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Bonding BisGMA to Dentin—a Proof of Concept for Hydrophobic Dentin Bonding
1 Department of Oral Biology & Maxillofacial Pathology, School of Dentistry, Medical College of Georgia, Augusta, GA 30912-1129, USA; Correspondence: * corresponding author, FTAY{at}mcg.edu
The use of TEGDMA as a diluent comonomer in the formulation of hydrophobic adhesives for ethanol wet-bonding is a concern, due to its leaching potential, higher water sorption, and bio-incompatibility. This study tested the hypothesis that hydrophobic bonding to acid-etched dentin may be accomplished with the use of ethanol-solvated BisGMA only. Phosphoric-acid-etched, oxalate-occluded, deep coronal dentin bonded under 20 cm water pressure with experimental BisGMA adhesives by ethanol wet-bonding exhibited tensile strengths that were not significantly different from that achieved with OptiBond FL bonded according to the manufacturer-recommended protocol, with similar acid-/base-resistant hybrid layers, resin tags, and nanoleakage distribution. Ethanol replacement of water-saturated dentin produced wider interfibrillar spaces, more extensive shrinkage of the collagen fibrils, and narrower hybrid layers. Experimental BisGMA adhesives provide the proof of concept that relatively hydrophobic resins may be coupled to acid-etched dentin by increasing its hydrophobic characteristics via ethanol replacement. They should be further optimized before clinical application.
Key Words: dentin bonding • hydrophobic resin • Bis-GMA • acid-etched dentin • ethanol wet bonding
Attempts to couple hydrophobic resins to dentin date back to the introduction of acid-etching and the synthesis of 2,2-bis[4(2-hydroxy-3- methacryloyloxy-propyloxy)-phenyl] propane (BisGMA) (Brudevold et al., 1956; Laswell et al., 1971). Since BisGMA has no specific chemical groups for competing with water, surface-active comonomers were introduced to promote adhesion to dentin (Bowen, 1965). Traditionally, hydrophilic resin monomers are used in dentin adhesives to enhance their wetting properties and to avoid phase changes when hydrophobic dimethacrylates are added to water (Spencer and Wang, 2002). Adhesives containing hydrophilic resins exhibit high water affinity (Ito et al., 2005; Yiu et al., 2006), resulting in rapid deterioration of their mechanical properties (Yiu et al., 2004; Ito et al., 2005). To extend the longevity of resin-dentin bonds, it has been proposed that future dentin adhesives be rendered less hydrophilic (Tay and Pashley, 2003). Ideally, this should be accomplished with the use of hydrophilic resin monomers that become hydrophobic after polymerization; however, there is no evidence that such a chimeric property exists in polymer chemistry (B.I. Suh, personal communication). The alternative strategy is to make acid-etched dentin less hydrophilic. so that it is compatible with hydrophobic resin monomers. This may be achieved by replacing water in the demineralized collagen matrix with ethanol. Known as the "ethanol wet-bonding" technique (Pashley et al., 2007), this protocol prevents phase separation of the hydrophobic dimethacrylates as they are applied to ethanol-saturated instead of water-saturated dentin (Becker et al., 2007). Recent studies have shown that it is possible to coax an ethanol-solvated BisGMA/triethyleneglycol dimethacrylate (TEGDMA) comonomer blend into acid-etched dentin by means of chemical dehydration techniques that maintain the integrity of the interfibrillar spaces (Nishitani et al., 2006; Sadek et al., 2007b). BisGMA is highly viscous, due to its molecular stiffness and intermolecular hydrogen bonding, and must be diluted with glycol dimethacrylates, such as TEGDMA, to facilitate handling. The potential disadvantages with the use of a TEGDMA diluent include its leaching potential (Geurtsen, 1998; Ortengren et al., 2001) and higher water sorption (Pereira et al., 2002; Sideridou et al., 2003). Since ethanol-solvated adhesives may be placed close to vital dental pulps, the potential chemical-biological interactions of TEGDMA with oral tissues (Janke et al., 2003; Abebe and Maddux, 2006; Schweikl et al., 2006) may also be of clinical concern. It is anticipated that the viscosity of ethanol-solvated BisGMA is low enough to infiltrate ethanol-saturated acid-etched dentin without an adjunctive diluent. The resultant hybrid layer may also be rendered more hydrophobic and hydrolytically stable by elimination of the relatively hydrophilic TEGDMA comonomer (Engelmann et al., 2002). Thus, the objectives of this study were to examine the feasibility of bonding BisGMA to acid-etched dentin, and to analyze the ultrastructure of the collagen matrix in hybrid layers created with the ethanol wet-bonding technique. The null hypotheses tested were that there are no differences (1) between tensile strengths of resin-dentin bonds, and (2) among various dimensional attributes in hybrid layers created in deep coronal dentin with BisGMA-ethanol wet-bonding or a conventional three-step etch-and-rinse adhesive.
