Marginal Permeability of Self-etch and Total-etch Adhesive Systems Open Access

The development of “Generational” bonding systems that improve the adhesion of esthetic restorative materials to tooth structure (enamel and dentin) continues to evolve with the introduction of a seventh generation adhesive system in late 2002 (Freedman & Leinfelder, 2002). Changes in manufacturer component ingredients and techniques for treating dentin have resulted in the implementation of “simpler” self-etching adhesive systems that incorporate an etchant, primer and adhesive in one or two containers marketed as “all-in-one” adhesives. Earlier generation, multi-bottle, total-etch systems involving separate etching, priming and adhesive components were considered time consuming and technique sensitive (Christensen, 2002; Amarl & others, 2001).

Surgical removal of tooth structure by hand or rotary instruments produces a thin residue, or smear layer, composed of minute particles of mineralized collagen matrix. The thickness can vary appreciably (1 to 15 microns), depending on whether the dentin is wet/dry, the size/shape of the cavity and the type of instrumentation used. This smear layer of cut tissue debris exists at the tooth surface-material interface, which is soluble in organic acids, can have beneficial characteristics and can function as a natural cavity liner that reduces dentin permeability. Conversely, the smear layer can interfere with adhesion of materials to the dentin substrate, serving as a reservoir for caries producing microorganisms, and potentially promotes pulpal inflammation and possible restoration replacement (Pashley, 1984).

Total-etch adhesive systems rely on various concentrations (30% to 40%) of phosphoric acid etchants to condition the enamel and dentin surfaces prior to application of separate primer and adhesive monomer agents (Perdigão, Geraldeli & Hodges, 2003).

Etching enamel is efficient in removing the smear layer, demineralizing the inorganic enamel surface, creating microporosities for a patent and mechanical bond (Perdigão & others, 2003; Van Meerbeek & others, 2003). Etching dentin also completely removes the smear layer, demineralizes the dentin substrate and opens the dentinal tubules (Van Meerbeek & others, 2003; Nakabayashi, Kojima & Masuhara, 1982; Nakabayashi, 1985; Eick & others, 1997; Eliades, Palaghius & Vougiouklakis, 1997). With total-etch systems, phosphoric acid etchants can cause over-conditioning of the dentin surface (organic [collagen] and inorganic [hydroxyapatite] components), collapsing the collagen fibers with absolute demineralization of the dentin substrate. Dentinal tubules are also denatured and funneled, increasing the flow of dentinal fluids with possible post-treatment sensitivity (Eliades & others, 1997; Eick & others, 1997; Van Meerbeek & others, 2003; Leinfelder & Kurdziolek, 2003; Perdigão & others, 2004).

Self-etch systems, composed of aqueous mixtures of acidic functional monomers, which are generally phosphoric acid esters, do not require a separate acid etch component and subsequent rinsing procedures (Van Meerbeek & others, 2003; Perdigão & others, 2003). Use of these systems has challenged the previous thought that ideal phosphoric acid enamel etching patterns and classical dentin surface hybrid layer formation are necessary (Hobson & McCabe, 2002; Miyazaki & others, 2001). Acidic monomers partially dissolve the hydroxyapatite constituent, incorporating the smear layer into the demineralized dentin substrate (collagen fibers and resin monomers), while simultaneously infiltrating the collagen network with primers and eventual resin monomer attachment with consequential occlusion of the dentinal tubules and decreased levels of post-treatment sensitivity. Self-etch systems also facilitate complete infiltration and penetration of the resin monomers into the collagen network of demineralized dentin, purportedly enhancing marginal integrity, and reducing or eliminating patient symptoms (Van Meerbeek & others, 2003; Leinfelder & Kurdziolek, 2003; Perdigão & others, 2004). However, a recent in vivo study by Perdigão and others (2003) concluded that “self-etch adhesive systems did not differ from total-etch systems in restoration sensitivity and marginal discoloration.”

Additional advantages associated with self-etch systems include reduced technique sensitivity, with moist bonding not necessarily being required as with total-etch systems (Perdigão & others, 2004; Christensen, 2002). While bonding to enamel has been reported with consistently predictable results, challenges have been encountered with dentin surface bonding, because of the complex composition and histologic nature associated with dentin (Van Meerbeek & others, 2003; Eick & others, 1997; Marshall & others, 1997; Swift, 2002). Resin adhesion to dentin involves the formation of a “hybrid zone,” where micromechanical porosities are formed from the demineralization of the dentin surface (intertubular, peritubular dentin) and opening of the dentinal tubules by acid etchants are followed by the introduction of hydrophilic primers and subsequent adhesion of monomer tags (Van Meerbeek & others, 2003; Walshaw & McComb, 1996; Eick & others, 1997; Swift, 2002; Marshall & others, 1997).

