Improving the effect of NaOCl pretreatment on bonding to caries-affected dentin using self-etch adhesives

Abstract

Objective

To evaluate the effect of sodium hypochlorite pretreatment on adhesion to normal and caries-affected dentin using self-etch adhesives.

Methods

Forty extracted human molars with coronal carious lesions were used in this experiment. The occlusal dentin surfaces including the caries-affected dentin in each group were treated as follows: group 1, rinsed with water; group 2, treated with 6% NaOCl for 15 s; group 3, treated with 6% NaOCl for 30 s; group 4, application with Accel for 30 s after NaOCl-30 s pretreatment. After rinsing with water and air-drying, the treated dentin surfaces were applied with self-etch systems (Bond Force and Clearfil Protect Bond) according to the manufacturers’ instructions, and built-up with resin composite. After 37 °C water storage for 24 h, the bonded normal or caries-affected dentin areas were isolated to create an hourglass configuration with a cross-sectional area of approximately 1 mm². The specimens were subjected to tensile stress at a cross-head speed of 1.0 mm/min.

Results

NaOCl-15 s pretreatment significantly improved the μTBS of both self-etch adhesives to caries-affected dentin, while the 30 s pretreatment did not affect them. For normal dentin, NaOCl-30 s pretreatment significantly reduced the μTBS of both self-etch adhesives although the 15 s pretreatment did not alter them. Furthermore, the application of Accel with a reducing effect increased the μTBS to normal and caries-affected dentin treated with NaOCl for 30 s.

Conclusions

The effects of NaOCl pretreatment on bonding of both self-etch adhesives were dependent upon type of dentin (normal and caries-affected dentin) and the treatment time.

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.