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RESEARCH REPORT |
1 Division for General Dentistry, Center for Dental Clinics, Hokkaido University Hospital, Kita 13 Nishi 7, Kita-ku, Sapporo 060-8586, Japan;
2 Leuven BIOMAT Research Cluster, Department of Conservative Dentistry, School of Dentistry, Oral Pathology and Maxillo-facial Surgery, Catholic University of Leuven, Kapucijnenvoer 7, B-3000 Leuven, Belgium;
3 Department of Conservative Dentistry, Hokkaido University Graduate School of Dental Medicine, Kita 13 Nishi 7, Kita-ku, Sapporo 060-8586, Japan;
4 Department of Biomaterials, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Science, and Research Center for Biomedical Engineering, Okayama University, 2-5-1 Shikata-cho, Okayama 700-8525, Japan; and
5 Department of Biomaterials Science, Hiroshima University Graduate School of Dentistry, 1-2-3 Kasumi, Minami-ku, Hiroshima 734-8553, Japan;
* corresponding author, bart.vanmeerbeek{at}med.kuleuven.ac.be
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: hydrolytic stability • self-etch adhesive • durability • functional monomer • dentin
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Besides micro-mechanical interlocking through hybridization, the potential benefit of additional chemical interaction between the functional monomer and residual hydroxyapatite has regained attention (Yoshida et al., 2004). Specific functional monomers as part of ‘mild’ two-step self-etch adhesives were shown to interact chemically, within a clinically reasonable time, with hydroxyapatite that remains available within the submicron hybrid layer. The specific molecular nature of the functional monomer and the subsequent dissolution rate of its calcium salt have been shown to determine actual chemical bonding efficacy and stability. From the 3 functional monomers investigated, 10-methacryloxydecyl dihydrogen phosphate (10-MDP) appeared not only to interact most intensively with hydroxyapatite, but also to have the most hydrolytically stable bond with calcium, as compared with 4-methacryloxyethyl trimellitic acid (4-MET) and 2-methacryloxyethyl phenyl hydrogen phosphate (phenyl-P). We hypothesize that improved monomer-tooth substrate interaction enhances the degradation resistance of the adhesive-dentin bond and thus extends the bond longevity. We therefore determined the hydrolytic stability of 3 self-etch adhesives that each contains one of the 3 functional monomers, through measurement of their micro-tensile bond strength (µTBS) to dentin, and characterization of the aged adhesive-dentin interface by transmission electron microscopy (TEM), both after long-term thermocycling.
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Fifty-four extracted human third molars (gathered following consent approved by the Commission for Medical Ethics of Hokkaido University), stored at 4°C in an aqueous solution of 0.5% chloramine, were used. The occlusal enamel was removed by means of an Isomet low-speed diamond saw (Buehler, Lake Bluff, IL, USA), perpendicular to the long axis of the tooth, to expose a flat mid-coronal dentin surface. A standard smear layer was manually produced by wet-sanding the dentin surface with 600-grit silicon carbide sandpaper for 60 sec. The dentin surface was thoroughly washed with water, and immediately dried with moisture-free air. The 3 adhesives were then applied strictly according to the manufacturer’s instructions, after which the bonded surfaces were incrementally built up with composite to a height of 5–6 mm, with Z-100 (3M, St. Paul, MN, USA).
Thermocycling and Micro-tensile Bond Strength Testing
After storage overnight at 37°C, the specimens were sectioned into 3–6 slabs, each approximately 0.7 mm thick, with the diamond saw used with water-cooling, and further trimmed into an hourglass shape by means of a high-speed super-fine diamond bur, ensuring that the narrowest portion was located at the bonding interface (interface area of approximately 1 mm2). The specimens were left untouched (control) or were thermocycled (60 sec of immersion, alternatively, in a 5 and 55°C water bath) during 10,000, 20,000, 30,000, 50,000, or 100,000 cycles (3 teeth each adhesive, per thermocycling session).
