Tendons are surrounded by a slippery sac called a tenosynovium; this allows the tendons to glide smoothly, even when they cross other structures or bend around corners. In conditions like intersection syndrome the tenosynovium becomes inflamed (called tenosynovitis). When the tenosynovium is inflamed the gliding motion of the tendons is impaired, and movement of the tendons becomes painful.
For more information: Wrist Tendonitis

