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Effects of HEMA/Solvent Combinations on Bond Strength to Dentin
1 Department of Operative Dentistry, Endodontics and Dental Materials, University of São Paulo, Bauru School of Dentistry, FOB USP, Depto. Dentística, Al. Otávio P. Brisola 9-75, Bauru, SP, 17012-101, Brazil; Correspondence: * corresponding author, ricfob{at}fob.usp.br
Re-expansion of dried demineralized dentin is required to optimize resin adhesion. This study tested the hypothesis that bond strengths to dentin depend upon the ability of experimental HEMA(2-hydroxy-ethyl-methacrylate)/solvent primers to re-expand the matrix. Dentin surfaces were acid-etched with 37% phosphoric acid for 20 sec, air-dried for 30 sec, primed with either 35/65% (v/v) HEMA/water, HEMA/methanol, HEMA/ethanol, or HEMA/propanol for 60 sec, and bonded with 4-META-TBBO(4-methacryloyloxyethyl trimellitate anhydride-tri-n-butyl borane) adhesive. After storage in water for 1 day at 37°C, the samples were prepared for microtensile bond strength testing. We used transmission electron microscopy to measure the width of interfibrillar spaces in the hybrid layers. The HEMA/ethanol primer and the HEMA/propanol primer produced the highest and the lowest bond strengths, respectively (p < 0.05). Bond strengths were directly correlated with the width of the interfibrillar spaces (p < 0.05). Bond strengths are related to the ability of the primer to maintain the re-expansion of collapsed demineralized dentin matrix.
Key Words: microtensile bond strength • dentin • solvents
When demineralized dentin matrix is air-dried, it may collapse up to 65% in volume (Carvalho et al. 1996). During dehydration, collagen fibrils are brought into closer contact, facilitating a variety of weak molecular associations between polypeptide chains that were not possible in the presence of water. Such interpeptide forces stabilize the structure of dry dentin and make it stiffer (Maciel et al., 1996). This shrinkage phenomenon reduces the interfibrillar spaces that serve as diffusion channels for resin infiltration, and ultimately compromises the bonding of adhesive systems to dentin (Gwinnett, 1994). One way to overcome the shrinkage problem is to re-wet the dentin surface with water before bonding (Gwinnett, 1994). Water is the strongest known hydrogen bonding (H-bonding) solvent (Barton, 1991). It forms clusters around the collagen residues that prevent interpeptide H-bonding. Water breaks the interpeptide bonds in dried collagen due to its higher H-bonding capacity, plasticizing the collagen fibrils and filling the interfibrillar spaces. This causes the matrix to re-expand to its full extent (Carvalho et al., 1996; Pashley et al., 2001) and re-creates the interfibrillar spaces that are necessary for resin infiltration (Pashley et al., 2000). Another approach is to use a bonding agent that contains water or other strong H-bonding solvents that are capable of breaking any interpeptide collagen H-bonds in dried dentin. The ability of water-based primers to re-expand dried, demineralized dentin, originally described by Sugizaki (1991), has been recently confirmed and described as a self-expansion phenomenon (Van Meerbeek et al., 1998). While some adhesives are water-based, others use ethanol or acetone as solvents. If such water-free adhesives are applied to demineralized, dried, collapsed dentin, the re-expansion will be dependent on the ability of solvents or monomers to associate with collagen and break interpeptide H-bonds (Pashley et al., 2001). Ideally, a primer solution that is applied to demineralized, dried dentin should contain solvents that have a higher ability to H-bond with collagen than the peptides have for themselves, to permit adequate opening of the interfibrillar spaces for resin penetration. Incomplete expansion may impair resin infiltration and compromise bonding. This study tested the effects of several HEMA(2-hydroxy-ethyl-methacrylate)/solvent mixtures as primer on the bond strength of resin applied to demineralized, dried dentin surfaces. The rationale was that different solvents will produce different degrees of re-expansion, depending upon their H-bonding capacity, thus allowing for different degrees of resin infiltration that ultimately result in different bond strengths. The null hypothesis tested is that there is no effect of HEMA/solvent mixtures on the resin/dentin bond strength.
Flat dentin surfaces were created on mid-coronal dentin of extracted human third molars by means of a slow-speed diamond saw (Isomet, Buehler Ltd., Lake Bluff, IL, USA). The teeth were collected after the patient’s informed consent was obtained under a protocol approved by the Medical College of Georgia Institutional Review Board. The flat dentin surfaces were polished with wet 600-grit silicon carbide paper and then etched with 35% phosphoric acid (3M ESPE, St. Paul, MN, USA) for 20 sec, rinsed with water, and air-dried for 30 sec with oil-free compressed air. The dried surface was then immediately primed with 20 µL of one of the 35 vol% HEMA/65 vol% solvent solutions. The solvents used were: water (control), methanol, ethanol, and n-propanol. The primed surfaces were left undisturbed for 60 sec, gently air-dried with air (5 sec), and subsequently bonded with 4-META/TBBO(4-methacryloyloxyethyl trimellitate anhydride-tri-n-butyl borane) adhesive agent (Amalgambond, Parkell, Farmingdale, NY, USA) without using the primer in the adhesive kit. Composite buildups were constructed with Z-100 resin composite (3M ESPE) in five 1-mm increments. The bonded teeth (3 per group x 4 groups = 12 teeth) were stored in 37°C distilled water for 24 hrs before being tested.
Tensile Bond Strength Testing and Failure Mode Analysis The fractured specimens were examined under 40X and the failure mode classified as being cohesive within the material or substrate, adhesive at the interface, or mixed failures.
