| Sign In to gain access to subscriptions and/or personal tools. |
Minimizing Dentinal Fluid Flow Associated with Gap FormationDepartment of Restorative Dentistry, School of Dental Science, The University of Melbourne, 720 Swanston Street, Melbourne, Victoria 3010, Australia Correspondence: * corresponding author, palamara{at}unimelb.edu.au
The relationship between gap formation and outward fluid flow and procedures to minimize both phenomena were investigated in extracted human premolars restored in vitro with MOD composite restorations. We hypothesized that either glass-ionomer cement (GIC) liners or low-shrinkage composite could reduce fluid flow related to gap formation. Two groups restored with bonding agents with either high- or low-shrinkage resin composites, and 2 groups restored by either conventional or light-cured GIC liner plus resin composite were compared (8 teeth/group). Fluid flow was measured with an automated apparatus. Baseline fluid flow was low and unchanged after bonding, but increased sharply (though transiently) after teeth were lined with GIC. Outward flow was significantly greater with conventional than with light-cured GIC. Inward fluid flow occurred during light-curing, followed by extensive, prolonged outward flow after curing. Low-shrinkage composite or GIC liners reduced gap formation and limited outward fluid flow. GIC liners promoted outward fluid flow during their setting reactions. Abbreviations: GIC, glass-ionomer cement; CEJ, cemento-enamel junction; MOD, mesio-occluso-distal; SEM, scanning electron microscopy.
Key Words: polymerization shrinkage • dentinal fluid flow • bonding • liner • GIC
Polymerization shrinkage and gap formation remain a problem in clinical dentistry (Davidson and Feilzer, 1997). The gap can permit the invasion of oral fluids and bacteria (Koran and Kurschner, 1998), resulting in postoperative sensitivity, secondary caries, and pulpal damage (Eick and Welch, 1986; Garberoglio et al., 1995). Transudation of dentinal fluid through dentin adhesives can result in the presence of water-filled regions within the hybrid and adhesive resin layer, contributing to degradation of adhesive-dentin bonds and interference with infiltration of the bonding agent (Tay and Pashley, 2003). 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.
Preparation of Specimens Thirty-two intact, human maxillary premolars extracted for orthodontic reasons were collected. Informed consent was obtained under a protocol approved by the Ethics in Human Research Committee, University of Melbourne. Teeth were stored in 1% Chloramine T solution and used within 1 mo of extraction. Roots were removed 3 mm below the cemento-enamel junction (CEJ), by means of a slow-speed diamond saw with water coolant (Struers, Ballerup, Denmark). Pulp tissue was removed, then the teeth were immersed in 1% sodium hypochlorite (Milton’s solution, Procter & Gamble, Parramatta, Australia) for 5 min, for the removal of remaining pulp tissue. 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 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 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
Data Analysis
One specimen in the conventional GIC group was excluded from analysis, due to technical problems with fluid flow measurement. Fluid flow was observed at all stages of the restorative procedure (e.g., inward fluid flow during light-curing), but is not discussed in this report. Slow outward fluid flow (approximately 0.04 to 0.05 nL/sec) occurred in all specimens during baseline (Table 1  ). 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).
Following light-curing, all groups demonstrated outward fluid flow, beginning approximately 50 to 100 sec after curing was completed (Table 2 ). 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).
Gaps occurred in all specimens in the bonding groups, in the range 3 to 20 µm (Fig. 2 ), 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.
Clinically, composite restorations are placed in teeth which have a positive intrapulpal pressure and slow outward fluid flow. As a result, restorations in deep dentin are often placed under conditions of excessive moisture, which can impede resin infiltration, reduce bond strength, and create water blisters, impairing the effectiveness of the seal (Prati and Pashley, 1992; Sano et al., 1999). Inadequate bonding will also lead to gap formation at the adhesive-dentin interface (Davidson and Feilzer, 1997; Abbas et al., 2003). In this study, we investigated the relationship between gap formation and outward fluid flow, and procedures to minimize both phenomena. If it occurred clinically, the large, sustained outward fluid flow into the gap at the restorative-tooth interface may be one factor contributing to the post-operative pain frequently reported with composite restorations (Brännström, 1984; Eick and Welch, 1986; Ciucchi et al., 1997; Orchardson and Cadden, 2001). The duration of the outward fluid movement was not measured in this study, as a consequence of technical limitations, but no diminution in rate was observed, even 15 minutes’ post-curing. 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 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.
This study was supported by the Cooperative Research Center for Oral Health Science (CRC-OHS) and by the School of Dental Science, University of Melbourne, Australia.
A supplemental appendix to this article is published electronically only at http://www.dentalresearch.org. Received for publication June 23, 2005. Revision received May 4, 2006. Accepted for publication July 6, 2006.
Journal of Dental Research, Vol. 85, No. 11,
1027-1031 (2006)
|
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
|
|
 |
|
Sign In to gain access to subscriptions and/or personal tools.
TOP
ABSTRACT
INTRODUCTION
