Fossa closure is a technique to prevent dental caries in children’s permanent teeth, one tooth fossa closure costs about 100 RMB, there are 8 permanent molars in children’s upper and lower jaw, all need to do fossa closure. Because the permanent molars just erupted in the mouth, the cusps are high and the bottom of the sulcus is deep, the mineralization of the bottom of the sulcus of the newly erupted molars is not yet perfect, plus the children often brush not clean and thorough, so the sulcus is a good area for caries, so the sulcus needs to be closed. Fluoride resin is applied to the bottom of the sulcus, because the fluoride ion has the function of anti-caries, so it can protect the sulcus of permanent molar from caries, usually the first permanent molar of upper and lower jaw erupts at the age of 6 weeks, and the second permanent molar erupts at the age of 12 weeks, when the occlusal surface of the tooth is completely erupted into the mouth, and there is no gum flap covering the tooth surface, then the sulcus can be closed.