Periodontitis is an inflammation of the supporting tissues of the teeth, typically manifested by recession of the gums, and resorption of the alveolar bone, which eventually leads to loosening and even loss of teeth. After gum recession and tooth loosening, the periodontal gap will tend to increase significantly, and there will be a significant displacement of the teeth, and the gap will be even larger after the displacement. In this case, the first thing that should be done is to control periodontitis, and a basic and better treatment should be carried out for the periodontium. This usually includes scaling, or what we normally call supragingival scaling, but for periodontitis, supragingival scaling alone is not enough, and it needs to be supplemented with subgingival scaling root planing and a series of other operations. The periodontal plaque and calculus will be completely removed. After removal, the periodontitis should be closely monitored to see if there is any further periodontal pathology. If it is determined that the periodontitis is no longer progressing and is under control, then orthodontic treatment can be considered at this time to minimize the gap between the teeth. However, if the periodontitis is not under control, the application of orthodontic treatment is very ineffective.