|Year : 2011 | Volume
| Issue : 3 | Page : 495
|Comparative evaluation of marginal adaptation between nanocomposites and microhybrid composites exposed to two light cure units
Ruchi Dhir Sharma1, Jaideep Sharma2, Anuradha Rani1
1 Department of Conservative Dentistry and Endodontics, Himachal Institute of Dental Sciences, Poanta Sahib, Himachal Pradesh, India
2 Department of Orthodontics, Himachal Institute of Dental Sciences, Poanta Sahib, Himachal Pradesh, India
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|Date of Submission||17-Nov-2010|
|Date of Decision||14-Feb-2011|
|Date of Acceptance||27-Jun-2011|
|Date of Web Publication||3-Nov-2011|
| Abstract|| |
Background: Recent advances in resin adhesives and restorative materials, as well as an increased demand for esthetics, have lead to the introduction of newer resin-based composites like nanocomposites and light-curing units like light emitting diodes (LEDs).
Aim: The present study was conducted to evaluate the effect of conventional quartz tungsten halogen (QTH) curing unit and blue LED on marginal adaptation of microhybrid composite (Filtek Z250) and nanocomposite (Filtek Z350) resins.
Materials and Methods: Eighty Class V cavities were prepared on, extracted human premolars and were divided into four groups of 20 each. The four groups were designed according to the restorative resin and light cure unit used microhybrid/QTH, nanocomposites/QTH, microhybrid/LED and nanocomposites/LED. After thermocycling and immersion in 2% Basic Fuschin dye solution, the teeth were sectioned and dye penetration was observed under a stereomicroscope at 20X magnification.
Statistical Analysis: All the samples were scored and results were analyzed using the "Kruskal Wallis and Mann Whitney tests" with "Wilcoxone correction."
Results: The results revealed least microleakage in microhybrid composites exposed to QTH and maximum in nanocomposites exposed to LED.
Conclusion: Microhybrid composites exhibited lesser microleakage than nanocomposite resins.
Keywords: Light emitting diode, microleakage, marginal adaptation
|How to cite this article:|
Sharma RD, Sharma J, Rani A. Comparative evaluation of marginal adaptation between nanocomposites and microhybrid composites exposed to two light cure units. Indian J Dent Res 2011;22:495
An important milestone in the history of modern restorative dentistry is the development of cured composite resins.  Their development in the 1970s heralded a period of rapid progress in the field of colored restorations.  Despite the inability to control polymerization shrinkage, resin composites are widely used in restorative dentistry because of patients' demand for better esthetics.  All composites undergo 0.6-1.4% shrinkage during polymerization depending on the type of composite, the rate of cure and the amount and nature of the filler.  This shrinkage can result in a gap formation between the composite material and the tooth structure, particularly if the restoration margin is placed in the dentine or cementum  (as is seen in root caries and cervical defects that are prevalent mostly due to aging, gingival recession, and dentin exposure).  Bacteria, fluids, molecules, or ions can pass through the gap between the resin composite and the cavity wall, a process called "microleakage." Microleakage is thought to be responsible for hypersensitivity, secondary caries, pulpal pathoses, and failure of restorations.
|How to cite this URL:|
Sharma RD, Sharma J, Rani A. Comparative evaluation of marginal adaptation between nanocomposites and microhybrid composites exposed to two light cure units. Indian J Dent Res [serial online] 2011 [cited 2013 Dec 6];22:495. Available from: http://www.ijdr.in/text.asp?2011/22/3/495/87082
Poor marginal adaptation (leading to microleakage at the restoration tooth interface) can, therefore, limit the overall lifespan of the restoration, leading to degradation, discoloration, and marginal staining of the restoration.
Marginal breakdown has been attributed to many factors, including differences in the coefficients of thermal expansion between the tooth structure and the restorative material, inadequate adhesion to dentine, and polymerization shrinkage of the resin material.  In the oral cavity, the restorations are subjected to both thermal and mechanical stresses, which also contribute to the increase in marginal deterioration leading to microleakage.  Thus, obtaining a good marginal seal for composite resin to the tooth structure still remains a major challenge in restorative dentistry!
The degree of polymerization shrinkage of composite resins depends not only on the material but also on the intensity, exposure time, spectral output, and direction of curing unit.  This addresses the need to characterize properties like (a) polymeric component to minimize the deleterious effects of contraction stresses developed during polymerization and (b) candidate curing unit to improve the extent of polymerization of the restorative resin. 
