Tenosynovitis is inflammation of the lining of the sheath that surrounds a tendon. The synovium is a lining of the protective sheath that covers tendons. Tenosynovitis is inflammation of this sheath. The cause of the inflammation may be unknown, or it may result from infection, injury overuse, and strain. The wrists, hands, and feet are commonly affected, but the condition may occur with any tendon sheath.