| The emergence of dentin bonding technology and its development allow the dream of directly gluing restorative materials and tooth structure together to become a reality. It has laid a solid foundation for cosmetic dentistry. The bonding system has been developed from the first generation to the seventh generation. The concepts of dentin bonding has gone from dry adhesive to wet bonding, and accordingly the adhesive has gone from hydrophobic adhesive to hydrophilic adhesive. With the deepening of the study, the researchers have found that at present the immediate bonding effects of each adhesive system are satisfied. However, the long-term durability is poor, which has become a major challenge to the development of dentin bonding technology.The gold standard to evaluate the long-term durability of dentin bonding is the randomized clinical trial (RCT), which is time-consuming and laborious. For this reason, the artificial aging method has been established as the most popular model to evaluate the durability of adhesive in adhesive research. The main purpose of artificial aging is to simulate the usage of restoration in the patient’s mouth. Although it can not completely simulate the in vivo situation, it plays important roles in predicting the long-term effects of dentin bonding restoration. The quality of bonding interface directly determines the durability of adhesive restoration. At present, there are many methods to evaluate the aging of bonding interface and each method has its own advantages. They can simulate many factors that prompt the aging of bonding restoration to some extent. However, the oral environment is so complex that none of them can fully simulate it. In addition, different aging methods may have various effects on the adhesive systems through different mechanisms. Until now, there is no systematic evaluation of the effects of various aging methods on the bonding interface. In order to understand the effects of various aging methods on the bonding interface and the differences between them, five kinds of commonly used aging methods have been selected in this study:water aging, thermal cycling aging, enzyme aging, NaOCl aging, and PH cycling aging. Through comparative study the effects of various aging methods on the micro tensile strength, nano-Ieakage and section analysis of bonding interface, we attempted to reveal the characteristics of different aging methods and provide the theoretical and experimental bases for their applications. Our study is divided into two parts:the first part is to determine the effects of five aging methods on dentin bonding durability; the second part is to evaluate the anti-aging abilities of chlorhexidine (CHX) in different bonding system by using PCR thermal cycling method, combined with the detection of enzymes to further explore the mechanism of its anti-aging mechanisms.PART ONE Effects of different aging method on dentin bonding durabilityExp.1Effects of different aging methods on microtensile bond strength of dentin bondingMaterials and methods:Seventy freshly extracted intact human third molars were ground and bonded with an etch-and-rinse adhesive Adper TM Single Bond2and a self-etching adhesive G-Bond. Specimens were then stored in37℃water for24hours after resin restoration and sectioned into beams by a slow saw. Specimens in each group were randomly divided into7groups:the control group (Control), water aging (H2O), thermal cycling aging (TC), NaOCl aging (NaOCl), pH cycle aging (pH), Clostridium histolyticum collagenase aging (COL), and cholesterol esterase (EST) aging. The microtensile bond strength test was perfomed.Result:Two-factor variance analysis showed that, compared with the control group, the μTBS in water aging (H2O) group showd no significant difference (P>0.05), whilst the μTBS in other five groups (TC, pH, NaOCl, COL, EST) were significantly decreased (P<0.05). Additionally, the μTBS showed no significant difference among the five aging groups (P>0.05).Exp.2Effects of different aging methods on failure modes of dentin bonding interfaceMaterials and methods:Seventy freshly extracted intact human third molars were ground and bonded with an etch-and-rinse adhesive Adper TM Single Bond2and a self-etching adhesive G-Bond. Specimens were then stored in37℃water for24hours after resin restoration and sectioned into beams by a slow saw. Specimens in each group were randomly divided into7groups:the control group (Control), water aging (H2O), thermal cycling aging (TC), NaOCl aging (NaOCl), pH cycle aging (pH), Clostridium histolyticum collagenase aging (COL), and cholesterol esterase (EST) aging. The micro-tensile bond strength test was perfomed. After the tests, light microscope and field emission scanning electron microscopy (FESEM) were used to determine the failure modes of dentin bonding interface.Result:The failure modes of dentin bonding interface are mainly composed of adhesive failure and mixed failure through stereoscopic light microscope observation. However, after further analysed the representative failed teams, there were signicant differences among different bonding systems and different aging methods. Most failures were found on the top of hybrid layer treated with water aging, which was similar to the control group. After thermocycling aging, failures were mainly located at the bottom of the hybrid layer bonded with Single-Bond, whilst failures were mainly located in the adhesive layer bonded with G-Bond. The failures located in dentin appeared in NaOCl aging group in both two kinds of adhesive system. In pH cycling group, mixed failures were significantly increased. Two kinds of bonding systems