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RESEARCH REPORT |
1 Faculty of Dentistry, University of Hong Kong, Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong SAR, China;
2 Department of Dental Science, University of Bologna, Italy; and
3 Department of Oral Biology and Maxillofacial Pathology, Medical College of Georgia, Augusta, GA, USA;
* corresponding author, kfctay{at}hknet.com
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: osmosis • water blisters • enamel • self-etch • water trees
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Because of the high concentrations of hydrophilic and acidic resin monomers, low film thicknesses, and lack of more hydrophobic coupling resin layers (Cheong et al., 2003), one-step self-etch adhesives have been reported to behave as permeable membranes after polymerization (Tay et al., 2002a). This is due to the presence of water-attracting hydrophilic domains (Zaikov et al., 1988) and water trees (i.e., interconnecting water-filled channels) within the polymerized adhesives (Tay et al., 2002b) that permit water to move from the underlying dentin through the adhesive. Water droplets that collect along the adhesive surface are trapped by the hydrophobic coupling composite as water blisters. Fluid flow through porous films (Zentner et al., 1985) does not occur when completely dehydrated dentin (Tay et al., 2003b) or relatively impermeable substrates such as composites are bonded with these adhesives (Tay et al., 2003a).
Osmotic blistering (Pommersheim and Nguyen, 1998) is a well-recognized problem in the resin paint-coating industry. Classic osmotic blistering occurs in the opposite direction when resin-coated, impermeable substrates are immersed in water (e.g., boat hulls). In the presence of contaminating ions on a substrate surface, osmotic cells are initiated between the coating and substrate (van der Meer-Lerk and Heertjes, 1975). The osmotic gradient causes water to flow from the outside into the interface. Blister formation occurs, since there is no channel for the relief of the osmotic pressure in an impermeable substrate. With time, the blisters enlarge to a point where adjacent blisters coalesce, eventually resulting in the delamination of the resin coating. Blister initiation may be prevented by reversing the concentration gradient, such as when a concentrated salt solution is placed outside the coating, to counterbalance the hypertonicity of the resin-enamel interface. This reversed phenomenon parallels what has been observed along adhesive-dentin interfaces.
Although dentin adhesives are usually coupled to composites when they are applied to dentin, direct exposure of adhesive-coated enamel to the oral environment may occur when these adhesives are used as dentin desensitizers (Prati et al., 2001), as adhesive flash during the bonding of orthodontic brackets (Bishara et al., 2001; Korbmacher et al., 2002), for sealing of ground enamel surfaces (Glasspoole et al., 1999; Kuhar et al., 1999) following interproximal stripping, or with the use of filled self-etch adhesives as alternatives for conventional pit-and-fissure sealants (Simonsen, 2002). Since enamel is less permeable than intertubular dentin (ten Bosch et al., 2000; De Munck et al., 2003), it is anticipated that the classic features of osmotic blistering may occur when enamel coated with a one-step self-etch adhesive is placed in water. Thus, the objective of this study was to test the hypothesis that osmotic blistering occurs when one-step self-etch adhesives are used as enamel coatings in the absence of additional composite coupling.
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Experimental Design
Five one-step self-etch adhesives were examined in this study. They included 4 two-component systems (Prompt L-Pop and Adper Prompt, 3M ESPE, St. Paul, MN; One-Up Bond F, Tokuyama Corp., Tokyo, Japan; Xeno III, Dentsply DeTrey, Konstanz, Germany) and one single-component system (iBond, Heraeus Kulzer, Hanau, Germany). Four teeth were used for each adhesive. The numbers of coats of adhesives that were applied to the tooth halves and light-cured were in accordance with the manufacturers’ instructions.
Optical Microscopy
The bonded tooth surfaces were examined without additional composite coupling. Since it was impossible to differentiate between adhesive-coated enamel and dentin under a scanning electron microscope, we first determined the location of osmotic blistering from one half of each tooth using a stereomicroscope (Nikon SMZ10, Tokyo, Japan) at 20-30X magnification.
For each adhesive, 3 bonded specimens were immersed in distilled water for 30 min before examination. After examination, they were replaced in distilled water for 30 min, followed by immersion in 4.8 M CaCl2 for 10 min to reverse the osmotic gradient. The last specimen was immersed for 30 min in the CaCl2 solution only, without prior immersion in distilled water, and was used as a control.
