Influence of glutaraldehyde priming on bond strength of an experimental adhesive system applied to wet and dry dentine

Abstract

Objectives

This study tested the following null hypotheses: (1) there is no difference in resin–dentine bond strength when an experimental glutaraldehyde primer solution is added prior to bonding procedures and (2) there is no difference in resin–dentine bond strength when experimental glutaraldehyde/adhesive system is applied under dry or wet demineralized dentine conditions.

Methods

Extracted human maxillary third molars were selected. Flat, mid-coronal dentine was exposed for bonding and four groups were formed. Two groups were designated for the dry and two for the wet dentine technique: DRY: (1) Group GD: acid etching + glutaraldehyde primer (primer A) + HEMA/ethanol primer (primer B)-under dried dentine + unfilled resin; (2) Group D: the same as GD, except for primer A application; WET: (3) Group GW: the same as GD, but primer B was applied under wet dentine condition; (4) Group W: the same as GW, except for primer A application. The bonding resin was light-cured and a resin core was built up on the adhesive layer. Teeth were then prepared for microtensile bond testing to evaluate bond strength. The data obtained were submitted to ANOVA and Tukey's test (α = 0.05).

Results

Glutaraldehyde primer application significantly improved resin–dentine bond strength. No significant difference was observed when the same experimental adhesive system was applied under either dry or wet dentine conditions. These results allow the first null hypothesis to be rejected and the second to be accepted.

Conclusion

Glutaraldehyde may affect demineralized dentine properties leading to improved resin bonding to wet and dry substrates.

Introduction

The concept of adhesion to dentine of most of the current bonding agents relies on micromechanical retention, formation of a hybrid layer¹ and use of the wet bonding technique.² In this approach, water is a compulsory component during pre-bonding procedures since it maintains the demineralized collagen network in an expanded state, due to its higher solvation capacity.3, 4 Although this situation is important to further resin penetration and collagen envelopment, ideally water should be removed from the surface of demineralized dentine during bonding, in order to prevent several irreversible drawbacks.

Residual water can hamper hydrophobic monomer interdiffusion and might combine with hydrophilic monomers.5, 6 Consequently, the formation of a resin-deficient, underlying zone of unprotected collagen fibrils at the bottom of the hybrid layer is likely to occur. This situation might lead to premature hydrolytic/enzymatic degradation of collagen and resin, due to oral/dentinal fluid penetration7, 8 into the nano-sized spaces of this layer (nanoleakage).9, 10

Air-drying the dentine collagen network, results in its shrinkage that may reach values up to 65% by volume.³ As bound water is removed, dehydrated collagen fibrils establish a variety of weak chemical bonds with each other, leading to the state of collapse of the demineralized matrix. This process is concomitant with a gradual increase in collagen rigidity.¹¹ However, if the matrix is re-wet with water, collagen interfibrillar bonds are broken and the matrix is able to re-expand to its original volume.³ Water presents a high modulus of Hansen's solubility parameter for H-bonding forces (δh = 37.3 (J/cm³)^1/2), which denotes its strong ability to establish hydrogen bonds among collagen molecules.12, 13

In order to substitute water as a promoter/stabilizer of demineralized dentine matrix expansion, it is desirable that such a substance presents a Hansen's solubility parameter for H-bonding (δh) higher than 18.2, the δh of dried collagen.¹⁴ The so-called ‘water-free’ or ‘non-aqueous’ solvents have been studied in this connection.12, 13, 14 In addition, Carvalho et al.¹³ suggested that maintaining collagen matrix expansion during bonding (i.e. during resin infiltration) is as important as it is in the pre-bonding procedures. That is, as solvents are evaporated from the dentine surface, the collagen network must preserve its original structure to allow resin infiltration.

Glutaraldehyde, a substance that has been used in the adhesive dentistry field, appears to be a potential element to improve demineralized dentine properties. Its capacity to fix proteins irreversibly¹⁵ and to increase the modulus of elasticity of collagen fibrils12, 16 is of great interest to maintain the collagen structure in position during bonding. In addition, its well-reported ability to react chemically with collagen and resin components, such as HEMA,17, 18 may also contribute to facilitating adhesive system penetration into and wettability of the dentine substrate.

The purpose of this study was to investigate the contribution of glutaraldehyde priming of demineralized dentine on the bond strengths of experimental adhesive systems, under dry and wet substrate conditions. The following null hypotheses were tested: (1) there is no difference in resin–dentine bond strength when an experimental glutaraldehyde primer solution is added prior to bonding procedures; (2) there is no difference in resin–dentine bond strength when experimental glutaraldehyde/adhesive system is applied under dry or wet demineralized dentine conditions. The rationale is that glutaraldehyde-fixed collagen network might act as a substrate more receptive to water-free primers under the dry condition.