| Part One Three-dimensional finite element stress analysis of uneven-threaded titanium dental implantObjectives: Thread shapes of the implant play very important roles in biomechanical optimization designs,which can increase the implant-bone contact surface,enhance the primary stability of the implants,and improve the stress distribution of the bone interface.At present,the surface of the implant used in the bone usually is even-threaded in one shape.However,some literatures documented that uneven-threaded implant—micro thread with smaller pitch and depth in the cortical bone and bigger pitch and depth in the cancellous bone—shows better mechanical effect.There are a few reports about the biomechanical property of the implants with different neck micro threads,however.To design the implant properly so as to rationalize the stress on the bone interface with the implant,especially to reduce the stress concentration on the superior border of the cortical bone in the neck,will help reduce bone absorption,decrease the possibility of implant loosening,and prolong the implant life.In order to improve the long time success rate of implantation,it's necessary to prioritize the design of the implant thread,especially the micro thread in the neck and the combination of the threads.This study is to explore the biomechanical property of the implants with different neck micro threads.Methods:1 Implant groupingFour different neck forms: uniform-threaded implant(UT),wide-neck implant(WN),implant with single-microthreaded neck(SMN),and implant with double-microthreaded neck(DMN).2 Mandibular models2.1 Preliminary modelingBased on the CT scanning image of the mandible,a corresponding new project was established in Mimics 16.0.According to the gray values of the CT images,the bony part was extracted.Then the bony masking materials were edited,and an accurate void-free masking model that only included mandible pixels was obtained.The 3-dimensional(3D)model was then automatically calculated by using the intra-masking pixels.The final generated preliminary 3D geometric model of the mandible was output using the STL format(Standard Template Library).2.2 Model optimizationThe STL-formatted preliminary 3D geometric model of the mandible was introduced into Geomagic studio 2013(Geomagic Inc.,USA),which was followed by preliminary smoothing using several rapid fairing operations.The polygonal surface performance was further improved,and a mandibular polygon model was ultimately obtained.The polygon model was then transmitted into the accurate surface stage,and the mandibular patch model was ultimately obtained.After computer-aided design(CAD)object conversion,the mandibular CAD model was finally obtained.2.3 Generation of cortical and cancellous bonesAt the Geomagic polygon phase,the model of the interface of mandibular cortical and cancellous bones was obtained by drawing the shell 1.5mm inward and removing the crossing surface;the above steps were then re-operated step-by-step,so that the cancellous bone CAD model could then be obtained with the interface as the outer contour.The overall mandibular CAD model and the cancellous bone CAD model were then introduced into Solidworks 2015 to obtain the cortical bone model by a Boolean subtraction operation.2.4 Assembly of implant and mandibleThe 4 types of implants were then fixed at the same place in the mandibular first molar region.One 3.2mm high abutment was established on the implant top using Solidworks,on top of which one 2mm thick ceramicdental prosthetic restoration was then established,followed by importing into the Ansys Workbench 14.5 with the SAT format.2.5 MeshingThe hexahedron-based and tetrahedron-supplemented automatic meshing method was used,with a unit size of approximately 0.5mm,and the densities of the nodes and units at and around the implant were properly increased.2.6 Material properties and constraintsAll the materials used in this study were assumed to be homogeneous and isotropic linear elastic materials,with small elastic deformation.The degree of freedom of all the nodes along the top of the bilateral mandibular condyle was set as a rigid constraint.The implant and the bone tissues were set at 100% of osseointegration.The ceramic restoration and the implant were set as the immobilized contact.2.7 Loading conditionsUniformly distributed loads under the following three conditions were applied onto the implant: axial 100-N load,horizontal 50-N load from the buccal side to the lingual side,and oblique 100-N load from the buccal side downward to the lingual side and forming a 