DeQuervain's Tenosynovitis is an inflammation of the tissue on the thumb side of the wrist. This is a result of the tendons (cord like fibers that attach muscle to bone) and synovium (a slick membrane that allows the tendons to move easily) becoming irritated and causing pain. This usually involves the abductor pollicis longus and the extensor pollicis brevis tendons. Inflamed tendons cause the tendon sheath or lining to swell and thicken thereby pinching the tendons and making them unable to slide easily. Pain on the thumb side of the wrist is the most common symptom. It may also be tender to the touch and a small "knot" may be present. DeQuervain's can be caused by repetitive motions that irritate the tendons or an injury to the thumb side of the wrist.