The normal torque angle of the maxillary anterior teeth is an important factor in the aesthetics and function of the anterior teeth, and torque control of the front teeth is an extremely important aspect of the correction process. At present, the normal torque angle of the front teeth is among the phase Ⅲ clinical test items recognized by the American orthodontic professional committee; consequently, good control of front teeth torque is of great significance to the aesthetics of the upper anterior teeth. In this paper, the influence of a lip appliance on the bad torque of upper anterior teeth and the associated methods of control are reviewed in detail. The advantages and disadvantages of various control methods for the anterior teeth and the significance of correct anterior teeth torque angle are summarized. The existing research results indicate that the torsion of a straight arch wire applied directly to individual teeth is too great, making it difficult to enter the groove. Although the bending of the arch wire overcomes these shortcomings, the procedure is cumbersome; it stimulates the soft tissue of the vestibular groove and increases the patient’s discomfort. The bending mechanism of the rocking chair is more complicated; it is greatly affected by the friction between the arch wire and the bracket and is not conducive to closing the tooth extraction gap using the sliding method. The portal auxiliary arch and the single bending torque are suitable for correcting the torque angle of a single tooth. Auxiliary arch torque can be used to correct the upright upper anterior teeth during the process of closing the extraction space and after adduction; therefore, this procedure is worth popularizing. However, the accuracy of orthodontic control of anterior teeth torque requires further study.

Objective : To analyze the initial displacement of the upper central incisor and stress distribution of periodontal ligament under different torque values of upper incisors under the action of a four-curved auxiliary arch to provide a reliable basis for the safety of clinical application of four-curved auxiliary arches. Methods : A three-dimensional finite element model for torque control of upper anterior teeth with a homemade quadrilateral auxiliary arch was established. Four different states were analyzed: molar ligation without extraction space (group A), microimplant ligation without extraction space (group B), molar recovery with extraction space closure (group C) (the adductive traction force was set at 115 g) and microimplant recovery with extraction space closure (group D) (the adductive traction force was set at 115 g). When four types of torque (0.5 N, 1.0 N, 1.5 N, and 2.0 N) were applied. The initial displacement of upper central incisors and the stress distribution of periodontal ligament in 16 groups (A1-A4, B1-B4, C1-C4, D1-D4) were observed. Results : Under different conditions, as the strength of the four-curve auxiliary arch increases, the maxillary anterior teeth has crown labial inclination and a root lingual inclination. The displacement of the incisor tip increases with the increase in the loading force of the torque auxiliary arch, and the displacement of the incisor root apex increases as the force increases. The difference in incisor-apex displacement distance in A1-A4, B1-B4, C1-C4, D2 and D4 groups increased as the torque force increases, while the difference between the D3 group and D1 and D2 groups decreased slightly. The stress of the cervical periodontal ligament of the upper central incisor did not exceed the stress of the periodontal ligament in the following groups: A1, A2, B1, B2, B3, C1, C2, D1, and D2. The stress of the lip side of the upper central incisor did exceed the stress of the periodontal ligament in the following groups: A3, A4, B4, C3, C4, D3, and D4. In other words, when using the four-curved auxiliary arch as an implant anchorage, the force applied in the absence of extraction space should not exceed 1.5 N, and the force applied in the adduction of extraction space should not exceed 1.0 N. When using the nonimplant anchorage, the force applied in the absence of extraction space and the adduction of extraction space should not exceed 1.0 N. In addition, the range of force should not exceed the maximum stress of the periodontal ligament in the cervical region such that the effective and safe torque movement can be achieved. Under other stress conditions, the stress of the labial and cervical periodontal ligament of the upper central incisor exceeded the stress value (2.6 × 10-2MPa). The stress value of periodontal ligament was 2.6 × 10-2MPa in all groups. Conclusion A four-curved auxiliary arch has a significant effect on the upper anterior teeth, and the use of microimplants can better control root movement such that the crown of upper central incisors cannot be excessively lip inclined.