Effect Of Different Attachments On The Mesially Movement Of Upper Molar With Plastic Aligner

PURPOSE: The inclination of the upper molar often occurs when closing the premolar extraction space with a plastic aligner.Three-dimensional(3D)finite element models(FEMs)were used to analyze the initial force system in the mesial translation of an upper left first molar with no attachment,traditional root-control attachment and optimized attachment with different mechanical loading in plastic aligner-based orthodontic technology.METHODS: A three-dimensional(3D)finite element model was derived from a cone-beam CT(CBCT)scan data and then built in these softwares of Mimics,Geomagic,Solidwords and Ansys.A model with an upper left first molar and its periodontal apparatus,including periodontal ligament(PDL)and alveolar bone,different types of light-cured composite attachments and thermoformed plastic aligners were constructed.Seven FE models were constructed: model A without composite attachments,model B with a vertical rectangle attachment,model C with a horizontal rectangle attachment,models D and E with one semiterete attachment with inclined plane,and model F and G with two semiterete attachments with inclined plane.Model A simulates the situation with no attachnent,model B and C simulate the situations with traditional attachments,and model D,E,F,G simulate the situations with optimized attachments.Seven FE models were used to analyze the effects of the initial force system and displacement pattern of the molar in plastic aligner-mediated simulating mesially movement with different attachments.The initial tooth displacement,tension-compression pattern at the PDL and Von Mises stress in mesial alveolar fossa of different FE models were compared.RESULTS: The tooth in the model G with a moment imposed on the entire tooth and attachments,a broad compression area throughout the mesial surface,a broad tension area in the entire distal surface at the PDL,and a broad Von Mises stress distribution over all the inner surfaces of the mesial alveolar fossa were observed.In the other models,a compression area in the cervical area of the mesial root surface,a tension area in the apical of the distal root surface of the PDL,and a Von Mises stress concentrated in the cervical inner surface of mesial alveolar fossa were observed.In the model B with buccal vertical rectangular,the compression stress of mesial periodontal ligament closer to buccal was obviously wider than that of the palate.The rotation center of the buccal side was obviously near the apical foramen than the palatal side.In the model C with one semiterete optimized attachment,the stress pattern of the periodontal ligament in the mesial surface and the displacement patterns were not significantly different from those of model A with no attachments.But the maximum displacement of model C was greater than that of model A.CONCLUSIONS: Through this 3D FE analysis of the plastic aligner system,we can conclude the following conclusions: 1.The addition of attachments increased the molar's movement efficiency.2.The models can't achieve bodily movement with no attachment,traditional attachments,one semiterete attachment with inclined plane and two semiterete attachments with inclined plane.3.The disto-cervical attachment has contribution to root-control with the existence of mesio-occlusion attachment.4.The optimized attachments combined with a moment on the entire tooth generate a force system that can approximately achieve the bodily movement of the upper molar.This research provides a theoretical basis for the clinical application of the plastic aligner.