Improving the effect of NaOCl pretreatment on bonding to caries-affected dentin using self-etch adhesives
Introduction
Carious dentin consists of two distinct layers (caries-infected dentin and caries-affected dentin), which have different morphological and chemical structures. Caries-infected dentin is infected and quite demineralized, and must be removed. On the other hand, caries-affected dentin is partially demineralized and should be preserved in clinical treatment because it is uninfected and remineralizable. Dental adhesives are routinely applied to caries-affected dentin after removal of caries-infected dentin in clinical practice. However, previous studies have reported that bond strengths to caries-affected dentin were significantly lower than those to normal dentin1, 2, 3, 4, 5, 6 and the hybrid layer created with caries-affected dentin was thicker and more porous compared with that with normal dentin.1, 4, 6 It was speculated that this was the result of differences in morphological, biochemical and mechanical characteristics between normal and caries-affected dentin.7
When the tooth structure is cut, a smear layer is created on the surface. Surface preparation methods have been shown to vary the thickness, roughness, density and degree of attachment of the smear layer to the underlying tooth structure.8 Dentin smear layer is composed mostly of submicron particles of mineralized collagen debris.9, 10 It is speculated that there would be differences in the morphological and chemical structures of the smear layer created on normal and caries-affected dentin, because the composition of smear layer is similar to the underlying dentin substrates.11
Self-etch adhesives are widely used for bonding to enamel and dentin, because of the simplified bonding procedures and reduced technique sensitivity.12 Self-etch adhesives are able to demineralize the smear layer and underlying dentin, while simultaneously allowing resin monomers to penetrate into the demineralized zone, which results in the creation of a hybrid layer.12 However, it has been reported that the characteristics of the smear layer variously affect the bond strength of self-etch adhesives to dentin.13, 14, 15, 16 Therefore, the characteristics of smear layer covering the caries-affected dentin might be one reason why self-etch adhesives exhibit reduced adhesion to caries-affected dentin compared with normal dentin.
Sodium hypochlorite (NaOCl) solutions are widely used in the chemomechanical treatment of root canals due to their antibacterial properties and effective dissolution of organic substances from biological materials.17 Some researchers have reported that, when NaOCl was applied to smear layer-covered dentin, the mineral to matrix ratio at dentin surface increased and the smear layer was thinned due to dissolution of the organic phase.18, 19 On the other hand, NaOCl treatment significantly reduces the bond strength to dentin, since remnants of super-oxide radicals generated by NaOCl within the dentin surface inhibit polymerization of resin monomers.20, 21, 22 In addition, this compromised bonding ability can be reversed by the application of an antioxidant/reducing agent prior to the bonding procedures.20, 23, 24 Recently, the product, Accel (Sun Medical, Kyoto, Japan), has been introduced for reducing agent, which contains p-toluenesulfinic acid sodium salt, as a pretreatment before a NaOCl-irrigated root canal is filled with an adhesive root canal sealer (Super Bond Sealer, Sun Medical, Kyoto, Japan). However, there is little published information available on the effect of pretreatment with NaOCl and reducing agent with regards to the adhesion of self-etch adhesives to caries-affected dentin.
The purpose of this study was to evaluate the effect of pretreatment with NaOCl aqueous solution with or without a subsequent application of a reducing agent on the adhesion of one- and two-step self-etch adhesives to normal and caries-affected dentin and to observe the morphological alterations of smear layer-covered normal and caries-affected dentin surfaces after treatment with NaOCl solution. The null hypothesis tested was that NaOCl pretreatment does not affect the bond strength to normal and caries-affected dentin and morphological characteristics of their smear layer-covered surfaces.
Section snippets
Materials and methods
Forty extracted human molars with coronal carious lesions, stored frozen, were used in this study, according to a protocol approved by the Human Research Ethics Committee, Tokyo Medical and Dental University, Japan. The inclusion criteria were that the caries be limited to the occlusal face, that it extends at least half the distance from the enamel–dentin junction to the pulp chamber, and that there is enough surrounding normal dentin to serve as a control bonding site. The occlusal enamel was
Statistical analysis
The microtensile bond strengths (μTBS) obtained in this study were analyzed by three-way ANOVA to test the effect of materials, pretreatment methods and type of dentin (normal dentin or caries-affected dentin). The Bonferroni and t-test were used for post hoc multiple comparisons at α = 0.05.
SEM observation of normal and caries-affected dentin surface after NaOCl pretreatment
Further eight teeth were used for SEM observation. After treatment with or without NaOCl for 15 and 30 s, the dentin surface was rinsed off with water and then dehydrated in ascending concentrations of ethanol in the following steps: 25% ethanol for 20 min, 50% for 20 min, 75% for 20 min, 95% for 30 min and 100% for 60 min. After the final ethanol step, the specimens were immersed in hexamethyldisilazane (HMDS) for 10 min, and dried by placement on filter paper inside a covered glass vial at room
Results
The μTBS bond strengths of protect bond (PB) and bond force (BF) to normal and caries-affected dentin are shown in Table 2. ANOVA analysis revealed that there was no significant difference among the materials (p = 0.32) and there was significant difference among the pretreatment methods (p < 0.001) and type of dentin (p < 0.001) and there was a statistically significant interaction between the type of dentin and the pretreatment method on each adhesive system (PB, p < 0.001: BF, p = 0.002). For both PB
SEM observation
In the case of the control group, the smear layer of normal dentin exhibited a smooth texture with a compact crust of cutting debris (Fig. 1a), while the caries-affected dentin was covered with a thicker and irregular smear layer with sludge-like formation, in which fibril-like structures were observed (Fig. 2a). When NaOCl was applied to the smear layer-covered normal dentin for 15 and 30 s, alteration in the surface morphology and smear layer thickness of normal dentin after 15 and 30 s
Discussion
The smear layer on dentin is composed of disorganized collagen debris binding mineral particles,9, 10 and differs little in composition from the underlying dentin substrate.11 The disorganized collagen within the smear layer is not denatured, however after acidic exposure, it forms a gelatinous matrix around the mineral.10 The disorganized collagen and/or the mineral trapped within the gelatinized collagen cannot be easily removed even when etched with phosphoric acid.28 The disorganized
Conclusions
Within the limitations of this study, the bond strengths of the tested one- and two-step self-etch adhesive systems (Bond Force and Clearfil Protect Bond), were significantly lower to caries-affected dentin than normal dentin, in which there were no significant differences between the adhesive materials. In addition, the smear layer on caries-affected dentin layer exhibited a different morphology, which was thick and irregular with fibril-like structures, compared to the smear layer on normal
Acknowledgement
This work was supported by the grant from the Japanese Ministry of Education, Global Center of Excellence (GCOE) Program, “International Research Center for Molecular Science in Tooth and Bone Diseases”.
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Effect of smear layer deproteinization with enzyme solutions on bonding efficacy of one-step self-etch adhesives
2020, International Journal of Adhesion and AdhesivesCitation Excerpt :Smear layer deproteinizing with HOCl solution can promote the infiltration of the resin monomer of self-etch adhesives into the underlying dentin with elimination of the hybridized smear layer and [7,8] and enhance chemical interaction of acidic functional monomer with hydroxyapatite in dentin [9.10]. Additionally, this smear layer deproteinizing pre-treatment has more positive effects for the bonding of self-etch adhesives to caries-affected dentin than normal dentin because caries-affected dentin produces thicker smear layers with an enriched organic phase than normal dentin [5,6]. Enamel substrate is mainly composed of hydroxyapatite (-85 vol%) and water (-12 vol%), with little organic matrix (-3 vol%).