Children generally start to replace their teeth from the age of 6 to 7, but some children’s milk teeth are always late to fall out and permanent teeth are late to erupt, why is this? I drew the following pictures to explain the common causes of “retained milk teeth” The congenital lack of permanent teeth embryo due to the absence of permanent teeth development, the root of the milk teeth can not be pressed to stimulate the resorption phenomenon does not occur, it will naturally remain on the dental bed for a longer period of time. Sometimes these remaining milk teeth are not lost in adulthood and still have some chewing function. Milk tooth disease Infection around the root secondary to milk tooth disease, resulting in necrosis of the permanent tooth germ; or injury to the permanent tooth germ due to trauma to the milk tooth. Adhesion between the roots of the baby teeth and the alveolar bone Due to inflammation, the roots of the baby teeth are not fully resorbed or not resorbed at all, and the baby teeth cannot be lost. Misalignment of the permanent tooth germ, i.e., it does not grow underneath the baby tooth, resulting in the permanent tooth not coming into contact with the baby tooth root. Since the roots of the milk teeth are not stimulated by the pressure of the permanent teeth, the roots are not resorbed, so they do not loosen in the dental bed, resulting in delayed loss of the milk teeth. The soft tissues of the missing teeth are often rubbed with food to thicken the gingival mucosa and make the texture tough, thus increasing the resistance to the eruption of permanent teeth, resulting in the late eruption of permanent teeth. In addition, malnutrition and endocrine metabolic disorders in children can also cause late eruption of permanent teeth.