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RESEARCH REPORT |
1 Department of Operative Dentistry and Endodontics, Iwate Medical University, School of Dentistry, 1-3-27, Chuo-douri, Morioka 020-8505, Iwate, Japan;
2 Department of Dental Materials Science, School of Dentistry, Health Sciences University of Hokkaido, Ishikari-Tobetsu 061-0293, Hokkaido, Japan;
3 Division of Pediatric Dentistry, Hokkaido University, Graduate School of Dental Medicine, Kita 13, Nishi 7, Kita-ku, Sapporo 060-8586, Hokkaido, Japan; and
4 Division of Cariology and Endodontology, Hokkaido University, Graduate School of Dental Medicine;
*corresponding author, masanori-h{at}mue.biglobe.ne.jp
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: laser-Raman analysis • fractography • microtensile bond test • wet bonding technique
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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The null hypothesis to be tested in this study was that imperfect hybridization induced failure at the demineralized dentin or within the hybrid layer under bond testing.
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Bonding Procedure
Two commercially available acetone-based adhesives—One-Step (Bisco Inc., Schaumburg, IL) and Prime & Bond NT (Dentsply DeTrey, Konstanz, Germany)—were investigated in this study. Specimens were randomly chosen to be used for the following 2 acid-conditioned dentin experiments. For the wet bonding, the prepared dentin surfaces were acid-conditioned for 15 sec (Uni-Etch, 32% phosphoric acid, Bisco Inc. for One-Step group or Conditioner 36, 36% phosphoric acid; Dentsply DeTrey for Prime & Bond NT group). The acid-conditioned dentin surface was thoroughly washed under a water spray. Excess water was blot-dried from the dentin surface with a cotton pellet, leaving the surface visibly moist. At least 2 consecutive coats of bonding resin (One-Step - Bis-GMA, BPDM, HEMA, acetone [Bisco Inc.]; or Prime & Bond NT - PENTA, UDMA, bisphenol A dimethacrylate, acetone, nanoscale filler cetylamine hydrofluoride [Dentsply DeTrey]) were then applied and light-cured for 10 sec with a light-curing unit (Curing Light XL 3000; 3M, St. Paul, MN, USA). Following the surface treatment, each of the 6 1-mm increments of the resin composite (AELITEFIL - Bis-GMA, TEGDMA, filler [Bisco Inc. for the One-Step group]; or Dyract AP - Bis-GMA, TEGDMA, filler [Dentsply DeTrey for the Prime & Bond NT group]) was built up and light-cured for 60 sec. For the dry bonding, the acid-conditioned dentin surfaces were dried for 5 sec with oil-free compressed air from an air syringe, the tip of which was kept 10 cm from the dentin surface. The following bonding procedures were conducted as previously described.
Micro-tensile Bond Test
After the bonded specimens had been stored in sterilized water at 37°C for 24 hrs, those to undergo micro-tensile testing were sectioned, with a diamond saw (Isomet; Buehler Ltd., Lake Bluff, IL, USA), perpendicular to the adhesive interface to produce a beam (adhesive area: 0.9 mm2) (Shono et al., 1999). Four beams were obtained per tooth. These specimens were then attached to a testing apparatus with a cyanoacrylate adhesive, and a tensile load was applied by a material tester (EZ Test, Shimadzu Co., Kyoto, Japan) at a crosshead speed of 1.0 mm/min. Twenty beams, which were obtained from 5 teeth, were tested from each of the 4 groups. The bond strengths obtained were subjected to two-way ANOVA and Fisher's PLSD test (p < 0.05).
Fractographic Analysis
After the micro-tensile bond test, all fractured surfaces were observed with an FE-SEM (S-4000, Hitachi Ltd., Tokyo, Japan). To evaluate the failure pattern for each group, we calculated the area fractions of the failure modes per total fractured surface (%) of all specimens from the SEM photomicrographs, using an image analyzer (Digitizer, KD4030B; Graphtec, Tokyo, Japan). The failure modes were classified into the following 5 groups: failure in the resin composite, failure in the bonding resin, failure at the top of the hybrid layer, failure within the hybrid layer, and failure in the demineralized dentin. We used simple regression analysis on the plot of the percentage of the failure mode vs. the bond strength.
Laser-Raman Analysis
Twelve resin-dentin-bonded slabs were cross-sectioned perpendicular to the adhesive interface for each group. Two slabs were obtained per tooth. The samples were then polished by means of SiCs (600, 1200, 2000-grit) and soft cloths with 1-µm alpha-alumina powder with distilled water. Subsequently, the specimens were placed in 4.0% phosphoric acid solution for 2 sec to remove polishing debris and paste. To evaluate the degree of resin impregnation within the hybrid layer, we analyzed the central portion of the hybrid layers formed by wet and dry conditions and the bonding resin layer with a laser-Raman spectroscope (NR-1800, Nihon Bunkou, Tokyo, Japan). The analyzed points were studied on the X-Y-Z stage of an optical microscope. The specimens were examined with 514.5-nm argon ion laser excitation at the focus of the microscope's objective (x100). The output power was 200 mW, and the laser spot size was approximately 1 µm in diameter. Raman spectra were obtained in the range of 1400-1800 cm-1 with 30 accumulations and an integration time of 2.0 sec.
