Effect Of Different Loading Places Of Micro-implant On The Movement Of Anterior Teeth By Using Three-dimensional Finite Element Analysis

Objectives: For each orthodontic practioners, the anchorage which is adevice that resists the counterforce caused by the force making tooth move hasalways been their primary concern. The therapeutic effect of some traditionalanchorage appliances, such as Nance arch, transpalatal arch, headgear,facebow and so on has been restricted because of their foreign body sensation,appearance, comfort and compliance of the patient. However, themicro-implant as an anchorage appliance in the treatment brings conveniencefor both physicians and patients. Especially for the patients with bimaxillaryprotrusion, gummy smile, deep overbite, the role of implant anchorage can notbe underestimated.Finite element model is usually built on the CT scan images to restitutethe shape, which has good geometrical similarity. And then it is used tocalculate the stress, displacement and so on of each node by functions. This isthe3-dimensional finite element analysis which can simulate thebiomechanical response, and now is one of the most popular method in manyfields, with high reliability. The model of the research can be reused ormodified for other research which cost savings, so that to provide a foundationfor comparison of different experiments.In the cases of maxillary protrusion or gummy smile, it is particularlyimportant to control the position of anterior teeth. Therefore, how to use theanchorage in the treatment to achieve maximum retraction or intrusion ofanterior teeth is the key to obtain the best treatment effect. Application ofmicro-implant in controlling anterior teeth has achieved good curative effect inclinics, but there is no clear theoretical basis for how load on themicro-implants in different positions to affect the movement of the anterior teeth currently. This study is aimed to provide a theoretical guidance forclinical application of micro-implant in controlling the movement of anteriorteeth by simulating the biomechanical effects of anterior teeth after loadedwith micro-implant at different positions on the established finite elementmodel of maxillary-dentition-appliances complex and analyze the influence ofthe two kinds of retraction hooks with different height on the stress andmovement.Methods：1laboratory equipment:Hardware: W2000CT (multi-slice spiral CT machines, Hitachi, Japan),computers, Finite Element Analysis Workstations.Software: Mimics10.1(3D reconstruction software, Materialise companyfrom Belgian), Solidworks2011(three-dimensional solid model CAD designsoftware, American Dassault Systemes SA company), Geomagic11.0(reverseengineering software, Geomagic U.S. company), ANSYS Workbench13.0,(large general-purpose finite element analysis software ANSYS, Inc. USA).2Establishment of experimental model2.1Obtaining the experimental dataWith the informed consent, one adult male patient of first premolarsextraction from the maxillary was adopted for Spiral CT scanning, from thetop of the skull to the lower edge of the jaw. The CT scans are stored inDICOM3.0format.2.2Processing of cancellous bone and cortical boneMimics10.0was applied to read CT scan images in Dicom format, andthen extract the cortical bone, cancellous bone, and tooth structure to build theinitial three-dimensional model of maxillary which needed modifying byreverse engineering software Geomagic and finally was imported into ANSYSWorkbench.2.3Establishing the model of SWA brackets, arch wire and retraction hookAccording to the requirements of OPA-K and the data standards of MBTstraight wire bracket, a maxillary arch wire with the size of0.018×0.025 inches and a set of brackets whose slot is0.022inches was established. On thearch wire fixed two retraction hooks bilaterally in the middle of lateral incisorand the canine.2.4Establishment of the solid model of periodontal ligamentThe model of upper dentition was imported in Solidworks(CADsoftware). The periodontal ligament was established by expanding the modelalong with the outer boundary of root by shell modeling and was set0.25mmin thickness.2.5Establishment of the finite element modelAssemble the model of cancellous bone, cortical bone, upper dentition,SWA brackets, arch wire and periodontal ligament together by Booleanoperations after they were imported in the finite element software ANSYSWorkbench according to the correct position. The finite element model neededmeshing and be set the material properties of each part.3Calculation3.1loading conditionsLink the three mini-implants of different positions to the anteriorretraction hook in one side respectively to simulate the force system loaded onthe teeth.Load of