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RESEARCH REPORT |
1 School of Dentistry, Medical College of Georgia, Augusta, GA, USA;
2 Paediatric Dentistry and Orthodontics, Faculty of Dentistry, The University of Hong Kong, Pokfulam, Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong SAR, China;
3 Osaka University Graduate School of Dentistry, Osaka, Japan; and 4 Research and Development, Ivoclar-Vivadent AG, Schaan, Liechtenstein;
* corresponding author, franklintay{at}gmail.com
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: fluid transudation • one-step self-etch • water-treeing • sound dentin • caries-affected dentin.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Ultrastructually, water entrapment in one-step self-etch adhesives is visualized as water-filled channels (i.e., water trees; Tay and Pashley, 2003). With single-bottle versions of such adhesives, additional discrete water droplets (Tay et al., 2003b; De Munck et al., 2005) were identified, and were surmised to represent phase separation of water after evaporation of volatile solvent (Van Landuyt et al., 2005). Together with nanoleakage within the hybrid layers of self-etch adhesives, these water-rich interfacial regions represent potential sites from which bond degradation may initiate (Hashimoto et al., 2004a; Carvalho et al., 2005).
Since all the above observations were obtained from sound coronal dentin, it was difficult for us to resolve whether the entrapped water originated from water-containing self-etch adhesives or from the hydrated dentin. Theoretically, water emanating from dentinal tubules (i.e., convective water flux; Pashley et al., 1981) should be minimal when mild self-etch adhesives are applied to smear-layer-covered dentin. However, evaporative water flux may still occur through the adhesive-infiltrated smear plugs as adhesive solvents are removed with air-blasts (Goodis et al., 1990, Matthews et al., 1993). We hypothesized that by eliminating convective and evaporative water fluxes (Sidhu et al., 2004) from dentin substrates (Lee et al., 2003), any water entrapment within the adhesive should be attributed to the retention of water derived from one-step self-etch adhesives.
This hypothesis is testable by the use of an adhesive with known water concentration, and transparent carious dentin as a bonding substrate, since the dentinal tubules in the latter are heavily occluded with intratubular mineral deposits (Zheng et al., 2003). Although completely dehydrated sound dentin could be used, we considered hydrated caries-affected dentin to be a more clinically relevant bonding substrate, since the results could be compared with the adjacent hydrated sound dentin derived from the same specimen. Thus, the null hypothesis tested was that there is no difference in the manifestation of water-rich interfacial regions stained by silver in resin-dentin interfaces created with an experimental single-bottle one-step self-etch adhesive in sound or transparent carious dentin.
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Tooth Preparation
Access to carious coronal dentin was achieved with a tungsten carbide bur in a high-speed handpiece under water cooling. Since the effects of caries-indicating dyes on water flux are unknown, soft caries-infected dentin was removed by means of a slow-speed handpiece equipped with a polymer bur (SmartPrep, SS White Burs, Inc., Lakewood, NJ, USA) that reportedly has a hardness similar to that of caries-affected dentin (Boston, 2003). Two or 3 polymer burs were used for each tooth, until the burs dulled and vibrated on reaching the hard, discolored caries-affected dentin.
The bulk of the occlusal hard tissue above the cavity floor was removed with a slow-speed saw (Isomet, Buehler Ltd., Lake Bluff, IL, USA) under water irrigation. Additional abrading of the remaining hard tissues was performed with 180-grit silicon carbide (SiC) paper. This created a flat surface with a zone of hard caries-affected dentin surrounded by peripheral sound dentin, at a level that was slightly below the original cavity floor generated by the polymer burs. Teeth with pulpal exposures were discarded, resulting in 11 intact specimens for the study.
Experimental Adhesive
An experimental unfilled, single-bottle, one-step self-etch adhesive with known water and acetone concentrations was formulated, since it is impossible to control the hydrolysis of ester-linkages in 2-hydroxyethyl methacrylate (HEMA) or acidic resin monomers when water is pre-included in single-bottle self-etch adhesives (Moszner et al., 1999; Nishiyama et al., 2004).
