The inability to straighten the thumb may be caused by tendon rupture, dislocation of joints due to trauma, tenosynovitis, etc. 1. Trauma-induced tendon rupture: the patient’s thumb is injured by contusion or other traumatic injuries, which causes complete rupture of the tendon, resulting in unfavorable flexion and extension of the thumb, and then the thumb will not be straightened. 2. Trauma leading to joint dislocation: joint dislocation due to the violence at that time is relatively large, there may be joint dislocation after the dislocation of some of the joints of the distal end of the dislocated joints will be stabbed local soft tissues, resulting in the thumb can not be straightened. 3. Tenosynovitis: If the patient has tendinitis of the flexor tendon of the little finger, it may lead to flexion deformity around the little finger, which will affect the flexion and extension activities of the little finger, and lead to the situation that the thumb can not be straightened, and even popping sound will occur. There are other reasons why the little thumb is not straight, it is recommended to go to the regular hospital, improve the examination, clear diagnosis, and treatment under the guidance of the doctor.