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Electric-current-assisted Application of Self-etch Adhesives to Dentin
1 Division of Dental Sciences and Biomaterials, Department of Biomedicine, University of Trieste, Via Stuparich, 1, I-34125 Trieste, Italy; Correspondence: * corresponding author, lbreschi{at}units.it
The use of electric current during the application of etch-and-rinse adhesive systems has been recently claimed to increase bonding of etch-and-rinse adhesives by enhancing substrate impregnation. The null hypothesis tested in this study was that electrically assisted application has no effect on bond strength of self-etching bonding systems. Three self-etch adhesives (Protect-Bond, Xeno III, and Prompt L-Pop) were applied with the aid of an electric signal-generating device (ElectroBond) and tested vs. controls prepared with the same disposable sponges but without electric current. Specimens bonded under the influence of electric current exhibited increased microtensile bond strength compared with the controls (p < 0.05). High-resolution SEM analysis showed that bonding under the influence of electricity reduced interfacial nanoleakage. It is speculated that resin infiltration may be improved by the attraction of polar monomers by an electric current or by modification of the dentin surface charges, resulting in better water substitution or evaporation.
Key Words: dental bonding systems • electric current • microtensile bond strength • nanoleakage
Despite the simplicity and improved user-friendliness of one-step self-etch adhesives (Van Meerbeek et al., 2005), they have been shown to exhibit guarded laboratory and clinical performance when compared with that of multi-step adhesives (Peumans et al., 2005). These all-in-one adhesives are prone to degradation, due to their increased hydrophilic monomer content (Tay et al., 2003, 2004), which expedites water sorption (Li et al., 2001). Due to their intrinsic permeability, they have also exhibited relatively extensive nanoleakage (Tay et al., 2002; Suppa et al., 2005) after water storage (Tay et al., 2003). Different modifications to the application protocols of these simplified self-etch adhesives have been reported. They include the application of multiple layers (Pashley et al., 2002; Hashimoto et al., 2004), the use of an additional layer of hydrophobic resin agent (King et al., 2005), enhanced solvent evaporation (Hashimoto et al., 2005; Van Landuyt et al., 2005), and prolonged curing time (Cadenaro et al., 2005). An adhesive application protocol, based on the use of an electric signal to enhance monomer infiltration in dentin, has recently been reported (Pasquantonio et al., 2006). This device (ElectroBond; Seti, Rome, Italy) consists of a handpiece that carries an adhesive-filled disposable sponge. Release of the adhesive is triggered by the electric potential difference between the tooth surface and the adhesive. Similar to an apex locator, the second electrode (i.e., lip clip) is placed intra-orally and connected via an electric circuit that creates an electrical current via a digitally controlled current modulator. The rationale for applying a dentin adhesive under an electric current is to improve adhesive infiltration of the demineralized dentin (Pasquantonio et al., 2006) by altering the surface charges and hydrogen bonding potential of the dentin substrate (Vaidyanatha et al., 2001; Pashley et al., 2003). The efficacy of applying self-etching adhesives under the influence of an electric signal has not been investigated. Since these adhesives contain high concentrations of polar monomers, it is speculated that pronounced dentin surface interactions with an electric current may occur during the self-etching, self-priming process. This study examined the bonding efficacy and interfacial nanoleakage of 2 simplified self-etch adhesives and a two-step self-etch primer adhesive under the application of an electric current. The null hypothesis tested was that the application of an electric current during the bonding of self-etch adhesives to dentin has no effect on their microtensile bond strengths.
Thirty intact, caries-free, human third molars were selected, after informed consent had been obtained from the donors under a protocol approved by the Human Assurance Committee of the University of Bologna, Italy. The teeth were stored in a 0.5% chloramine T solution at 4°C and used within one month after extraction. Mid-coronal dentin was exposed with a low-speed diamond saw under water irrigation (Micromet, Remet, Bologna, Italy). A standardized smear layer was created on the exposed coronal dentin with 180-grit wet silicon carbide paper. Each tooth was longitudinally sectioned into 2 halves, creating 2 bonding substrates similar in terms of tubular density and orientation. The teeth (N = 10 for each group) were bonded with 3 adhesives: a two-step self-etching primer system (Clearfil Protect Bond, Kuraray Medical Inc., Tokyo, Japan), and 2 one-step self-etching systems (Xeno III, Dentsply DeTrey, Konstanz, Germany; and Adper Prompt L-Pop, 3M EPSE, St. Paul, MN, USA). For one half of each tooth, the adhesive was applied with the use of the electric device, ElectroBond ver. 1.2004 (Pasquantonio et al., 2003), which created an electric potential difference between the dentin substrate and the adhesive applicator tip. Voltage was maintained constant with a voltmeter. Each specimen to be bonded was fitted into a wet sponge, to simulate periodontal tissues (Pethig, 1987), and the sponge was subsequently wired to the electric circuit. For the other half of each tooth (control group), the adhesive was applied in the same manner, but with the electric generator switched off. A single-blind study design was used, in which the operator performing the bonding procedure was not aware of the operating state of the electrical device (i.e., switched-on mode or switch-off mode). The electric applicator was used with a continuous brushing motion. For the two-step self-etch primer adhesive, Clearfil Protect Bond, application of both the priming solution and the bonding agent was conducted with the electric device in the switched-on mode in the experimental group, and in the switched-off mode in the control group. A 4-mm-thick layer of microhybrid resin composite (Filtek Z250, 3M ESPE) was incrementally placed over the bonded dentin surface and polymerized for 20 sec.
