The tendon sheath is a double-layered, sleeve-like, closed synovial tube that wraps around the tendon. It is a synovial sheath that protects the tendon. It surrounds the tendon in two layers, and the cavity between the two layers is the synovial cavity, which contains synovial fluid of the tendon sheath. The inner layer is closely attached to the tendon, and the outer layer is lined inside the tendon fiber sheath. Together with the bone surface, they have the functions of fixing, protecting and lubricating the tendon and protecting it from friction or compression. If the tendon is excessively rubbed here for a long time, damaging inflammation of the tendon and tendon sheath may occur, causing swelling, which is called tenosynovitis. If not treated, it may develop into permanent inconvenience in movement.