| Objective:In the current clinical orthodontic treatment,the resistance control is crucial,which directly affects the final treatment effect.Compared to taditional implants,mini inplants have many advantages,which therefore is widely applied to clinical treatment of orthodontic doctors.Although mini implant is widely used in clinical orthodontic,excessive bone-implant interface stress often causes bone damage and implant failure.The success rate is approximately 90%.One of the main factor is the bone quality around the implant parts.The bearing capacity of the implant is normally related to the bone quality of the implant position.Many studies have shown that the stability of the implant is associated with the thickness of cortical bone and the bone density.Researches have shown that many bone parameters have important influence on the stress around the mini implant:1,cortical bone mineral density,2,cortical bone thickness 3,cancellous bone mineral density.Studies have also shown that great cortical bone thickness,the density of cortical bone and cancellous bone spirit will reduce the stress concentration around the implant.However,some clinical studies have shown that cortical bone mineral density is too high and the rate of abscission is higher.And few research based on the real parameters value.Three-dimensional finite element method are often used to evaluate the stress distribution of different types of bone around the implant,and it can help researchers predict implant after loading stress distribution of implant bone.In short,mini-implant can provide firm and effective anchorage,while the premise is the stability of the implant itself.But whether the thickness of cortical bone,cancellous bone can affact the density of mini implant stabilityis not clear.In this study,through the spiral CT measurement,we establish a three-dimensional finite element model of different types of bone,analysing different cortical bone thickness,density,and cancellous bone mineral density mini implant-bone interface stress distribution characteristics,discussing the influence of different bone conditions on the implant stability,providing theoretical basis for clinical work,improving the success rate of mini implant,then patients can reduce pain and the level of treatment can be raised.The clinical application of implants has certain guiding significance.Second,the main research contentMethods:1 material and equipment1.1 The experimental equipmenthardware: Windows 7 system,spiral CTSoftware: Mimics,ANSYS17.0 finite element analysis software1.2 Material: pure titanium,the mini implant,thread shape2 experiment method2.1 Maxillary and mandibular spiral CT scanning and measurementA total of 50 cases of mandibular spiral CT scanning in patients with13-45 years of age data from the second hospital of Hebei Medical University Department of imaging data.Requirements: 1)in the Han population;2)without severe crowding,no detention without deciduous teeth,supernumerary teeth and tooth loss;3)no craniofacial malformations,side to side,on the relationship between the normal mandible;4)without oral and maxillofacial trauma and surgery;5)without bone metabolism disease;6)without periodontal disease and alveolar bone lesions,not for periapical surgery;7)without serious abnormal root morphology;8)image clear.Scanning thickness 0.625 mm,scan results carry on,three-dimensional reconstruction of the mandible.The use of spiral CT were measured on the mandibular micro implant implantation site before surgery(in the second premolar and the first molar buccal,first and two molar buccal alveolar crest,distance 6mm)cortical thickness,cortical bone density of cancellous bone thedensity of each measuring three times the average.2.2 Set a bone parameters and material propertiesBased on 5% and 95% of the data,determine the thickness of cortical bone is thick or thin,maxillary cortical bone thickness of 1.0 mm and 1.3 mm,mandible cortical bone thickness is 1.4 mm and 2.7 mm,because of the bone mineral density arge span,according to the experiment purpose the cortical bone mineral density defined as a high,middle and low three numerical,maxillary cortical bone 1200 hu,1000 hu and 600 hu respectively,defined the mandible cortical bone mineral density of 1400 hu,1200 hu and 960 hu respectively,cancellous bone mineral density is defined as a high,low,jaw cancellous bone mineral density of 520 hu and 820 hu respectively.Hu will be measured by CT value translated into Grayvalue through formula: Grayvalue= Hu + 1024,On the basis of the Mimics provide empirical formula:Density=47+1.122* Grayvalue,E-Modulus=-172+1.92* Density,we calculate the elastic modulus of different bone mineral density and define the poisson's ratio of different bone and implant according to the elastic modulus.3 Establish the finite element models of mandible with mini-implant.3.1 Setting up the finite element model of maxillary and mandible.A young adult male was selected from 50 patients who underwent helical CT scanning,and the CT images of the patient were selected.Establishing the model on the basis of a male volunteer's maxillary and mandible,and then setting between maxillary second premolar and the first molar as implant implant site,outside surface is cortical bone,for the rest of the cancellous bone,in the ANSYS software the section is reduced to a trapezoid.The upper surface of section size: 15 mm wide,lower surface width 13.5 mm,9.5 mm high.Import it into ANSYS software,stretching the cross section along the mandibular transverse into a 20 mm long hexahedron,forming a model of the maxillary.In the mandible between the second premolar and the first molar implant site,choosing the bone CT section here,outside surface is cortical bone,and the rest is the cancellous bone.in the ANSYS software we reduced the section to a hexagon which the size is:the upper surface width is10.89 mm,the lower surface width is 18.2mm,and the hight is 39 mm,then tensile the hexagon in to a octahedron with the length of 20 mm.3.2 Established the mini implant three dimensional finite element model.According to the clinical commonly size: the total length is 12 mm,the length in the bone is 8 mm,the neck diameter is 1.6 mm,the tip diameter is1.4 mm,thread tip height is 0.3 mm,the cutting edge thread apex angle of60 degrees,and the pitch is 0.6mm,Shoulder platform height is 2m.In the ANSYS17.0 