CMC OA of the thumb is a degenerative joint disease. Although the exact cause remains unclear, there is a general consensus among researchers about some of the mechanisms leading to CMC OA of the thumb. It is believed that laxity of the ligaments surrounding the CMC1 joint is the main cause of CMC OA. Especially laxity of the most significant ligament, the palmar beak ligament, will lead to joint instability. This instability causes misalignment of the joint bones, which will then rub against each other. As a consequence, the cushioning cartilage, which covers the surfaces of the joint bones, wears down. The synovial membranes get irritated and synovitis occurs. Eventually, the bare surfaces of the joint bones are exposed to each other. The gap between the joint bones decreases, as there is hardly any cartilage left. In reaction of this process the joint bones thicken at the surface, resulting in subchondral sclerosis. Also bony outgrowths, called osteophytes, are formed at the joint margins. In advanced stages of CMC OA, misalignment of the joint bones causes the tendons attached to thumb to force the base of the thumb into radial deviation and extension and the thumb phalanges in hyperextension. As a consequence the ‘zigzag’ deformity becomes visible. The large forces, which the CMC1 joint has to withstand, are considered to be another cause of CMC OA. During daily manual activities, such as grabbing and pinching, the thumb is constantly being used. The forces generated by these movements have an enlarged impact on the CMC1 joint because of the leverage within the thumb. This makes the joint even more sensitive to wear and tear. Showing the normal forces of the thumb Risk factors Several risk factors of CMC OA of the thumb are known. Each of these risk factors does not cause CMC OA by itself, but acts as a predisposing factor influencing the process of OA in some way. Risk factors include: female gender, suffering from obesity, repetitive heavy manual labor, familial predisposition and hormonal changes, such as menopause.
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