Three-dimensional Finite Element Study Of Maxillary Anterior Teeth Retraction Treated Witn Invisible Appliance Combined With Micro-implant Anchorage

[Objective] The purpose of this study is to establish3D-finite element model of maxillary dentition, alveolar bone, periodontal membrane of maxillary anterior teeth, invisible appliance and micro-implant. In this study, we combined the invisible appliance and micro-implant anchorage and analyzed the maxillary protrusion extraction cases treated by invisible appliance combined with micro-implant anchorage at biomechanical aspect. The periodontal tissue stress, stress distribution and the approximate location of the center of resistance was also investigated. This study is to explore the feasibility and effect of these two kinds of new technology combined in the maxillary anterior teeth protrusion cases, so as to provide the theoretical basis for clinical application and scientific guidance.[Materials and Methods]1. Establish the overall three-dimensional finite element model of maxillary dentition, alveolar bone, periodontal membrane of maxillary anterior teeth invisible appliance and the micro-implant. Based on the DICOM image files of the maxillary dentition and alveolar bone through spiral CT scanning, the geometry and three-dimensional finite element model of maxillary dentition, alveolar bone, periodontal membrane of maxillary anterior teeth, invisible appliance and micro-implant was established by using MIMICS software and ANSYS finite element software.2. The first part is to establish a three-dimensional finite element model with a contact relation of teeth and teeth, teeth and invisible appliance, cement relation of micro implant and maxillary. The magnitude and distribution of the stress of periodontal membrane of maxillary anterior teeth was investigated in the maxillary anterior teeth retraction case treated by invisible appliances and micro-implant by using the non-linear three-dimensional finite element method, so as to speculate the direction of the anterior teeth and the approximate location of the center of resistance.[Results]1. The3D finite element model of maxillary dentition, alveolar bone, periodontal membrane of maxillary anterior teeth invisible appliance and the micro-implant was successfully established.2. The magnitude and distribution of the stress of periodontal membrane of maxillary anterior teeth was similar with a loading force of150g or300g in anterior teeth retraction when the invisible appliance was loaded by the micro implant. The high stress concentration zone of canine was observed on the mesial and distal surface of the neck of the PDL with more stress on the distal surface. The stress concentration zone of the labial and apical surface was relatively small with less stress on the lingual surface. The high stress concentration zone of the lateral incisor was observed on the distal and partly labial surface of the neck of the PDL with less or even no stress on the mesial surface. The stress zone of the central incisor was observed as the same on both of the mesial-distal and labial-distal surface of the neck of the PDL with less or even no stress.3. The results through the nonlinear finite element stress analysis showed that The magnitude and displacement of the periodontium stress were higher in the300g group compared with the150group, with a steep curve and rotation trend of the maxillary dentition. When towing hook located invisible appliance canine distal labial surface of the neck edge (0mm), their stress force and displacement is the largest.4. Significant changes of the magnitude of the displacement and direction of the maxillary dentition were observed when applying a loading force of150g with the draw hook2mm above the canine labial marginal and the cervical margin in the invisible appliance. The change of the rotation direction was significant with relatively uniform stress of the PDL compared with300g group, which indicating that the center of resistance was9mm to11mm above the occlusal plane on the vertical or called Z direction.[Conclusion]1. The geometry and overall three-dimensional finite element model of maxillary dentition, alveolar bone, periodontal membrane of maxillary anterior teeth, invisible appliance and micro-implant were successfully established through spiral CT scanning combined with using MIMICS software and ANSYS finite element software, thus providing a good platform for the study of the mechanical properties of invisible appliance with a better similarity and simulation.2. The conclusions could be drawn according to the magnitude, distribution and the displacement of the stress under different mechanical condition in the anterior teeth retraction case treated by invisible appliance combined with the micro-implant. The trend of the displacement of the overall maxillary dentition is at accordance in the anterior teeth retraction case treated by invisible appliance combined with the micro-implant. A clockwise rotation was observed in the maxillary anterior teeth with the draw hook Omm above the canine labial margin and the cervical margin. While a counter-clockwise rotation was observed in the maxillary anterior teeth with an increasing height of the draw hook to the cervical margin as2mm,4mm and6mm and the rotation trend is increasing as well.3. Under different mechanical condition, the stress concentration zone of the anterior teeth was significant on the mesial and distal surface of the neck of the PDL in the maxillary anterior teeth retraction case treated by invisible appliance combined with micro implant. No significantly increasing stress value was observed in the apical area which indicating little possibility of root resorption of the maxillary anterior teeth.