INTRODUCTION
Restorative filling materials used in dentistry are required to have long-term durability in the oral cavity1). Tooth repair is increasingly performed with tooth-colored restorative materials2) that may have varying degrees of durability. It is noteworthy that in the oral cavity, the absence of good mechanical strength coupled with chemical processes or dissolution can increase the surface roughness of a restorative — which is the first sign of erosion3,4).
Recently, attention has shifted to the role of diet, particularly fruit juices, in the etiology of dental erosion5). For example, Gedalia et al.6) reported that enamel microhardness was reduced after just one hour of exposure to Coca-Cola. As for surface hardness, it has been used as an indirect method to measure polymerization adequacy. This is because it is a relatively simple method which also shows good correlation to the degree of polymerization measured by means of infrared spectroscopy7,8).
For restorative composites, the standard method of polymerization is curing in the oral cavity under visible light. An inherent disadvantage of resin composites is that polymerization adequacy depends on factors that are not standardized, including light intensity, curing time, and material thickness1). Moreover, compomers (or polyacid-modified composites) are dimethacrylate resins with carboxyl groups grafted into the molecule. After placement, the materials absorb water from the oral environment over a number of months. This process activates the
acid, which then reacts with the basic glass particles in the compomer in an ionomer-type reaction4).
It is well established that adequate polymerization plays an important role to the success of dental restorations. Against this background, surface hardness was used in this study to evaluate the performance of different light-cured composite resins.
To fulfill the objective of this study, the effects of different solutions, immersion times, material surfaces (top or bottom), and roughness on the surface hardness of composite resin materials were investigated.
MATERIALS AND METHODS Composite resin specimens
Table 1 lists the restorative materials used in this study, which comprised composites from five different groups. For each group, 36 specimens were made using fiberglass die molds of 5 mm diameter and 2 mm height (hence a total of 180 specimens). The specimens were light-cured for 40 seconds at 450−500 mW/cm2 (Hilux Dental Curing Light Unit, Benlioğlu Dental Inc., Turkey). The light tip was in close contact with the restoration surface during polymerization. After curing, the cured specimens were separated from the molds.
Half of the specimens were polished with Sof-Lex disks (3M, St Paul, MN, USA) with a light-orange aluminum grit (30-μm slurry; 3M ESPE Dental Products 2385P), the other half of the specimens were unpolished. Sof-Lex disks were used in this Technical Report
Effects of different solutions on the surface hardness of composite resin materials
Nuran YANIKOĞLU, Zeynep Yeşil DUYMUŞ and Baykal YILMAZ
Department of Prosthodontics, School of Dentistry, Atatürk University, Erzurum, Turkey Corresponding author: Nuran YANIKOĞLU; E-mail: ndinckal@atauni.edu.tr
In this study, the surface hardness of five light-cured composite resins were evaluated, namely: filled (Estelite), nanofil (Ælite), unfilled (Valux Plus), hybrid (Tetric ceram), and Ormocer-based (Admira) composite resins. The microhardness values of composite specimens were measured at the top and bottom surfaces after 24 hours or 30 days of immersion in different solutions (tea, coffee, Turkish coffee, mouthwash, cola, and distilled water). Comparisons were made with univariate analysis of variance and Duncan’s multiple range test. It was found that rough specimens of reinforced nano-hybrid composite material immersed in cola for 30 days had the lowest surface hardness (33.20), whereas rough specimens of hybrid composite material immersed in cola for 24 hours had the highest surface hardness (156.00). In both tea and coffee, the top surfaces tended to be harder than the bottom ones. In conclusion, the five different materials exhibited different hardnesses, and that the hardness values of composite materials were statistically different in different immersion solutions.
Keywords: Composite resins, Surface hardness, Solution
Received Dec 19, 2007: Accepted Oct 30, 2008
Table 1 Materials used in this study Type of
Composite Material Composition Brand name Lot number Manufacturer Filled
microhybrid Submicron filled composite, silica-zirconia filler, 0.1−0.3 μm.
Estelite Σ E511 2009-05 Tokuyama Dental Corporation, Tokyo, Japan Hybrid BIS-GMA, Urethane
dimethacrylate, triethylene glycol dimethacrylate, Inorganic fillers: barium glass, 0.7 μm.
