| Objectives:This study aims to evaluate the immediate and long-term bonding strength of universal adhesive to dentin and the microstructure of dentin-resin interface under the experimental conditions of simulated pulpal pressure in vitro and Nd:YAG laser irradiation pretreatment with different energy density parameters.This is to explore the feasibility of clinical application of Nd:YAG laser irradiation pretreatment to improve the dentin bonding effect.Methods:Select 70 freshly extracted molars without caries(patients are 20-40 years old).Expose the dentin of the occlusal surface and pulp cavity perpendicular to the long axis of the tooth and control the thickness of the remaining dentin to be between 1.5mm and 2.5mm.Then use sandpaper grind the surface of the dentin to form a smear layer.The molar samples were randomly divided into 5 groups according to the Nd:YAG laser pretreatment energy density parameters(Group 1:no laser treatment,control group;Group 2:82.94J/cm2;Group 3:103.67J/cm2;Group 4:124.40J/cm2;Group 5:155.50J/cm2).The molar samples were cemented on the simulated pulpal pressure model,and the natural pulpal pressure of human teeth of 1.47kPa was simulated with 15cmH2O.Use the Nd:YAG laser with the corresponding parameters of each group to pretreat the dentin bonding surface,and then use the self-etching light-curing mode of 3M Single Bond Universal(SBU)adhesive,and FiltekTM Z350XT composite resin was built up to 4mm layer by layer to prepare dentin-resin bonding specimens.According to whether the samples were aged or not,the samples in each group were randomly divided into immediate subgroup and aging subgroup.The samples in immediate subgroup were immersed in deionized water at 37℃ for 24h.The samples of the aging subgroup were immersed in deionized water at room temperature for 30 days,and then cooled and heated in deionized water at 5℃-55℃for 5000 times.After the treatment,all samples were tested for micro-tensile bonding strength,observed fracture mode and observed by scanning electron microscopy(SEM)of the bonding interface.SPSS software was used for statistical analysis of data.Results:1.Scanning electron microscope observation showed that there was a smear layer on the dentin bonding surface without laser treatment,and the dentin tubule was blocked.After laser irradiation,the smear layer was removed and the dentin tubule was opened.In the control group,the hybrid layer is thinner,the resin tags were less and shorter,and the bonding interface is flatter.After laser treatment,the hybrid layer was slightly thicker,and the resin tags into the dentin tubule was more and longer.After aging,cracks can be seen in the bonding interface.2.In the immediate and after aging,different energy densities of Nd:YAG laser pretreatment have significant differences in the bonding strength of dentin.Under the immediate condition,103.67J/cm2 Nd:YAG laser treatment had the highest micro-tensile bonding strength,while under the aging condition,124.40J/cm2 Nd:YAG laser treatment had the highest micro-tensile bonding strength,and there was no significant statistical difference between the 103.67J/cm2,124.40J/cm2 and 155.50J/cm 2 groups.The micro-tensile bonding strength of the control group was the lowest before and after aging.Except for the 155.50J/cm2 Nd:YAG laser treatment group,the micro-tensile bonding strength of all aging groups was significantly lower than that of the immediate group with the same energy density(P<0.05).3.The main fracture mode before and after aging is interfacial fracture,but the relative proportion of interfacial fracture increases after aging.The interface fracture ratio of 103.67J/cm2 Nd:YAG laser treatment group was the lowest under immediate condition,the interface fracture ratio of 124.40J/cm2 Nd:YAG laser treatment group was the lowest under aging condition,and the interface fracture ratio of control group was the highest.Conclusions:1.Under the conditions of simulated pulpal pressure,Nd:YAG laser melting and recrystallization,can remove smear layer on the bonding interface,make the most of the dentin tubules show the open state,which is beneficial to the penetration of the adhesive into the dentin tubules,increase the number and length of the resin tags.And Nd:YAG laser can make the dentin surface more rough,these conditions are conducive to form good mechanical retainer.2.Under the conditions of simulated pulpal pressure,Nd:YAG laser irradiation pretreatment on dentin surface can improve the bonding strength of dentin,and Nd:YAG laser treatments with different energy densities have different effects.Among the energy density parameters of Nd:YAG laser selected in this experiment,the energy density value of 103.67J/cm2 is the best parameter for improving the dentin bonding immediately and after aging. |
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