Orthodontists have long recognized that after moving teeth within the jaws or adjusting the maxillomandibular relationship to normal with orthodontic appliances, there is a tendency to return to the original position, i.e., relapse. Retention is necessary to prevent relapse and to stop this tendency. However, in order to prevent recurrence, it is necessary to know the cause of recurrence. The reasons are as follows: 1, periodontal and gingival fiber reconstruction: teeth moved by orthodontic treatment, the tension of the stretched or compressed gingival and periodontal fibers have not yet established a new balance, the position of the teeth is unstable, easy to relapse. In particular, the reconstruction of the fibers on the alveolar ridge and the transverse septal fibers is very slow, and the resulting tension is likely to cause the moved tooth to return to its original misaligned state. In addition, the alveolar bone from the transitional bone reconstruction of normal alveolar bone also needs some time. 2, the influence of perioral muscle power: after the treatment, due to the position of the teeth arch and jawbone has changed the muscle power balance has also changed, due to malocclusion morphology changes often preceded by functional and muscle power modification, that is, the original power balance of the orthodontic treatment of the teeth, the arch and jawbone position will have a certain negative impact on the position of the instability of the state. 3, occlusal balance is not fully established: in the process of orthodontic treatment, due to the change of teeth, dental arch and jaw position relationship, thus destroying the original abnormal relationship that has been accustomed to. The newly constructed occlusal relationship has not yet reached the maxillofacial coincident, and the contact relationship with the oblique surface of the tooth tip, but also must be adjusted constantly, in the adjustment process, there is a tendency to relapse. 4, the influence of bad habits: oral bad habits is an important cause of malocclusion, if at the end of orthodontic treatment, still not broken oral bad habits, easy to lead to the recurrence of malocclusion, such as abnormal swallowing tongue habits lead to open jaw malocclusion recurrence. 5, after orthodontic need to keep time: wearing a retainer period is not specified, generally should be related to the cause of malocclusion, malocclusion type and other factors. Generally people think that the retention time is about 2 years. The longer the retention time, the more stable the effect.