Tetric Ceram J04946 2010-02 Ivoclar, Vivadent Schaan, Liechtenstein
Ormocer Aliphatic and aromatic dimethacrylat micro particle , 0.7 μm
Admira 651615 2008-10 Voco, P.O Box 767 27457, Cuxhaven, Germany Hybrid Zirconia silica, BIS-GMA,
TEGDMA resins, 3.5-0.01 μm
ESPE Valux
Plus 6KY 2009-05 3M , ESPE , Dental Products St. Paul, MN, 55144-1000 Reinforced
Nano hybrid Methacrylate based resin,
5-100 nanometers Ælite Aesthetic
Enamel 0600000151
2009-01 Voco, Germany
Table 2 Univariate analysis of variance
SOURCE df Sum of
Square Mean
Square F P
MATERIAL 4 39376.476 9844.119 119.033 .000
SURFACE TREATMENT 1 18313.364 18313.364 221.441 .000
SOLUTIONS 5 16935.576 3387.115 40.956 .000
TIME 2 228773.047 114386.52 1383.138 .000
MATERIAL SURFACE 1 1768.915 1768.915 21.389 .000
MATERIAL * SURFACE TREATMENT 4 77549.170 19387.29 234.427 .000
MATERIAL * SOLUTIONS 20 67739.485 3386.974 40.955 .000
SURFACE TREATMENT * SOLUTIONS 5 5583.484 1116.697 13.503 .000
MATERIAL* SURFACE TREATMENT * SOLUTIONS 20 64682.839 3234.142 39.107 .000
MATERIAL* TIME 8 59332.128 7416.516 89.679 .000
SURFACE TREATMENT * TIME 2 11683.691 5841.845 70.638 .000
MATERIAL *SURFACE TREATMENT * TIME 8 13305.002 1663.125 20.110 .000
SOLUTIONS * TIME 10 29659.845 2965.984 35.864 .000
MATERIAL * SOLUTIONS * TIME 40 62061.001 1551.525 18.761 .000
SURFACE TREATMENT * SOLUTIONS * TIME 10 6079.957 607.996 7.352 .000
MATERIAL * SURFACE TERATMENT* SOLUTIONS * TIME 39 5959055 1527.950 18.476 .000
MATERIAL * MATERIAL SURFACE 4 1735.013 433.753 5.245 .000
SURFACE TREATMENT* MATERIAL SURFACE 1 1.927 1.927 0.023 .879
MATERIAL * SURFACE TREATMENT* MATERIAL SURFACE* 4 2995.806 748.952 9.056 .000
SOLUTIONS * MATERIAL SURFACE 5 313.758 62.752 0.759 .580
MATERIAL * SOLUTIONS * MATERIAL SURFACE 20 674.566 33.728 0.408 .991
SURFACE TREATMENT* SOLUTIONS * MATERIAL SURFACE 5 217.987 43.597 0.527 .756 MATERIAL*SURFACE TREATMENT* SOLUTIONS *
MATERIAL SURFACE 20 939.562 46.978 0.568 .936
TIME* MATERIAL SURFACE 2 267.201 133.600 1.615 .199
MATERIAL * TIME * MATERIAL SURFACE 8 2529.353 316.169 3.823 .000
SURFACE TREATMENT * TIME * MATERIAL SURFACE 2 151.148 75.574 0.914 .401
MATERIAL*SURFACE TREATMENT * TIME * MATERIAL
SURFACE 8 2617.173 327.147 3.956 .000
SOLUTIONS * TIME *MATERIAL SURFACE 10 932.582 93.258 1.128 .337
MATERIAL* SOLUTIONS * TIME *MATERIAL SURFACE 40 1979.804 49.495 0.598 .978
SURFACE TREATMENT* SOLUTIONS * TIME *MATERIAL
SURFACE 10 125.293 12.529 0.152 .999
MATERIAL* SURFACE TREATMENT* SOLUTIONS * TIME
*MATERIAL SURFACE 39 1639.192 42.031 0.508 .995
ERROR 1442 119254.44 82.701
TOTAL 1800 9214281.79
study because they have been shown to provide the smoothest surfaces9).
Immersion solutions
Six specimens of each material were immersed in one of the six different solutions (distilled water, mouthwash, cola, tea, coffee, or Turkish coffee).
Distilled water served as a control solution.
Immersions lasted for 1 day and 30 days, except for the immersions in mouthwash. Based on the assumption that most people use mouthwash only once per day, each test material was immersed in 20 ml of the mouthwash (Kloroben, Drogsan, Ankara, Turkey) for 2 minutes or 1 hour (equivalent to 2 minutes for 30 days). The latter immersion period was based on a previous study which reported that a period of 12 hours was equivalent to 2 minutes of daily mouthwash for 1 year10).