Field Emission-Environmental Scanning Electron Microscopy (FE-ESEM)
The other half of each tooth was bonded similarly when it was scheduled for FE-ESEM examination, with the same sequence of fluid immersion as described previously. The bonded specimens were examined with a FE-ESEM (Philips XL-30, Eindhoven, The Netherlands) under wet conditions (Cowan et al., 1996). Examination was conducted without coating at 15 kV, with the temperature of the Peltier (cooling) stage fixed at 4°C and the vapor pressure of the vacuum chamber varying between 5.8 and 6.3 Torr, to generate a relative humidity of 99–100%.
Transmission Electron Microscopy (TEM)
Prompt L-Pop and iBond were selected for TEM examination. The adhesives were similarly applied to the sectioned tooth halves but were coupled with a microfilled composite (EPIC-TMPT, Parkell, Farmingdale, NY, USA) to prevent osmotic blistering from damaging the resin-enamel interfaces. The bonded interfaces were immersed in a 50 wt% ammoniacal silver nitrate tracer solution for 24 hrs, followed by immersion in a photodeveloping solution to reduce the diamine silver ions into metallic silver grains, according to the nanoleakage protocol described by Tay et al.(2002b). Undemineralized, unstained, epoxy-resin-embedded, 90-nm-thick sections were prepared and examined under a TEM (Philips EM208S) operated at 80 kV.
In vivo Study
To validate the clinical fidelity of the in vitro results, we applied the adhesives to the labial enamel surfaces of the central incisors of two co-authors, after obtaining their informed consent to a protocol that was reviewed and approved by the institutional review board of the Medical College of Georgia (Augusta, GA, USA). Documentation was performed by both direct and indirect techniques. For the direct technique, clinical photographs were taken at different periods after the application of the adhesive to labial enamel. High-magnification clinical photographs were also taken with a camera attached to an endodontic microscope (OPMI pico, Carl Zeiss, Oberkochen, Germany). For the indirect technique, polyvinyl siloxane impressions (Affinis LightBody; Colténe AG, Altstätten, Switzerland) were taken of the adhesive-bonded enamel, from which epoxy resin replicas were prepared, according to the replica technique reported by Itthagarun and Tay (2000). The replicas were coated with gold/palladium and examined under a SEM (JEOL, Model 5400, Tokyo, Japan) at 5–10 kV.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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) and iBond (Fig. 1B). For Prompt L-Pop, osmotic blistering in enamel was so severe that delamination of the adhesive layer could be seen within 30 min of water immersion. No osmotic blistering could be observed when the bonded specimen was immersed in the control CaCl2 solution (not shown). The effect of reversing the osmotic gradient on existing water blisters is illustrated by specimens bonded with One-Up Bond F. Osmotic blisters that were located in enamel after 30 min of water immersion (Fig. 1C) were almost completely collapsed after re-immersion in the CaCl2 solution (Fig. 1D).
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. Both individual blisters and large irregular blisters that were formed by the coalescence of smaller blisters could be identified (Fig. 2A). Subsurface adhesive damage could be seen around the blisters when these specimens were examined under wet conditions (Fig. 2B).
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). Similar features were also observed in specimens bonded with iBond, with subsurface cratering occurring beneath the thin, adhesive film derived from the collapsed blisters (Fig. 2D).
TEM of the resin-enamel interface in Prompt L-Pop (Fig. 3A) revealed two distinct modes of nanoleakage (Tay et al., 2002b) that appeared as isolated silver grains and silver-impregnated water channels (water trees). These pathways for water movement within the adhesive were also seen in the less aggressive adhesive iBond (Fig. 3B), in which the enamel smear layer was retained as part of the hybridized enamel complex (Fig. 3C).