The control and thermocycled specimens were then pulled apart, following a µTBS protocol, with the use of a desktop material tester (EZ-test, Shimadzu, Kyoto, Japan) with a cross-head speed of 1 mm/min. The µTBS was expressed in MPa, as derived from dividing the imposed force (in N) at the time of fracture by the bond area (in mm2). One-way ANOVA and the Tukey-Kramer test were used to analyze the µTBS data statistically at a significance level of 
= 0.05.
TEM
Two specimens per adhesive, of which one was not (control) and the other was subjected to 100,000 thermocycles, were processed for TEM analysis of the adhesive-dentin interface, according to established specimen preparation procedures, including fixation, dehydration, and embedding in epoxy resin (Van Meerbeek et al., 1998). Ultrathin sections (70–90 nm) were cut by means of a diamond knife (Diatome, Bienne, Switzerland) in an ultra-microtome (Ultracut UCT, Leica Microsystems, Vienna, Austria). Unstained as well as stained (5% uranyl acetate [UA] for 20 min and saturated lead citrate [LC] for 3 min) non-demineralized sections were observed with a transmission electron microscope (H-800, Hitachi Ltd., Tokyo, Japan) operating at 75 kV.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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). The 4-MET-based adhesive (UFB) revealed a µTBS statistically similar to that of the control (37.9 ± 5.9 MPa), up to 50,000 cycles, but after 100,000 cycles, its µTBS was significantly lower (22.5 ± 7.7 MPa, or a reduction of 41%). For the phenyl-P-based adhesive (LBII), the µTBS was already, after 30,000 cycles, significantly lower (reduction of 28%) than the control µTBS (44.7 ± 13.2 MPa), and further decreased to 23.2 ± 7.6 MPa after 100,000 cycles (reduction of 48%).
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), followed by UFB (Fig. 2a), and were only at the bottom of the hybrid layer formed by LBII (Fig. 3a). After staining, the submicron hybrid layers of all self-etch adhesives stained heavily electron-dense (Figs. 1b, 2b, 3b). Banded collagen fibrils could more clearly be detected within the hybrid layer produced by LBII (Fig. 3b).
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), as compared with non-thermocycled CMB interfaces (Figs. 1a, 1b). For UFB, voids were observed at the bottom of the hybrid layer in non-stained sections (Fig. 2c), and fewer collagen fibrils (lacking cross-banding) were observed in stained sections (Fig. 2d), as compared with non-thermocycled UFB interfaces (Figs. 2a, 2b). Although there was no clear difference in interfacial morphology between the unstained LBII sections prepared without and those prepared after thermocycling (Fig. 3c), the stained sections clearly revealed a more vaguely organized and significantly less stained hybrid layer with fewer cross-banded collagen fibrils (Fig. 3d). |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Micro-tensile bond strengths to dentin and TEM observations after long-term thermocycling in this study suggest that chemical bonding at the interface, as demonstrated by Yoshida et al.(2004), may contribute to the long-term stability of the adhesive bond. The data well reflect the differences in chemical bonding efficiency of the respective functional monomers to HAp, as previously measured by x-ray photoelectron spectroscopy (XPS) and atomic absorption spectroscopy (AAS) (Yoshida et al., 2004). That study revealed that, within a clinically relevant application time of 30 sec, the functional monomer 10-MDP was capable of chemically interacting with an intensity that did not further increase with a longer application time. The chemical bonding capacity of 4-MET was doubtful in a short application time, while phenyl-P appeared hydrolytically unstable and showed very weak chemical interaction with HAp, even when applied for 30 min. Besides having an intense chemical bonding capacity, the resultant binding should be stable with time. From the 3 functional monomers tested, the Ca-salt of 10-MDP was hardly soluble, indicating its superior stability.
