Causes

Thumb arthritis commonly occurs with aging. Previous trauma or injury to the thumb joint also can cause thumb arthritis.

In a normal thumb joint, cartilage covers the ends of the bones — acting as a cushion and allowing the bones to glide smoothly against each other. With thumb arthritis, the cartilage that covers the ends of the bones deteriorates, and its smooth surface roughens. The bones then rub against each other, resulting in friction and joint damage.

The damage to the joint might result in growth of new bone along the sides of the existing bone (bone spurs), which can produce noticeable lumps on your thumb joint.