Clear Aligner Orthodontic Therapy Of Rotated Mandibular Teeth With Different Shapes:A Three-dimensional Finite Element Analysis

Objective:To investigate aligner activation influencing on biomechanics of clear aligner orthodontic therapy of rotated mandibular teeth with different shapes,and provide a reference for clinical orthodontic treatment.Methods:Establish a three-dimensional finite element model of mandibular dentition-periodontium-alveolar bone-appliance with highly simulated crown shape based on cone-beam computed tomography and intraoral optical scanning data of an adult who had rotated lower incisor and premolar by Mimics、Geomagic Studio、3-matic and Abaqus.The initial displacement,movement trend and von Mises stress distribution of periodontium and alveolar bone of target teeth and adjacent teeth were analyzed when the loading of three different aligner activation of 1.0°,1.5°and 2.0° was applied to rotated the left lower central incisor(group A)and second premolar(group B).Results:The maximum displacement and periodontium stress of target teeth increased with aligner activation.Target tooth in group A was 1.4-2.0 times asthe maximum displacement as group B.Meso-adjacent teeth are significantly higher than disto-adjacent teeth on the tooth maximum displacement and periodontium stress,and the maximum displacement of meso-adjacent tooth was 65%-105%as much as target toothin group A.During the rotation movement,mesial tipping and slightly intrusive movement of target teeth were observed,and adjacent teeth showed the trend of tipping movement,which was mainly lip-lingual tipping in group A and was mainly mesial and distal tipping in group B.The stress of alveolar bone was both distributed in alveolar fossa neck of target teeth and adjacent teeth,and the stress also obviously concentrated in alveolar fossa neck of mandibular incisors in group B.Conclusion:Increasing the aligner activation can improve the efficiency of tooth de-rotation,but the risk of periodontal tissue injury is also increased.It is suggested that aligner activation should not exceed 1.5° for the lower incisor and 2.0° for the lower premolars.For incisor derotation,we should be paid to the adverse effect on adjacent teeth.Sufficient spaces is not only beneficial to correct rotated teeth,but also reduce the adverse effect on adjacent teeth during rotation.Anterior teeth seem to work as anchorage units during rotation of a second premolar with clear aligners.