The effect of thermocycling regimens on the nanoleakage of dentin bonding systems
Introduction
With the increasing number of dentin bonding systems on the market, critical evaluation of their performance is necessary. Whereas a clinical trial is the best way to provide valuable information about the effectiveness of different dentin bonding systems [1], it is time-consuming and expensive. Instead, laboratory tests are often carried out to evaluate the efficacy of dentin bonding systems and predict their clinical performance. A micro-leakage test of the tooth/restoration interface is among the most frequently used laboratory tests to indicate the sealing ability of dentin bonding systems. This is considered to be a major factor influencing the longevity of dental restorations [2]. The consequences of micro-leakage include post-operative sensitivity, marginal staining and secondary caries [3]. Although the clinical relevance of the leakage tests has not yet been clarified [4], and laboratory results do not always correlate precisely with the clinical situation, a micro-leakage test is a useful method in the investigation of dentin adhesives [5].
Poor adhesion to tooth structure and the polymerization contraction of restorative materials are the main causes of micro-leakage, which can be greater if there is a mismatch in coefficients of thermal expansion of teeth and restorative material [6]. Many leakage tests have used thermal stress to simulate the clinical situation. The results of the effects of thermocycling on micro-leakage are variable, because the methods employed, the thermocycling regimens adopted, and the restorative materials tested were quite different [6], [7], [8], [9].
The use of dentin bonding systems with their micro-mechanical adhesion to tooth structure has greatly reduced micro-leakage [7], [10], [11]. However, the ideal situation where the adhesive resin completely penetrates the demineralized dentin is rarely achieved [12], leaving some porous regions within the exposed collagen fibers. These porosities can be penetrated by solutions such as silver nitrate, even in gap-free restorations, and this has been termed ‘nanoleakage’ [13], [14]. This phenomenon may cause the breakdown of the bond and the subsequent failure of restorations. The effects of thermal stress have often shown increased micro-leakage [2], [9], but in the case of nanoleakage, there is a lack of information. The technique employed for nanoleakage measurement has used polished specimens [13], [14]; however, there is some controversy with this method, since it is possible that sectioning and polishing of the specimens may cause a smearing of silver across the specimen surfaces. The results would then show a greater deposition of silver than is actually occurring, thus giving misleading results.
The purpose of the study was: (1) to compare nanoleakage associated with polished specimens with that of fractured specimens based on the hypothesis that polished specimens may show more nanoleakage than fractured specimens; (2) to investigate the effect of different thermocycling regimens on nanoleakage of PermaQuick, a three-step dentin bonding agent, and Prime and Bond NT (PBNT), a one-step dentin bonding agent, based on the hypothesis that different thermocycling regimens with different dentin bonding systems may result in different nanoleakage; (3) to compare the nanoleakage of PBNT after treatment with either non-rinse conditioner (NRC) or 34% phosphoric acid.
Section snippets
Materials and methods
Forty-eight freshly extracted human third molars were stored for a maximum of one month at 4 °C in physiological saline containing 0.1% thymol. Three combinations of etchant/dentin bonding agents were selected for evaluation (Table 1).
Comparison of fractured and polished specimens
Silver penetration was found along all the dentin/adhesive interfaces. The fractured specimens showed similar leakage patterns to those of polished samples for each dentin bonding group (Fig. 1, Fig. 2, Fig. 3).
Thermocycled specimens
The thermocycled specimens and controls had similar leakage patterns as polished flat surface specimens in each identical dentin bonding group. The leakage patterns of representative resin dentin interfaces are illustrated in the scanning electron micrographs shown in Fig. 1, Fig. 2,
Discussion
When specimens are cut and polished in silver penetration studies, the question arises regarding whether the cross-sectioning and polishing procedures can result in silver particles being smeared across the specimen surface and filling porosities. Such smearing may introduce an apparently greater deposition of silver than actually occurred. This study compared polished and fractured specimens, the latter to avoid any smearing effects. The SEM observation of both types of specimens demonstrated
Acknowledgements
The research was supported by the Australian Dental Research Foundation Inc., St Leonards, NSW 2065, Australia. The assistance of Jocelyn L. Carpenter, School of Botany, The University of Melbourne, with the FE-SEM imaging is appreciated.
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