The experimental adhesive (pH = 1.58) consisted of (wt%): 4-methacryloxyethyltrimellitic anhydride [4-META] (17%), 1,6-bis-[2-methacryloyloxyethoxycarbonylamino]-2,4,4-trimethylhexane [UDMA] (14%), glutaraldehyde (4%), camphorquinone (0.3%), tertiary amine [ethyl 4-(dimethylamino) benzoate] (0.5%), butylated hydroxytoluene (0.03%), acetone (39.17%), and water (25%). Glutaraldehyde was included and HEMA was excluded to simulate a commercially available adhesive (iBond, Heraeus-Kulzer, Hanau, Germany). To prevent hydrolysis of 4-META and UDMA into lower-molecular-weight, non-functional components and methacrylic acid during shipment, we initially segregated the glutaraldehyde and water from the other components. The reconstituted single-bottle adhesive was allowed to stand for 60 min, to convert the 4-META into its functional form, 4-methacryloxyethyltrimellitic acid. The adhesive was used within 1 mo after reconstitution and was stored at 4°C after each use.
Substrate Characterization
Variations in caries-affected dentin exist in their degree of mineralization, porosities, and nanohardness. Since tubular occlusion was the primary concern, the carious dentin that remained after polymer bur excavation and SiC paper abrasion was examined with transmission electron microscopy (TEM). Substrate characterization was based upon the absence of structurally disorganized (i.e., caries-infected) dentin containing bacteria in distorted tubules with minimal intratubular mineral deposits, and the presence of intact tubules occluded with intratubular minerals that are characteristic of the transparent zone in carious human dentin (Marshall et al., 2001).
Three specimens were randomly selected, and a 2-mm-thick section was prepared from each tooth by an additional cut made parallel to the abraded surface. The 2 undemineralized disks were sonicated in 0.1 M cacodylate buffer (pH 7.4), and then fixed, dehydrated, and embedded in epoxy resin for TEM (Tay et al., 1999). Regions containing the carious dentin were retrieved for the preparation of 90- to 120-nm-thick sections. Unstained sections were examined under a TEM (EM208S, Philips, Eindhoven, The Netherlands) operating at 80 kV.
Bonding Procedures
The remaining specimens were sectioned 3 mm below the cemento-enamel junction. After the pulpal tissues were removed, 4 crown segments containing carious dentin surrounded by adjacent sound dentin were cemented to Plexiglass slabs penetrated by a stainless steel tube. This permitted filling of the pulp chamber with water via a piece of plastic tubing connected to a syringe barrel (Pashley and Depew, 1986). To permit evaporative water flux without convective water movement, we positioned the crown segment at the same level as the water syringe, producing 0 cm of water pressure. When air blasts are used to evaporate adhesive solvents, any water movement is due only to evaporation. The remaining 4 crown segments were bonded under dentin perfusion to permit both convective and evaporative water fluxes during bonding, by raising the water syringe to deliver 20 cm of water pressure.
Three consecutive coats of the experimental adhesive were applied to each crown segment with agitation and a dwelling time of 30 sec. The adhesive solvent was evaporated with moisture-free air blown 0.25 m above the bonding surface. The adhesive was light-cured for 20 sec in a light-emitting-diode curing unit (LE Demetron 1, Sybron Kerr, Orange, CA, USA) operated at 925 mW/cm2. To enable the bonded carious dentin to be discerned from bonded sound dentin during laboratory processing, we coupled a 2-mm-thick layer of translucent, microfilled composite (EPIC-TMPT, Parkell Inc., Farmingdale, NY, USA) to the bonded dentin and light-cured it for 40 sec.
One-mm-thick sections containing only composite-coupled sound dentin, or regions with composite-coupled carious dentin, were made perpendicular to the resin-dentin interfaces. One section of each category was retrieved from each tooth. The sections were immersed in a 50 wt% ammoniacal silver nitrate tracer solution for 24 hrs, for detection of potential water-rich regions within the resin-dentin interfaces via the reduction of the silver-ion complexes into silver deposits, according to a tracer-impregnation protocol (Tay and Pashley, 2003). The silver-impregnated specimens were processed for TEM examination.
Statistical Analysis
The silver distribution within the adhesive layers was examined with an image analysis program (Scion Image, Scion Corp., Frederick, MD, USA), with 8 sections used from each of the 4 bonded groups (sound vs. carious dentin; 0 vs. 20-cm water pressure). Since the data were not normally distributed, they were analyzed by Kruskal-Wallis ANOVA on ranks and Dunn’s multiple-comparison tests at 
= 0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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), dentinal tubules were occluded with fine intratubular mineral deposits (Fig. 1B) or in combination with discrete caries crystals (Daculsi et al., 1987) (Fig. 1C).