The coffee solution was prepared by adding 15 g of coffee powder (Nescafe Classic, Nestlé, Société des Produits, S.P.N., Switzerland) into 500 ml of boiling distilled water. The tea solution was prepared by immersing five prefabricated bags of tea (Lipton Yellow Label, Corlu, Turkey) into 500 ml of boiling water for 10 minutes. Turkish coffee was prepared by adding 5−7 g of a commercial brand (Kurukahveci Mehmet Efendi, TS 3117,Y, Dudullu, Istanbul) into 65 ml of cold water that had been taken off the heat once it came to a boil.
All the immersions solutions were refreshed every week.
Surface hardness measurement
Vicker’s diamond indenter was used in a microhardness tester (Micromet 2001, Buehler, Illinois, USA) for specimen indentation. For each microhardness test,
two indentations were made randomly on the top and bottom surfaces of each specimen using a load of 10 g for 15 seconds. The effect of material surface was also assessed in this study, because Sharkey et al.11) stated that the interaction between the curing lamp and composite surface significantly influenced the results of the top and bottom surfaces of disk-shaped specimens.
All hardness values were expressed in Vickers hardness, where 1 HV=1.854 P/d2, with P being the indentation load and d the diagonal length.
Statistical analysis
The univariate analysis of variance was used to compare the microhardness values among the five dental composite resins, between two different surface treatments (rough and smooth), among six different immersion solutions (tea, coffee, Turkish coffee, distilled water, mouthwash, cola), between two different immersion times (1 and 30 days), and between two different material surfaces (top and bottom). In addition, combinations of these factors were assessed for significant effects on surface hardness. This was followed by Duncan’s multiple comparison test which compared the effects of immersion time. Statistical analysis was performed using the Statistical Package for Social Sciences (SPSS) software, version 10.0 (SPSS Inc., Chicago, IL, USA).
RESULTS
The univariate analysis of variance indicated that all the five factors considered in this study had statistically significant effects on the surface hardness of composite resin specimens (p<0.001)
Table 3 Mean Vickers hardness values for composite resin materials before immersion in solutions
TEA COFFEE TURKISH
COFFEE COLA MOUTHWASH DISTILLED
WATER
MATERIALS A B A B A B A B A B A B
ÆLITE S 75,08 71,04 75,60 71,04 75,10 70,98 75,80 71,04 75,08 71,02 75,00 71,12 R 47,10 45,28 47,18 45,22 47,10 45,24 47,10 45,28 47,10 45,28 47,12 45,26 ESTELITE S 109,74 102,80 109,76 102,80 109,90 102,76 109,72 102,78 109,70 102,80 109,74 102,80 R 74,90 58,90 74,94 58,98 74,98 58,96 74,98 58,96 74,90 58,94 74,68 58,70 TETRIC
CERAM S 75,64 67,18 75,58 67,16 75,64 67,18 75,72 67,20 75,52 67,08 75,60 67,08 R 61,80 74,02 61,84 74,10 61,80 74,02 61,58 74,00 61,84 74,00 61,80 74,02 VALUX
PLUS S 70,60 78,46 70,62 78,44 70,60 78,46 70,58 78,42 70,48 78,66 70,50 78,78 R 76,52 73,04 76,54 73,06 76,52 73,04 76,56 73,00 76,52 73,04 76,54 73,02 ADMIRA S 53,58 46,92 53,60 46,84 55,31 50,84 53,58 46,92 53,60 46,92 53,58 46,94 R 74,90 58,90 53,84 50,48 53,76 50,56 53,86 50,46 53,88 50,46 53,84 50,48 S: Smooth surface; R: Rough surface; A: Top surface; B: Bottom surface.
(Table 2).
Table 3 shows the mean Vickers hardness values of composite resin specimens before immersion in any of the solutions. On the other hand, Figs. 1 to 6 show the mean Vickers hardness values and standard deviations after immersion in each solution for 1 and 30 days.
Results in this study revealed that the mean surface hardness values before immersion were lower than those after 1 day of immersion. In particular,
hybrid composite resin (Tetric Ceram) and Ormocer (Admira) with rough surfaces (unpolished) showed significant increases in surface hardness after 1 day in cola.
In most conditions, surface hardness significantly decreased after 30 days of immersion (p<0.001).
