| The good relationship between lips and teeth,normal overbite and overjet are vital goals which orthodontists pursuit. To treat an adult patient with gummy smile and deep overbite, intruding anterior teeth should be a prioritized option.With the development of MIA method,more and more orthodontists choose MIA to intrude upper anterior teeth. Compared with traditional method,such as headgear and segmental arch,intruding anterior teeth with MIA has many advantages, such as non-compliance from patients and no extrusion of molars. However, loading method, loading angle, working archwire and position of the miniscrew are various when MIA is used to intrude upper anterior teeth. How these differences affect the result of intruding has not been clarified.The three dimensional finite element method, as an effective theoretical method used in stress analyzing, has been widely used in biomechanical study. In our study, finite element models including upper anterior teeth, anterior maxilla and straight wire appliance were constructed. After designating different loading methods, loading angles and working arch wires, stress distribution and initial displacement of anterior teeth were analyzed in order to provide biomechanical reference for clinical application.The study is composed of four experiments as follows:Experiment 1:The construction of 3D finite element model including anterior teeth, anterior maxilla and straight wire appliance. Objective: To provide operation platform for following experiments through constructing 3D finite element models including anterior teeth, anterior maxilla and straight wire appliance. Method: A 23-year old female volunteer with individual normal occlusion was scanned from occlusal plane to anterior nasal spine by CT. Import the CT data into Mimics8.1 to calculate 3D models of anterior teeth and maxilla. Smooth and substantiate the models with Geomagic Studio10.0. The software UG NX5.0 was used to contribute brackets and arch wires. Assemble all parts of models into 3D finite element models using ANSYS10.0 software. Result: 3D finite element models including anterior teeth, anterior maxilla and straight wire appliance were constructed. Conclusion: On the basis of CT data, it was feasible to construct high quality 3D finite element models through general application of Mimics, UG NX and ANSYS.Experiment 2:Finite element analysis of the effect of two commonly used loading approaches on upper anterior teeth. Objective: To analyze the effect of two commonly used loading approaches on stress distribution and initial displacement of maxillary anterior teeth through finite element method, and to provide biomechanical reference for clinical application. Method: Loading 1 was the intruding force which loaded from center of archwire to the alveolar bone surface below anterior nasal spine. The archwire was 0.019×0.025 inch stainless steel, and force magnitude was 100g. Loading 2 was intruding force which loaded from center of archwire between center and lateral incisor to the alveolar bone surface between center and lateral incisors. The archwire was 0.019×0.025 inch stainless steel, and force magnitude was 50g per side. The stress distribution on PDL and initial displacement of upper anterior teeth were calculated. Result: The stress on PDL of loading 2 was more uniform than loading 1. Upper incisors tipped labially in both of the loading methods. Teeth beside the loading point tended to tip away from the point. Result: It is more reasonable to insert miniscrew between center and lateral incisors than below the anterior nasal spine and to intrude the anterior teeth on both sides.Experiment 3: Finite element analysis of the effect of loading angle in sagital direction to upper anterior teeth. Objective: To analyze the influence of loading angle on stress distribution and initial displacement of maxillary anterior teeth through finite element method, and to provide biomechanical reference for clinical application. Method: The loading method was the same as loading 1 in experiment 2. Force magnitude was 100g,with loading angles between intruding force and occlusal plane was 600,650 and 700 respectively. The stress distribution on PDL and initial displacement of upper anterior teeth were analyzed. Result: Labial tipping degree and compressive stresses near the labial alveolar crest increased along with loading angles. Conclusion: The more intruding force far away from the center of resistance, the anterior teeth crown tipped more labially. The distance between intruding force and CRes should be controlled and the labial torque should be increased when necessary.Experiment 4: Finite element analysis of the effect of archwire size on upper anterior teeth. Objective: To analyze the effect of archwire size on stress distribution and initial displacement of maxillary anterior teeth through finite element method, and to provide biomechanical reference for clinical application. Method: The loading method was the same as loading1 in experiment 2. Force magnitude was 100g, with 0.018×0.025 inch and 0.019×0.025 inch stainless steel archwire were selected respectively. The stress distribution on PDL and initial displacement of upper anterior teeth were analyzed. Result: Compared with 0.018×0.025-in archwire, there were more uniform compressive stress distribution on PDL and less tipping of central incisors with 0.019×0.025-in archwire. Conclusion: 0.019×0.025-in archwire was more qualified for intruding upper anterior teeth than 0.018×0.025-in archwire. Larger size archwire should be chosen in order to reduce unfavorable tooth movement and to optimize intruding effect. |
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