Normally, the tendon runs within a smooth sheath along a groove at the wrist joint. It is held in this position by a ligament. Sometimes after an injury such as a wrist fracture, this tendon sheath can become unstable. If the ECU tendon is not held in place, it may 'snap' over the bone as the wrist is rotated. This snapping can be painful and cause tendonitis of the ECU tendon.