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RESEARCH REPORT |
Department of Restorative Dentistry, School of Dental Science, The University of Melbourne, 720 Swanston Street, Melbourne, Victoria 3010, Australia
* corresponding author, palamara{at}unimelb.edu.au
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: polymerization shrinkage • dentinal fluid flow • bonding • liner • GIC
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Since pulpal interstitial fluid is under positive tissue pressure, dentinal fluid flows from the pulp through exposed tubules that are not sealed peripherally by restorative materials (Pashley, 1991; Orchardson and Cadden, 2001). We have previously observed that prolonged rapid outward fluid flow occurred after the light-curing of resin composite in extracted teeth (unpublished observations), beginning approximately 1–2 min after light-curing and persisting for at least 20 min. It was assumed that a gap between the restoration and cavity wall induced outward fluid movement.
To overcome problems associated with gap formation due to polymerization shrinkage, investigators have advocated several techniques, including the use of dentinal bonding agents (Douvitsas, 1991), low-shrinkage resin composite (Yap and Soh, 2004), glass-ionomer cement liners (Kemp-Scholte and Davidson, 1990), and different application techniques (Tolidis et al., 1998; Alomari et al., 2001). Most studies have attempted to minimize gap formation; however, none has investigated fluid flow resulting from gap formation. We undertook this study to investigate the efficacy of bonding agents used with either high- or low-shrinkage composite, and liners (conventional and light-cured GIC), in minimizing outward fluid flow after light-curing. The null hypothesis tested was that no difference in dentinal fluid flow would occur in teeth restored with a bonding agent with either high- or low-shrinkage composite or with GIC liners.
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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We used a plastic block (4 cm square, 5 mm thick) with a central hole to mount each tooth. An 18-gauge needle was glued into the hole, and the tooth was attached to the block by means of cyano-acrylate cement (M-Bond 200, Micro-Measurements Group Inc., Raleigh, NC, USA), and subsequently covered by epoxy resin (Araldite, Vantico AG, Basel, Switzerland). Before each experiment, the intact tooth was pressure-tested at 13 kPa to ensure that there was no leakage (Ciucchi et al., 1997). Afterward, an MOD cavity was prepared with a bucco-palatal width one-third of the inter-cuspal distance, and proximal boxes approximately 1 mm above the CEJ. Occlusal depth was approximately 3 mm.
Restorative Procedures
Teeth were randomly assigned to 4 groups (2 bonding and 2 liner groups), of 8 teeth each. High- and low-shrinkage composites were selected based on published values for shrinkage (Watts et al., 2003; Stavridakis et al., 2005). Differences in volumetric shrinkage and cuspal displacement during light-curing were confirmed in preliminary experiments [high-shrinkage composite—volumetric shrinkage, 2.89 ± 0.39%, cuspal contraction, 10.9 ± 2.7 µm; low-shrinkage composite—volumetric shrinkage, 1.92 ± 0.36%, cuspal contraction, 4.0 ± 1.2 µm (mean ± SD, n = 10); details in the APPENDIX]. For each composite material, the etchant, bonding agent, and resin were from the same manufacturer. Detailed composition of materials is given in the APPENDIX, and all materials were used according to manufacturers’ directions.
Group A teeth (bonding group + high-shrinkage composite) were etched with phosphoric acid, 37.5% (Gel Etchant, Kerr Corp., Orange, CA, USA; batch no. 401056), then rinsed with water for 10 sec, and dried with the use of a triplex syringe for 20 sec. One coat of bonding agent (Optibond Solo Plus, Kerr Corp.; batch no. 403078) was applied and gently air-dried, then light-cured (Coltulux 75, Coltene/Whaledent Inc., Mahwah, NJ, USA) for 40 sec. Cavities were restored with high-shrinkage composite (Point 4 Optimized Particle Composite System, Kerr Corp.; batch no. 402692) placed as a single increment and cured for 120 sec (40 sec/side).
Group B teeth (bonding group + low-shrinkage composite) were restored as were those in group A, but with etching agent (37% phosphoric acid; Super Etch, SDI Limited, Bayswater, Australia; batch no. 030648), bonding agent (Stae adhesive, SDI Limited; batch no. 021073), and composite (Glacier Restorative System, SDI Limited; batch no. 020560) from the same manufacturer.