Duncan’s multiple comparison test showed that water caused the least reduction in mean surface hardness after 30 days, followed by mouthwash (p<0.001). At the end of 30 days, Ælite registered its
Fig. 1 Mean Vickers hardness values of composite resin materials in tea (analyzed by Duncan’s test).
Fig. 2 Mean Vickers hardness values of composite resin materials in coffee (analyzed by Duncan’s test).
Fig. 3 Mean Vickers hardness values of composite resin materials in Turkish coffee (analyzed by Duncan’s test).
Fig. 4 Mean Vickers hardness values of composite resin materials in cola (analyzed by Duncan’s test).
Fig. 5 Mean Vickers hardness values of composite resin materials in distilled water (analyzed by Duncan’s test).
Fig. 6 Mean Vickers hardness values of composite resin materials in mouthwash (analyzed by Duncan’s test).
lowest surface hardness in cola and Estelite in coffee.
DISCUSSION
The variability in our results was consistent with other studies which showed that several factors — including type of composite, polishing time, and polishing technique — had significant influences on surface roughness, hardness, and microleakage12). On the surface roughness of resin composites, it is related to the composition and porosity of the material as well as the instruments and procedures used in polishing13−15). In this study, Sof-Lex was used for all the composite specimens because it provided the smoothest final surface for composites16).
Resin composites are heterogeneous materials that are composed of three major components: resin matrix, filler particles, and silane coupling agent17). The resin matrix and filler particles have different levels of hardness that cause variations in removal efficiency during polishing, and that these variations can lead to differences in surface roughness13,18). Therefore, some of the variability in our results might be attributed to the varied polishing responses of the composite materials tested.
In general, the surface hardness values of composite resin specimens after 24 hours of immersion are higher than those after 30 days. This is because the materials deteriorate by way of water absorption. As for surface hardness comparison before immersion and after 24 hours of water storage, it is not meaningful at all as it boils down to the polymerization reaction19). On polymerization reaction kinetics, Murchison and Moore20) stated that application of the curing light for at least 40 seconds resulted in significantly higher Knoop hardness values than light-curing for 20 seconds. Furthermore, increased microhardness was found for composite resin materials after water storage, which was attributable to a post-cure period where chemical bonds continue to be made21).
In addition, curing light is absorbed and scattered by composite resins, resulting in higher light intensity at the top than the bottom surface.
For this reason, Bayindir and Yildiz22) found significantly different top and bottom surface hardness values, whereby those of the top surface were consistently higher than those of the bottom surface. However, in the present study, there were no statistically significant differences in hardness between the top and bottom material surfaces (p>0.05), although there was a tendency towards higher surface hardness values on the top surface when compared to the bottom surface.
It has been shown that the curing light generally cures the macrofill and heavy filled hybrids better than the other composites23,24). As depth of cure
affects hardness, and where a significant negative correlation exists between surface hardness and wear resistance, Say et al.25) showed that there were indeed significant differences in wear resistance among the different resin composites. In the present study, the results were consistent with those of Say et al.25) whereby the hybrid composite material exhibited the highest surface hardness (p<0.001).
According to Tsuruta and Viohl26), another factor that influenced the hardness of light-cured polyalkenoate cements stored in air was humidity.
In their report, hardness increased with time when test specimens were stored in dry conditions, but no increase occurred when stored in high humidity and in water. As for Bayindir and Yildiz22), they found that there were no statistically significant differences in Vickers hardness among the different composite resins tested when they were stored in water.
Similarly in the present study, the hybrid composite material exhibited higher surface hardness than the microhybrid composite material, but they did not differ significantly from each other when stored in distilled water.
Hybrid systems are not a mere, simplistic combination of glass ionomers and resins. As BisGMA-type resins are very hydrophobic and are not compatible with the aqueous environment of the glass ionomer cements, other resin matrices such as hydroxyethyl methacrylate (HEMA) and UDMA are used in hybrid materials27). Geurtsen et al.28) stated that the higher organic matrix of hybrid materials may be the reason of higher susceptibility to water absorption and material disintegration. Conversely, the hydrophobic matrix of the resin composite material may have prevented water absorption, thus contributing to the microhardness of the material itself.
Microhybrid composite materials incorporate a high volume fraction of filler particles with a narrow particle size distribution, and that the particles had a mean size below 1 μm20). Besides Bis-GMA, the matrix of this composite resin contains UEDMA and modified urethane (Bis-EMA) to reduce the polymerization shrinkage and intrinsic stresses of the material. Furthermore, to enable a higher volume of fillers to be incorporated in the polymeric matrix, the microhybrid composite has 60% of small silica particles30).
