For patients with moderate and severe periodontitis, surface tomography is required to understand the extent and degree of alveolar bone resorption damage in the entire mouth. The degree of alveolar bone resorption is an important indicator of the prognosis of the tooth. If the alveolar bone resorption exceeds 1/2 of the root length, the prognosis of the tooth is poor. If the resorption is more than 2/3, the tooth may need to be extracted, but of course, whether or not to extract the tooth should be considered in conjunction with the results of the intraoral examination. The relationship between the alveolar bone and the tooth is equivalent to the relationship between the soil and the tree, without the support of the soil the tree will collapse, similarly the destruction of the alveolar bone will cause the tooth to loosen and shift, and at the same time the gap between the teeth will become larger. With the current level of medicine, the resorption of the alveolar bone in most cases is irreversible, so periodontal treatment in most cases can only control the development of the disease and prevent further aggravation. Therefore, periodontitis should focus on early prevention and treatment.