Although technological advances in materials and techniques have been developed in adhesive dentistry, shortcomings persist, since long-term microleakage occurs with all restorations (Van Meerbeek & others, 2003). Perfect adhesion to tooth structure is the primary objective; however, several contributing factors, such as material physical characteristics, polymerization source, cavity location and configuration (C-factor), morphological and histologic composition of dentin, occlusion components and lack of strict adherence of manufacturers' instructions and inconsistent clinical techniques by the practitioner, can be variables that diminish restorative success (Van Meerbeek & others, 2003; Heymann & Bayne, 1993; Perdigão & others, 2003; Leinfelder & Kurdziolek, 2003; Nakabayashi, Nakamura & Yasuda, 1991; Walshaw & McComb, 1996; Eick & others, 1997; Gordan & Mjör, 2002; Santini & others, 2004).

This in vitro study evaluated the microleakage of self-etch and total-etch adhesive systems at the coronal (enamel) and apical (dentin) margins of Class V resin composite restorations.

Ninety-six extracted, non-carious human molars were cleaned of calculus, soft tissue and other debris. The teeth were stored in a 1% Chloramine-T solution (Fisher Chemical, Fair Lawn, NJ, USA) which consisted of 12% active chlorine diluted in distilled water prior to usage. The teeth were then divided into eight equal groups of 12.

In all groups, circular, Class V cavity preparations were cut on the facial or lingual surfaces at the cementoenamel junction (CEJ) with the coronal margins located in enamel and the apical margins located in cementum (dentin). The preparations were cut with a #56 carbide bur in a high-speed handpiece cooled with an air-water spray. A bevel was placed on the enamel margin (0.5-mm width) with a #257 diamond bur. Each carbide bur was discarded following preparation of each group of teeth. Preparation dimensions of 3.0 mm × 3.0 mm × 1.5 mm (depth) were measured with a periodontal probe to maintain uniformity. One operator cut all preparations to ensure a consistent calibrated size and depth.

The teeth were randomly assigned to eight groups (n=12). All materials were used strictly following manufacturer's instructions (Table 1).

The preparation surfaces (enamel and dentin) were conditioned for 15 seconds with Ultra-Etch 35% phosphoric acid etchant gel. Following application of the etchant, the preparations were thoroughly rinsed to ensure all etchant was removed using an air/water syringe and then gently air-dried. OptiBond Solo Plus was applied to both surfaces with an applicator tip for 15 seconds using a light brushing motion. The enamel/dentin surfaces were gently air thinned for three seconds and polymerized with a halogen light source for 20 seconds. The resin composite Z-100 (shade A4) was placed in the preparations in one increment, with careful manipulation of the material, then light polymerized for 40 seconds.

The self-etching, self-priming, adhesive bonding agent iBond was applied to preparation surfaces (enamel and dentin) with an applicator tip. Two additional coats of iBond were applied followed by a 30-second waiting time. The surfaces were gently air dried until no movement of the adhesive film was detected (visibly glossy surface), followed by light polymerization for 20 seconds. The resin composite Z-100 (shade A4) was placed in the preparations as in Group 1.

The Adper Prompt L-Pop disposable reservoir system was activated per manufacturer's instructions. The self-etching adhesive was applied to the preparation surfaces (enamel and dentin) and massaged, applying pressure for 15 seconds. A gentle stream of air thoroughly dried the surface to a thin film followed by light polymerization for 10 seconds. The resin composite Z-100 (shade A4) was placed in the preparations as in Groups 1 and 2.

An equal amount of Xeno III Liquid B was dispensed into Liquid A and thoroughly mixed for five seconds. The adhesive mixture was applied to the enamel/dentin surfaces and left undisturbed for 20 seconds. A gentle stream of air was applied to the adhesive for at least two seconds to remove the solvent. The adhesive was then light polymerized for 10 seconds, followed by placement of the Z-100 resin composite (shade A4) as in Groups 1 through 3.

Equal portions (1 drop) of Simplicity 1 and 2 were dispensed in separate wells prior to application. Simplicity 1 was applied to the preparation surfaces (enamel and dentin) and gently agitated for 10 seconds followed by three applications of Simplicity 2 directly onto the Simplicity 1 saturated surfaces. The preparations were gently dried to evaporate the solvent (approximately 5 to 10 seconds) and light polymerized for 10 seconds. The resin composite Z-100 (shade A4) was placed in the preparations as in Groups 1 through 4.

