Tendon sheaths - these are tubes of connective tissue filled with a lubricating fluid that surround the tendons like a tunnel. The tendons themselves are covered by a layer of membrane, the epitendineum. If this becomes inflamed or changes due to persistent overstrain, a dragging pain develops along the tendon. The cause is mainly overstraining of the tendon, but an inflammatory joint disorder can also cause the symptoms. The tendons of the wrist are affected most often. However, the symptoms can occur in all other tendons, if they run through a tendon sheath. As a rule, inflammation of the tendon sheath is not caused by an infection, but is the result of repetitive overstrain.