| Objective: Anchorage control is the key factor influencing the treatmenteffect in orthodontic process. Comparing with traditional anchorage such aspalatal arch, lingual arch, extraoral bow et al, mini-implant anchorage with itsadvantages such as simple operation, smaller trauma, lower cost, and thereliable curative effect, become a research hotspot in the field of orthodonticsin recent years and more and more applied in the clinical.The stability of the implant itself is the premise of implant strongresistance for orthodontic treatment，and its stability is closely related to thequality of jaw bone. Jaw bone quality has a great deal of variation in thecrowd, teenagers with a thinner cortex bone were often finding in clinicalapplication which has lower primary stability. Whether prolong the healingtime and adjust the load force value can improve the success rate ofmini-implants have few reports.The retention forms of mini-implant mainly include mechanical forceembedded and biological combination. Mechanical force embedded means atthe beginning of the mini-implant implantation, without osseointegration,provided by the micro friction between the mini-implant and bone tissue.Biological combination is divided into fiber combination (there is a layer ofnon-mineralization of fiber connective tissue between implant and bone tissue),as well as the osseointegration (direct contact with the mini-implant and bonetissue, which does not exist other tissue, to reach the direct contact on thestructure and function) and the mixed combination.Studies show that mini-implant healing time before loaded will have animpact on the stability of it, but the best loading time remains controversial.The traditional view is that, there should be a certain time before loading afterthe implant implantation to improve the bone-implant contact rate, in order to guarantee its stability. In recent years, a new idea is that a micro-motion withincertain limits can make advantages to the osseointegration of mini-implantswith good primary stability, which provides the basis for mini-implantimmediate loading.There are many methods of mini-implants research. In recent years thethree-dimensional finite element analysis was proved to be an effectivemethod. Application of three-dimensional finite element analysis can be moreaccurate of the real internal structure and the histological changes. Thus thethree-dimensional finite element method is now widely applied in the field oforthodontics.Experiments in this paper using three-dimensional finite element method,establish the models for mini-implants and the type Ⅲbone, analysis thestress through the difference implant-bone interface of different healing timebefore loading, in order to observe and compare the influence of differenthealing time on the mini-implant stability for type Ⅲbone, provide theoreticalbasis for guiding clinical application, improve the success rate of planting.Methods：1ExperimentComputer: desktop (Intel(R) Xeon(R) CPU E5-1650，3.20GHz:8Gmemory, windows7,64bit operating system)Packages: Catia V5，Hyperworks11.0，Abaqus6.102The model of mini-implant and jaw2.1The model of jawEstablish the model on the basis of a female volunteer’s maxillary; theplacement site is on the left side between the second premolar and the firstmolar. The surface part of the jaw model is cortical bone, the thickness is0.7mm, and inside part of the jaw is cancellous bone. According to the actualsize, the jaw model was simplified to a isosceles trapezoid, the upper surfacewidth is15mm, the lower surface width is13.5mm, and the hight is7.5mm，then tensile the trapezoid in to a hexahedron with the length of20mm. 2.2The model of mini-implantEstablish the model according the clinical commonly size: the totallength is12mm, bone segment length is8mm, diameter is1.6mm, threadheight is0.2mm, and the blade-like thread apex angle of60degrees, the pitchis0.6mm.2.3Assemble the implant-bone modelThe jaw and mini-implant were combined with the correspondingposition.Forces of1N and2N were loaded on the neck of mini-implant. Setthe gingiva side for Y-axis positive side, the distance side for the X-axispositive side, the implants axis for the Z-axis. Loading direction is parallel tothe X-Y plane (perpendicular to the implant axis), and opposite to the Y-axispositive direction (occlusal side).2.4Experiment grouping design2.4.1Immediate Load group（IL）The non-osseointegration condition was simulated by the coulombfriction model, the coefficient of friction μ=0.2, then use the method ofinterference fit to simulate the inital stress of the implant-bone interface.Immediate Loading1group (IL1): interference=0.03;Immediate Loading2group (IL2): interference=0.05;Immediate Loading3group (IL3): interference=0.1;2.4.2Early Loading group (EL) and Delay Loading group(DL):EL and DL group simulated the condition of loading after3weeks and7weeks, respectively.Define the implant-bone interface with the bone-implantcontact rate (BIC):EL group: BIC=34%DL group: BIC=44%The different BIC was simulated by different combination rates betweenthe implant-bone interface.In the combination regions, the relativedisplacement is0under the external force, and in the non-combination regions,the relative sliding is allowed under the external force, and the coefficient offriction μ=0.2. 3Materials3.1Material propertiesAssuming that implant, cortical bone and cancellous bone are continuous,homogeneous and isotropic linear elastic material, the material deformation issmall.3.2Entity modelingEstablish and assemble the three-dimensional model of mini-implant andjaw with the computer technology.3.3MeshingImport the three-dimensional model into Hypermesh module of the finiteelement modeling software Hyperwork11.0, and then refine the model grids.3.4Parts connection and boundary conditionsImported the mesh finite element model into the Abaqus6.10, establish allparts of the corresponding interface.4The observation indexesUse Hyperworks11.0Hyperview module to view results. Collect theVon-Mises stress value and the displacement value, analysis the regulation ofstress and displacement distribution.Result:1Establish the implant-bone models of different healing time.2The stress and displacement distribution on mini-implant: Von-Misesstress is mainly distributed in the area contacted with cortical bone;displacement is concentrated in the part out of bone, and in the bone, itmainly concentrated in the area contacted with the cortical bone.3Implant-bone interface stress and displacement distribution: theVon-Mises stress distribution in all conditions are mainly concentrated in therange of the cortical bone and reduce quickly within the scope, in the range ofcancellous bone the stress is smaller; In the EL and DL group, thedisplacement is concentrated within the range of cortical bone, in cancellousbone the displacement decrease rapidly; In the IL group, the displacementpeak value occurred in cortical bone area, but the change of displacementthroughout the jaw is wavy, wave peak value appeared in each blade of the thread.4The impact of healing time on the stress and displacement ofimplant-bone interface: IL group shows a high stress; the stress anddisplacement distribution of EL and DL group are basically the same. Thisshows that in the type Ⅲbone with thinner cortical bone, the BIC of34%and44%can provide sufficient stability for mini-implant.5The impact of interference on the stress and displacement ofimplant-bone interface: IL1~IL3group show that the interference and thestress peak value were positively correlated, and the peak value shows nodifference between no loading,1N and2N force loading. This means theinitial stress of the interface was provided by mechanical force embedded, andit shows no difference between1N～2N．Conclusion:1The stress distribution of implant-bone interface is mainly concentratedin cortical bone; within the scope of cancellous bone the stress is smaller;2In the type Ⅲ bone，primary stability provided by the mechanical forceembedded, and in this case immediate loading is safe;3A BIC of34%and44%can provide enough stability for mini-implant;4In the type Ⅲ bone, smaller loading value can reduce the peak value ofstress and displacement, make the distribution of implant-bone interface moreeven, therefore, reduce the loading force properly will gain a greater stabilityof mini-implant. |
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