Microtensile Bond Strength Evaluation
Nanoleakage Evaluation
Significant increases in microtensile bond strength were observed for the 3 adhesives when they were bonded to dentin by the electric-current-assisted application technique, compared with controls (Protect Bond, 48.9 ± 10.3 MPa vs. 36.6 ± 8.4 MPa for controls; Xeno III, 39.3 ± 6.6 MPa vs. 25.5 ± 5.9 MPa for controls; Adper Prompt L-Pop, 41.5 ± 7.0 MPa vs. 22.2 ± 5.7 MPa for controls; p < 0.05; Fig. 1  ). No difference in failure modes was observed between the electric-impulse-assisted application groups and the control groups (data not shown).
Representative FE-SEM images for nanoleakage expression within hybrid layers of the 3 adhesives applied with the electric-current-assisted technique and the control technique are presented in Figs. 2 –4. Homogenously scattered silver grains (from 10 to 30 nm in diameter) were found throughout the entire thickness of the hybrid layer, and could be clearly identified in micrographs taken by the secondary electron mode. Nanoleakage patterns tended to decrease in the ElectroBond-assisted application group (especially for simplified adhesive systems; Figs.2a, 3a) vs. control application (Figs. 2b, 3b). At a higher magnification, these fine silver grains were found to be close to exposed collagen fibrils within the hybrid layer (Figs. 3c, 3d). Within the hybrid layers, fine silver deposits were scattered through the entire thickness for all tested adhesives (Fig. 2b). We confirmed the nature of the silver grains by examining the specimens from the same location using the back-scattered electron mode, along with energy-dispersive x-ray analysis (EDX data not shown).
Although the principles of electric current conduction throughout a human tooth have not been fully elucidated, several devices that rely on the tooth’s electrical properties are used in dentistry. Electronic root apex locators (Sunada, 1962), a pulp vitality tester (Daskalov et al., 1997), and early caries lesion detectors (Huysmans et al., 1995) are some of the tested devices. Since dentin is not an ideal component of electric circuits (i.e., it is not a pure capacitor or resistor), the flow of electricity depends upon dentin thickness, the presence of water and solutes (Eldarrat et al., 2003), and the relative humidity of the environment (Krizaj et al., 2004). Eldarrat et al.(2004) showed that resistance and impedance of a dentin surface are related to the presence or absence of a smear layer and its thickness. Etching of the surface and subsequent exposure of the organic matrix in a wet environment increase the electric flow by reducing resistance (Eldarrat et al., 2003). Thus, to ensure consistency of results, we subjected all adhesive applications to the same electrical experimental conditions, regardless of the tooth dimensions. For this reason, preliminary data on the resistance and impedance of tooth specimens were recorded, so that the electric device could be adjusted to electrical values that are compatible with in vivo use (Daskalov et al., 1997; Krizaj et al., 2004). Since microtensile bond strength was significantly improved in the ElectroBond-assisted technique vs. the control application technique, regardless of the tested adhesives, we had to reject the null hypothesis examined in this study. In particular, simplified adhesives exhibited bond strengths that were almost double those of the control groups, suggesting a potential use of the electric-current-assisted technique during the application of these adhesives. Apart from bond strength enhancement, the electric current also improved the quality of the hybrid layer. This was manifested as a qualitative reduction in the extent of nanoleakage for teeth bonded with the electric-current-assisted technique, particularly for the 2 simplified adhesives, Xeno III and Adper Prompt L-Pop. Considering that extensive nanoleakage was seen when these simplified adhesives were bonded to sound dentin (Tay et al., 2002; Suppa et al., 2005), its reduction should be considered a potential improvement in the bond quality. Since resin infiltration of self-etch adhesives occurs concomitantly with demineralization of the smear-layer-covered dentin (Van Meerbeek et al., 2005), electric current may influence either of the 2 processes. Since hybrid layers created by the experimental and control techniques were similar in thickness (data not shown), demineralization was unlikely to be affected by the generation of an electric signal. Conversely, the marked reduction in nanoleakage with the electric-current-assisted technique suggests that the latter was involved with the process of resin infiltration. We