If a child’s teeth are in the process of permanent tooth replacement and the secondary new teeth have erupted without the milk teeth falling out, it is important to take the child to the hospital immediately for extraction of the milk teeth, which is clinically known as retained milk teeth and is a pathological change. Under normal circumstances, as permanent teeth erupt, the roots of the milk teeth will resorb and gradually fall out slowly, after which the secondary permanent teeth will erupt. If the permanent teeth have already erupted and the baby teeth have not fallen out, it is likely that in the future there will be a double layer of teeth, or even teeth that are not aligned properly. Therefore, if new teeth grow before they fall out, the baby teeth must be extracted immediately, so that the permanent teeth can be restored to their original position as much as possible and reduce the chances of uneven teeth alignment in the future.