A person has two sets of teeth in his life. A group of teeth that grow in childhood are called deciduous teeth and have 20 teeth. At school age (about 6 years old), deciduous teeth gradually fall off and are replaced by permanent teeth. From the appearance of teeth, teeth are composed of crown, root and neck. Crown is the part of the outer layer of teeth covered by enamel, and it is also the main part of chewing function. Under normal circumstances, most of the dental crowns are exposed in the oral cavity, which is called clinical dental crowns. The root of a tooth is the part of the outer layer of the tooth covered by cementum, and it is also the supporting part of the tooth. Some teeth are single teeth, some are multi-teeth, and the tip of each root is called root tip; Each root tip has a small hole, which passes through the blood vessels and nerves of the pulp, and is called the root tip hole; Under normal circumstances, the root of the tooth is completely embedded in the alveolar bone. The joint between the crown and the root of the tooth is curved, which is called the neck, also known as the neck edge or neck line.

From the side of the tooth, the tooth body is composed of enamel, cementum, dentin and pulp. The enamel, which constitutes the surface layer of the crown, is a translucent white hard tissue, which is the hardest and highly calcified tissue in the tooth tissue. Cementum is the yellow hard tissue that constitutes the surface layer of root. Dentin is the main part of teeth, which is located in the inner layer of enamel and cementum. It is not as hard as enamel, and there is a hole in it called pulp cavity. Dental pulp is a honeycomb filled with blood vessels, nerves and lymph. It is the only soft tissue in tooth tissue. According to the morphological and functional characteristics of teeth, teeth are divided into four categories: incisors, canines, bicuspids and molars. The incisors are located in the front of the mouth, and there are 8 incisors, including left, right, upper and lower incisors. Seen from the adjacent surface, the crown is wedge-shaped, with a thick neck and a thin edge. Their main function is fasting and they are single. Fangs, commonly known as canine teeth, are located at the corners of the mouth, with four teeth on the left, right and up. The crown is still wedge-shaped and the cutting edge has prominent sharp corners. Its main function is to pierce and tear food, and it is a stout and growing single root. The bicuspids, also known as premolars, are located in the left, right, upper and lower * * 8, and have a cubic crown and an occlusal surface, which usually has bicuspids. The second bicuspid of mandible has three points, the main function of which is to help bicuspid tear food and grind food. The roots are flat, and there are also people with forks. There are 12 molars behind bicuspids. The crown is large, cuboid, with wide occlusal surface and 4-5 cusps. Its main function is to grind food. Generally there are three maxillary molars and two mandibular molars. Teeth are divided into deciduous teeth and permanent teeth according to the time they exist in the mouth. The deciduous teeth begin to erupt at 7-8 months after birth, and all the deciduous teeth at the age of 2.5 erupt, 20 * * *. From 6 to 7 years old to 12 and 13 years old, deciduous teeth gradually fall off and are replaced by permanent teeth. This period is called mixed dentition. Therefore, the time of deciduous teeth in the mouth is 5 ~ 10 years. The occlusal period of deciduous teeth is about 2.5 ~ 6 years old. Permanent teeth are the second dentition after deciduous teeth fall off. Unless due to illness or accidental injury, they will not fall off, and no teeth can erupt to replace them after falling off. The first permanent molar began to develop in April and erupted at the age of 6. It is the first permanent tooth to erupt and does not replace any deciduous teeth. After 12 ~ 13 years old, all deciduous teeth fall off, which is called the occlusal period of permanent teeth.