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RESEARCH REPORT |
1 Pediatric Dentistry and Orthodontics, Faculty of Dentistry, The University of Hong Kong, Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong SAR, China;
2 Restorative Dentistry, School of Dental Science, University of Newcastle, Newcastle upon Tyne, UK;
3 Department of Conservative Dentistry & Biomaterials, Guy’s, King’s & St Thomas Dental Institute, King’s College London, Guy’s Hospital, London, UK; and
4 Department of Oral Biology and Maxillofacial Pathology, Medical College of Georgia, Augusta, GA, USA;
* corresponding author, kfctay{at}netvigator.com
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: RMGIC • absorption layer • hydrated dentin • water movement
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Confocal scanning optical microscopy is a valuable tool for demonstrating dynamic water fluxes between RMGICs and dentin (Watson et al., 1998), and the use of fluorescent dyes enables the absorption layer to be imaged (Sidhu et al., 2002). However, resolution of the hybrid layer that is created when dentin is conditioned with polyacrylic acids (Nakanuma et al., 1998; Abdalla, 2000) is beyond the scope of light microscopy. Moreover, the clinical implication of the absorption layer in RMGIC/dentin interfaces remains unknown.
Since RMGIC absorption layers were observed predominantly in deep hydrated dentin (Sidhu and Watson, 1998), the objective of this study was to examine the ultrastructural characteristics of the absorption layer created with two RMGICs in polyacrylic-acid-conditioned dentin. The hypotheses examined were that the absorption layer is formed only in the presence of water derived from hydrated dentin, and that the absorption layer facilitates better bonding of RMGICs to deep dentin.
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Experimental Design
Bonding surfaces were treated with 10% polyacrylic acid (Dentin Conditioner, GC Corp., Tokyo, Japan) for 20 sec, then were rinsed thoroughly (Tanumiharja et al., 2000). The dentin substrates were divided into 2 categories: hydrated and dehydrated dentin. For hydrated dentin, the teeth were bonded in their normal hydrated status with their roots intact (i.e., with the pulp and dentinal tubules filled with water). To determine if the absorption layer could form in dehydrated dentin, we severed the roots to remove the contents of the pulp chamber. After being acid-conditioned, these teeth were dehydrated through an ascending ethanol series (70%, 80%, 95%, three changes in 100%) for 2 hrs each prior to being bonded.
Two light-cured, machine-mixed RMGICs (Fuji II LC, GC Corp.; Photac-Fil Quick Aplicap, 3M-ESPE, St. Paul, MN, USA) were investigated. They were applied to dentin in 2 two-mm-thick layers and light-cured within 60 sec of being mixed, to minimize the release of the 2-hydroxyethyl methacrylate (HEMA) component (Palmer et al., 1999). All of the teeth that had been bonded, including the roots or exposed pulps, were immediately sealed with 2 coats of light-cured surface sealant (BisCover, Bisco, Schaumburg, IL, USA) to prevent either desiccation or water-sorption via external sources (Chuang et al., 2001). Thus, the only available water source in hydrated specimens was of pulpal origin, accessible via the dentinal tubules. The specimens were stored at 37°C and 100% relative humidity for 24 hrs before being processed further.
Transmission Electron Microscopy
Five specimens were used for each of the 4 groups investigated (i.e., 2 RMGICs bonded to either hydrated or dehydrated dentin). Two blocks (2 x 2 x 2 mm) containing the RMGIC/dentin interfaces were retrieved from each tooth and sealed immediately with BisCover resin. The specimen blocks were embedded in epoxy resin for ease of handling during ultramicrotomy, according to the protocol described by Tay et al.(2001a). Unstained sections (90 nm thick) were examined with a transmission electron microscope (Philips EM208S, Eindhoven, The Netherlands) operating at 80 kV.
We used energy-dispersive x-ray analysis further to determine the elemental composition of structural phases that were initially identified, similar to the protocol described by Hatton and Brook (1992) for elemental analysis of glass-ionomer cements. Grids were further coated with carbon, and spot analyses were performed with another microscope (Philips Technai 12) equipped with an x-ray analyzer (EDAX Inc., Mahwah, NJ, USA) at 80 kV.
To locate potential capillary pore spaces within RMGICs that may harbor retained water (Yap and Lee, 1997), we immersed additional 2-mm-thick slabs in 50 wt% ammoniacal silver nitrate, according to the silver impregnation protocol reported by Tay and Pashley (2003). After reduction of the diamine silver ion complexes to metallic silver, the specimens were processed in the manner previously described.
