The role of etching in bonding to enamel: A comparison of self-etching and etch-and-rinse adhesive systems
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).
Section snippets
Materials and methods
Three SEA systems were examined for bonding resin composite to human enamel: Adper Prompt L-Pop (3M ESPE, St. Paul, MN, USA)-(PLP), Clearfil SE Bond (Kuraray Medical Inc., Okayama, Japan)-(CSE) and Clearfil S3 (Kuraray Medical Inc., Okayama, Japan)-(CS3). These were compared with a series of ERA systems, comprised of Adper Single Bond Plus adhesive (3M ESPE, St. Paul, MN, USA)-(SB) with PA solutions from 40 wt% to 0.025 wt% [PA(x%)/SB]. These materials are described in Table 1. In addition three
Results
The ANOVA demonstrated significant differences in mean bond strength values among adhesive systems (p = 0.0001). Results for all bond strength measurements are shown in Fig. 1 and Table 3, where statistical differences between groups are indicated. Fig. 1 shows the curve for bond strength as a function of the PA concentration used in the ERA systems, where three distinct regions are identified (A–C). In region A, the bond strengths are statistically equivalent (p < 0.05) for PA concentration of
Discussion
This study examined the role that etching of enamel and resin impregnation into the etched enamel played in providing micro-mechanical bonding for the three SEA adhesive systems examined. The study design utilized ERA model systems, having varied PA concentrations for etching the enamel coupled with Single Bond Plus adhesive. To cover the range of etching expected for the SEA systems, the range of PA concentrations was 40.0–0.025 wt%. Results for the bond strength measurements using these PA/SB
Conclusions
In conclusion, it was found that the two SEA systems, CSE and CS3, produce bond strengths to enamel that are greater than might be predicted by micro-mechanical bonding alone. This may be a confirmation of some degree of chemical bonding, as has been proposed for these materials. Even so, the bond strength provided is insufficient to match that of ERA systems using PA etching.
Acknowledgment
This study was supported in part by Bisco, Inc.
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