After 2-3 years of orthodontic treatment (mainly fixed orthodontic treatment), every patient enters the retention phase, and wears a retainer. This is because through 1-2 years of orthodontic treatment, the teeth in the mouth have shifted, and if the aligners are removed and not maintained, the teeth will have a tendency to change back to their original shape, a situation we call “relapse”. There are many reasons for “relapse”, and I will briefly introduce them below. (1) The final alteration of the facial muscular balance has not been completed During the formation of malocclusion, many tissues of the muscular system produce a muscular dynamic balance that is compatible with the malformation. The completion of orthodontic treatment of malocclusion is often faster than the remodeling of the muscular tissues, and the remodeling of the muscular tissues is often not completed when the orthodontic treatment is completed, so that the new tooth position may still be affected by the old muscular balance and be in an unstable state leading to relapse. (2) The tension of the muscle fibers around the teeth has not been restored to equilibrium After orthodontic treatment of malocclusion, widening of the periodontal gap and twisting of the periodontal fibers are normal, and before they have established a new balance, the teeth are unstable in their new position, especially the twisted teeth, which are especially prone to relapse. (3) Eruption of wisdom teeth The anterior inclination of wisdom teeth (third molars) and horizontal interruption have the force of forward pushing, which can lead to the recurrence of malocclusion. (4) The upper and lower teeth occlusion has not yet been established in balance The occlusal relationship of the teeth is the strongest factor for the stability of the teeth in their new position. The new occlusal state requires continuous tooth wear or manual adjustment and grinding to achieve a good contact relationship between the cusp bevels of the teeth. During its self-adjustment process, the malocclusion has a tendency to recur. (5) Bad oral habits are not broken Bad habits can lead to abnormal muscle power balance, which is an important pathogenic factor of malocclusion. If the original bad oral habits persist after orthodontic treatment, it may cause relapse. (6) Influence of growth type Different individuals have their own specific growth trend, and this trend is the growth type (commonly known as the influence of genetics). Patients who undergo orthodontic treatment in the early stages of permanent dentition are still in the growth phase when the treatment is over, and the jaws still grow in the same way, and the continuation of this growth pattern causes instability in orthodontic results. Therefore, we emphasize that the end of active orthodontic treatment is only half of the orthodontic treatment, the other half depends on how well the patient cooperates with the retainer. If the retainers are not worn as required, the teeth will slowly become unaligned again, especially within a year after the orthodontic treatment is completed and the aligners are removed, and the relapse of the teeth is the fastest. Unfortunately, the number of patients who wear their aligners as required by their doctors is relatively low, so I hope this will be taken seriously.