Hydrophobic Adhesives We formulated an experimental ethanol-based, light-cured BisGMA dentin primer by blending 9.85 g BisGMA, 0.05 g camphorquinone, and 0.1 gethyl-4-N, N dimethyl-aminobenzoate with 10 g of 100% ethanol (all from Sigma-Aldrich, St. Louis, MO, USA). To prevent moisture contamination, we added 1 g of molecular sieves (M514-500, Fisher Chemical, Fairlawn, NJ, USA) to the dentin primer as a dehydrant. We calculated the Hoy’s solubility parameters (Hoy, 1989) for dispersion forces (  d), polar forces (p), and hydrogen bonding (h) of the dentin primer (Table 1 ) by summing the molar attraction constants of its chemical components (Computer Chemistry Consultancy, Singen-Friedingen, Germany). Similarly, we estimated the Hoy’s solubility parameters for water-saturated and ethanol-saturated acid-etched dentin (Table 1) by assuming a composition of 30 wt% liquid and 70 wt% collagen peptide content in these matrices (Miller et al., 1998), using a previously reported fractional contribution algorithm (Agee et al., 2006). We determined miscibility between ethanol-solvated BisGMA (solvent) and the ethanol-saturated collagen matrix (polymer) by calculating their Hoy’s solubility parameter differences [  5(MPa) ] (Table 1; Fig. 1). Although BisGMA concentrations below 50 wt% were conjectured to be miscible with the ethanol-saturated collagen matrix (data not shown), the highest BisGMA concentration (ca. 50 wt%) was selected to achieve a balance between substrate-primer miscibility and resin content.
Two-step and three-step hydrophobic etch-and-rinse adhesives were designed based on the experimental BisGMA primer. For the two-step system, 2 consecutive coats of the unfilled primer were used. The first coat was applied to the dentin with agitation for 15 sec. A second application of fresh adhesive was made, giving a total application time of 30 sec. Excess solvent was evaporated with a gentle air stream for 10 sec. The adhesive was then light-cured for 20 sec. For the three-step system, the first coat of unfilled primer was applied as previously described. After solvent evaporation, a coat of a low-viscosity flowable composite (ÆliteFlo LV, Bisco Inc., Schaumburg, IL, USA) was applied, air-thinned, and light-cured for 20 sec prior to composite placement. This technique simulated the use of an unfilled hydrophilic primer and a filled bonding resin that is currently used in OptiBond FL (Sybron-Kerr, Orange, CA, USA), a hydrophilic three-step etch-and-rinse adhesive that is regarded as an excellent reference dentin adhesive (De Munck et al., 2006).
Experimental Design Each crown segment was attached to a Plexiglass platform that was perforated by a piece of 18-gauge stainless steel tubing. For all groups, the dentin surface was etched with 37% phosphoric acid gel (Etch 37, Bisco Inc.) for 15 sec, rinsed, and left moist to prevent collapse of the collagen matrix. To prevent subsequent water contamination of the resin-bonded interface by outward movement of dentinal fluid after ethanol replacement (Sadek et al., 2007a), we treated the etched dentin with 2.8% half-neutralized oxalic acid (pH 2.2; BisBlock, Bisco Inc.) for 1 min, followed by water rinsing, to occlude the patent dentinal tubules with subsurface calcium oxalate crystals (Pashley et al., 2001). Tubular occlusion was universally adopted for all experimental and control groups, so that an additional variable would not be introduced into the experimental design.
Bonding Procedures
Tensile Testing
Scanning Electron Microscopy (SEM)
Transmission Electron Microscopy (TEM)
The remaining 4 central slabs from each group were completely demineralized in 17% EDTA and embedded in epoxy resin. From 70- to 80-nm-thick sections were stained in 1% phosphotungstic acid and 2% uranyl acetate. Images taken at 50,000X magnification were analyzed with Scion Image software (Scion Corp., Frederick, MD, USA) for the percentage area occupied by interfibrillar spaces, and the diameter and D-spacing of the collagen fibrils. Since the data from the 2 experimental groups were similar, they were combined and compared with the control group by Student’s t test, with
A significant difference was found between the tensile bond strengths of two-step BisGMA (38.7 ± 3.7 MPa) and three-step BisGMA (46.5 ± 9.2 MPa) to deep coronal dentin (p < 0.05) (Table 2 ). There were no differences between the tensile bond strengths of OptiBond FL (43.2 ± 8.1 MPa) and the two-step BisGMA (p > 0.05), and between the three-step BisGMA and OptiBond FL (p > 0.05). Mixed failures were predominantly observed in all groups.
