Inability to extend the thumb is one of the symptoms of flexor tendon tenosynovitis, which mostly occurs in the thumb and middle finger. The dysfunction of the flexion and extension of the affected finger is particularly pronounced in the early morning and is relieved or disappears after activity. The pain sometimes radiates to the wrist. There may be pressure pain in the metacarpophalangeal joint flexion, and sometimes thickened tendon sheaths and pea-sized nodules may be palpable. When the affected finger is bent, it suddenly stays in the semi-bent position, and the finger can neither be straightened nor flexed, as if it is suddenly stuck, with unbearable pain and soreness, but after using the other hand to assist in triggering, the finger can move again, producing a trigger-like action and popping sound, so it is also called trigger finger or popping finger. The following diseases are also causes of the inability to extend the thumb: 1, radial stenosis tendovaginitis is a chronic sterile inflammatory change of the tendon sheath caused by mechanical friction. Tenosynovitis is a common orthopedic disease, mostly seen in manual workers, especially those who repeatedly do extension, flexion, pinching and grip operations with their fingers are susceptible to this disease, generally more women than men. Tendon sheath is a kind of tendon auxiliary device, is the connective tissue around the tendon to adapt to the sliding of the tendon and differentiate the formation of a double layer of tubular structure surrounding the tendon, mostly seen in the wrist, ankle, fingers, toes and other tendons long and active parts. The tendon sheath is divided into two layers, the outer layer is the fibrous tendon sheath, which is thickened by the transverse and oblique fibers of the deep fascia, attached to the bone and joint capsule, and plays a role in restraining, supporting, sliding and enhancing the tension of the tendon. The inner layer is the synovial tendon sheath, which is located within the fibrous tendon sheath. The synovial sheath is divided into dirty and wall layers. The wall layer is lined with the inner surface of the fibrous tendon sheath, and the folded part formed on the bone surface is called the tendon lamina, and the layer wrapped around the surface of the tendon is the dirty layer. The two ends of the synovial membrane of the dirty and wall layers are closed as a blind cavity, which contains a small amount of synovial fluid, which plays the role of lubrication and maintaining the mobility of the tendon. 2.Septic tenosynovitis is mostly caused by stab wounds on the transverse part of the palm of the finger, while hematogenous infection is less common. The sheath is rich in synovial fluid, moist and less blood, which provides favorable conditions for infection. Once infection occurs, inflammation quickly spreads to the entire sheath canal. Infection of the built-up sheath of the thumb and little finger may also spread to the radial or ulnar bursa, and may spread to the forearm. Within the narrow lumen of the fibrous sheath canal, the inflammation itself and the tension of the exudate can disrupt the blood circulation of the tendon, leading to tendon necrosis. Even if the tendon is not necrotic, adhesions often occur after the inflammation heals, seriously affecting the function of the finger. 3, acute septic tenosynovitis hand tendinitis often caused by deep stabbing infection, but also by the spread of infection in nearby tissues. The causative organism is mostly Staphylococcus aureus. Infection of the tendon sheath of the dorsal extensor muscle of the hand is rare.