A Finite Element Analysis Of En-mass Retraction Of Maxillary Anterior Teeth With Vertical Traction

The position and protrusion of maxillary are vital factors that affects facial aesthetics. The protrusive upper jaw and relatively retrusive lower jaw have a negative effect on patients’ beauty and mental health. To improve this situation, we chose to extract the first premolar then retract the anterior teeth. Anchorage preservation during space closure is important for maximum retraction of anterior teeth. In tradition, various auxiliaries such as headgear、transpalatal arch 、intermaxillary elastics are used to reinforce anchorage, but these are complicated and need patients cooperation, meanwhile, fail of torque control of anterior teeth and vertical control of posterior teeth make the aesthetics worse, The magical thing, micro-implant anchorage(MIA) occur and can solve the problems above perfectly.The three dimensional finite element analysis(3D FEA) has been widely used in orthodontics to solve biomechanical problem.Currently, there are some reports about using this method to analysis en-mass retraction of anterior teeth with MIA, but few reports take the vertical intrusion into account, and the biomechanism is not clear.We try to construct a finite element model for en-mass retraction of anterior teeth with MIA, the initial displacement of maxillary teeth was calculated when micro-implant height increase and when adding vertical traction between anterior teeth, wishing to provide guidance for clinical use.The study was composed of three parts as follows:Experiment 1: The construction of 3D finite element model. Objectives: To construct 3D finite element models including the alveolar bone, upper teeth, bracket、arch wire and MIA using CT scanning and Mimics software, providing operation platform for following research. Methods: An adults, whose treatment plan was extraction of bilateral first premolar and need en-mass retraction of anterior teeth to improve aesthetics was chosen. After aligning and leveling of upper teeth, the patient was scanned from occlusal plane to zygomatic arch with CT, and the data was imported into Mimics17 in the format of DICOM, then “Erase” interference information such as the unnecessary part、bracket、metal artifacts, use the “Calculate 3D”to establish a primary model. Periodontal Ligament(PDL) was considered to have a uniform thickness of 0.2mm around the optimized root; An appliance with 0.022-inches bracket slots and a 0.019*0.025-inches stainless steel arch-wire was generated use Unigraphics NX 8.5, then assemble all parts of models in Ansys Workbence15.0. All parts were given different Young’s modulus and Poisson’s ratios according to reports in the literature and then meshing them. Result: A finite element models was been constructed,which comprised a total of 952372 elements and 1704979 nodes. Conclusion: It was feasible to construct a high-quality 3D finite element model in the cooperation of CT image、Mimics、 Unigraphics NX, 、and Ansys workbence.Experiment 2: FEA of en-mass retraction of upper dentition used different hight of MIA. Objectives: To investigate the biomechanical effects of maxillary teeth in en-mass retraction with different height of implants. Methods: According to clinical application, put the implant between the second premolar and the first molar, the height was assumed to be 2mm、5mm、8mm above the alveolar crest, The retraction hook was placed bilaterally between the lateral incisor and the canine at a height of 2 mm, a 200-g retraction force was applied between the micro-implant and the retraction hook through the simulated nickel titanium spring. Result: As the height of micro-implant increase, sagittal displacement of anterior teeth decrease and vertical displacement increase, the lingual tipping of anterior teeth decreased; the first molar and the second molar moved in a way of distal tipping and distal-lingual rotation. Conclusion: High position of micro-implant could control the torque of anterior to some extent; Anchorage of posterior teeth was reinforced and the width between the first molar decrease。Experiment3: FEA of en-mass retraction of upper dentition adding vertical traction. Objectives: To investigate the biomechanical effects of maxillary teeth in en-mass retraction with different height of implants when adding vertical traction. Method: On the basis of experiment 2, adding vertical traction between the incisors、between the incisor and the lateral incisor、and between the lateral incisor and the canine, the force was 50 g for both sides. Results:For example, the micro-implant of posterior area was 5mm above the alveolar crest,adding vertical traction in three different position as planned, Lingual tipping of anterior teeth decreased with vertical traction; The different location of vertical force cause different effects on anterior teeth; The first molar tend to move distally, and this tendency decrease as the micro-implant between anterior teeth moved back. Conclusion: Torque of anterior teeth could be easily controlled when adding vertical traction; A midline vertical force cause a more obvious tendency of intrusion of anterior teeth and distal tipping of the first molar.