Thumb CMC (carpometacarpal) joint arthritis is a condition that causes inflammation and degeneration of the joint at the base of the thumb. Arthritis is a common medical condition that causes inflammation in the joints, leading to pain, stiffness, and difficulty moving. Thumb arthritis, also known as basal joint arthritis or carpometacarpal (CMC) joint arthritis, is a type of osteoarthritis that affects the joint at the base of the thumb. The CMC joint is a hinge joint that allows the thumb to move up, down, and away from the hand, and it is essential for many everyday tasks such as grasping and holding objects. Symptoms of CMC joint arthritis may include pain and stiffness in the affected joint, swelling and tenderness, reduced range of motion in the thumb, and difficulty performing tasks that require the use of the thumb.