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, 2B). At 20 cm of water pressure, the adhesive layers were also devoid of silver deposits, although some nanoleakage could be identified in hybrid layers (Fig. 2C). Conversely, the underlying dentin substrates were highly porous and exhibited extensive silver deposition.
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), and occasionally throughout the adhesive layer (Fig. 3B). With the application of water pressure, additional water droplets emanating from patent tubular orifices were observed (Fig. 4A). The convective flux also induced through-and-through water movement, resulting in the manifestation of large blisters within the adhesive-composite interface (Fig. 4B).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Our results indicated that evaporative water flux was responsible for the formation of water trees, while convective water flux additionally contributed to the expression of water droplets from dentinal tubules. Although water-treeing could be identified throughout the adhesive layer in bonded sound dentin (Fig. 3B
), they were predominantly seen as a relatively thick layer over the hybrid layer surface. These water trees were also continuous with silver deposits that occupied the interfibrillar spaces of hybrid layers (Fig. 3A). Although air-drying is mandatory for removing adhesive solvents, it also induces evaporative water loss from dentin before self-etch and etch-and-rinse adhesives are polymerized (Hashimoto et al, 2004b). In the presence of smear plugs within tubular orifices, water probably passes through the channels of least resistance, along the smear plugs and peritubular dentin (Carvalho et al., 2005) into the interfibrillar spaces of intertubular dentin, and is subsequently trapped as water vapor within the adhesive. In the case of convective water flux, 20 cm H2O water pressure plus evaporative water loss, more water would pass through the smear plugs and adhesive, resulting in water blebbing in the adhesive layer, wherein they undergo phase separation into spherical water droplets within the solvent-sparse, comparatively hydrophobic "all-in-one" adhesive. Concomitant evaporative water flux accounts for primary water-treeing over the hybrid layer surface, as well as secondary water-treeing around the circumference of these water droplets (Fig. 4A).
It is remarkable that, in the absence of water fluxes from transparent carious dentin, an excellent seal could be established along the bonded interfaces. Even when the transparent carious dentin was bonded under 20 cm of water pressure, far fewer silver deposits were seen within these hybrid layers when compared with those in sound dentin. The result is an excellent resin seal on the surface of a highly porous, incompletely infiltrated bonding substrate. Although these hybrid layers were thicker than those created in sound dentin (Yoshiyama et al., 2002), the expected bond strength to the caries-affected dentin would be lower than the corresponding bond strength to sound dentin, since it is unlikely that any adhesive can diffuse through a zone of partially demineralized carious dentin that may be several hundred micrometers thick. In the broadest sense, this scenario resembles a much amplified version of an incompletely infiltrated hybrid layer produced by an etch-and-rinse adhesive in sound dentin. Similar to incompletely infiltrated sound dentin, the non-infiltrated caries-affected dentin may undergo self-destruction via the release of endogenous matrix metalloproteinases (Pashley et al., 2004). There is also immuohistochemical evidence suggesting that the antigenicities of type I collagen and proteoglycans are masked in caries-affected dentin (Breschi et al., unpublished results) that may prevent the potential for intrafibrillar remineralization of the partially demineralized collagen matrices in caries-affected dentin.
In conclusion, the results of this work provide reassurance of the validity of the concept of simultaneous etching and priming in self-etch adhesives. Although tubular occlusion did prevent water-treeing and through-and-through fluid movement in our one-step self-etch adhesive model, it is unrealistic that, clinically, one can bond only to transparent carious dentin without involving sound dentin. A likely scenario is that one may encounter a gradual transition from a strongly bonded but permeable interface to a weakly bonded (Yoshiyama et al., 2002) but relatively impermeable interface (Lee et al., 2003), as the bonding substrate shifts from sound to caries-affected dentin along the cavity floor. Thus, the use of two-step self-etch adhesives that exhibit higher bond strengths and less permeability appears to be a more prudent approach in the execution of the self-etching technique.
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ACKNOWLEDGMENTS |
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Received October 27, 2004; Last revision May 25, 2005; Accepted June 30, 2005
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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