For this reason, several new materials have been developed with modifications in filler technology, filler distribution, filler loading, and alterations in the matrices.  The basic formula is higher the filler content, lower is the resin content which causes less shrinkage. In the quest for a higher filler load, several new materials like microhybrid, packable composites, and, more recently, nanocomposites have been introduced.
Microhybrids have been popular in restorative dentistry. They involve tightly clustered spheres of the same size with gaps being filled by smaller-sized spheres. These materials exhibit reduced polymerization shrinkage and offer improved strength, but at the cost of esthetic quality. However, the latest innovations make use of nanotechnology that has become the most popular discipline in science and technology with the introduction of "nanocomposites" in the vocabulary of restorative dentistry.  Nanocomposites consist of two fillers: (a) nano particles that allow high polish ability and (b) nanoclusters that allow higher filler loading, thereby, exhibiting high strength of microhybrids.
Optimum photoactivation of composite resin restorative materials is fundamental for achieving enhanced physical, mechanical, and chemical properties.  Because the material properties cannot be altered by the operator, one can choose the appropriate light-curing unit to attain adequate polymerization of the former in order to achieve best clinical performance. This elevates the importance of light-curing units.  Newer curing units have been introduced recently in an effort to minimize polymerization shrinkage.
Over the past two decades, halogen curing lights (quartz tungsten halogen [QTH]), with an output radiation of λ400-500 nm, have reigned supremacy over various curing lights for polymerizing composite resins. However, light-emitting diodes (LEDs) are becoming increasingly popular in dental practice as they convert electronic energy into light energy more efficiently, produce less heat, and are battery-powered. In addition, LEDs can last for thousands of hours in contrast to the 300-500-h lifespan of conventional QTH bulbs, which are fragile and costly to replace.  Although halogen-curing lights have been popular for polymerization, they present certain disadvantages like heat generation that can possibly harm the dental pulp, bulb silvering that reduces the intensity of the emitted light, gradual loss of light output, and frequent bulb replacement.  Thus, introduction of LEDs, based on gallium nitride technology, in 1995 is the latest innovation to address the shortcomings in composite materials and light-curing units. LEDs use doped semiconductors for the generation of light and present a spectral bandwidth of 440-500 nm. They produce minimal heat.
Although manufacturers claim their respective latest advancements to be the best, however, the literature still lacks sufficient scientific evidence to determine the most favorable technology for restorative resins and curing units. Hence, this in vitro study will investigate the degree of dye penetration as an estimation of marginal adaptation (microleakage) in Class V cavities restored with the microhybrids and nanocomposite resins when exposed to the QTH curing unit and LEDs.
The null hypothesis (H0) for the study considered that both the restorative resins presented equal degree of dye penetration (microleakage) at enamel and dentin margins on exposure to QTH and LED.
| Materials and Methods|| |
The specifications for composite resins, dental adhesives, and light cure units used in the study are shown in [Table 1], [Table 2] and [Table 3], respectively.
Eighty extracted human premolar teeth (both maxillary and mandibular) free of caries, cracks, abrasions, attrition, and restorations were selected for the study. Any extrinsic stains or calculus deposits on the teeth were cleaned and specimens were stored in isotonic saline until used. Box-type Class V cavities were prepared on the buccal surface of the premolars with gingival margins below the cemento-enamel junction (CEJ). The preparations measured 2 mm inciso-gingivally (1 mm below the CEJ) and extended from the mesial line angle to the distal line angle. The cavities measured 1.5 mm in depth in the center and 1 mm at the margins (due to a convex contour of the facial surface of the tooth/axial wall). The enamel margins were given 0.5 mm bevel at a 45-degree angle [Figure 1].
In order to compare microleakage in different groups, teeth were randomly divided into four experimental groups of 20 samples each [Figure 2] and [Figure 3].
The specimens in all the groups were restored with the respective composite resin and cured with the respective curing unit by the following method:
The dentin bonding agent (ALL BOND SE, BISCO, USA) was applied to the prepared cavity as per the manufacturer's instructions. The prepared cavity was gently air dried. Equal number of drops of "ALL BOND SE Parts I and II" (1:1) were dispensed into the mixing well. The components were mixed well until a uniform pink color was obtained. Two coats of dentin bonding agents were applied to the dried preparation and agitated for 10 s using an applicator. It was then light cured with the respective curing unit for 10 s.
The cavity was then restored with the respective composite resin with a teflon-coated composite filling instrument. Because each preparation was 1.5 mm deep, a single bulk increment of resin-based composite was placed into the prepared cavity. The inserted composite resin was shaped. After placing the mylar strip, the restored cavity was light cured using the respective light curing unit for 20 s (as per the manufacturer's instructions). The tip of the light cure unit was placed approximately 1 mm away from the surface of the restoration.