Environmental Scanning Electron Microscopy
Another 2-mm-thick slab was produced from each bonded tooth and polished under wet conditions with 1200-grit silicon carbide paper, followed by 1 µm alumina (Buehler Ltd.). These surfaces were brought into relief by being etched with 10% phosphoric acid (Bisco Inc.). They were placed on the Peltier (cooling) stage of a field-emission/environmental-scanning electron microscpe (Philips XL-30 ESEM-FEG) and examined at 20 kV wet and without being coated. The temperature was fixed at 4°C, and the vapor pressure of the specimen chamber was maintained at 6.1 Torr to achieve a 100% relative humidity (Stokes et al., 2002).
To evaluate the effects of dehydration stresses on RMGIC/dentin interfaces, we further examined the wet slabs at different humidities (95%, 90%, 85%, and 75%) by a gradual reduction of the chamber pressure, following the method described by Neubauer and Jennings (1996) for examination of water-based Portland cements. This enabled us to determine specific sites along the interfaces that are susceptible to micro-crack initiation.
Microtensile Bond Testing and Fractographic Analysis
Five teeth were used for each group and bonded with the respective RMGICs. They were sectioned into 0.9 x 0.9 mm beams containing the interfaces. From the array of beams sectioned from each tooth, 3 beams, each located at 2 mm central to the dentino-enamel junction along the corner of the array, were selected for bond testing. Twenty beams from each group were subjected to tensile stress until failure, with the use of a universal testing machine (Model 4440; Instron Inc., Canton, MA, USA) at a crosshead speed of 1 mm/min. The data were analyzed by Kruskal-Wallis one-way ANOVA on ranks and by Dunn’s multiple-comparison tests at 
= 0.05.
The dentin sides of 4 fractured beams were air-dried and sputter-coated with gold/palladium for examination with a scanning electron microscope (Cambridge Stereoscan 360, Cambridge, UK) operating at 15 kV.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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). Both the resin matrix and the absorption layer (Fig. 1B) in Fuji II LC contained additional multilocular phases that were absent in Photac-Fil Quick. Elemental analyses revealed that calcium was present in both the absorption layer and the multilocular phases. The relative concentrations of silicon and aluminum in the multilocular phases were higher than those in the absorption layer (Fig. 1C), but lower than those in the resin matrix (not shown). The absorption layer was absent when both RMGICs were bonded to dehydrated dentin, with the cement in direct contact with the hybrid layer (Fig. 1D).
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) and around the glass fillers in the resin matrices (Fig. 2B). Unlike air voids, these capillary pore spaces were evident only after silver impregnation. A siliceous hydrogel layer, indicative of GI reaction (Tay et al., 2001b), could also be identified around the periphery of the basic glass fillers (Fig. 2B).
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). Micro-cracks began to appear between the absorption layer and the RMGICs at 90% relative humidity (Fig. 3B), developing into larger continuous cracks at 85% relative humidity (not shown). Although vertical dehydration cracks began to form within the absorption layer at 75% relative humidity (Fig. 3C), integrity was still maintained between the absorption layer and the hybrid layer. Conversely, continuous cracks were seen adjacent to the hybrid layer in specimens bonded to dehydrated dentin, even at 100% relative humidity (Fig. 3D), indicating that these were true gaps that developed autogenously before the specimens were subjected to dehydration stresses.
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) showed significant differences between specimens that were bonded to hydrated dentin and those bonded to dehydrated dentin (P < 0.001). Premature failure occurred in all 20 of the beams of Fuji II LC and in 19 of the beams of Photac-Fil Quick that were bonded to dehydrated dentin. Fractographic analysis revealed the presence of artifactual cracks that were expected from air-dried, water-containing specimens. In specimens bonded to hydrated dentin, fractured RMGICs were identified on the surfaces of absorption layers (Fig. 4B). Conversely, in specimens bonded to dehydrated dentin, remnant RMGICs were seen along the surface of the hybrid layer, and in specimens with the dentinal tubules exposed by polyacrylic acid-etching (Fig. 4C).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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).
In this study, the RMGICs were stored under isolated conditions, with no possible water gain or loss to the external environment. When cured under similar conditions, conventional GICs exhibited spontaneous fracture due to the development of internal stresses (Feilzer et al., 1995). Although autogenous shrinkage occurred in RMGICs (Watts et al., 2000), no spontaneous fractures were observed (Feilzer et al., 1995). In this study, spontaneous fractures occurred only when the RMGICs were bonded to dehydrated dentin (Fig. 3D), in the absence of the absorption layer (Figs. 1D), and with direct contact of remnant RMGICs with the surface of the hybrid layer (Fig. 4C). This confirmed our hypotheses that the absorption layer is formed only in the presence of water derived from hydrated dentin, and is crucial for mediating the bond between RMGICs and dentin. The RMGIC absorption layer has been thought to act as a stress-breaking layer (Sidhu et al., 2002) and may provide a function similar to that of a dentin adhesive layer in relieving polymerization shrinkage stresses (Ausiello et al., 2002). Such a layer was most prominent when dentinal tubules were cut ‘end-on’ in deep dentin (Fig. 1A), and was either thin (1–2 µm) or absent when hydrated superficial dentin with sparsely distributed, or obliquely oriented, tubules was examined (Tay and Pashley, unpublished results). Although our use of a dehydrated dentin model is far-removed from clinical reality, the correlation between the absence of the absorption layer and poor bonding in laboratory-dehydrated dentin explains why dentin surfaces should be kept moist to promote the bonding of RMGICs (Wilder et al., 1998).
