Osteoarthritis is the gradual breakdown of the material, called cartilage, that covers the joint. Cartilage is a tough elastic material that acts as a shock absorber. In osteoarthritis, the cartilage becomes thinner and may even disappear in some places. When this happens, bone rubs against bone, causing pain. Under this weakened cartilage, the bone tries to regenerate and forms growths, called spurs or osteophytes. The joint may become deformed, larger, and lose its mobility.

Although we still have a lot to learn about the causes of osteoarthritis of the base of the thumb, some predisposing factors are associated with it:

• Instability of the joint leading to osteoarthritis in the long-term
• Misuse of the thumb with repetitive movements

Osteoarthritis of the base of the thumb is very common and mainly affects women. Onset is generally after the age of 40. Most often, it is the dominant hand that is affected. This type of osteoarthritis may be associated with osteoarthritis of the fingers.

Normal	Diseased
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Pain on mobilization (movement) is characteristic and is located at the base of the thumb. Its intensity is generally moderate, but it may be incapacitating at times. Fine motor skills and proximal thumb-index pinch movements (e.g., using scissors) may all be affected. There may also be some deformity.