Retention of teeth after orthodontic treatment: The periodontal tissues and occlusal relationships of a person are always changing, whether or not they have received orthodontic treatment. Maintenance after active orthodontic treatment needs to be done both mechanically and biologically. Biological retention is usually the maintenance of the health of the teeth and periodontium. For example, gingival circumcision of twisted teeth can prevent the recurrence of twisted teeth. Orthodontic retention usually refers to mechanical retention. Clinically used retainers are: 1. Hawley Retainer This retainer is strong and durable, basically does not affect the bite, and allows the teeth to make a small amount of upward and downward and buccal and lingual adjustments during the retention period to make the bite better. It is suitable for long-term continuous wear. The disadvantages are: there are abutments, foreign body sensation is more obvious; a wire is visible in the anterior area, which affects the aesthetics. There are two kinds of materials for fabrication: self-coagulation and thermal coagulation. Now, due to the progress of materials, Harley retainer is a better retainer. 2.Pressing film retainer (Positioner) Pressing film retainer is made of transparent elastic plastic film, and its shape matches with the patient’s dental alignment at the end of treatment. It is suitable for patients whose teeth are neatly aligned at the end of orthodontic treatment and whose posterior teeth have a good occlusal relationship. It is popular because of its transparent and beautiful appearance, compact size and comfortable taste. The disadvantage is that it is easy to be damaged and more than one retainer needs to be made; the occlusal relationship of the posterior teeth cannot be fine-tuned and the chewing adaptation period is prolonged. 3, lingual bonded orthodontic retainer (Bonded orthodontic retainer, BOR) BOR is an aesthetic, efficient long-term retainer, which is popular with patients because it does not depend on the patient’s cooperation to stabilize the treatment effect. The most clinically used are: cuspid to cuspid retainers and flexible multi-strand archwire retainers. BOR is a preferred retainer after orthodontic treatment because it is fixed to the inner side of the teeth by a single archwire and is therefore comfortable, aesthetically pleasing and does not need to be removed.