30° angle with the implant axis.2.8 Evaluation indexesThe observation indexes were the Von-Mises equivalent stress(ES)and maximum principal stress(MPS).Results:1 General features of the stress distribution on the implant-bone interfaceDifferent neck morphologies of an implant would affect the stress value and stress distribution on the implant-bone interface;the overall stress concentration areas were all located at the neck and the apical part around the implant.2 When axial 100-N load was appliedThe maximum ES in the cortical bone area(CB-MES)in UT,WN,SMN,and DMN was 46.58 MPa,35.25 MPa,20.98 MPa,and 58.01 MPa,respectively,and in the cancellous bone area was 3.64 MPa,7.45 MPa,2.60 MPa,and12.38 MPa respectively.MPS at the superior border of the cortical bone was0.97 MPa,0.89 MPa,0.79 MPa,and 1.22 MPa,respectively;MPS at the junction of cortical and cancellous bones was 1.72 MPa,3.89 MPa,0.45 MPa,and8.07 MPa,respectively;MPS at the end of the implant was 0.73 MPa,0.67 MPa,0.64 MPa,and 0.57 MPa,respectively.MPSs at the mid-point of the cancellous bone area were relatively lower.3 When horizontal 50-N load from the buccal side to the lingual side was appliedCB-MES in UT,WN,SMN,and DMN was 53.21 MPa,41.93 MPa,36.95 MPa,and 64.18 MPa,respectively,and in the cancellous bone area was3.72 MPa,6.57 MPa,3.77 MPa,and 4.07 MPa respectively.MPS at the superior border of the cortical bone was 41.77 MPa,39.61 MPa,29.67 MPa,and46.99 MPa,respectively;MPS at the junction of cortical and cancellous bones was 9.45 MPa,2.80 MPa,1.73 MPa,and 9.49 MPa,respectively;MPS at the end of the implant was 0.07 MPa,0.09 MPa,0.15 MPa,and 0.26 MPa,respectively.MPSs in the cortical bone area were relatively higher.4 When oblique 100-N load was appliedCB-MES in UT,WN,SMN,and DMN was 49.37 MPa,47.78 MPa,34.02 MPa,and 78.52 MPa,respectively,and in the cancellous bone area was3.74 MPa,3.72 MPa,3.81 MPa,and 7.33 MPa,respectively.MPS at the superior border of the cortical bone was 29.35 MPa,21.28 MPa,15.34 MPa,and34.69 MPa,respectively;MPS at the junction of cortical and cancellous bones was 3.50 MPa,2.71 MPa,0.97 MPa,and 8.58 MPa,respectively;MPS at the end of the implant was 0.16 MPa,0.11 MPa,0.15 MPa,and 0.10 MPa,respectively.MPSs in the cortical bone area were relatively higher.Part Two Study on the fatigue performance of the uneven-threaded titanium dental implants treated by surface rougheningObjectives: The material of the dental implant and its surface characteristics and shapes make great effects on the osseointegration.Particle blast and acid etching,a new non-coating method,is an improved way to treat the surface,through which the implant surface shows apparent changes,notonly improving the roughness of the surface,but prioritizing the micro structure of the surface.After acid etching,large amount of small secondary cavities appear in the big caves,which is of great importance for the mechanical closure of the implant and the bone.Particle blast can reduce the contamination of different elements,and heated combined acid etching of HCL/H2SO4 after particle blast can obtain fairly satisfactory surface morphology feature.Nonetheless,there are no reports about the effects of this physical and chemical treatment on the fatigue property.This study aims to probe the effects of different neck threads and surface roughening on mechanical fatigue performance.Methods:1 Implant groupingThe implants were divided into 5 groups: uniform-threaded implant(UT),wide-neck implant(WN),implant with single-microthreaded neck(SMN),implant with double-microthreaded neck(DMN),and implant with rough surface and double-microthreaded neck(RSDMN).2 Surface morphological observationAfter FSEM(field-emission scanning electron microscopy)observation,the surface structure and morphology were analyzed in DMN and RSDMN.3 Fatigue experiment3.1 experiment standardLoads were applied according to ISO14801:2007.3.2 experiment environmentThe experiment was carried out in the air with the temperature of 20±5?.3.3 Loading waveform and frequencyOne-way load was used,and the load showed sinusoid change between nominal peak and 10% nominal peak.The loading frequency was 15 Hz.3.4 Steps3.4.1 Assembly of the implant parts: the lower part and the abutment were connected through inner hexagon and fastened 8-10 Ncm with a center screw.3.4.2 Adhesion of the semispherical load-bearing part3.4.3 Imbedding and fixing of the implant3.4.4 Connection of the dynamic and static electronic universal material testing machine INSTRON3.4.5 The primary loading was 80% static load,followed by decreased loads.At least 4 