Quick Check: is your pain from CMC joint arthritis or De Quervain's tenosynovitis? Carpometacarpal (CMC) joint arthritis occurs at the base of your thumb (where the thumb and wrist meet). As cartilage wears down, the bones rub together causing pain and difficulty using your thumb. De Quervain's tenosynovitis occurs when the tendons around the base of the thumb are irritated or constricted. It may hurt when you turn your wrist, grasp or make a fist. Need help picking the right knee brace? ask our kinesiology team.