Ultrastructural Analysis of Untested Resin-Dentin Interfaces and Silver Uptake
Statistical Analyses
The highest bond strength was obtained after dry dentin was treated with HEMA/ethanol primer (p < 0.05) followed by HEMA/methanol and HEMA/water, which were not different between each other (p > 0.05). Lower bond strengths were observed after dentin was treated with HEMA/propanol primer (p < 0.05) (Table  ). Most of the bond failures were either adhesive or mixed. The HEMA/propanol group showed only adhesive failures. No cohesive failures in dentin or resin were observed.
Regression analysis demonstrated that the correlation between average bond strength and Hansen’s triple solubility parameters of the solvents (  d, p, h for dispersive, polar, and hydrogen bonding forces, respectively) and the primer mixtures was not significant (R2 values ranging from 0.19 to 0.49, p > 0.05; data not shown). However, when bond strengths were plotted against the average width of the interfibrillar spaces, a moderate to strong correlation was observed (R2 = 0.84, r = 0.92, p < 0.1; Fig. 1).
Transmission Electron Microscopic (TEM) Analysis of the Interfaces Transmission electron micrographs of the hybrid layers formed with the 4 experimental primers (Fig. 2 ) show that interfibrillar space widths and fibril diameters in both the ethanol and methanol groups were significantly wider (Table , p < 0.05) than those in the water and propanol groups. The diameters of the collagen fibrils were not significantly different in the water, methanol, and ethanol groups (p > 0.05), but they were significantly smaller in the propanol group (p < 0.05). Silver uptake was minimal in the methanol group, slight in the ethanol group, moderate in the water group, and heavy in the propanol group. For the ethanol, water, and propanol groups, there was a predilection for silver uptake along the base of the hybrid layer.
The results showed that the type of solvent in the primer solutions significantly affected resin/dentin bond strengths. This requires the rejection of the tested null hypothesis. The differences in bond strengths can be explained by the differences in the ability of the respective primers to re-expand the previously dried demineralized matrix. Collagen is a fibrous biopolymer that normally exists in an aqueous environment (Lazarev et al., 1992). Interpeptide H-bonding is important for maintaining the helical structure of collagen (Lee, 1986). In an aqueous environment, collagen peptides form H-bonds with water clusters, due to the very high h value of water, 37.3 (J/cm3)1/2, relative to the collagen peptides (Pashley et al., 2001). In demineralized dentin matrix, the presence of rinsing water maintains the matrix in a fully expanded state (Carvalho et al., 1996; Pashley et al., 2000, 2001). When dentin surfaces were dried with an air-stream for 30 sec, much of the water located in the matrix was removed (Nakaoki et al., 2000). This caused the matrix to collapse, approximating collagen fibrils. As collagen peptides touch, they form new interpeptide H-bonds that stabilize and stiffen the matrix in a shrunken state (Maciel et al., 1996). Air-dried, shrunken dentin has few interfibrillar spaces for resin penetration, thereby compromising the bonding of resins to dentin (Pashley et al., 1993; Gwinnett, 1994).
For such an undesirable situation to be reversed, primers must be able to re-expand the collapsed matrix. Since HEMA alone is not capable of re-expanding dry dentin matrix (Carvalho et al., 1996; Nakaoki et al., 2000), the solvents used in this study were chosen based on their graduated ability to re-expand demineralized, dry dentin matrix (Pashley et al., 2001). It was expected that the HEMA/water primer would induce the highest degree of expansion and would therefore result in the highest bond strength, assuming that the wide interfibrillar spaces are preserved during bonding procedures. Although it has been demonstrated that solvents with higher
The significantly higher bond strengths obtained when ethanol was used may be due to its It has been generally accepted that only air-drying was responsible for the shrinkage of the demineralized dentin during bonding procedures (Gwinnett, 1994). However, a recent study showed that solvents and monomers are also responsible for dimensional changes in the demineralized dentin matrix, and that those changes are related to the solubility parameters of the solvents commonly used in the bonding agents (Pashley et al., 2001). The current study confirms that the solubility parameters of HEMA/solvent mixtures can significantly affect bond strengths to dentin, by modifying the final degree of expansion of dried matrix. These findings reflect the use of experimental, single HEMA/solvent mixtures. Although results with the use of more complex mixtures such as HEMA/solvent/water cannot be predicted from this study, we may speculate that the degree of expansion will be proportional to the amount of water present in the mixture (Pashley et al., 2001). This advances our understanding of the mechanism(s) of dentin bonding that should challenge manufacturers to devise new products that include solvents capable of maintaining the structure of the demineralized dentin matrix in an expanded configuration during and after resin infiltration.
This study was partially supported by grants #300481/95-0 from Conselho Nacional de Desenvolvimento Cient’fico e Tecnol—gico (CNPq, Brazil), #01/06140-1 and 02/06682-1 from Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP, Brazil), and #DE 014911 from the National Institute of Dental and Craniofacial Research (NIDCR, Bethesda, MD, USA). The authors are grateful to Mrs. Michelle Barnes for her secretarial support. Received for publication September 19, 2002. Revision received April 21, 2003. Accepted for publication May 27, 2003.
Journal of Dental Research, Vol. 82, No. 8,
597-601 (2003) This article has been cited by other articles:
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ABSTRACT
INTRODUCTION
= 0.05. Regression analysis was used to obtain the best least-squares fit of bond strengths vs. Hansen’s solubility parameters (