One coat of Nano-Bond self-etch primer was applied to the preparation surfaces (enamel and dentin) followed by a 15 to 20 second waiting time. A gentle stream of air was applied to the surfaces for 2 to 3 seconds to remove excess primer. One coat of Nano-Bond Adhesive was applied to the primed surfaces, followed by a light stream of air to evaporate the solvent. The preparations were light polymerized for 10 seconds, followed by placement of Z-100 resin composite (shade A4) as in Groups 1 through 5.

Adper Scotchbond 35% phosphoric acid etchant gel was applied to the preparation surfaces (enamel and dentin) followed by a wait time of 15 seconds. The surfaces were rinsed, removing all etchant, and gently dried, leaving a slightly moist surface. Adper Scotchbond Multi-Purpose Primer was applied to the enamel and dentin surfaces and, again, gently dried for 5 seconds. Adper Scotchbond Multi-Purpose Adhesive was applied to the enamel and dentin surfaces and light polymerized for 10 seconds. The resin composite Z-100 (shade A4) was placed in the preparations as in Groups 1 through 6.

A Touch & Bond blue activator pledglet was placed into a dispensing well and thoroughly saturated with Touch & Bond Adhesive liquid. The mixture was applied to the preparation surfaces (enamel and dentin) for 20 seconds, then gently air-dried for 3 to 5 seconds to evaporate the solvent. A second coat of the mixture was applied and dried for 3 to 5 seconds, followed by light polymerization for 10 seconds. The resin composite Z-100 (shade A4) was placed in the preparations as in Groups 1 through 7.

All restorative materials were polymerized with a Schein (Sullivan-Schein, Melville, NY, USA) halogen light. The light had previously been monitored with a radiometer and, thus, provided adequate intensity (≥600mW/cm²). The resin composites were polished with Sof-Lex (3M-ESPE, St Paul, MN, USA) flexible aluminum oxide disks of decreasing abrasiveness (course to superfine). The teeth were stored in distilled water at room temperature for seven days prior to leakage assessment.

The teeth were thermocycled for 1000 cycles in separate distilled water baths of 5°C and 55° C ± 2°C with a dwell time of 60 seconds in each bath and a transfer time of 3 seconds. The root apices then were sealed with utility wax, and two coats of nail varnish were applied to the entire tooth surface to within 1.0 mm of the restoration. The teeth were immersed in 1% methylene blue dye for 24 hours at room temperature, removed and thoroughly rinsed. They were then invested in clear acrylic auto-polymerizing resin and labeled. A Buehler Isomet (Buehler Ltd, Evanston, IL, USA) low-speed diamond saw, cooled with water, sectioned each tooth block transverse to the tooth through the center of the restoration from the facial to the lingual surface. Two sections were obtained from each block: each side of the cut yielding dye penetration (leakage) readings. Each section (24 sections per group) was examined at 20× magnification under a Meiji (Meiji-Labax Co, Tokyo, Japan) binocular microscope. Based on an ordinal ranking system, the degree of leakage was determined as follows: 0 degree = no dye penetration; 1 degree = dye penetration up to one half the cavity wall length; 2 degree = dye penetration up to the full length of the cavity wall, not including the axial wall; 3 degree = dye penetration to the full length of the cavity wall, including the axial wall. Two readings (averaged) were taken at the enamel and dentin margins of each tooth block.

To determine statistically significant differences in the quantity of leakage at the enamel and dentin margins among the adhesive groups, non-parametric data were analyzed using Kruskal-Wallis one-way Analysis of Variance and Mann Whitney-U multiple comparison tests corrected for ties. An intergroup comparison was performed by a Wilcoxon signed rank test evaluating microleakage at the enamel and dentin margins of combined adhesive groups. All tests were conducted at a p<0.05 level of significance. The statistical calculations were executed using Statview 5.0 (SAS Institute, Cary, NC, USA).

Table 2 lists the distribution of microleakage ordinal scores at the coronal (enamel) and apical (dentin) margins.

The Kruskal-Wallis Analysis of Variance revealed a significant difference among adhesive groups at the enamel margin (p=0.0138). A multiple comparison of the eight adhesive groups by a Mann Whitney test showed, statistically, three groupings: 1) the restored teeth treated with Adper Scotchbond Multi-Purpose exhibited significantly less leakage than the other adhesive groups, except iBond; 2) among the self-etch adhesive groups, the restored teeth treated with iBond exhibited significantly less leakage than the Nano-Bond group and 3) the other adhesive groups clustered together, experiencing intermediate leakage.