For Group C teeth (conventional GIC liner), the same procedures were followed as in group A, except that dentin was conditioned with polyacrylic acid 10% (Ketac Conditioner, 3M ESPE Dental Products, St. Paul, MN, USA; batch no. 180742) for 10 sec before conventional GIC liner (Ketac Bond Conventional Glass Ionomer Liner/Base, 3M ESPE; batch no. 168389 for liquid and 176982 for powder) was applied, with a thickness of approximately 1 mm.
In group D (light-cured GIC liner), we applied approximately a 1-mm thickness of light-cured GIC (Vitrabond Light-Cured Glass Ionomer Liner/Base, 3M ESPE; batch no. 3EA for liquid and 3FC for powder) to the entire pulpal floor and light-cured it for 40 sec. Subsequent bonding procedures were similar to those for group A. Cavities were then restored with high-shrinkage composite.
Fluid Flow Study
Each tooth was connected by silicone tubing, filled with phosphate-buffered saline, to a capillary tube (30 cm long, internal diameter 0.84 mm), placed horizontally in an automated apparatus for the measurement of fluid flow (Flodec, De Marco Engineering, Geneva, Switzerland). An infrared-emitting diode and a sensing element located opposite the diode on a screw mechanism permitted the linear displacement of an air bubble to be measured. This device can trace linear displacement as small as 5 µm (2.8 nL) (Ciucchi et al., 1995). The device was modified to allow for the measurement of fluid flow over very short time intervals, and the sampling rate was set at 25 points/sec. Data were fed into spreadsheet software (Microsoft, Excel 2003). A hydrostatic pressure of 1.3 kPa was applied throughout all procedures to imitate physiological pulp pressure (Ciucchi et al., 1995).
Before restorative procedures began, baseline flow rate was established for 5 min. Fluid flow was monitored continuously, and fluid flow rates for the bonding groups were evaluated in detail at three stages: after acid etching, after curing of the bonding agent, and after the composite was light-cured. For the liner groups, dentinal fluid flow was measured after the application of liner, and after light-curing of the composite. For each step, fluid flow was measured over a five-minute interval before we proceeded to the next step. Following light-curing of the composite, outward flow was measured for 15 min. Each tooth was covered by a small plastic cup filled with moist cotton wool to minimize evaporation throughout all measurements (Sidhu et al., 2004).
SEM Observation
After restoration, all teeth were stored at 100% relative humidity for 24 hrs, then sectioned vertically in a mesio-distal plane with the use of a slow-speed diamond saw (Struers). The cut surfaces were polished, and 3 specimens of each group were randomly selected for observation under SEM. Both an epoxy replica and the original teeth were viewed. The specimens were replicated with the use of vinyl polysiloxane impression material (Affinis, Coltene/Whaledent Inc.), and left for 24 hrs before epoxy resin was poured. Replicas and tooth sections were mounted, sputter-coated, and examined by SEM (Philips XL 30 FEG, Eindhoven, The Netherlands). We used replica specimens to measure the size of any gap at the resin-dentin interface.
Data Analysis
We used repeated-measures ANOVA (General Linear Model) or one-way ANOVA to compare effects of the 4 materials on fluid flow. Post hoc comparisons were conducted by Tukey’s test. All analyses were performed at the 0.05 level of significance (Minitab Inc., State College, PA, USA).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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). Neither etching nor bonding caused a significant increase in outward flow, for both bonding groups (p > 0.05). Conversely, after GIC liner was applied, outward flow was significantly greater than during baseline for both groups (p = 0.0003 for conventional GIC; p = 0.008 for light-cured GIC), but was less pronounced with light-cured GIC (Table 1).
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). No significant differences were found in the lag period between curing and beginning outward flow (p > 0.05). The low-shrinkage composite group had the lowest initial outward flow rate (p = 0.001) (Fig. 1, Table 2). Both bonding groups showed continuing outward flow for at least 15 min (the longest time period tested). In contrast, outward flow ceased in both liner groups after 8–11 min. Teeth restored with high-shrinkage composite demonstrated the greatest final volume of outward fluid flow, while the group lined with light-cured GIC showed the lowest final volume (p = 0.001).
