De Quervain’s tenosynovitis is a painful condition that occurs when two of the thumb’s tendons—located on the thumb side of your wrist—become overused or irritated.These tendons are covered by a slick coating called synovium that allows them to smoothly slide back and forth through the sheaths that enclose them. Occasionally, overuse of the hand or wrist causes swelling and inflammation in these tendons or in the sheaths themselves.
Source: Verywell health