The prepared specimens were subjected to 1000 cycles of thermocycling at 5°C-55±2C with a dwell time of 25 s.
After immersion in water for 10 days, the surfaces of teeth were painted with two coats of nail varnish, leaving only the restoration and the surrounding 1 mm area unpainted. To prevent any dye leakage through the apical foramen, it was sealed with yellow sticky wax [Figure 4].
|Figure 4: Teeth coated with nail varnish and sealed with yellow sticky wax|
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All the specimens were immersed in a freshly prepared 2% solution of Basic Fuschin dye  for 24 h in separate containers and were labelled for identification.
After drying the samples at room temperature, the teeth were sectioned buccolingually with the help of a diamond disc held in a straight hand-piece. The samples were observed under a binocular optical zoom stereomicroscope (NIKON SMZ 1000 ; Excel technologies, Enfield, CT) at 20X magnification [Figure 5]. Two observers scored the degree of dye penetration to avoid any bias.
The ranking system used to score the degree of dye penetration is shown in [Figure 6]. 
Score for degree of dye penetration in the samples was obtained [Figure 7].
The statistical analysis for marginal adaptation was performed using the Kruskal Wallis test followed by the Mann Whitney u-tests with the Wilcoxone correction for pair-wise comparisons at a significance level of P < 0.05.
| Results|| |
[Table 4] represents the scores for dye penetration in each sample at the enamel and dentin margins in all the four groups.
|Table 4: Score for dye penetration in each sample at the enamel and dentin margins in all the four groups|
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The data are represented graphically in [Figure 8].
|Figure 8: Graph showing mean scores for dye penetration in all the groups at the enamel and dentin margins, cured with quartz tungsten halogen and light emitting diode|
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[Table 5] and [Figure 8] show that microleakage in either of the composite resins at the enamel and dentin margins was less when cured with QTH as compared with LED.
|Table 5: Mean scores and comparison of dye penetration in composites, at the enamel and dentin margins, cured with QTH and LED|
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[Table 6] represents distribution of dye penetration scores at the enamel and dentin margins in all the groups.
|Table 6: Distribution of dye penetration scores at the enamel and dentin margins in all the groups|
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The data are represented graphically in [Figure 9].
|Figure 9: Graph showing distribution of dye penetration scores at the enamel and dentin margins in all the groups|
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The Kruskal Wallis test showed that dye leakage in the microhybrid and nanocomposites on exposure to QTH and LED curing units were different. When pair-wise comparisons were made by the Mann Whitney U-test, the microhybrid composites showed significantly lesser dye leakage (P < 0.05) than the nanocomposite along both the enamel and the dentin margins. Thus, the null hypothesis was rejected for the study.
However, statistically, for all the groups at the enamel and dentin margins P > 0.05. Hence, dye leakage for the all the four groups was significant at 5%. Thus, it can be concluded that although all the experimental groups showed dye leakage at the enamel and dentin margins, statistically, there was no significant difference in leakage at both the margins.
The results for dye leakage are obtained in the ascending order as:
Microhybrid composite exposed to QTH < nanocomposite exposed to QTH < microhybrid composite exposed to LED < nanocomposite exposed to LED.
| Discussion|| |
The dye leakage method was used in this study because it was simple, effective, inexpensive, and did not require the use of complex laboratory equipment. The degree of dye penetration indicates the inert space between the tooth margin (enamel and dentin or cementum) and the restorative material interface that could allow the ingress of bacterial endotoxins and their inflammatory products. Class V cavities were prepared on these teeth because cervical lesions are frequently observed and indicated for composite restorations. They involve both the enamel and the dentin margins and therefore the nature of adaptation of composite resin could be compared at both the margins. The enamel margins were beveled in order to increase the surface area for composites to bond to enamel. The self-etch technique was employed using the "All-Bond SE" adhesive (sixth generation, dentin bonding agent) for all the restorations. These adhesives have recently been introduced and combine the functions of primer and adhesive components, which have eliminated the need for separate etch and rinse, thereby reducing the time for clinical procedure. The adhesive system was used strictly according to the manufacturer's recommendations. Because the cavities were only 1.5 mm in depth, the bulk technique was used to restore them with the respective composite resin.  Both the LED and the halogen curing lights were used at comparable high intensities above 500 mW/cm 2 (LED-508 mW/cm 2 and QTH-540 mW/cm 2 ).
