The common causes of inability to straighten the middle finger include congenital developmental abnormality, osteoarthritis, trauma, tenosynovitis and so on. 1. Congenital developmental abnormality: some patients are born with abnormal development of the bone structure, resulting in the middle finger not being able to straighten. 2. Osteoarthritis: The bones of the human body will undergo degenerative changes with age, which will cause pain in the middle finger at the beginning, and in severe cases, it will be manifested as stiffness of the joints, which will eventually lead to the inability to straighten the middle finger. 3. Trauma: If the patient suffers from external impact, resulting in a fracture of the middle finger, the patient will be accompanied by obvious pain and limited mobility, and need to go to the hospital in time for X-ray examination. 4. Tenosynovitis: After repeated friction of the tendon sheath produces aseptic inflammation, it is easy to cause adhesion between the tendon sheath and the tendon, which leads to joint pain and activity limitation, and in severe cases, the middle finger cannot be straightened. There are many reasons for the inability to straighten the middle finger, and it may also be related to rheumatoid arthritis, nerve damage and other diseases. If the symptoms continue to be unrelieved, it is recommended to go to the hospital in a timely manner, so that the doctor can find out the cause of the disease and carry out targeted treatment.