Thumb carpometacarpal (CMC) arthritis is a disorder where the cartilage of the thumb CMC joint erodes, causing pain, inflammation, stiffness, and swelling of the joint. The thumb CMC is located at the base of thumb where it meets the wrist. After the cartilage protecting the thumb CMC joint erodes, you are left with the bone of the thumb rubbing and grinding on the bone of the wrist. Due to the importance of this joint and its involvement in thumb movement, thumb CMC joint arthritis can be severely debilitating. Symptoms usually start with pain in the base of the thumb, but can also include swelling or weakness of the joint. You may notice difficulty manipulating small objects when using your thumb.