Department of Orthodontics, Hospital of Stomatology, Jilin University, Jilin University, Changchun, Jilin, China.
Prog Orthod. 2023 Nov 20;24(1):38. doi: 10.1186/s40510-023-00491-2.
To investigate the displacement of dentition and stress distribution on periodontal ligament (PDL) during retraction and intrusion of anterior teeth under different proclination of incisors using clear aligner (CA) in cases involving extraction of the first premolars.
Models were constructed, consisting of the maxilla, PDLs, CA and maxillary dentition without first premolars. These models were then imported to finite element analysis (FEA) software. The incisor proclination determined the division of the models into three groups: Small torque (ST) with U1-SN = 100°, Middle torque (MT) with U1-SN = 110°, and High torque (HT) with U1-SN = 120°. Following space closure, a 200 g intrusion force was applied at angles of 60°, 70°, 80°, and 90° to the occlusal plane, respectively.
CA therapy caused lingual tipping and extrusion of incisors, mesial tipping and intrusion of canines, and mesial tipping of posterior teeth in each group. As the proclination of incisors increased, the incisors presented more extrusion and minor retraction, and the teeth from the canine to the second molar displayed an increased tendency of intrusion. The peak Von Mises equivalent stress (VMES) value successively decreased from the central incisor to the canine and from the second premolar to the second molar, and the VMES of the second molar was the lowest among the three groups. When the angle between the intrusion force and occlusal plane got larger, the incisors exhibited greater intrusion but minor retraction.
The "roller coaster effect" usually occurred in cases involving premolar extraction with CA, especially in patients with protruded incisors. The force closer to the vertical direction were more effective in achieving incisor intrusion. The stress on PDLs mainly concentrated on the cervix and apex of incisors during the retraction process, indicating a possibility of root resorption.
探讨在拔除第一前磨牙的情况下，使用透明矫正器（CA）内收和压低前牙时不同切牙前倾角度对牙列和牙周膜（PDL）的位移及应力分布的影响。
构建包括上颌骨、PDL、CA 和无第一前磨牙的上颌牙列的模型。然后将这些模型导入有限元分析（FEA）软件。切牙前倾角度将模型分为三组：小转矩（ST）组 U1-SN=100°，中转矩（MT）组 U1-SN=110°，大转矩（HT）组 U1-SN=120°。在关闭间隙后，分别以 60°、70°、80°和 90°的角度在牙合平面施加 200 g 的内收力。
CA 治疗导致切牙舌倾和唇倾、尖牙近中倾斜和压低、后牙近中倾斜。随着切牙前倾角度的增加，切牙表现出更多的唇倾和少量的内收，从尖牙到第二磨牙的牙齿表现出更多的内收趋势。Von Mises 等效应力（VMES）峰值从中切牙到尖牙依次减小，从中切牙到第二磨牙依次减小，第二磨牙的 VMES 在三组中最低。当内收力与牙合平面之间的角度增大时，切牙表现出更大的内收，但少量的唇倾。
在使用 CA 进行前磨牙拔除的情况下，通常会出现“过山车效应”，尤其是在切牙前突的患者中。更接近垂直方向的力更有利于实现切牙内收。在回缩过程中，PDL 上的应力主要集中在切牙的颈缘和根尖，提示可能发生牙根吸收。
