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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 osteoarthritis 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 (see Figure 1). OA at the base of the thumb is more commonly seen in women over the age of 40. The exact cause is unknown, but genetics, previous injuries such as fractures or dislocations, and generalized joint laxity may predispose towards development of basilar thumb arthritis. |
Figure 1. Thumb Basal Joint
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The most common symptom of thumb arthritis 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 of thumb arthritis, 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 (see Figure 2). The next joint above the CMC may compensate by loosening, causing it to bend further back (hyperextension).
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Figure 2. In severe cases, the thumb collapses into the palm, causing a zig-zag deformity.
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The diagnosis of thumb arthritis is made by history and physical evaluation. Pressure and movement such as twisting will produce pain at the joint. A grinding sensation may also be present at the joint (see Figure 3). X-rays are used to confirm the diagnosis, although symptom severity often does not correlate with x-ray findings.
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Figure 3. Grind Test.
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