different loads were applied to each group until the ultimate tolerance was obtained at the cycle of 5×10~6,with 3 implants intact.3.4.6 The bending moment applied was calculated.3.4.7 The cycle number and the displacement under different loads were recorded,and the displacement of the implant under the same load of 5×10~6cycles was compared.Results:1 Surface morphological observationMa surface of DMN observed by naked eyes: smooth and clean,with even luster and bright color of silver.FSEM observation: under low power electron microscope,the surface is flat with micro roughness and strikes of the same direction.Under 300 power electron microscope,the micro-rough surface with fairly many paralleled groove-like cuttings was observed.Under1000 power above electron microscope,the surface was observed fairly flat and neat,with a lot of regular and shallow groove-like cuttings of the same direction,but with pits or heaves scattered locally.SLA(sand-blasted,large grit and acid-etched)surface of RSDMN observed by naked eyes: rough,even grayish white with no metal luster.FSEM observation: under low power electron microscope,the surface is neat,even and rough.Under 300 power electron microscope,the irregular rough surface and cracks with rather small dents were seen.Under 1000 power above electron microscope,on the irregular dents there are large quantity of5-30 um pits.The pits are sharp-edged with partial fuse and with scattered micro cracks occasionally seen.Under 10,000 power electron microscope,in the big caves and dents,there are large amount of irregular small dents,about200-500 nm,with different size and depth,with the bottom semi-round and the edge round and blunt.2 Dynamic and repeated fatigue performance2.1 The limit load and maximum bending moment when 5×10~6 was reachedIn groups of UT,WN,SMN,DMN,and RSDMN,the limit load at 5×10~6cycles was 250 N,300N,300 N,300N,and 275 N respectively;and the according maximum bending moment was 137.50 Ncm,165.00 Ncm,165.00 Ncm,165.00 Ncm,and 151.25 Ncm respectively.2.2 Dynamic fatigue displacementIn groups of UT,WN,SMN,DMN,and RSDMN,when 5×10~6 cycles were reached under the load of 250 N,the according displacement was0.391 mm,0.251 mm,0.257 mm,0.258 mm,and 0.272 mm respectively.Part Three Study on the osseointegration of uneven-threaded titanium dental implant treated with surface rougheningObjectives: Study one explores the effects of different neck threads on stress distribution,study two probes the effects of different neck threads and surface roughening on mechanical fatigue performance,and this study is aimed to study the effects of different neck threads and surface roughening on osseointegration,and finally to provide experimental basis for optimizing the implant neck micro thread structure and roughening the surface of the implants.Methods:1 Animal groupingNinety adult New Zealand white rabbits,male,were randomized into 5groups,with 18 in each: UT,WN,SMN,DMN,and RSDMN.2 ImplantationSix hours before the implantation,the rabbits were forbidden to take any food or drink any water.1% pentobarbital sodium was injected through ear vein to apply general anesthesia.On the femur far from the heart,the skin was shaved and sterilized,and sterile drape was applied.The 0.5% lidocaine hydrochloride was used to apply local infiltration anesthesia.Skin,fascia,and periosteum were cut open in order,and the bone was exposed after stripping the periosteum.The implanting caves were made by a set of drills.The 5different implants were randomly implanted into the rabbit femur far from the heart,with the implanting torque of 10 Ncm and the top of the implant at the same level of the bone.The cut was washed locally with 0.9% sodium chloride,and the periosteum,fascia and skin were sutured successively.3 X-ray observationAt week 12 post-implantation,X-ray was taken to observe the imaging features of the specimens in 5 groups.4 Removal torque testAt week 4,8,and 12,rabbits were killed to take out the femur with the implant,and the maximum torque value was measured by the portable digit torque testing instrument when the implant was backed out.5 Implant-bone FSEM observationAt week 4,8,and 12,the backed out DMN and RSDMN implants were fixed,dehydrated,dried,adhered,and sprayed with gold powder,and then the surface morphological features were observed with FSEM.6 Implant-bone EDX analysisAt week 4,8,and 12,the backed out DMN and RSDMN implants were dried,adhered,and sprayed with gold powder,and then the chemical element composition was measured with EDX(energy-dispersive X-ray microanalyzer).7 Histological observationAt