TEM did not reveal any ultramorphologic difference in interfacial ultrastructure after 100,000 thermocycles for CMB, while some voids and less distinct collagen in the hybrid layer were clearly observed within the 100,000 thermocycled interfaces of UFB and LBII. These results also confirmed the so-called AD-concept (Adhesion-Decalcification concept), meaning that the less soluble the calcium salt of the acidic molecule, the more intense and stable the molecular adhesion to a HAp-based substrate (Yoshida et al., 2001; Yoshioka et al., 2002). At the interface of CMB, numerous HAp crystals were observable (Fig. 1). This must be attributed to its functional monomer, 10-MDP, that bonded to HAp and kept it there, thus resulting in a less distinct demineralization effect. Even after 100,000 thermocycles, the stained sections (Fig. 1d) showed many HAp crystals within the submicron hybrid layer. This phenomenon indicates that resin appeared to have infiltrated the partially demineralized dentin very well, keeping HAp crystals around collagen fibrils, and therefore preventing collagen fibrils from being stained (and disclosing their typical cross-banding pattern). Keeping HAp around collagen may better protect collagen against hydrolysis, e.g., degradation of the bond (Sano et al., 1999; Hashimoto et al., 2000, 2002; Van Meerbeek et al., 2003).
The other 2 adhesives clearly presented hybrid layers that do not contain many residual HAp crystals as compared with CMB. This means that they demineralized dentin much more and had a lower adhesion capacity (Yoshida et al., 2004). After long-term thermocycling, some voids at the bottom of the hybrid layer (Fig. 2c) and fewer and non-banded collagen fibrils within the hybrid layer (Fig. 2d) were observed for UFB. For LBII, collagen fibers could still be observed, but were less organized and much less present at the bottom of the hybrid layer (Fig. 3d). This indicates that HAp and collagen must have been less protected by the monomers, and more prone to degradation.
To date, a demineralized dentin zone that was insufficiently infiltrated by resin at the bottom of the hybrid layer produced by etch-and-rinse adhesives is thought to be the site where interfacial degradation is expected to begin (Nakabayashi and Takarada, 1992; Sano et al., 1994, 1995). However, with two-step self-etch adhesives (as in this study), the risk of leaving demineralized dentin that was insufficiently infiltrated by resin is thought to be much smaller. Following a self-etch approach, acidic monomers simultaneously demineralize and infiltrate dentin (Nakabayashi and Pashley, 1998). Recent studies, however, have called this assumption into question, since some discrepancies between the depth of demineralization and the depth of resin infiltration were reported for some mild self-etch adhesives (Carvalho et al., 2005). Nevertheless, any remaining insufficiently resin-infiltrated demineralized dentin is considered as the weak point of the resin-dentin bond. Unprotected and incompletely resin-coated collagen fibrils are more easily affected by hydrolysis, enzymatic attack, and functional and thermal stress, eventually resulting in the degradation of the bond (Pashley et al., 2004). Moreover, because the preservation of HAp crystals within the hybrid layer is reported to serve as a receptor for additional chemical bonding (Van Meerbeek et al., 2003; Yoshida et al., 2004), the chemical interaction potential of functional monomers to HAp around collagen fibrils within the hybrid layer may better protect collagen against degradation, as was confirmed by means of bond strength determination and interfacial TEM characterization in this study.
In conclusion, the hypothesis advanced was rejected, because the long-term durability of the dentin-adhesive interface of two-step self-etch adhesives differed, depending on the particular adhesive, and appeared to be related to the hydrolytic stability of the functional monomer itself and of its interaction with dentin. The adhesive that contained the functional monomer 10-MDP, which effectively interacts chemically with HAp within a clinically reasonable time, and the calcium salt of which is hardly soluble, showed no signs of degradation in bond strength and interfacial ultrastructure. Intimate monomer-dentinal tissue interaction is therefore expected to extend bond longevity.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Received May 2, 2005; Last revision August 3, 2005; Accepted August 28, 2005
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REFERENCES |
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