Resin-based restoratives were found to undergo greater micromorphological damage following an acid challenge than after storage in either distilled water or artificial saliva31). While acids adversely affect the surface integrity of resin-based restoratives, it must be pointed out that this erosive loss of material subsequently led to an increase in the pH of the acidic storage solutions32,33).
Modern tooth-colored dental restorative
materials have been shown to behave differently in different solutions such as cola, apple juice, and orange juice, with the latter two proving more aggressive than the cola34). Orange juice is known to contain citric acid, and cola to contain phosphoric acid. These acids are both erosive, and hence it is not clear why cola was less aggressive35). In the present study, Tetric Ceram, Valux Plus, and Admira exhibted the highest surface hardness values in cola.
On the other hand, Ælite exhibited the lowest surface hardness value in cola, followed by Estelite. These observations could be considered to be consistent with another study which found that composite resin specimens immersed in cola showed modest increases in hardness up to 1 month, but had statistically significant reductions in hardness after 1 year34). Apart from a significant interaction between time and storage solution in the present study, the univariate analysis of variance also showed statistically significant interactions between time and surface treatment (rough or smooth), and between time and material (p<0.001).
All specimens displayed color changes after immersion in tea, coffee, and Turkish coffee, although these discolorations were not measured with a spectrophotometer. Crispin and Angelo36) stated that the tea-coffee solution caused the greatest amount of darkening over a 30-day period.
In clinical conditions, the effects of mouthwash use on restorative materials may depend on many in vivo factors that cannot be replicated in vitro. Saliva, salivary pellicle (a thin, naturally occurring bacterial film from salivary proteins that regularly forms on teeth and other surfaces in the oral cavity), as well as foods and beverages consumed may have additive or mitigating effects on the effects of mouthwashes37). Gürgan et al.38) reported that both alcohol-containing and alcohol-free mouthwashes affected the hardness of resin composites. Similarly, Penugonda et al.39) stated that the alcohol content in mouthwash could affect composite hardness, whereby this softening affect was directly related to the percentage of alcohol in the mouthwash. However, Gürdal et al.37) argued that alcohol content had no effect on the microhardness of esthetic restorative materials. In the present study, the mouthwash that was used contained alcohol, but the effects of the mouthwash on surface hardness were not different from the effects of distilled water. In this sense, this result was similar to that reported by Gürdal et al.37).
Concerning the differences in experimental results between the present study and those of previous studies37−39), it was most probably due to differences in specimen preparation method. In the study of Gürgan et al.38), the specimens were finished⁄polished and then immersed in distilled water to complete post-irradiation polymerization.
However, in the present study, the specimens were finished⁄polished and then immersed in mouthwash.
As the curing reaction of photopolymerized composite resins continues for a period of 24 hours, premature finishing⁄polishing procedures may cause the resins to be susceptible to the heat generated during the finishing⁄polishing procedures. Consequently, this results in decreased surface hardness38). Further on the effects of mouthwash on hardness and wear, Yap et al.10) suggested that the restorative material itself also had a contributing influence.
In addition, the ethanol content of the mouthwashes in the previous studies37−39) might be higher than that tested in the present study.
Furthermore, their storage periods in the mouthwash were much longer than the immersion periods used in this study (2 minutes and 1 hour). Therefore, the detrimental effects of ethanol might not be duly manifested at the end of both immersion periods used in the current study. Since the effects of the mouthwash were not different from those of distilled water, it was most probably the water component of the mouthwash that affected the microhardness changes28).
CONCLUSIONS
Within the limitations of this study, the following conclusions were drawn:
1. Material type, storage solution, surface treatment, material surface, and immersion time were significant factors that influenced surface hardness.
2. Decrease of surface hardness was time- dependent.
3. Tetric Ceram (hybrid composite resin material) had the highest surface hardness (on the bottom surface of the specimen) after immersion in cola for 1 day.
4. Specimens stored in distilled water had less reduction in surface hardness over time than those stored in other solutions.
5. Tea, cola, and coffee solutions significantly affected surface hardness.
6. A rough composite surface affected the surface hardness for some composite materials when stored in tea, coffee, or cola.
ACKNOWLEDGMENTS
This work was presented as a poster at the 31st Annual Conference of the European Prosthodontic Association, Athens, Greece, 11−13 October 2007.
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