Arthritis at the base of the thumb is more common in women and
usually starts after age 40. The cause of this form of arthritis
is unknown in most cases. - Hand & Upper Extremity Service
Arthritis: Base of the Thumb What is arthritis at the base of
the thumb? In a normal joint, cartilage covers the ends of the
bones and allows them to move smoothly and painlessly against
one another. In osteoarthritis (or degenerative arthritis), the
cartilage layer wears out, resulting in direct contact between
the bones. In the hand, the second most common joint to develop
osteoarthritis is the joint at the base of the thumb. The thumb
basal joint, also known as the carpometacarpal (CMC) joint, is a
specialized saddle-shaped joint that is formed by a small wrist
bone (trapezium) and the first of the three bones in the thumb
(metacarpal). The specialized shape of this joint allows the
thumb its wide range of movement—up and down, across the palm,
and the ability to pinch with the fingers (see Figure 1). Who
gets arthritis at