Results at the dentin margin were different: none of the eight adhesive systems stood out statistically as experiencing more or less leakage (p=0.5397).

A Wilcoxon signed rank test comparing all 96 enamel versus dentin surfaces, pairwise, confirmed there was significantly (p<0.0001) greater leakage along the dentin than the enamel margins of the Class V restorations.

This study evaluated the microleakage of eight different adhesives comparing self-etch (sixth/seventh generation) and total-etch (fourth/fifth generation) systems, all of which demonstrated dye penetration (leakage) at both the enamel and dentin margins. According to the Wilcoxon signed rank test, significantly less leakage was exhibited at the enamel margin compared to the dentin margin of the adhesive groups.

In several studies that evaluated self-etch and total-etch adhesive systems, less microleakage was also reported at the enamel margin compared to the dentin margins (Alavi & Kianimanesh, 2002; Amarl & others, 2001; Pradelle-Plasse & others, 2001; Santini & others, 2004; Koliniotou-Koumpia, Dionysopoulos & Koumpia, 2004). Analysis of the data from this study revealed significantly lower microleakage values with a total-etch adhesive (Adper Scotchbond Multi-Purpose) compared to the other adhesives (except iBond) at the enamel margin. This finding was in agreement with studies reporting decreased leakage associated with total-etch (especially at the enamel margin) compared to self-etch systems (Koliniotou-Koumpia & others, 2004; Pradelle-Plasse & others, 2001; Gagliardo & Avelar, 2002; Alavi & Kianimanesh, 2002; Santini & others, 2004). However, some contradictory results have been reported by other researchers (Amarl & others, 2001).

Also at the enamel margin comparison of the self-etch systems, iBond demonstrated significantly less leakage than Nano-Bond. Microleakage of restorations using self-etch adhesives could have resulted from incomplete etching of the enamel surface by acidic monomers, allowing for higher values than the total-etch systems using a separate phosphoric acid etchant. Scanning electron microscopy (SEM) studies have shown that the use of phosphoric acid as an enamel etchant improves enamel penetration and the subsequent attachment of adhesive monomers (Kanemura, Suno & Tagami, 1999; Perdigão & others, 1997).

Self-etch and total-etch adhesive systems both showed significantly higher leakage at the dentin margins; however, no significance was revealed between the individual adhesive systems. These results are in accordance with Pontes, de Melo and Monnerat (2002), Santini and others (2004) and Gagliardi and Avelar (2002); however, other studies showed a significant difference between self-etch and total-etch adhesives at the dentin margins (Pradelle-Plasse & others, 2001; Alavi & Kianimanesh, 2002; Koliniotou-Koumpia & others, 2004; Amarl & others, 2001). The Adper Scotchbond Multi-Purpose total-etch system showed lower (not significant) leakage scores than the other adhesive groups, except iBond, at the dentin margin, which disagreed with results from several reports (Amarl & others, 2001; Alavi & Kianimanesh, 2002). Reasons for the increased leakage scores associated with total-etch systems (phosphoric acid etchants) of dentin compared to enamel surface substrate include hypermineralization of the dentin surface and subsequent collapse of the collagen fibrillar network (Leinfelder & Kurdziolek, 2003).

iBond was the only self-etch system marketed as a one-bottle, no mixing system containing a gluteraldehyde component specifically for sensitivity. It showed the lowest leakage scores (not necessarily always significant) at the enamel and dentin margins of self-etch adhesive systems. Possible explanations for this occurrence include multiple applications (manufacturer's instructions) of the iBond adhesive onto the preparation surfaces and increased waiting periods prior to light polymerization.

Microleakage has been defined as the “marginal permeability to bacterial, chemical and molecular invasion at the tooth/material interface” and is the result of a breakdown of the tooth-restoration interface, causing discoloration, recurrent caries, pulpal inflammation and possible restoration replacement (Gordon & others, 1998; Brännström, 1984; Going, 1972; Fusayama, 1987). The interface between the restorative material and tooth surface of a cavity preparation is 10 to 20 microns wide, permitting bacterial access (Pashley, 1984).