speculate that 3 mechanisms are potentially responsible for the improved infiltration of self-etch adhesives bonded with the experimental technique. They include a direct electrostatic effect on polar monomers present in the adhesives, modification of dentin matrix wettability, and enhanced water removal. Polar resin monomers present in the self-etch adhesives can interact with the electric current. Since these molecules diffuse toward the demineralization front during adhesive application, this polarization effect may increase the penetration of polar resin monomers through the smear layer and the underlying sound dentin. Since simplified self-etch adhesives contain higher concentrations of ionic and hydrophilic monomers, they should be more susceptible to the passage of electric currents. This possibly explains their higher bond strength increase over that of the two-step self-etch adhesive, Clearfil Protect Bond. Enhanced resin infiltration may also be due to transient biophysical modification of the dentin organic matrix when it is exposed to an electric current, with enhancement of the wettability of the dentin surface. Demineralization of the smear layer and underlying intact dentin by acidic resin monomers creates a water-based acidic environment, consisting of a complex polyelectrolyte hydrogel with a diffusion coefficient of ions that approximates the diffusion coefficient of free, unbound water (Comper and Laurent, 1978). Pethig (1987) demonstrated that collagen fibrils are polar, due to the 3.7-Debye dipole moment present in the peptide unit of the triple helix, and to the presence of dipoles derived from the water molecules (Jayasuriya et al., 2003) attached to proteoglycan-associated lateral chains (Breschi et al., 2003). Large glycosaminoglycans that are present in the proteoglycan lateral chains regulate the biophysical properties of the dentin organic matrix, as well as its three-dimensional appearance. Glycosaminoglycans not only have the ability to fill space, they also bind and organize water molecules. Being highly negatively charged and strongly polar, they are strongly influenced by electric signals (Scott, 1988). Thus, proteoglycans assume a fundamental role in determining the properties of the dentin matrix (Perdigão and Lopes, 1999). Since collagen fibrils and proteoglycans are highly polar, they are susceptible to three-dimensional modifications when in electric fields. Electric current may also modify intra- and interfibrillar hydrogen bonds among the collagen fibrils (Jayasuriya et al., 2003) inducing temporary changes in the quaternary protein structure. Since Adper Prompt L-Pop and Xeno III are more acidic than Clearfil Protect Bond, they expose more dentin organic matrix than does Protect Bond, by more aggressive dissolution of the apatite phase (Van Meerbeek et al., 2005). The piezoelectric characteristics of collagen fibrils may also enable them to alter their three-dimensional arrangement (Marino and Gross, 1989; Jayasuriya et al., 2003). Thus, the application of electric current may induce subtle orientation changes within the organic fibrillar network that favors adhesive infiltration. Application of an electric current may also increase the water substitution rate by modifying water dipoles, thereby favoring water/solvent exchange during resin infiltration (Pashley et al., 2003; Van Landuyt et al., 2005). In conclusion, this study reports, for the first time, improvements in bond strengths and reduction in nanoleakage when self-etch adhesives are use in conjunction with an electricity-assisted application technique for bonding to dentin. Further in vivo studies are currently ongoing to validate the potential mechanisms that are proposed for the improvement in resin infiltration that is associated with the use of the ElectroBond-assisted bonding technique.
The authors thank Mr. Aurelio Valmori for extensive technical assistance. Drs. Breschi and Pasquantonio are inventors of US Patent (2003) 6,641,396, cited in the manuscript, and this fact may be considered as a potential conflict of interest. The investigation was supported with grants from MIUR, Italy. Dental adhesive systems were generous gifts from the manufacturers. Received for publication October 7, 2005. Revision received June 21, 2006. Accepted for publication September 22, 2006.
Journal of Dental Research, Vol. 85, No. 12,
1092-1096 (2006)
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ABSTRACT
INTRODUCTION
= 0.05. 
Values are mean ± standard deviation [number of premature failed sticks/number of intact sticks tested]. Groups with the same superscripts are not statistically different (p > 0.05).