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), whereas liner groups exhibited infrequent gaps from 3 to 8 µm wide. A good contact between dentin and restorative material was an infrequent finding in the high-shrinkage composite group, but the majority of low-shrinkage composite specimens exhibited only intermittent gaps. Specimens with light-cured GIC exhibited more intimate contact between liner cement and dentin than that of conventional GIC, and SEM observations of original teeth demonstrated only an occasional gap between liner and dentin.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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As in similar previous studies, variability among teeth was high (Hashimoto et al., 2004; Sidhu et al., 2004). The level of variation appears to be an inevitable consequence of individual differences in tooth morphology and variability in cavity preparation, remaining dentin thickness, tubule diameter, etc. Compared with previous findings from studies where dentin disks rather than conventional cavity preparations were used (Hashimoto et al., 2004), acid etching in this study did not produce a significant increase in outward flow. In both studies, however, bonding was ineffective in reducing fluid flow. The subsequent lack of adhesion of the restorative material to dentin led to gap formation of 5 to 20 µm (Brännström, 1984; Lim et al., 2002).
The adhesive bond will not be damaged if shrinkage stresses generated at the initial stage in the curing process can be fully compensated for by viscous flow of the resin. The higher the bond strength and viscous flow, the longer the restoration can withstand gap formation, and the smaller the resulting gap (Feilzer et al., 1990). This phenomenon may explain why a lag period of 50 to 100 sec occurred before initial outward fluid flow appeared. When outward flow starts, it indicates that gap formation is present. Low-shrinkage composite may take longer to reach its gel point, permitting the resin to flow before becoming rigid, resulting in the longest lag period and lowest initial flow rate (Table 2
). Alternatively, thermal effects may also contribute to the delay. Inward fluid movement occurs during light-curing (Hashimoto et al., 2004), which will be reversed as the tooth cools after light-curing is completed.
The reduced outward fluid flow associated with the low-shrinkage composite may be the result of several factors. The resins used in the low-shrinkage material were different from those in the high-shrinkage material (see APPENDIX). Both bonding agent and resin used with the low-shrinkage composite contained urethane dimethacrylate resin (UDMA), which showed improved toughness and flexibility after polymerization (Glenn, 1982; Rawls, 2003) compared with BisGMA, the major resin in the high-shrinkage material. The greater flexibility may permit the resin to continue flowing while shrinkage stress occurs (Youngson and Grey, 1992), with less tendency to gap formation and retained residual stresses. The lower cuspal displacement associated with the low-shrinkage composite is also likely to minimize gap formation.
The use of a liner can prevent gap formation by decreasing the mass of composite restoration and by absorbing some polymerization stress (Tolidis et al., 1998; Alomari et al., 2001). Hence, outward fluid flow in the liner groups immediately after placement may be due to the GIC cements drawing water during the acid-base setting reaction (Mount, 1994; Yiu et al., 2004), which ceases after the cements set. After light-curing of composite, outward flow was slower and stopped earlier in the light-cured than in the conventional GIC, probably due to the lower diffusion rate through light-cured GIC (Sidhu and Watson, 1995). Light-cured GIC had better adaptation to the dentin surface than did conventional GIC, with minimal gap formation. Under clinical conditions, the bond between GIC liner and dentin may deteriorate over time (Yiu et al., 2004); fluid movement as a result of this breakdown is likely to be very gradual.
In conclusion, under the conditions of this study, gap formation and the consequent outward fluid flow could be minimized by two strategies. The use of a low-shrinkage composite resulted in much-reduced fluid flow. The use of GIC liner led to an immediate but transient fluid flow, which appeared to be associated with water uptake during the setting reaction. Clinically, the effectiveness of a GIC liner will depend on the durability of the bond, the assessment of which is beyond the scope of this study.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Received June 23, 2005; Last revision May 4, 2006; Accepted July 6, 2006
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REFERENCES |
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