Optimum Design Of Implant Diameter And Length In Type â…£ Bone: A Three-dimensional Finite Element Analysis

Background:Anchorage control is of paramount importance for successful orthodontictreatment. There are some traditional methods having been developed toobtain anchorage such as Nance arch, TPA bar, headgear chin and extraoralbow et al. But all of these techniques depend on patient compliance and havea low comfort level, thus the development of the orthodontic treatment islimited. The use of mini-implant as orthodontic anchorage has become verypopular nowadays with its rapid development in the field of orthodontictreatment. As a temporary bone anchorage device, mini-implant has manyadvantages over other anchorage devices. And the application of implant canexpand the scope of orthodontic treatment including molar distalization andmolar intrusion, providing a conservative and convenient treatment for thosecases that require surgical treatment.In clinical work, we often encounter the patients with weaker bone whoseimplants has low insertion torque and poor stability, which could affect itsapplication. How to improve the stability of the implants for these patients isthe difficulty of the current implant anchorage applications. At present, thereare more researches of implant stress distribution in typeⅠ o r Ⅱbone than ofthe improving of the stability in type Ⅳbone. It is researched that the stressdistribution in type Ⅰ or Ⅱbone are significantly different from the stressdistribution in type Ⅳbone. There is a clear difference of prosthodonticsimplant in insert location and load direction. In type IV bone, the mechanicsdistribution characteristics of the diameter and length have not been reported.In the application of mini-implant, the mechanical compatibility ofimplant-bone interface is the key factor to guarantee the stability of implants.The three-dimensional finite element method is an important theoreticalmechanics analysis methods and widely used in recent years in the mechanical analysis of the implant. It includes the researches of implant size, loading time,the thickness of the cortical bone, implant placement and loading directionand other factors. It made great progress. But nowadays most of thethree-dimensional finite element method using a discrete variable, which cannot accurately reflect the characteristics of bone tissue stress distribution withtwo continuous variable.Objective：This experiment set the mini-implant diameter and length as continuousvariable with three-dimensional finite element method, and explored theoptimal design of the mini-implant diameter and length for type Ⅳbone, toprovide the theoretical basis for implant production, selection and clinicalapplication and also improve its success rate.Methods：1Experimental materialComputer: the desktop (Intelp (R) Core (TM) i7-2600Cpu,3.4GHz:16Gmemory, windows7operating system)Packages: ANSYS13.0(AnsysInc.Houston)2Experimental Method2.1model of jawUsing computer software to establish the simplified jaw model whichsimplified into a trapezoid, then stretched it into a hexahedron with referenceto the shape of bone cross-section between the maxillary second premolar andthe first molar. Set the surface part as cortical bone and the inside part ascancellous bone, and then use workbench in the Design Modeler module toestablish the jaw model.2.2Implant modelWith reference to clinical implant, utilize the Design Modeler module inworkbench to establish implant, set threaded height of0.15mm and pitch of0.5mm. Implant diameter and intrabony length were set as parameters,diameter varing from1.2mm to2.0mm and intrabony length from6.0mm to10.0mm. 2.3Assembly of modelCancellous bone, cortical bone and the implant were operated, accordingto the corresponding position for Boolean operations, then obtained thecalculation model.2.4Material Properties:Set the elastic modulus and Poisson’s ratio of cortical bone, cancellousbone and implant.2.5Meshing the modelsCortical and cancellous bone were divided into hexahedron, the implantwas divided into tetrahedron, then meshing refinement the implant. Thedividing unit number was133396.2.6Loading force and directionSize: On the top of the implant applied orthodontic force of2N.Direction: Set the occlusal side as positive Y-axis, the distal side as theX-axis positive side, the implants axis as the Z-axis, loading point is the top ofthe implant. Loading direction is parallel to the X-Y plane (perpendicular tothe implant axis), and30°tilted to the X-axis positive direction (30°tiltedocclusally).2.7Solution and calculation projectsApplication the jaw Von Mises peak stress (Maximum Equivalent Stress,abbreviated as Max EQV Stress) and the peak displacement with mechanicalassessment of the different designs. Analyze the stress sensitivity of the valueof the variable.Results：1The established implant dimensional finite element model (which) hasgood geometric similarity and biological similarities.2Stress distribution and displacement of the implant: the stress anddisplacement of the implant is mainly concentrated on the neck of the implant.Take the implant (length of8mm, diameter of1.6mm) for example, themaximum stress is19.38Mpa, and the minimum value is2.27e-6Mpa; Thecloser to the implant head is, the larger the displacement value is. The maximum displacement is0.0068mm, and the minimum displacement value is7.5276e-6mm.3Stress distribution of the implant-bone interface: the stress of the boneis mainly concentrated in the contact area of the implant-bone interface, and ismainly located in the cortical bone. Stress in the cortical bone decays faster,and less in cancellous bone area. Take the implant (length of8mm, diameterof1.6mm) for example, the maximum stress on the cortical bone is16.43Mpa,while the maximum stress on the cancellous bone is0.91Mpa. The maximumstress on the cancellous bone was reduced by94.47%when compared withcortical bone.4The influence of implant diameter on the bone interfacial stress: whenthe implant diameter changes from1.2mm to2.0mm with same implant length,cortical bone stress decreased by77.66%to78.37%, and the stress within thecancellous bone reduced by60.70%to61.95%, indicating that the influenceof implant diameter on the stress change of bone tissue is obvious.5The influence of implant length on the bone interfacial stress: When theimplant length changes from6mm to10mm with the same implant diameter,cortical bone stress is reduced by0.67%to3.84%, and the cancellous bonestress reduced by-2.99%to0.29%, indicating that the influence of implantlength on the stress change of bone tissue is not obvious, and when theimplant length is larger, the influence is less.6Sensitivity analysis shows that the output variables (included stress anddisplacement of the cortical bone and cancellous bone) are more sensitive tothe diameter than the length.Conclusions1Implant stresses and displacements are mainly distributed on the neckof the implant, the bone tissue stress is mainly concentrated in the implant-bone contact area, and the cancellous bone has a larger stress attenuation.2Under the conditions setting in this study, the influence of the diameterchange on the stress changes of the bone tissue is more obvious than that ofthe of length. 3Under the conditions setting in this study, the change of implantdiameter is more sensitive when compared to the length in the IV type bone.As a biomechanical point of view, the diameter of the implant in the IV typebone should be greater than1.5mm.