Raymond Severt, M.D.

In a normal joint, cartilage covers the end of the bones and serves as a shock absorber to allow smooth, pain-free movement. In osteoarthritis (OA, or “degenerative arthritis”) the cartilage layer wears out, resulting in direct contact between the bones and producing pain and deformity. One of the most common joints to develop OA in the hand is the base of the thumb. The thumb basal joint, also called the carpometacarpal (CMC) joint, is a specialized saddle-shaped joint that is formed by a small bone of the wrist (trapezium) and the first bone of the thumb (metacarpal). The saddle shaped joint allows the thumb to have a wide range of motions, including up, down, across the palm, and the ability to pinch 

The most common symptom is pain at the base of the thumb. The pain can be aggravated by activities that require pinching, such as opening jars, turning door knobs or keys, and writing. Severity can also progress to pain at rest and pain at night. In more severe cases, progressive destruction and mal-alignment of the joint occurs, and a bump develops at the base of the thumb as the metacarpal moves out of the saddle joint. This shift in the joint can cause limited motion and weakness, making pinch difficult. The next joint above the CMC may compensate by loosening, causing it to bend further back (hyperextension).