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Biomaterials & Bioengineering |
The Extent to which Resin can Infiltrate Dentin by Acetone-based Adhesives
M. Hashimoto*,1,
H. Ohno2,
M. Kaga3,
H. Sano4,
K. Endo2 and
H. Oguchi3
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;
Correspondence: *corresponding author, masanori-h{at}mue.biglobe.ne.jp
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ABSTRACT
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The combined methodologies of fractography and laser-Raman spectroscopic analysis were used for evaluation of the resin-dentin bonds made with wet and dry bonding. Resin-dentin-bonded beams were produced by means of 2 acetone-based adhesives (One-Step and Prime & Bond NT). The micro-tensile bond test was conducted, and the fractured surfaces of all specimens were examined by SEM and an image analyzer. The amount of resin infiltration within the hybrid layer was quantified by means of a laser-Raman spectroscope. In Raman analysis, the amount of resin impregnation within the hybrid layer of the dry bonding was found to be significantly lower (approximately 50%) than that in the wet one. Under fractographic analysis, a correlation was found between the bond strength and the failure mode. Based on those findings, it was suggested that the integrity between the bonding resin and the top of the hybrid layer played a major role in bond strength.
Key Words: laser-Raman analysis • fractography • microtensile bond test • wet bonding technique
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INTRODUCTION
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It has been suggested that the bond strength obtained under wet conditions for an etched dentin surface is greater than that under dry conditions (Kanca, 1992; Swift and Triolo, 1992; Tay et al., 2000). This was suggested to be because loss of the interfibrillar space is induced by desiccation, leading to a decrease in the resin impregnation within the demineralized dentin. However, morphologic studies such as SEM or TEM cannot allow for the quantification of the amount of resin impregnation within the hybrid layer. The resolution limit of Raman analysis (approximately 1 µm in diameter) is suitable for the evaluation of the hybrid layer with a thickness of a few microns. Moreover, the amount of resin impregnation can be determined by laser-Raman analysis (Suzuki et al., 1991; Van Meerbeek et al., 1993; Spencer et al., 2000). However, no combined bond-strength study incorporating both micromorphologic analysis and the spectroscopic approach has been reported. Therefore, the purpose of the present study was to quantify how much resin infiltrated within the demineralized dentin and its relation to the bond strength and the failure mode, for bonds made under both wet and dry conditions, by means of the micro-tensile bond test, fractography, and laser-Raman analysis.
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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Tooth Preparation
Twelve non-carious human premolars were extracted for orthodontic reasons with the informed consent of the patients, under a protocol approved by the appropriate institutional review board. The teeth were stored in normal saline solution at 4°C for less than one month after extraction and then used in this study. Using a model trimmer, We prepared flat dentin surfaces of the mid-coronal portion perpendicular to the long axis of the tooth to expose a flat dentin surface and then ground them with 600-grit SiCs under wet conditions for 30 sec.
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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There was a significant difference in bond strength between wet (55.1 ± 12.3 MPa) and dry bonding (11.9 ± 5.6 MPa) for One-Step (p < 0.05). However, no significant differences were observed between wet (39.1 ± 21.1 MPa) and dry bonding (32.7 ± 17.9 MPa) for Prime & Bond NT (p > 0.05) (Fig. 1A , 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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Figure 1. Bond strength results and fractographic analysis. (A) Tensile bond strength (mean ± standard deviation) (upper) and area fractions of failure modes on fractured surfaces for each resin system (lower). Groups with tensile bond strengths that were significantly (p < 0.05) different are indicated by different lower-case letters: two-way ANOVA and Fisher's PLSD test. n = 20 for each group. (B) Simple regression analysis comparing the bond strength and the percentage of each failure mode. OS, One-Step; PB, Prime & Bond NT; C, resin composite; B, bonding resin; TH, top of hybrid layer; WH, within hybrid layer; DD, demineralized dentin; BS, bond strength.
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Fig. 2A 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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Figure 2. Laser-Raman analysis. (A) Raman spectra of the bonding resin layer, the central portion of the hybrid layer made with wet bonding and that made with dry bonding. Aromatic ring (1614-1620 cm-1); C=C (1642-1644 cm-1). (B) The amount of resin infiltration within the hybrid layer calculated by comparison based on the relative intensity of the peak area of the aromatic ring (1548-1624 cm-1). The same letter indicates that there is no significant difference: Mann-Whitney U test. P < 0.05 was considered significant. n = 24 for each group.
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The boundaries between the top of the hybrid layer and within the hybrid layer at the fractured surface, made with wet bonding of One-Step (a, dentin side; b, resin side), are shown in Fig. 3. 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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Figure 3. SEM micrographs showing the fractured surface of One-Step. (A,B) Failure of the top of (TH) and within the hybrid layer (WH) at the dentin side (A) and the opposing resin side (B) of a specimen made with wet bonding. Typical failure of the top of the hybrid layer is shown in A, where white arrows point to scratches.
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In Fig. 4A, 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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Figure 4. SEM micrographs showing the fractured surface of Prime & Bond NT. (A) Total fractured surface at the dentin side of a specimen made with wet bonding. Higher magnification of the sections of the fractured surface areas marked (B) and (C) in (A). (C) The widening of the dentinal tubules at the demineralized dentin was caused by the effect of acidification without impregnation of the bonding resin at this portion. B, bonding resin; TH, top of the hybrid layer; WH, within the hybrid layer; DD, demineralized dentin.
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DISCUSSION
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We could assess the degree of polymerization by comparing the band area of the aromatic ring (1614-1620 cm-1) with that of C=C (1642-1644 cm-1). For One-Step, the guide peak of C=C was too weak for the degree of polymerization to be evaluated. In Fig. 2A , 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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This work was supported, in part, by a Grant-in-Aid for Scientific Research No. 11470401 and by High Performance Biomedical Materials Research from the Ministry of Education, Science, Sports and Culture, Japan.
Received for publication November 20, 2000.
Revision received November 26, 2001.
Accepted for publication November 28, 2001.
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Journal of Dental Research, Vol. 81, No. 1,
74-78 (2002)
DOI: 10.1177/154405910208100116
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ABSTRACT
INTRODUCTION