| Objectives In patients with malocclusion who were missing the first molar,an invisible bracketless orthodontic technique was commonly used to move the second molar mesially to take the place of the first molar.However,the anterior movement of the second molar was often accompanied by a mesial inclined movement,which not only resulted in a dislocation during orthodontic treatment but also had an orthodontic lag effect,thereby prolonging the treatment period.Moreover,during the mesial movement,the mandibular second molars with normal roots tended to move differently from those with bent roots.To achieve as much overall movement as possible when the second molar was moved mesially,a threedimensional finite element model of the alveolar bone-periodontal membrane-mandibular dentition and the bracketless invisible orthodontic appliance for the missing left mandibular first molar was developed using the three-dimensional finite element analysis research method.The results of displacement trend and stress distribution during the mesial movement of the lower molar in two root systems were obtained by performing tests with different posterior inclination angles in the left mandibular second molar appliance preset,and on this basis,the effect of posterior inclination angle on the magnitude and distribution of alveolar bone and periodontal membrane stress during the mesial movement of the molar were also analyzed.This not only provided a theoretical basis for selecting the right posterior inclination for the mesial movement of the molar during invisible orthodontic treatment but also served as a reference for orthodontists when drawing up their orthodontic treatment plans.Methods The patient’s dental-related data of tooth and jaw was obtained using CBCT scans and a three-dimensional finite element model of the alveolar bone-periodontal membrane-mandibular dentition and bracketless invisible orthodontic appliance for the missing left mandibular first molar was constructed using Mimics,Geomagic,and UG software.Two test groups for the left mandibular second molar were designed concerning the needs of the trial: Normal root group(Group A),A1 aligner without preset posterior inclination;A2 aligner was pre-prepared for 0.5° posterior inclination;A3 aligner was preprepared for 1° posterior inclination;A4 aligner was pre-prepared for 1.5° posterior inclination;And A5 aligner was pre-prepared for 2° posterior inclination.Bent root group(group B),B1 aligner without preset posterior inclination;B2 aligner was pre-prepared for0.5° posterior inclination;B3 aligner was pre-prepared for 1° posterior inclination;B4aligner was pre-prepared for 1.5° posterior inclination;And B5 aligner was pre-prepared for2° posterior inclination.All groups forced the aligners to move 0.25 mm mesially,and the three-dimensional model was imported into ANSYS software for analysis and solution,and the stress distribution in the left mandibular second molar and periodontal tissues,as well as the displacement trend of the molar,were obtained.Results 1 The crown movement was essentially greater than the root movement when the molar moved mesially in both root cases,and the maximum displacement of the molar decreased with an increase in the preset posterior inclination angle in both models A and B.2 Comparative analysis of each model in group A,the molar crown and root movement was relatively consistent in group A5 among the subgroups,and the Von-Mises stress in the alveolar bone and periodontal membrane and the principal stress in the molar and periodontal membrane was most evenly distributed when the preset posterior inclination angle is 2°,with no obvious stress concentration point.3 Comparative analysis of each model in group B,the axis of rotation of the teeth in group B3 was essentially horizontal in all subgroups.The buccal,lingual,proximal and distal mesial movement of the crown was essentially equal,and Von-Mises stress in the periodontal membrane and alveolar bone and principal stress in the molar and the periodontal membrane was more evenly distributed when the bent root molar was preset at a posterior inclination angle of 1°.Conclusions 1 The mesial movement of the molar during invisible orthodontic treatment is tipping movement.2 Invisible aligner with a pre-set posterior inclination which,although it reduces the efficiency of the movement of the molar,helps the molar to move closer to the overall movement when moving mesially.3 With a preset posterior inclination of 2° for the molar with normal root,the movement of the molar is closest to the overall movement;With a preset posterior inclination of 1° for the molar with bent root,the movement of the molar is closest to the overall movement.Figure 19;Table12;Reference 95... |
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