The existence of capillary pore spaces (i.e., water trees) within the absorption layers probably provided the channels for continuous water flux from dentin across the RMGIC during its maturation (Sidhu et al., 1997; Watson et al., 1998). Despite the differences in setting mechanisms between RMGICs and Portland cements, both are equally susceptible to moisture gain and loss, and water movement can occur within the sealed cements during setting (Bentz and Hansen, 2000). Portland cements with low water/cement ratios exhibit the characteristics of ‘self-desiccation’ and ‘autogenous shrinkage’, and water is critical for their hydration via capillary pore spaces within the cement (Bentz et al., 2001).
Two distinct glass-ionomer reactions were reported in RMGICs (Young, 2002), with the first corresponding to the consumption of intrinsic water in the RMGICs, and the second attributing to subsequent extrinsic water sorption. Compared with conventional glass-ionomer cements, the water content in RMGICs is reduced. With photopolymerization, the acid-base reaction is also reduced during the early setting stage of RMGICs (Kakaboura et al., 1996). Water that resided within the capillary pore spaces may eventually be used up in subsequent acid-base reaction. We speculate that, under sealed conditions, RMGICs may self-desiccate if external water is not available to replenish the consumed water, by drying out the fine capillaries’ spaces within the maturing cement. The menisci that develop along the interface between the water-filled and air-filled capillaries can create capillary pressures that generate high tensile stresses and strains, leading to autogenous shrinkage of the RMGIC (i.e., shrinkage apart from those generated by free radical polymerization or dehydration shrinkage resulting from environmental water loss). In the absence of a stress-relieving mechanism, these strains may concentrate along the junction between the RMGIC and dentin, due to a stiffness-toughness mismatch between the restorative material and dentin. The observed spontaneous fractures resulted in the recorded null bond strengths in both dehydrated groups, despite the presence of remnant glass-ionomer inclusions on the surface of the hybrid layer and within the dentinal tubules (Fig. 4C). Conversely, autogenous shrinkage may be minimized when an absorption layer functions as a stress-relieving, semi-permeable membrane that permits water to enter the RMGIC matrix, under an osmotic gradient created by the unpolymerized HEMA (Hamid et al., 1998) and hydrophilic salts within the matrix. Theoretically, continuous water flux should occur until there is a balance of osmotic pressure. This hypothesis should be further examined with the complementary use of confocal microscopy and environmental scanning electron microscopy.
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ACKNOWLEDGMENTS |
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Received May 28, 2003; Last revision May 25, 2004; Accepted June 2, 2004
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Ausiello P, Apicella A, Davidson CL (2002). Effect of adhesive layer properties on stress distribution in composite restorations—a 3D finite element analysis. Dent Mater 18:295–303.[ISI][Medline]
Bentz DP, Hansen KK (2000). Preliminary observations of water movement in cement pastes during curing using x-ray absorption. Cem Concr Res 30:1157–1168.
Bentz DP, Geiker MR, Hansen KK (2001). Shrinkage-reducing admixtures and early age desiccation in cement pastes and mortars. Cem Concr Res 31:1075–1085.
Chen YC, Chirila TV, Russo AV (1993). Hydrophilic sponges based on 2-hydroxyethyl methacrylate, II: effect of monomer mixture composition on the equilibrium water content and swelling behaviour. Mater Forum 17:57–65.
Chirila TV, Chen YC, Griffin BJ, Constable IJ (1993). Hydrophilic sponges based on 2 hydroxyethyl methacrylate, I: effect of monomer mixture composition on the pore size. Polym International 32:221–232.
