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 soreness. Differential diagnosis of thumb extension: 1, thumb deformity: radial nerve injury in the movement of clinical manifestations: radial nerve injury in the upper arm, the extensor muscles are widely paralyzed, triceps brachii, brachioradialis, radial wrist long and short extensors, posterior rotator, extensor digitorum generalis, ulnar carpal extensors and index and little finger intrinsic extensors are paralyzed. Therefore, there is wrist ptosis, thumb and fingers ptosis, inability to extend the metacarpophalangeal joint, anterior rotation deformity of the forearm, inability to rotate back, thumb inversion deformity. 2, thumbs syndrome: Mafang syndrome is also known as congenital mesodermal dysplasia, Marchesani syndrome, spider finger syndrome, limb slenderness, which is characterized by peripheral connective tissue dystrophy, skeletal abnormalities, internal eye disease and cardiovascular abnormalities, and is a genetic disease with basic defects in connective tissue. Thumb sign (thumbsign): The thumbs are closed inward, the remaining four fingers make a fist, and the tip of the thumb extends beyond the lower edge of the palm, accounting for about half of the patients with this disease.