A cystic protrusion of the wrist is one of the symptoms of a tendon sheath cyst. Tenosynovial cyst (thecal cyst) is the most common type of mass on the dorsal side of the wrist and can also occur on joint capsules and tendon sheaths in other parts of the body. I. Pathogenesis The etiology is unclear and may be related to trauma and chronic strain. Clinically, there are two types: unicompartmental and multicompartmental, with unicompartmental being the most common and multicompartmental in a few cases. The outer wall of the capsule is composed of dense fibrous tissue, and the inner wall consists of a smooth white membrane of synovial cells. It is controversial whether it is connected to the synovial cavity of the joint or tendon sheath. II. Pathogenesis Currently, most people believe that the connective tissue on the joint capsule, ligament, and tendon sheath is degenerated and forms cysts due to local malnutrition. Some cases are related to trauma. In tendon sheath cysts, the wall of the cyst is dense fibrous connective tissue with no lining cells, and the cyst is filled with colorless and transparent gummy mucus, and the cavity is mostly single-roomed, but there are also multiroomed cases. The cyst is closely associated with the joint capsule or the tendon sheath. Some people believe that the cyst cavity is connected to the joint cavity or the synovial cavity of the tendon sheath, while others believe that it is only connected to the root and not to each other.