Chuang SF, Jin YT, Tsai PF, Wong TY (2001). Effect of various surface protections on the margin microleakage of resin-modified glass ionomer cements. J Prosthet Dent 86:309–314.[ISI][Medline]
Feilzer AJ, Kakaboura AI, de Gee AJ, Davidson CL (1995). The influence of water sorption on the development of setting shrinkage stress in traditional and resin-modified glass ionomer cements. Dent Mater 11:186–190.[ISI][Medline]
Hamid A, Okamoto A, Iwaku M, Hume WR (1998). Component release from light-activated glass ionomer and compomer cements. J Oral Rehabil 25:94–99.[ISI][Medline]
Hatton PV, Brook IM (1992). Characterisation of the ultrastructure of glass-ionomer (poly-alkenoate) cement. Br Dent J 173:275–277.[ISI][Medline]
Kakaboura A, Eliades G, Palaghias G (1996). An FTIR study on the setting mechanism of resin-modified glass ionomer restoratives. Dent Mater 12:173–178.[ISI][Medline]
Nakanuma K, Hayakawa T, Tomita T, Yamazaki M (1998). Effect of the application of dentin primers and a dentin bonding agent on the adhesion between the resin-modified glass-ionomer cement and dentin. Dent Mater 14:281–286.[ISI][Medline]
Neubauer CM, Jennings HM (1996). The role of the environmental scanning electron microscope in the investigation of cement-based materials. Scanning 18:515–521.
Palmer G, Anstice HM, Pearson GJ (1999). The effect of curing regime on the release of hydroxyethyl methacrylate (HEMA) from resin-modified glass-ionomer cements. J Dent 27:303–311.[ISI][Medline]
Pereira PN, Yamada T, Tei R, Tagami J (1997). Bond strength and interface micromorphology of an improved resin-modified glass ionomer cement. Am J Dent 10:128–132.[ISI][Medline]
Sidhu SK, Watson TF (1998). Interfacial characteristics of resin-modified glass-ionomer materials: a study on fluid permeability using confocal fluorescence microscopy. J Dent Res 77:1749–1759.
Sidhu SK, Sherriff M, Watson TF (1997). The effects of maturity and dehydration shrinkage on resin-modified glass-ionomer restorations. J Dent Res 76:1495–1501.
Sidhu SK, Sherriff M, Watson TF (1999). Failure of resin-modified glass-ionomers subjected to shear loading. J Dent 27:373–381.[ISI][Medline]
Sidhu SK, Pilecki P, Cheng PC, Watson TF (2002). The morphology and stability of resin-modified glass-ionomer adhesive at the dentin/resin-based composite interface. Am J Dent 15:129–136.[ISI][Medline]
Stokes DJ, Rea SM, Porter AE, Best SM, Bonfield W (2002). Characterization of biomedical materials, cells & interfaces using environmental SEM (ESEM). Mater Res Soc Symp Proc 711:113–118.
Tanumiharja M, Burrow MF, Tyas MJ (2000). Microtensile bond strengths of glass ionomer (polyalkenoate) cements to dentine using four conditioners. J Dent 28:363–366.
Tay FR, Pashley DH (2003). Water treeing—a potential mechanism for degradation of dentin adhesives. Am J Dent 16:6–12.[ISI][Medline]
Tay FR, Pashley EL, Huang C, Hashimoto M, Sano H, Smales RJ, et al. (2001a). The glass-ionomer phase in resin-based restorative materials. J Dent Res 80:1808–1812.
Tay FR, Smales RJ, Ngo H, Wei SH, Pashley DH (2001b). Effect of different conditioning protocols on adhesion of a GIC to dentin. J Adhes Dent 3:153–167.[Medline]
Watson TF, Sidhu SK, Griffiths BM (1994). Ionomers vs. composites at the tooth interface. In: Proceedings of the Second International Symposium on Glass Ionomer Cements. Hunt P, editor. Philadelphia: International Symposia in Dentistry, pp. 123–130.
Watson TF, Pagliari D, Sidhu SK, Naasan MA (1998). Confocal microscopic observation of structural changes in glass-ionomer cements and tooth interfaces. Biomaterials 19:581–588.[ISI][Medline]
Watts DC, Kisumbi BK, Toworfe GK (2000). Dimensional changes of resin/ionomer restoratives in aqueous and neutral media. Dent Mater 16:89–96.[ISI][Medline]
Wilder AD, Boghosian AA, Bayne SC, Heymann HO, Sturdevant JR, Roberson TM (1998). Effect of powder/liquid ratio on the clinical and laboratory performance of resin-modified glass-ionomers. J Dent 26:369–377.[ISI][Medline]
Yap A, Lee CM (1997). Water sorption and solubility of resin-modified polyalkenoate cements. J Oral Rehabil 24:310–314.[ISI][Medline]
Young AM (2002). FTIR investigation of polymerisation and polyacid neutralisation kinetics in resin-modified glass-ionomer dental cements. Biomaterials 23:3289–3295.[ISI][Medline]
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