De Quervain's tenosynovitis is an inflammation of the sheaths of the tendons (tenosynovium) that move the thumb away from the hand. It is technically referred to as a stenosing tenosynovitis. Both the tendons and their surrounding tissue sheaths are involved in the area where the tendon has to go through a small tunnel of bony and fibrous tissue at the wrist. The inflamed tenosynovium becomes painful at the tunnel area.

De Quervain's Tenosynovitis
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De Quervain’s is a mechanical problem caused by repetitive use of the thumb and wrists. A direct blow to the area may also cause the condition.