How long has it been since the burn? If it is not improper functional exercise or nerve injury, it should be caused by hand scar contracture. The effective treatment is plastic surgery, which can improve the function of the hand.

(Dr. Chen Xin from Jishuitan Hospital solemnly reminds me that because I can't see the patient in person, I can't fully understand the condition. The above suggestions are for reference only. Please go to the hospital for specific diagnosis and treatment under the guidance of a doctor! )