Normal joints have cartilage that covers the ends of bones. This allows them to move easily and painlessly between each other. The cartilage layer becomes thinner in osteoarthritis, or degenerative arthritis. This causes direct contact between the bone and the cartilage. The thumb’s base joint is the most common place in the hand to develop osteoarthritis. The thumb basal joint (also known as the carpometacarpal or CMC joint) is a special saddle-shaped joint. It is made by the trapezium, a small wrist bone, and the metacarpal, the first of three bones in your thumb. This joint is unique because it allows the thumb to move up and down across the palm and can pinch with the fingers.

It is caused by:
The majority of this type of osteoarthritis can be attributed to years of repetitive use of the thumb and hand. This condition is more common if you do repetitive tasks that place a lot of stress on your thumb joint. The most common form of arthritis at the base or thumb is found in women, and it usually begins after the age 40. In most cases, the cause of this type of arthritis is unknown. This form of arthritis can be caused by past injuries, such as fractures and severe sprains. Also, generalized joint laxity could increase your chances of developing it at an earlier age.