There are many reasons why the thumb cannot be bent, the most common ones are the following: 1. If there is stenosing tenosynovitis, it will affect the flexion and extension activities of the thumb, and some patients may have a popping sound and limited flexion and extension activities on their own, requiring the help of the healthy hand to bend. This means that stenosing tenosynovitis is serious and requires active surgical treatment, including tenosynovectomy or partial resection; 2. If the thumb is locally traumatized and causes fracture, dislocation, or tendon rupture, the thumb cannot be bent. Patients need to combine physical examination with auxiliary tests, such as taking general X-ray to clarify the diagnosis.