De Quervain's disease is a term used to describe a painful disorder affecting the tendons at the base of the thumb. This is one of the most common kinds of tendon lining inflammatory diseases or tenosynovitis. These tendons are encased in sheaths, or sleeves, through which the tendons slide. The inner wall of the sheaths contains cells that produce a slippery fluid to lubricate the tendons. With repetitive or excessive movements such as hand twisting and forceful gripping, the lubrication system may malfunction. Failure of the lubricating system allows friction to develop between the tendons of the thumb and their common sheath. The repetitive friction accounts for the abnormal thickening and the constriction of the sheath which interferes with the smooth gliding motion of the tendons.