We calculated the amount of resin impregnation on a relative basis by comparing the band height of the bonding resin layer at 1614-1620 cm-1 (aromatic ring) with that within the central portion of the hybrid layer. The amount of the resin impregnation was calculated according to the following equation: amount of resin impregnation (%) = (band area at 1584-1624 cm-1 of hybrid layer/ that of bonding resin layer) x 100. Twenty-four measurements were conducted for each group, and the data were analyzed by the Mann-Whitney U test (p < 0.05).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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, upper). The percentage of each failure mode on the fractured surface for all specimens is shown in Fig. 1A (lower). The major portion of the fractured surface exhibited failure of the top of the hybrid layer for all groups. Fig. 1B shows the correlation between the bond strength and the area fraction of each failure mode, determined by simple regression analysis. As the bond strength increased, the area percentage of the top of the hybrid layer decreased, whereas that of the other portions (the resin composite, the bonding resin, within the hybrid layer, and the demineralized dentin) increased.
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shows the Raman spectra obtained in the range of 1400-1800 cm-1 for all groups. The amount of the resin infiltration (%) was shown in Fig. 2B. The mean values of the infiltration ratio of the bonding resin within the hybrid layer were 56.3 ± 7.2% (Prime & Bond NT, wet), 50.1 ± 9.7% (One-Step, wet), 28.0 ± 6.2% (Prime & Bond NT, dry), and 27.2 ± 6.4% (One-Step, dry). Statistically significant differences were found between the wet and dry conditions for both systems (p < 0.05).
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. The scratches (white arrows) shown at both sides of the specimen indicate that the failure occurred at the boundary between the top of the hybrid layer and the bonding resin.
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, the overall fractured surface on the dentin side of a specimen of Prime & Bond NT (wet bonding) is shown. The sections marked (B) and (C) in the Fig. are shown at higher magnifications as SEM photomicrographs in Figs. 4B and 4C, respectively. All of the categorized failure patterns of the present study were found at the fractured surface in this specimen. The diameters of the dentinal tubules at the demineralized dentin were greater than those at the top of the hybrid layer (Fig. 4C).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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, the band area of the aromatic ring within the hybrid layer was lower than that of the cured bonding resin. In contrast, the band area of C=C showed no difference between the two for Prime & Bond NT. These findings suggested that a significant amount of unsaturation remained within the hybrid layer, due to the presence of monomer molecules that were unreacted. This may have been due to the residual water content within the exposed collagen web. Sano et al. (1999) observed microporosity between collagen fibrils within the hybrid layer in a monkey's oral cavity after 1 yr. They suggested that the rate of polymerization of the bonding resin within the hybrid layer was lower than that of the bonding resin layer, which caused the elution of uncured resin (monomer) from the hybrid layer. The results of the present study supported their findings.
The amount of resin infiltration within the hybrid layer can be measured for comparison of the peak area of the aromatic ring between the bonding resin and the hybrid layer (Fig. 2B). It has been suggested that the volume fraction of the collagen fibrils is approximately 30% within the hybrid layer (Marshall, 1993; Pashley et al., 1995). When perfect resin infiltration is achieved, the peak area of resin within the hybrid layer is approximately 70% compared with that in the bonding resin layer. Based on those findings, the 50-60% peak area of resin in this study (Fig. 2B) suggested that comparatively good resin impregnation occurred with wet bonding. However, there was a significant, nearly 50%, difference in the amount of resin infiltration between specimens made with wet and those with dry bonding.
The fractured surfaces shown in Fig. 3 demonstrate the failure within the hybrid layer and at the top of the hybrid layer. Failure at the top of the hybrid layer was typical of both systems, especially for the dry bonding (Fig. 1A). The area percentage of the top of the hybrid layer at the fractured surface increased with decreased bond strength. In contrast, the failure of the other portions (the resin composite, the bonding resin, within the hybrid layer. and the demineralized dentin) was decreased (Fig. 1B). The bonding resin did not penetrate the exposed collagen web far enough to reach the depth of acid conditioning which creates demineralized dentin within the bond structure (Fig. 4C). However, the area percentages of the demineralized dentin and within the hybrid layer were minimal at the fractured surface, despite the resin impregnation being decreased for dry bonding (Figs. 1A, 2B). Analysis of those findings suggested that the bonding at the boundary between the bonding resin and the top of the hybrid layer might be weaker than the mechanical properties of the demineralized dentin or within the hybrid layer. The exposed superficial collagen fibrils were entangled at the top part of the collagen web after acidification. It has been suggested that the unbinding of the triple helix of the collagen fibrils might create the membrane structure at the top of the collagen web, due to the breakdown of the cross-linking of the collagen fibers (Suzuki and Nakai, 1993; Van Meerbeek et al., 1996; Maciel et al., 1998). In addition, the shrinkage of the collagen web might induce a decrease in the sizes of the microspaces between the fibers, due to the brief air drying (Pashley et al., 1995; Perdigão et al., 1996). It was suggested that the resin infiltration was extremely reduced at the top of the collagen network. Hence, the area percentage of the top of the hybrid layer was increased proportionally at the fractured surface, because failure was easily initiated and developed at this portion. Thus, we rejected the null hypothesis for the two adhesives investigated. These findings indicated that the nature of the bonds between the top of the hybrid layer and the bonding resin exerted a profound influence on bond strength.
Recently, evidence of the depletion of collagen fibrils within demineralized dentin was demonstrated in the human oral environment over the long term (Hashimoto et al., 2000a, 2001). Therefore, the amount of resin infiltration may affect bond integrity in the long term (Kato and Nakabayashi, 1998; Hashimoto et al., 2000b). It is possible that bond strength may be decreased with long-term clinical use, although the bond strength of the dry bonding was not significantly different from that of the wet bonding for Prime & Bond NT after 24 hrs. Hence, further research is required to elucidate the relation between bond integrity and bond strength over the long term.
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ACKNOWLEDGMENTS |
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Received November 20, 2000; Last revision November 26, 2001; Accepted November 28, 2001
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REFERENCES |
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