Acid-/base-resistant hybrid layers and resin tags in the two-step BisGMA (not shown) and the three-step BisGMA groups (Fig. 2A ) were similar to those identified in OptiBond FL (Fig. 2B). Nanoleakage distribution within hybrid layers was comparable in the two-step BisGMA (Fig. 2C), three-step BisGMA (not shown), and OptiBond FL (Fig. 2D) groups. The mean hybrid layer thickness identified from the TEM images in the two-step BisGMA and three-step BisGMA groups (3.87 ± 0.25 µm) was 20% narrower (p < 0.05) than that in the OptiBond FL (4.82 ± 0.14 µm). High-magnification stained TEM images of the hybrid layers revealed wider interfibrillar spaces in the BisGMA groups (Fig. 2E) compared with the OptiBond FL group (Fig. 2F). Image analysis results of the different dimensional attributes within the hybrid layers are summarized in Table 2 .
The principle of infiltrating acid-etched dentin with BisGMA is based on: (1) increasing the hydrophobic nature of the bonding substrate as water is replaced by ethanol in the demineralized dentin, and (2) increasing the hydrophilicity of BisGMA as it is solvated in ethanol, so that the dentin primer falls within the miscibility range of the ethanol-saturated collagen matrix. After infiltration of the collagen matrix and evaporation of the ethanol, the BisGMA assumes its original neat hydrophobic characteristics, resulting in a relatively hydrophobic resin-dentin interface. The results of this study indicated that it is possible to utilize the solubility parameter theory to design relatively hydrophobic adhesives for bonding to acid-etched dentin via the ethanol wet-bonding approach, with bond strengths that are comparable with those of a conventional three-step hydrophilic etch-and-rinse adhesive. This led to the acceptance of the first null hypothesis. The degree of double-bond conversion of neat BisGMA (41.5%) is lower than that of 50/50 BisGMA/TEDGMA (67.3%) (Khatri et al., 2003). Thus, eliminating TEGDMA as a diluent comonomer from the BisGMA primer may be achieved at the expense of reducing the degree of conversion of the polymerized resin within the hybrid layer. This reduction could have resulted in the lower tensile strength observed in the two-step BisGMA adhesive compared with the three-step BisGMA adhesive. For the latter, a better degree of conversion could have been achieved, at least at the top of the hybrid layer, by intermixing with the TEGDMA component of the flowable composite before light-curing of the BisGMA primer. It may be possible to take advantage of more hydrophobic BisGMA diluents that exhibit comparable/higher degrees of conversion, flexural properties, and less water sorption than BisGMA/TEDGMA (Pereira et al., 2002; Khatri et al., 2003; Lu et al., 2005), to design hydrophobic dentin adhesives that are miscible with ethanol-saturated dentin, according to the solubility parameter theory. Water-sensitive fluorescent sensors may also be incorporated into these adhesives to monitor their water sorption characteristics.
The 20% reduction in hybrid layer thickness in the BisGMA groups corresponded well with the 15–20% shrinkage in a macro-hybrid layer model when water in the demineralized dentin was replaced with ethanol (Pashley et al., 2007). The results of this study further revealed that the shrinkage induced by ethanol replacement resulted in an 83% increase in the interfibrillar area and significantly more shrinkage (ca. 20%) in the lateral dimension, but not the axial dimension (i.e., D-spacing), of the collagen fibrils. Thus, the second null hypothesis—that there are no differences among the various dimensional attributes in BisGMA-ethanol wet-bonding or a conventional three-step etch-and-rinse adhesive—has to be rejected. Shrinkage of the collagen fibrils in their lateral dimensions was likely the result of the development of intrafibrillar hydrogen bonding within individual collagen fibrils, since ethanol has a lower capacity to break these hydrogen bonds [
This study was supported by R01 grant DE 014911 from the NIDCR, Bethesda, MD, USA (PI David Pashley). The authors graciously acknowledge the technical support of Kelli Agee, Penny Roon, and Robert Smith, and the secretarial support provided by Michelle Barnes. Received for publication March 23, 2007. Revision received July 13, 2007. Accepted for publication July 2, 2007.
Journal of Dental Research, Vol. 86, No. 11,
1034-1039 (2007) This article has been cited by other articles:
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ABSTRACT
INTRODUCTION
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