A1, load anterior mini-implant located in the interadicular spacebetween lateral incisor and canine bilaterally with retraction hook;Load of A2, load anterior mini-implant located in the interadicular spacebetween central and lateral incisor bilaterally with retraction hook;Load of B, load posterior mini-implant located in the interadicular spacebetween second premolar and first molar bilaterally with retraction hook.The retraction hook is set on the arch wire in the middle of lateral incisorand canine bilaterally which is2mm or5mm high.A1and A2is loaded with a force of0.5N,0.75N,1N respectively. B isloaded with a force of1N,1.5N,2N respectively.Each load conditions is summarized as follows:Case1: The retraction hook is5mm high, A1loaded with0.5N. Case2: The retraction hook is5mm high, A1loaded with0.75N.Case3: The retraction hook is5mm high, A1loaded with1N.Case4: The retraction hook is5mm high, A2loaded with0.5N.Case5: The retraction hook is5mm high, A2loaded with0.75N.Case6: The retraction hook is5mm high, A2loaded with1N.Case7: The retraction hook is5mm high, B loaded with1N.Case8: The retraction hook is5mm high, B loaded with1.5N.Case9: The retraction hook is5mm high, B loaded with2N.Case10: The retraction hook is2mm high, A1loaded with0.5NCase11: The retraction hook is2mm high, A1loaded with0.75NCase12: The retraction hook is2mm high, A1loaded with1N.Case13: The retraction hook is2mm high, A2loaded with0.5NCase14: The retraction hook is2mm high, A2loaded with0.75NCase15: The retraction hook is2mm high, A2loaded with1N.Case16: The retraction hook is2mm high, B loaded with1N.Case17: The retraction hook is2mm high, B loaded with1.5N.Case18: The retraction hook is2mm high, B loaded with2N.3.2ParametersUnder various cases, calculate the rad values in the Y axis, displacement,peak Von-mise stress value of six anterior teeth and peak Von-mises stressvalue in PDL. Stress distribution is received by finite element analysis innephograms.Result:1Finite element model of maxillary, upper dentition, periodontalligament, appliance complex with high geometric similarity was establishedsuccessfully. The finite model which was processed by CAD, Solidworks andGeomagic software after getting three-dimensional data of maxillary by SpiralCT scanning with good geometric and biomechanical similarity nice couldmeet the operational requirements.2Under the same loading condition, the rad values, maximumdisplacement values, peak Von-mise stress values increase as the force magnitude increases, but the distribution is the same.3Left and right anterior teeth’s movement trend is similar.4The maximum principal stress region in PDL was at the alveolar ridgecrest under the loading with A1or A2while which was around the cervical ofperiodontal ligament under the loading with B.5The maximum principal stress region of anterior teeth was in the neckon the labial side of lateral incisor or canine which was near the retractionhook.6When A1was loaded, labial crown tipping and intrusion of eachanterior teeth occurred. The height of retraction hook has little influence onthe results.7When A2was loaded, nearly all the anterior teeth were intruded.8Under the loading condition of B, when the retraction hook was at thelevel of2mm(bracket slot level), lingual crown tipping occurred to centralincisor and canine while lateral incisor’s crown tipped labially. Lateral incisorswere intruded more than central incisors, at the same time, the canines wereextruded. As the height of retraction hook increased to5mm, crown labiallytipping and intrusion occurred to both central and lateral incisor while thecanine’s crown tipped more lingually and was extruded much more.Conclusions:1A three-dimensional finite element analysis method can effectivelysimulate the teeth and PDL stress, displacement changes after application oforthodontic force, which is the most important means of orthodonticbiomechanics research methods. Read CT scan images in Dicom formatthrough Mimics, and processed by Solidworks, Geomagic, you can create afinite element model with high geometric similarity.2With both the anterior and posterior MIA, central and lateral incisorscould be intruded. A force of50g to100g for intruding anterior teeth and aforce of100g to200g for posterior micro-implant to retract the anterior teethdoesn’t cause stress centralization in the apex of root or around the cervicalmargin. 3Choose the optimum height of retraction hook in clinics according tothe requirements of patients. The retraction hook at the level of2mm is chosenfor the patients whose incisors tipped labialy and needed more retraction space.The retraction hook at the level of5mm is chosen for the patients whoseincisors needed uprighting or more intruding.