Painful inflammation of a tendon and surround sheath. The inflammation can result from: overuse or over stretching of the tendon, trauma, infection, or inflammatory diseases. Tendons are the fibrous attachments of the muscles to the bones. The sheath of the tendon can also become inflamed causing tenosynovitis. De Quervain's tenosynovitis is one of the most common types and occurs at the base of the thumb and lateral wrist. When tenosynovitis is caused by an infection prompt treatment with antibiotics is needed to prevent permanent loss of function.