| A mini dental implant is a type of dental implant with a diameter less than 3.0 mm.It is used to restore dentition defects with reduced interradicular bone or a thin alveolar crest when a patient is not suited for the standard diameter implant(3.75-4.1mm).However,many studies suggest that the survival rates of mini dental implants are relatively lower(approximately 83.9%-89%).The main reasons are implant fracture and implant loosening.The fracture of mini dental implants is due to their low mechanical strength.Implant loosening is mainly because of the small surface area for mini dental implant osseointegration,which leads to a low osseointegration strength and an unfavorable stress distribution at the implant-bone interface.Improving the mechanical properties of implants and increasing their surface area are the keys to solving the low survival rate of mini dental implants.In this study,ultrafine-grained pure titanium(UFG-Ti)material was prepared by equal channel angular pressing(ECAP),and its microstructure and mechanical properties were studied.The three-dimensional(3D)finite element method(FEM)was used to optimize the thread height and pitch of the UFG-Ti mini implant,and then UFG-Ti mini implants with optimized morphology were fabricated.In order to evaluate the feasibility of UFG-Ti mini implants,the fracture strength,fatigue strength and corrosion resistance of the mini implant were tested.After receiving the surface treatment,UFG-Ti mini implants were placed into the femur of rabbit,then bone tissue morphology around the mini implant,osseointegration performance and implant-bone bonding strength were analyzed,which provide the experimental foundation for the clinical application of UFG-Ti mini dental implants.The present study includes three parts: Part I: optimum design and fabrication of UFG-Ti mini dental implant1.The coarse-grained pure titanium(CG-Ti)was processed by ECAP through four passes of BC route and extruded at 4mm/min at room temperature,and its microstructure and mechanical properties were tested.The results of transmission electron microscopy(TEM)analysis showed that the grain refinement of UFG-Ti material was uniform and the average grain size was about 300 nm.The Vickers hardness of UFG-Ti was increased from 1499 MPa to 2488 MPa,and the tensile and compressive yield strengths were 682 MPa and 700 MPa respectively.The tensile and compressive strength of pure titanium are significantly increased,increased by 51% and 53%,respectively,while retaining good elongation and elastic modulus of CG-Ti.2.The 3D finite element software was used to optimize the thread height and pitch of UFG-Ti mini dental implant.The surface area for osseointegration of the mini dental implant,the maximum equivalent stress(Max EQV stress)in bone tissue and the estimation of the mini implant strength were set as criteria for getting the optimized design.The results showed that the micro morphology of mini implant had a significant effect on the stress in the surrounding bone and the surface area for osseointegration.When the thread height was 0.3mm and the thread pitch was 0.67 mm,the Max EQV stress value in the cortical bone and the cancellous bone was decreased by 28% and 33.1% respectively,and the surface area for osseointegration of the mini implant was increased by 19%.3.Mechanical processing figure of UFG-Ti mini implant with optimized micro morphology was drawn,and then UFG-Ti mini implants were fabricated using computer numerical control(CNC)machine.The processing accuracy was analyzed through the stereomicroscope and scanning electron microscopy(SEM)examination.The results showed that the precision of mini dental implant is consistent with the research requirements.Part II: mechanical and chemical properties of UFG-Ti mini dental implant1.The fracture strength test was carried out according to the ISO 14801 standard,and specific implant holders were fabricated in compliance with the ISO recommendations.Cobalt-chrome alloy crowns were prepared on mini implants to meet the experimental conditions.The loading direction was 30° and the loading speed was 0.5 mm/s.The results showed that the fracture resistance of 2.5mm diameter UFG-Ti mini implants was 327.85±21.18 N,which was significantly higher than that of CG-Ti mini implants control group(197.39±11.49 N,P = 0.003).The results indicated that the use of ultrafine pure titanium can significantly improve the fracture strength of mini dental implants.2.The fatigue strength test of UFG-Ti mini implants was performed according to ISO14801 standard.The load applied using sinusoidal curve,the minimum load is 10% the maximum load,the load frequency is 10 Hz.The results showed that the fatigue strength of UFG-Ti and CG-Ti mini dental implants was 150 N and 90 N respectively.The fatigue strength of UFG-Ti mini implants was better than that of CG-Ti mini implants.3.The corrosion resistance of UFG-Ti and CG-Ti at different pH and fluoride ion concentration was studied by electrochemical workstation.The corrosion resistance was tested by impedance spectroscopy and polarization curve experiment.Corrosion morphology and corrosion product were also analyzed.The results of corrosion current and potential showed that the corrosion resistance of pure titanium decreases with the increase of fluoride concentration in neutral saliva environment,and the corrosion resistance of UFG-Ti is similar to that of CG-Ti.In the acidic saliva environment with the increase of fluoride ion concentration,the corrosion resistance of pure titanium decreased.The corrosion resistance of UFG-Ti was better than CG-Ti in high fluoride ion concentration saliva.The results of corrosion morphology and product analysis show that the corrosion products on the surface of UFG-Ti were less than that of CG-Ti.4.According to the optimized parameters of sandblasting and acid-etching(SLA)of UFG-Ti and CG-Ti material,the specimens of UFG-Ti and CG-Ti were processed by sandblasting with Al2O3 particles of 110 ?m size under an air pressure of 0.6 MPa.The distance was 10 mm and the treatment time was 15 s.And then the specimens were acid-etched using mixed acid which was consisted of hydrochloric acid(37 wt.%),sulfuric acid(98 wt.%)and deionized water in accordance with 2:1:1 configuration.The acid etching temperature was 80 ?,and the treatment time was 5min.The surface characterization was analyzed using a 3D morphology scanner and SEM.The results showed that the surface of UFG-Ti and CG-Ti material have a primary structure formed by sand blasting and secondary structure formed by acid etching.The surface roughness Sa of UFG-Ti was 1.67±0.18?m,and the surface roughness Sa of CG-Ti was 1.74±0.15?m.Part III: osseointegration performance of UFG-Ti mini dental implant1.The New Zealand white rabbit was used to establish an implanted animal model of mini dental implant.The mini dental implant was placed in site of the femur.The UFG-Ti mini implants with surface treatment were implanted into the rabbit femur and then the implant surgery was evaluated by X-ray examination.The results showed that the implant sit determined by the metaphysis has strong technical operation and the implant surgery was successful.2.The femoral tissue including the mini implants were analyzed using Micro-CT scanning at 2 months after implantation.Osseointegration performance of UFG-Ti mini implants was examined using VG staining method with bone-hard tissue sections.The results showed that there was no significant difference between mini implants of UFG-Ti and CG-Ti materials with the optimized macro morphology,which was superior to UFG-Ti mini implants with standard macro morphology.The result of VG staining showed that a good osseointegration performance was performed between the mini implant and the surrounding bone tissue.3.The pull out strength of mini dental impalnts was experiment using fresh femoral tissue containing mini implants.The results showed that the pull out strength of UFG-Ti mini implants with optimized macro morphology(422.50±50.29N)was significantly higher than that of UFG-Ti mini implants with standard macro morphology(302.50±34.09 N,P<0.05).There was no statistically difference between the pull out strength of UFG-Ti mini implants with optimized macro morphology and the CG-Ti mini implants with the same design(413.17±36.06 N,p>0.05). |
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