Investigation Of Mechanism And Affecting Factors For Bond Strength Between Resin And Dentin

Adhesion to dental hard tissues is included to bonding to both enamel anddentin .Enamel is composed of 96% hydroxyapatite. Following acid etchingenamel, strong micromachanical bond can be achieved between adhesive resinand enamel. The dentin substrate is physiologically and morphologicallydifferent from enamel. Dentin contains 60% organic components and water,40% mineral components, and fluid liquid in dentin tubule. All these factorsmay compromise bond strength between resin adhesive and dentin, result inrestoration falling off and failure of therapy in clinical practice.The properties of resin-dentin interface are affected by many factors, suchas the structure and surface properties of dentin, treatment methods, choices ofadhesive, and environment of oral. This research is focus on four fields relatedclosely with clinical situation, and some uncertain and controversial argumentsalso exist in these fields. These investigations are: FT-Raman spectroscopicstudy of interaction between adhesive and dentin;effects of thermal cycles anddifferent locations on shear bonding strength;under the same treatment withdentin investigation the difference of microstructure and shear bondingstrength between sound dentin and dental fluorosis dentin using confocal laserscanning microscope. The purpose of this investigation is further to exploremechanism and affecting factors for interaction between resin and dentin.Interaction mechanism between resin and dentin is generally explained bymechanical theory or mechanical-chemical theory. The former consider thatbonding force comes from mechanical retention of resin with dentin. Themechanical-chemical theory thinks except the existence of mechanicalretention, chemical interaction between resin and dentin may also occur.Five adhesives in two bonding systems are applied on the dental surface.The disks absorbed adhesive are washed with solvent to remove extra adhesivefrom the surface. As a surface scattering technique, FT-Raman could detectsurface situation of samples without modification, so FT-Raman spectra wererecorded at each step to study the interaction between resin and dentin. It wasobserved that carbonyl stretching mode of HEMA was moved to 24cm-1 lower,considering effects of polymerization, actual shift of carbonyl was 26-34cm-1,which indicates that hydrogen bonding exist between adhesive and dentin.This conclusion supports that except the existence of mechanical retention,chemical interaction between resin and dentin may also occur.Temperature in oral often changes. Because of the different thermalexpansion coefficients between dental bonding agents and dental hard tissues,change of temperature will result in stress on the interface of resin and dentin.Thermal cycling machine is used to stimulate oral temperature change in vitro.In this paper, times of cycle is set no more than 1000. Two adhesives areapplied. Following shear bond test, the fracture surface of specimen is viewedusing scanning electron microscope to investigate the affecting factors ofthermal cycles on shear bond strength in short term . Applying Single Bond orAdper Prompt, no statistically significant differences were found in shearbonding strength based on different thermal cycles. Shear bond strength ofeach thermal treatment is significantly lower in Adper Prompt groups than inSingle Bond groups. Scanning electron microscope revealed that the quantitiesof resin tags are less in Adper Prompt groups than these in Single Bond groups.Gaps can be found between dentin tubule and resin tags in Adper Promptgroups. Resin tags are located in superfacial region of composite resin.Occlusal surface is the highest incidence of restoration and the less isproximal surface. And the restoration durability on occlusal surface aredifferent from restoration on proximal surface. Occlusal force imposingthroughout dentin is changed with the location of dentin. Structuralcomponents of dentin are not homology in different locations. The structure ofdentin is tubule, intertubular dentin, peritubular dentin. Fluid liquid within thetubule and odontoblastic are structural components of dentin tubule .Thesestructure of dentin directly affect bond strength. Studies confirmed that dentinwith tubules running parallel to dental surface has more amount of peritubulardentin than that in dentin with tubules running perpendicular, which will resultin a lesser area of intertubular dentin to form hybrid layer and a lower bondingstrength. To evaluate bonding strength of occlusal and proximal surface in thesame tooth, shear bond test is carried out. Morphology of resin-dentininterface was observed with scanning electron microscope. Shear bondstrength are no statistically significant difference between occlusal surface andproximal surface. Thickness of hybrid layer and resin tags in two locations ofdentin is homology. No statistically significant differences were observedbetween applying Primer & Bond and applying Xenon â…¢ . Significantdifference of hybrid layer and morphology of resin tags was observed betweenPrimer & Bond and Xenonâ…¢. Thickness of hybrid layer was thinner andlength of resin tags was shorter in Primer & Bond. The results shown that thethickness of hybrid layer and morphology of resin tags are not basic factorsaffect bond strength, may be other mechanism. There are few reports on theinfluence of different location. This research compared bonding strength onocclusal surface with that on proximal surface while the microstructure onboth surfaces was observed by scanning electron microscope. Experimentalresults shown that there was no significant difference of bonding strengths onocclusal and proximal surface even some difference in hybrid layers and tagswere observed by scanning electron microscope on both surface. Thisexperimental result is consisted with conclusion that emphasizes that qualityof hybrid layer is a key factor for bonding strength.Researches so far have been carried out on sound dentin. However, fewstudies have been conducted to focus on dental fluorosis to gain a betterunderstanding of bonding to dental fluorosis dentin. Chronic fluoride ingestionon formation and mineralization stage, of which tooth formation andmineralization results in dysphasic enamel and abnormal mineralization.Lesion involves in both enamel and dentin with hypo calcified and hyperactivecalcified. Studies for aesthetic restoration to dental fluorosis were carried outon bonding to enamel instead of dentin. Micro tensile test and shear bondingtest was carried out on moderate dental fluorosis dentin, which is classified byDean's Index of Fluorosis, from same region with normal dentin as control.Two different adhesives were used. Microstructure of dental fluorosis wasobserved by confocal laser scanning microscope. And applying Single Bond2or Xenoâ…¢, experimental results shown that shear bond and microtensilestrength of dental fluorosis dentin were significantly lower than these ofnormal tooth. No statistically significant differences were found in eachadhesive group which bond to normal dentin or dental fluorosis dentin.The effects of thermal cycles and different locations of dentin on shearbonding strength were investigate, then scanning electron microscope is usedto analyze the affecting factors in this study. FT-Raman was firstly introducedto study mechanism of five adhesive systems bonding to dentin. Thenmicrostructure and shear bonding strength of dental fluorosis dentin werestudied. All these results could be a theory reference in clinical performance.