When the normal upper and lower incisors and lateral incisors just erupted, their cutting ends were jagged and uneven. This kind of sawtooth is called tooth development nodule-crista tubercle, which belongs to the normal anatomical shape of teeth. Because the development of teeth is composed of multiple developmental leaves, in front teeth, it is usually composed of three developmental leaves. In the process of tooth formation, the three developing leaves gradually fuse, and some unfused parts form uneven incisor ridges. With the increase of age, teeth will gradually wear and flatten due to chewing and occlusion.

Teeth eruption is a slow process, in which the teeth themselves are constantly changing: enamel is also wavy, because the growth line on its surface forms tiny grooves, which is why we often see several fine lines like porcelain cracks on the surface of teeth, which are actually horizontal lines of glaze; The enamel surface of newly erupted young permanent teeth is covered with a thin organic film called enamel skin, which can be removed by occlusal, chewing, abrasion and brushing.

However, young permanent teeth are still in the process of eruption. The clinical crown (the part of teeth that we can see in the oral cavity) is relatively low, the root has not yet formed, the apical foramen is still funnel-shaped, the pulp cavity is wide as a whole, and the root canal wall of teeth is thin. Permanent teeth generally begin to erupt when the root of tooth germ forms about two thirds; After sprouting, the roots continue to develop and fully form within 2 ~ 3 years after sprouting. Most of us can see obvious marginal nodules and lingual marginal ridges in the anterior teeth because the young permanent teeth erupt soon, wear less and have clear shapes. A plurality of cusps are formed on the posterior teeth, and the cusps together form a dentofacial fossa, and the upper and lower cusps are opposite to each other for grinding food. Young permanent teeth are more difficult to clean than well-developed permanent teeth because of their obvious ridges, complex shapes and deep cracks. In addition, the attachment position of gingival margin is unstable, and it takes 3 ~ 4 years to complete with tooth eruption. It takes 8 ~ L2 months for most permanent teeth to reach the occlusal plane after eruption.

In addition, there is another situation: after adulthood, the front door teeth are still serrated, mostly because people like to knock melon seeds with their front teeth, causing tooth edge wear. We call it "melon seeds".