When the permanent teeth have erupted and the baby teeth are not lost on time, or when the permanent teeth do not erupt and the baby teeth are retained in the permanent dentition, it is called retained baby teeth. Why does retention occur? The common reasons are: 1. Abnormal eruption direction of permanent teeth, so that the root of the milk teeth is not completely absorbed. 2. The permanent teeth are congenitally missing, and there is no permanent tooth germ under the milk teeth, so that the roots cannot be induced to resorb. 3. The permanent teeth erupt weakly and the roots of the milk teeth are not absorbed. Clinical manifestations: 1. Most common in mixed dentition early mandibular central incisors, permanent teeth erupt on the lingual side and the milk teeth are retained on the labial side, showing a double row of teeth phenomenon. The most common 2, followed by the residual roots or crowns of the first premolar are retained on the buccolingual side of the first premolar or in the proximal and distal centers. The retained second premolar is mostly a congenitally missing permanent tooth germ or an ambiguous blockage. The milk teeth without inherited permanent teeth can perform chewing functions, but it is difficult to use them for life. 3. More than 4 retained milk teeth are called multiple retained milk teeth, which are seen in people with cranial and clavicular hypoplasia. Treatment principle: If there are double rows of teeth, the retained milk teeth should be extracted as soon as possible so as not to affect the alignment of permanent teeth; if there are no inherited permanent teeth, they can be left untreated for the time being. Specifically: 1, the permanent teeth eruption direction is abnormal, so that the root of the milk teeth can not be completely absorbed. It is common in the early mixed dentition mandibular incisors, where the permanent teeth erupt from the lingual side and the milk teeth are left on the labial side. The residual roots or crowns of the first and second molars are retained in the buccolingual or proximal and distal mesial space of the first permanent molar. In this case, the retained milk teeth can be extracted. 2. The permanent teeth are congenitally missing and the roots of the milk teeth cannot be resorbed and are retained. In this case, the presence of permanent teeth can be determined by X-ray. If there are no permanent teeth, the baby teeth can be retained to perform chewing function, but not for life. 3, there are permanent teeth under the milk teeth, but the permanent teeth eruption is weak, and the roots of the milk teeth are not absorbed and remain, after taking X-rays to determine the presence of permanent teeth, the retained milk teeth can be extracted, and observe whether the permanent teeth can erupt naturally, if they cannot erupt normally within six months, the gums can be cut to help eruption.