week 12,the femurs with the implants in DMN and RSDMN groups were taken out and specimen was decalcificated to make tissue slices.After HE dyeing,osseointegration was observed.8 Statistical analysisThe removal torque of the 5 groups was processed with SPSS20.0software package,and variance analysis was processed with repeated measure data,with P<0.05 of statistical significance.Results:1 X-ray observationAt week 12 post-implantation,there was no linear image transmission inthe implant-bone surface,with fairly similar density with the bone around.There was no obvious bone absorption around the neck of the implants.2 Removal torqueAt week 4,8,and 12,the removal torque in RSDMN was all higher than that in DMN.There was no obvious removal torque difference between SMN and DMN,both higher than those in UT and WN,with the latter lowest.3 FSEM observation3.1 At week 4 post-implantationThe surface of the implants in DMN was covered with super thin fibers,and the cutting made by the machine was not covered with the adherence tissue yet.In contrast,in RSDMN,there was fairly even adherence tissue on the surface of the implant,almost all covering the roughened porous structure and small amount of bone tissue was seen.3.2 At week 8 post-implantationThe surface of the implants in DMN was covered with thin fibers and small amount of bone tissues,and the cutting made by the machine was almost fully covered with the adherence tissue.In contrast,in RSDMN,there was cellular-like thick adherence bone tissue on the surface of the implant,fully covering the roughened porous structure.3.3 At week 12 post-implantationThe surface of the implants in DMN was covered with thin bone tissues,and the cutting made by the machine was not fully covered with the adherence tissue yet.In contrast,in RSDMN,there was fairly thick adherence tissue on the surface of the implant,and bone tissue was adhered to the roughened porous structure.4 EDX analysisAt week 4,in DMN the surface adherence tissue contained 26.45%calcium and 21.29% phosphor,while in RSDMN 52.26% and 32.99%respectively.At week 8,in DMN the surface adherence tissue contained37.72% calcium and 25.63% phosphor,while in RSDMN 53.97% and 34.39%respectively.At week 12,in DMN the surface adherence tissue contained48.06% calcium and 32.05% phosphor,while in RSDMN 54.23% and 34.56%respectively.5 Histological observationAt week 12,there was clear lamellar bone around the implants in DMN and RSDMN,the former showing fairly continuous but thin and less dense lamellar bone,while the latter showing continuous,thick,and dense lamellar bone with large amount of bone cells in the new bone.Conclusions:1 Different neck forms of cylindrical implants with V-shaped microthreads affect implant-bone interface stress and stress distribution;the overall stress concentration area is located at the cortical bone area and the apical part around the implant neck.2 SMN with the same thread form significantly reduces the Von-Mises ES and MPS in the cortical bone area when axial,horizontal,and oblique loads are applied.3 Compared with SMN,WN shows higher Von-Mises ES and MPS in the cortical bone area when axial,horizontal,and oblique loads are applied.4 Compared with WN,UT shows higher Von-Mises ES and MPS in the cortical bone area when axial,horizontal,and oblique loads are applied.5 DMN shows higher Von-Mises ES and MPS in the cortical bone area when axial,horizontal,and oblique loads are applied.6 WN,SMN,and DMN show similar fatigue tolerance,all higher than that of UT.7 Compared with Ma implants,the implants treated with titanium sand blast combined with HCL/H2SO4 heated acid etching show lower fatigue tolerance but still can meet the mechanical needs of clinical functions.8 At different observation time,RSDMN shows higher removal torque compared with DMN.There was no obvious removal torque difference between SMN and DMN,both higher than those in UT and WN,with the latter lowest.The changing tendency of the removal torque in each group is basically similar,higher at week 8 compared to week 4,and obviously itreaches the peak at week 12.9 At different observation time,compared with Ma implants,the surface of the implants treated with titanium sand blast followed by HCL/H2SO4 heated acid etching bear more bone tissues and more calcium and phosphor,with earlier formation.10 At week 12 post-implantation,compared with Ma implants,the implants treated with titanium sand blast followed by HCL/H2SO4 heated acid etching show wider lamellar bone with better density and continuity. |
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