Possible reasons for microleakage of contaminants at the dentin restoration margin include cavity configuration (C-factor), dentinal tubule orientation to the cervical wall (CEJ), organic content of dentin substrate and movement of dentinal tubular fluids, incomplete alteration/removal of smear layer by acidic primers (self-etch systems) for adequate demineralization and hybrid layer formation; inefficient infiltration/penetration of primer components into the demineralized collagen fibrillar network, dentin substrate hydration level (solvent carriers [water, alcohol, acetone] in the adhesive agent reacting differently with varying degrees of surface “moisture” [water]), incomplete evaporation of the solvent from the dentin surface prior to attachment of adhesive monomers, acid component composition (pH, osmolality, thickening agent), polymerization contraction of the resin composite, physical characteristics of restoration material (filler loading, volumetric expansion, modulus of elasticity), inadequate margin adaptation of the restorative material, polymerization source-photoinitiator incompatibilities and instrumentation and finishing/polishing effects (Santini & others, 2004; Van Meerbeek & others, 2003; Leinfelder & Kurdziolek, 2003; Crim & García-Godoy, 1987; Eick & others, 1997; Nelson, Wolcott & Paffenburger, 1952; Davidson, de Gee & Feilzer, 1984; Yap & others, 2003; Mullejans & others, 2003). Additional factors facilitating marginal leakage include inappropriate barrier protection (dental rubber dam), tooth location, occlusal stresses/tooth flexure and age of patient (Heymann & Bayne, 1993; Van Dijken & Horstedt, 1987). Since the hybrid layer morphology was not evaluated microscopically in this study, the specific nature of restoration failure (microleakage) for each adhesive system is unknown, although anecdotally four factors are strongly suspected: inefficiency of acidic monomers in alteration of the smear layer for classic hybrid layer formation, cavity C-factor, orientation of dentinal tubules to the CEJ and post-treatment stresses caused by polymerization contraction.

Because this was an in vitro study, subjective post-treatment sensitivity could not be evaluated. However, due to high leakage scores occurring at the dentin margins using total-etch and self-etch adhesives, one can extrapolate that leakage of saliva contaminants (microorganisms and microbial products) at the tooth surface restoration interface could proliferate and intensify patient symptoms, since bacteria has proven to be the primary etiologic factor in long-term post-treatment sensitivity (Brännström & Nyborg, 1972; Fusayama, 1987).

Microleakage studies provide adequate screening methods, possibly determining whether adhesive systems will be clinically acceptable. Although this study was conducted in vitro, which can be a reliable indicator for ensuing in vivo studies, previous reports by Barnes and others (1993), Sidhu and Henderson (1992) and Ferrari and others (1994) indicate that data obtained from in vitro microleakage testing may be useful, but not necessarily reproducible in clinical in vivo settings. Schneider and others (2000) concluded that during formation of a hybrid layer, microleakage did not differ significantly in vital and non-vital dentin. In conducting in vitro microleakage investigations, obtaining conclusive information can be problematical, since vast differences in research protocols are reported in the dental literature.

The results of this study suggest that none of the bonding systems tested were resistant to dye penetration (leakage), especially at the dentin margin. Compared to “conventional” total-etch techniques, self-etching adhesives (except iBond) exhibited significantly greater leakage in enamel as well as substantial leakage at the dentin margins. Reported advantages of self-etch systems include “simple” application procedures and a reduction and/or elimination of post-treatment sensitivity; however, because of numerous uncontrolled variables encountered during patient treatment, perceived advantages can potentially become disadvantages (Leinfelder & Kurdziolek, 2003; Duke, 2003). Although iBond adhesive demonstrated substantially (not necessarily always significant) lower leakage values than the other self-etch systems and results were comparable to total-etch systems (Adper Scotchbond MultiPurpose), technique protocols associated with iBond proved to be labor intensive and may not be realistic for some practitioners in a clinical setting.

The results of this study demonstrate that the “dynamic” nature of dentin substrate morphology is indeed an important factor and possibly an insurmountable impediment for perfect adhesion of restorative materials to tooth structure. Clinical trials should be performed to assess the performance of these adhesive systems before definitive conclusions are formulated.

Within the limitations of this in vitro study, the following conclusions were reached:

1. All adhesive system groups exhibited dye penetration (leakage) at both the coronal (enamel) and apical (dentin) margins.

2. At the enamel margin, statistically significant leakage was exhibited among the eight adhesive groups.

3. At the enamel margin, Adper Scothchbond Multi-Purpose revealed significantly less leakage compared to the other adhesive groups (except iBond).

4. A comparison of the self-etch adhesives at the enamel margin revealed iBond had significantly less leakage compared to the Nano-Bond group. No other significant differences were recorded between self-etch adhesives.

5. A comparison of the adhesive groups at the dentin margin showed no significant differences.

6. An inter-group comparison revealed significantly lower leakage at the enamel margin versus the dentin margin of all adhesive groups.