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–4C are clinical photographs documenting the appearance of osmotic blisters after iBond was applied to the labial enamel surface of one co-author. Similar results were also observed with the use of the indirect resin replica technique from impressions taken of another co-author following the application of Xeno III to labial enamel surface (Figs. 4D–4E).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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The clinical data complemented our in vitro observation of osmotic blistering in enamel. Although one-step self-etch adhesives are normally used for coupling with resin composites or luting cements, they may be present as enamel coatings either intentionally or inadvertently. A clinically relevant question is whether non-rinsing self-etch adhesives may be used in lieu of composite surface sealants for refurbishing cavosurface margins in direct and indirect restorations. The results of this study suggest that one-step self-etch adhesives, because of their high water permeability, should not be used for such purposes. Incompletely removed unbound water may exist as water trees (Fig. 1E), providing channels for rapid water movement within the polymerized adhesive (Tay et al., 2002b). Bound water may also exist in the form of primary or secondary hydration shells, due to hydrogen bonding between water molecules and hydrophilic/ionic monomers (Zaikov et al., 1988). These sites were represented by the isolated silver grains (Fig. 1D) and allow for water sorption via diffusion into the polymerized adhesive.
In this study, we did not attempt to quantify the number and size of the blisters in different adhesives, since a statistical analysis that involves the comparison of only the physical appearance of these water blisters will not truly reflect the total volume of water that was trapped between the adhesive and enamel (Davies et al., 1983). Quantification of such water movement should be contemplated in future studies, with Fourier transform infrared-multiple internal reflection (FTIR-MIR) spectroscopy (Nguyen et al., 1996a), or capacitance measurements of electrical impedance (Wittman and Taylor, 1995), that were used in other branches of science for assessing the water content at the organic coating/substrate interface.
It is remarkable that water blisters can emerge within resin-bonded enamel, despite the relatively strong bonds and enamel hybridization reported when these adhesives were applied to enamel (Pashley and Tay, 2001; Hannig et al., 2002; Ibarra et al., 2002). In this study, we immediately immersed the cured adhesives into different fluid media after light-curing to approximate what could have occurred in the oral cavity after rubber dam removal. FTIR-MIR examination revealed that the degrees of conversion of some of these adhesives were low immediately upon polymerization and that a longer period was required for bond maturation to occur (Mr. Byoung Suh, personal communication). This probably permitted the adhesive to have a relatively high permeability to water. Presumably, during self-etching, the acidic monomers form calcium salts that create a hypertonic environment at the resin-substrate interface. This caused osmosis of water into the hypertonic region. The increased pressure at the interface apparently induced delamination and creep of the resin, resulting in blister formation. Since water could drain way from resin-dentin interfaces through the dentinal tubules, no blister formation occurred, whereas water that accumulated within the relatively impermeable enamel allowed blisters to form with the continuous build-up of osmotic pressure. Nguyen et al.(1996a) further demonstrated that increasing the film thickness of a resin coating only prolonged the time required for water from the outside to reach the resin-substrate interface, but did not completely arrest the process of osmotic blistering. Conversely, the use of multiple coating may help reduce the propensity of osmotic blistering due to the variation in the courses of conductive channels within the different resin layers.
We had initially anticipated the expression of water droplets from bonded dentin (Tay et al., 2002a) following immersion of the specimens in CaCl2 solution, since water could have been withdrawn from the dentinal tubules through the permeable adhesive layer. However, no water droplets could be seen from bonded dentin. This was probably due to the rapid evaporation of these minute water droplets once the specimens were retrieved from the CaCl2 solution to be examined by FE-ESEM. Dentin fluid droplets were seen when impressions of both vital and non-vital dentin bonded in vivo with one-step self-etch adhesives were examined by a resin replica technique (Chersoni et al., unpublished results). Since no osmotic gradient is generated with the use of a water-insoluble impression material, the driving force for water extrusion from bonded dentin may additionally be caused by the bending moment produced by in-plane compression stresses (ca. 5 MPa) created within the film coating after water sorption (Chuang and Nguyen, 1997). It is possible that competition may exist between the osmotic induction of blister growth and the fluid extrusion that is caused by the bending moment created within a swollen adhesive layer, with the former process dominating in enamel and the latter in dentin. While these complex issues should be further clarified in future studies, their manifestations are indirect verification of the permeability that is associated with the use of contemporary one-step self-etch dentin adhesives.
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ACKNOWLEDGMENTS |
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Received March 17, 2003; Last revision February 6, 2004; Accepted February 9, 2004
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REFERENCES |
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