.

Any injury to the blood vessels in the area, blood clots, or even scar tissue can contribute to a tendon rupture. It’s not always clear what caused the problem. Your surgeon may be able to answer this question more specifically to your situation based on X-rays or CT scans.