In clinical work, parents often consult a problem —- children who have lost their milk teeth for several months or more than a year, the new teeth still have not erupted. The eruption time of permanent teeth exceeds the normal eruption time range and has not yet erupted is the late eruption of permanent teeth, the common causes are: 1. The late eruption of individual permanent teeth is mostly related to the lesion of milk teeth, premature loss or retention of milk teeth. Premature loss of milk teeth is the most common clinically, children because of trauma or serious caries, resulting in premature loss of milk teeth, children used to chew with gums, so that the local gum hyperplasia keratinization, become hard and thick, resulting in permanent teeth eruption difficulties. The gum is pale and protruding, and the hard crown can be felt by hand under the gum tissue. In addition, after the early loss of milk molar, the neighboring teeth tilt toward the gap of missing teeth, resulting in the reduction of the gap of missing teeth and the lack of gap of permanent teeth eruption can also make the permanent teeth erupt late. 2.Difficulty in eruption of permanent teeth due to the obstruction of multiple teeth, tumors or cysts. This condition can only be detected and diagnosed by X-ray examination. 3. Abnormal development of the permanent teeth themselves, commonly in the maxillary central incisors. Twisted, inverted or angled crowns and roots of the central incisors can cause delayed eruption or even non-eruption of permanent teeth. 4. Genetic factors causing delayed eruption of permanent teeth are extremely rare. Such as cranial clavicle hypoplasia and congenital lack of thyroid secretion.