The following methods are used to determine the nerve necrosis: 1. The crown has deep cavities or hidden fissures, the X-ray shows that the pulp cavity is exposed or open, and there is an obvious low-density shadow area at the root tip. 2. The color of the tooth is obviously different from the surrounding teeth, there are significant differences, it loses its luster and is dark gray or yellowish brown, the tooth is brittle and easily disintegrates. 3. The tooth does not respond to external stimuli such as cold, heat, acid and sweet, and there is numbness and dullness in comparison with the neighboring teeth and the jaws. There is numbness and sluggishness in comparison with the neighboring teeth and the opposite jaw teeth. 4.The pulp vitality test was conducted to confirm that the teeth had lost vitality and the nerves were gradually necrotic.