The restorative materials are constantly exposed to thermal variations when placed in the oral environment, due to intake of food and fluids at varying temperatures. Hence, all the specimens for microleakage evaluation were subjected to thermocycling procedures at temperatures of 5°-55°±20°C, with a dwell time of 25 s, for 1000 cycles, to simulate temperature changes that take place in the oral environment.  This particular temperature range was used according to the International Organization for Standardization (ISO) TR11405 standard, and this is the estimate of the range that has been reported on the surfaces of molar teeth in the mouth of the patient.  It aims at thermally stressing the junction at the tooth-restoration interface by subjecting the restored tooth to extreme temperature changes compatible with temperature changes encountered intraorally.  A 2% Basic Fuschin dye was used for dye penetration because it provided a simple, quantitative, economical, and comparable method of evaluating the various composite systems. 
Microhybrid composites exhibited least microleakage on exposure to QTH and LED at both the enamel and the dentin margins as compared with nanocomposites. These results can be attributed to higher filler loading with smaller particle size of nanocomposites in comparison with comparatively larger-sized filler particles and lesser filler loading in the microhybrid composites. High filler loading results in a high degree of stiffness, which can lead to high shrinkage stress  Another factor is the monomer size. Larger the molecule (microhybrid composite), lesser is the polymerization shrinkage and lesser is the microgap formation.  Also, smaller-sized particles in nanocomposites cause scattering of light and decrease its absorption, thereby reducing the overall polymerization and increasing the microleakage in the material.  Also, nanocomposites contain a high amount of tri-ethylene glycol dimethacrylate (TEGDMA) in comparison with the minor amounts present in the microhybrid composite. The low-molecular weight TEGDMA and resultant high number of double bonds per unit weight create a high degree of crosslinking, creating a rigid resin with a relatively high shrinkage. Also, the majority of TEGDMA monomers elute within few hours, which may again contribute to microgap formations and, ultimately, increased microleakage. 
It was observed that except for microhybrid composites exposed to QTH, the maximum number of samples in all other groups exhibited significant dye leakage (SCORE 3) at both the enamel and the dentin margins. This could be attributed to the high configuration factor (C-factor) of 5 in Class V cavities and use of self-etch adhesive used in the study. C-factor is the ratio of bonded surfaces to unbounded surfaces on a tooth preparation. Higher the C-factor, greater is the potential for bond disruption from polymerization effects. , Also, All-Bond SE is a mildly aggressive self-etch adhesive with a pH of 2.2. One-step self-etch adhesives are more commonly associated with lower bonding effectiveness to both enamel/dentin.  This is because of high hydrophillicity due to which one-step self-etch adhesives behave as semipermeable membranes, allowing fluids to pass through, and seriously jeopardizing bond durability.  Self-etch adhesives are found to result in shallow resin tag penetration due to inability to remove smear layer that gets incorporated into the bonded layers that plugs the opened dentinal tubules. Thus, an increased number of closed tubules interferes with the adhesion capabilities of the self-etch primer. 
The mean scores for dye penetration at the enamel and dentin margins showed that leakage at the dentin margin was slightly higher at the enamel margins in all the groups except for "nanocomposites exposed to LEDs' group, where the scores are almost equal. However, when the Wilcoxone correction for pair-wise comparison was used, no statistical significant differences in microleakage were found at the enamel and the dentin margins with different composite resins exposed to different light cure units (P>0.05). This is in accordance with the results of studies carried out by other researchers. ,
QTH (250-500 nm) was found to cure composite resins more efficiently as compared with LEDs for both the composite resins due to the narrow spectrum of emission in LEDs (320-360 nm), which is closer to that of the absorption spectrum of the photoinitiator (Camphorquinone) present in the nanocomposite but insufficient for the microhybrid composites. ,
Resin-based composites with higher filler loading and smaller filler particles (nanocomposites) exhibit higher microleakage compared with those with bigger filler particles (microhybrids). Also, light-curing units with a narrow spectrum of emission (LEDs) may result in higher microleakage values compared with the traditional curing units with a broader emission spectrum (QTH). Hence, use caution when photoactivating composite resins using narrow spectrum lights.
| Conclusion|| |
Within the limitations of this study, it can be concluded that microhybrid composites exhibited lesser microleakage than nanocomposite resins. Highest microleakage in nanocomposites can be attributed to higher filler load and decreased particle size. High leakage at both the enamel and the dentin margins can be attributed to the use of the self -etch adhesive used in the study. QTH was found to be a more effective curing unit as compared with LEDs, which can be attributed to the narrow spectrum of emission in LEDs.
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Ruchi Dhir Sharma
Department of Conservative Dentistry and Endodontics, Himachal Institute of Dental Sciences, Poanta Sahib, Himachal Pradesh
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]
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