The baby teeth usually start to fall off gradually around 6-7 years old and are replaced by the inherited permanent teeth until the end of replacement around 12-14 years old. As long as the permanent tooth embryo exists under the milk teeth, the decay of milk teeth is mild and does not involve the dental pulp and complicate the periapical inflammation, generally it will not affect the replacement of milk teeth. However, if the caries of the milk teeth develops further and affects the periapical tissues, it may affect the development of the permanent tooth embryo and cause the permanent teeth to have enamel underdevelopment, which is manifested as chalky plaque on the enamel of the tooth surface or even enamel defect. On the other hand, periapical inflammation may also lead to localized alveolar bone destruction, abnormal root resorption of milk teeth, and retention of residual roots, causing premature or late eruption of permanent teeth, or abnormal eruption direction. In summary, that is to say, as long as the permanent tooth embryo exists, the bad milk teeth can be replaced with new teeth regardless of the severity of the inflammation, but the heavy inflammation may cause the inherited permanent teeth to develop abnormally and position abnormally. There is another situation that if the permanent tooth germ is missing, the new tooth will not be replaced regardless of whether the decay of the baby tooth occurs or not, but this situation is relatively rare.