This condition is caused when tendons on the thumb side of the wrist are swollen or irritated. The irritation causes the lining (synovium) around the tendon to swell, which changes the shape of the compartment. This makes it difficult for the tendons to move as they should. Thickening of the tendons can cause pain and tenderness along the thumb side of the wrist. This is particularly noticeable when forming a fist, grasping or gripping things, or when turning the wrist.