software,through the extension of the cylindrical screw,through the rotation of the screw,and then by the Boolean operation screw and screw thread synthesis,the establishment of mini implant model,see Fig.1.3.3 Assemble the implant-bone model3.4 Implant location and angleThe mini-implant was placed in the second premolar and the first molar buccal side 6 mm from the top of alveolar crest with a 45-degree angle to alveolar bone surface.Loading force 1.96 N on implant neck and the force parallel is to the surface of alveolar bone.3.5 To establish the mandible model with different parameters:According to the thickness of the cortical bone set includes the establishment of mini implant,cortical bone thickness of the mandible on different models,a total of 4,according to the different cortical bone density and cancellous bone density,two sets of the four models were established,and a total of 24 cases were established.Hypermesh software was used to mesh the model,and the orthodontic force 1.96 N parallel to the jaw surface was applied at the top of the mini implant,see Fig.2.ANSYS software was used to simulate the threedimensional simulation.4 Define material propertiesAssuming that implant,cortical bone and cancellous bone are continuous,homogeneous and isotropic linear elastic material,and material deformation is small elastic deformation.Setting up a corresponding interface,and assuming that implant with jaw bone union.Mini implant and jaw bone interface aredefined as a fixed contact.5 Analysis indicatorsAlong the direction strength by the finite element model of center of buttock implants,andrecord the peak stress of implant-bone interface.6 Data acquisitionAlong the direction strength by the finite element model of center of buttock implants,and every 0.1 mm to extract the mini implant-bone interface stress value,and they will be collected by the numerical graph.7 Data analysisThrough the analysis of the collected data,weanalyze different cortical bone thickness and whether density of mini implant have any effect.Results:1 Establishing model of maxillary under different cortical bone thickness and density of micro implant-finite element,and its morphology and biomechanical similarity are high,which meet the requirements of the experiments;2 Mini implant-bone interface's stress and displacement distribution: the results show that all conditions of Von-Mises stress distribution are mainly concentrated in the cortical bone area,the stress decays quickly in cortical bone,and cancellous bone area stress is very small(Fig.3);The maximum stress value in cancellous bone is on the border of cancellous bone and cortical bone(Fig.4);Maximum stress peak value appears in U2 HL,minimum stress peak appears in U1 LH.Displacement is concentrated in the area of the cortical bone,and the displacement peak is located in the cortical bone,while in the junction of cortical and cancellous bone rapidly reduced,the values of the displacement in cancellous bone area is smaller(Fig.5).On the same conditions,the peak displacement of the maxillary are also large than the displacement peak of the mandible(Fig.13).The maximum displacement occurs in U1 LL,while the minimum displacement appears in L2HH;3 Cortical bone thickness on the influence of the displacement distribution around the implant: when bone mineral densities are same,the greater the cortical bone thickness,the more cortical bonedisplacement peak decreases,the mandible displacement is less than the maxillary(Fig.13).The greater the thickness of cortical bone,the smaller the displacement peak of the cancellous bone(Fig.15);4 Cortical bone thickness on stress distribution around implant impact: when cortical bone mineral density is high or medium,the peak stress of cortical bone is almost not influenced by the thickness of cortical bone;When the cortical bone mineral density is low,the peak stress of cortical bone is proportional to the thickness of cortical bone(Fig.14);The greater the thickness of cortical bone,the smaller the peak stress in the cancellous bone is(Fig.16);5 Cortical bone mineral density's impact on the displacement distribution around the implant: in four different models established by the mathematical cortical bone thickness,the higher cortical bone mineral density is,the less the peak relative displacement in cancellous bone is(Fig.19),and the less peak stress reduces in the cancellous bone as well(Fig.15);6 Cortical bone mineral density's influence on the stress distribution around the mini implant: when the thickness of cortical bone and cancellous bone density are constant,the density of cortical bone and cortical bone is closely relative to internal stress peak value(Fig.14).The density of cortical bone and cancellous bone stress peak are of negative correlation(Fig.16);7 Cancellous bone density on the influence of mini displacement distribution around the implant: when cortical bone thickness and density is constant,the higher the density of cancellous bone is,the smaller the cortical bone within the peak displacement is(Fig.17),and the smaller the displacement of the cancellous bone is relatively(Fig.15);8Cancellous bone mineral density's influence on stress distribution around the mini implant: when cortical bone thickness and density is constant,the higher the density of cancellous bone is,the smaller the peak stress of the cortical bone is(Fig.18),and the larger the cancellous bone on the peak stress is(Fig.19).Conclusions:1 The maximum stress of mini implant bone interface are mainly concentrated in cortical bone,and rapidly decay,the stress in cancellous boneis small.2 Stress distribution of mini implant bone interface closely associated with cortical bone thickness and density.When the cortical bone thickness increase,the peak stress in cortical bone is larger;The peak displacement decreases with the increase of cortical bone thickness.3 The implant bone mineral density is the key factor that influence the stability of the implant.Higher cancellous bone mineral density can reduce the stress and displacement,is conducive to the stress distribution of the implant bone interface.Cortical bone density and the peak stress are related,cortical bone mineral density is negatively related to the displacement peak,as a result,embedded parts of cortical bone density shoulds not be too low or too high. |
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