The role of etching in bonding to enamel: A comparison of self-etching and etch-and-rinse adhesive systems

Abstract

Objective

Etch and resin infiltration morphologies were compared for three self-etch adhesive (SEA) systems and eleven model etch-and-rinse (ERA) systems using various phosphoric acid (PA) concentrations with Adper Single Bond Plus (SB) adhesive. Matches for the morphologies were made between each SEA system and one of the PA/SB systems and bond strength measurements were made for all the systems. The hypothesis was that similar morphology would result in similar bond strength assuming micro-mechanical bonding is the mechanism of adhesion.

Methods

Three specimens were prepared on polished (4000 grit) human enamel for each adhesive system to examine etch and resin infiltration morphology by SEM. For the latter, the adhesive systems were bonded using recommended methods and the enamel was dissolved in acid to reveal the resin. The etch patterns for the SEA systems were determined by rinsing off the material with water and acetone. Polished (4000 grit) human enamel was used with each adhesive system to determine 24-h resin composite to enamel shear bond strengths (SBS). A minimum of 10 specimens were used for each group. Data were analyzed by a one factor ANOVA and Fisher's PLSD post hoc test.

Results

The SBS to polished enamel for two of the three SEA systems were statistically significantly greater (p < 0.05) than the PA/SB matched systems, indicating that chemical bonding might be partly responsible for the measured bond strength. All three SEA systems provided statistically lower (p < 0.05) SBS values than the PA/SB systems with PA concentrations between 2.5% and 40%.

Significance

Although chemical bonding may be present for some SEA systems it does not provide enough increase in bond strength to compete with the bond produced by ERA systems using phosphoric acid etching of enamel.

Introduction

Bonding to enamel by means of phosphoric acid (PA) etching has proven to be a useful and successful procedure in dentistry for several decades. However, in recent years the value of this technique has taken a secondary position in importance to that of dentin bonding with the introduction of self-etching adhesive (SEA) systems, but questions about the effectiveness of enamel bonding with these systems still need to be resolved [1]. SEA systems are not able to etch enamel as effectively as the PA used in etch-and-rinse adhesive (ERA) systems [2], [3] and most published work indicates that SEA systems provide lower composite to enamel bond strengths than ERA systems [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], which may be related to their lower etching capability.

The good results obtained with conventional acid etching with PA are believed by the authors to be attributable to the particular morphology of the interface that is obtained when etching prepared enamel with PA at concentrations in the range of 34–37%. The resin penetration is fairly extensive, the structure is quite three-dimensional (i.e. scalloped) and the transition from resin to sound enamel is distributed over a number of microns. Such an interface may be more resistant to crack propagation than the relatively planar interfaces obtained with SEA systems [2], [3]. This concept is supported by the observation that bond strengths for ERA systems begin to decrease when the PA concentration drops below about 2–5% [14], [15]. In a study by Zidan and Hill [14], when using a 0.5% PA solution for 60 seconds (s), the bond strength was statistically lower than those for 2–35% solutions and the failure mode switched from a mixed form to a single adhesive failure. Saito et al. [15] found statistically lower bond strengths with a PA concentration of 3%, applied for 30 s, compared with PA concentrations of 5–65%, and observed much shallower etching and resin penetration into the enamel surface.

The degree of etching with many SEA systems is minimal, resulting in shallow inter-crystallite infiltration of the resin and a lack of inter-prismatic resin tag formation [3]. As with the PA/SB systems with low concentrations of PA, micro-mechanical bond strengths for the SEA systems might be expected to be less than ERA systems using higher concentrations of PA. However, some acidic monomers used in SEA systems are believed to be capable of chemical bonding [16], [17], [18], [19], [20], [21] in a manner similar to the acidic polymers used in glass-ionomer materials. But, it is not always clear to what extent chemical bonding contributes to overall bond strength in comparison with potential micro-mechanical bonding, when acidic materials are placed on mineralized tissue. With glass-ionomer materials it was observed that chemical contributions may be overwhelmed by micro-mechanical bonding if acidic pre-treatments of the enamel are used [22].

In consideration of the above information, the present study was undertaken to examine the role that the degree of etching and attendant micro-mechanical bonding plays in determining the bond strength of resin composite to enamel for three SEA systems. This was done through comparison of the etching and resin infiltration morphologies of these SEA systems with those of a series of ERA systems where the PA concentration varied from 40% to 0.025%. The study model assumed that all systems bond by micro-mechanical means and the hypothesis proposed is that systems producing similar etch and resin infiltration morphology on enamel will have comparable bond strength values (α = 0.05).