performed similarly in enzymes aging group. Failures were mainly located at the bottom of the hybrid layer in the clostridium histolyticum collagenase group, whilst in cholesterol esterase group failures were mainly located in the hybrid layer.Exp.3Effects of different aging methods on nanoleakage of dentin bonding interfaceMaterials and methods:Seventy freshly extracted intact human third molars were ground and bonded with an etch-and-rinse adhesive Adper TM Single Bond2and a self-etching adhesive G-Bond. Specimens were then stored in37℃water for24hours after resin restoration and sectioned into beams by a slow saw. Specimens in each group were randomly divided into7groups:the control group (Control), water aging (H2O), thermal cycling aging (TC), NaOCl aging (NaOCl), pH cycle aging (pH), Clostridium histolyticum collagenase aging (COL), and cholesterol esterase (EST) aging. After the tests, interfacial nanoleakage was assessed by scanning electron microscopy.Result:Scanning electron microscope results showed that, compared with the control group, except for the water aging groups, the remaining five aging methods can significantly increase the nanoleakage, in which, NaOCl aging group and pH cycling aging group is the most remarkable, with silver particles throughout the bonding interface.PART TWO Effects of chlorhexidine on dentin bonding durability of different adhesive systemsExp.4Effects of chlorhexidine on dentin bonding durability of etch-and-rinse adhesiveMaterials and methods:Ten freshly extracted intact human third molars were ground and bonded with an etch-and-rinse adhesive (Single-Bond). Specimens were either left untouched or placed in PCR tubes filled with three thermocycling mediums: water, chlorhexidine or silicone oil. Thermocycling (5000cycles) was done using the PCR programme at temperatures of5and55℃. The microtensile bond strength (μTBS) was evaluated and interfacial nanoleakage was assessed by scanning electron microscopy before and after thermocycling.Results:Significant differences were detected among groups kept in different mediums after thermocycling. For Single-Bond, both the chlorhexidine and silicone oil groups could preserve the μTBS (P<0.001). No obvious increase in silver deposition was observed in specimens incubated in water after thermocycling, less silver penetration was found in specimens incubated in chlorhexidine.Exp.5Effects of chlorhexidine on dentin bonding durability of self-etching adhesiveMaterials and methods:Ten freshly extracted intact human third molars were ground and bonded with a self-etch adhesive (G-Bond). Specimens were either left untouched or placed in PCR tubes filled with three thermocycling mediums:water, chlorhexidine or silicone oil. Thermocycling (5000cycles) was done using the PCR programme at temperatures of5and55℃. The microtensile bond strength (μTBS) was evaluated and interfacial nanoleakage was assessed by scanning electron microscopy before and after thermocycling. Results:Significant differences were detected among groups kept in different mediums after thermocycling. For G-Bond, μTBS of the chlorhexidine and water groups were significantly decreased (P<0.05). No obvious increase in silver deposition was observed in specimens incubated in water after thermocycling, less silver penetration was found in specimens incubated in chlorhexidine.Exp.6:The inhibition of CHX on the dentin collagen matrix degradationMethods and materials:Thirty caries-free sound extracted teeth were collected. They were sectioned into1mm thick sheet according to their longitudinal axis by slow cutting machine with water cooling. Pulp tissue was scraped completely. The enamel was precisely removed along the enamel dentinal junction by using a diamond bur and the dentin slices immersed in liquid nitrogen were grinded into powder. The dentin powder was divided into six groups: Group1, blank (mineralised dentine powder); Group2, treated with1%phosphoric acid for1min; Group3, treated with1%phosphoric acid for1min+AdperTM Single Bond2; Group4, treated with1%phosphoric acid for1min+0.2%CHX+AdperTM Single Bond2; Group5, treated with G-bond; Group6, treated with0.2%CHX+G-bond. The presence of MMP-8in six groups was assayed using a colorimetric assay system (QuantisirTM, Epigentek, USA) that allows direct measuring of MMP-8levels. Results: Single factor analysis of variance showed significant differences amongst all groups (P<0.05). When compared with the blank group, MMP-8was significantly decreased after treated with1%phosphoric acid for1min (Group2, P<0.05). The application of AdperTM Single Bond2and G-Bond on acid-etched dentine (Group3and Group5) significantly increased MMP-8(P<0.05), whilst0.2%CHX treatment after the application of AdperTM Single Bond2and G-Bond (Group4and Group6), significantly decreased their levels (Group3and Group5, P<0.05). The Tukey’s post hoc tests showed there was no significant difference between different adhesives pre-treatmented or not with CHX (P>0.05). Conclusions:1. Compared with the control group, except the water aging method, all other aging methods can significantly reduce the microtensile bond strength. The most reduction of the microtensile bond strength is observed in the NaOCl and pH cycling aging groups.2. Section analysis and nanoleakage assays reveal that types of bonding interface degradation are different in each aging group.3. Chlorhexidine can effectively improve the dentin bonding durability of etch-and-rinse adhesive Single-bond, but not of self-etch adhesive G-Bond. Additionally, chlorhexidine can significantly inhibit the MMP8activity. |
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