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The effect of irradiation distance on microhardness of resin composites cured with different light curing units.

Objectives: The aim of this study was to compare the microhardness of five different resin composites at different irradiation distances (2 mm and 9 mm) by using three light curing units (quartz tungsten halogen, light emitting diodes and plasma arc). Methods: A total of 210 disc-shaped samples (2 mm height and 6 mm diameter) were prepared from different resin composites (Simile, Aelite Aesthetic Enamel, Clearfil AP-X, Grandio caps and Filtek Z250). Photoactivation was performed by using quartz tungsten halogen, light emitting diode and plasma arc curing units at two irradiation distances (2 mm and 9 mm). Then the samples (n=7/per group) were stored dry in dark at 37°C for 24 h. The Vickers hardness test was performed on the resin composite layer with a microhardness tester (Shimadzu HMV). Data were statistically analyzed using nonparametric Kruskal Wallis and Mann-Whitney U tests. Results: Statistical analysis revealed that the resin composite groups, the type of the light curing units and the irradiation distances have significant effects on the microhardness values (P<.05). Conclusions: Light curing unit and irradiation distance are important factors to be considered for obtaining adequate microhardness of different resin composite groups.

containing filler particles ranging from 0.5 to 4 μm, and 0.02 to 0.09 μm, respectively. 8 More recently, nanofilled and nanohybrid composites have been introduced with a filler size ranging from 5 to 100 nm in an attempt to have enhanced properties in both aesthetics and mechanical performance. 4 The curing light technology has advanced with the introduction of high intensity quartz tungsten halogen (QTH), light emitting diodes (LED) and plasma arc (PAC) curing units. 9,10 Until recently, conventional QTH LCUs were widely used to cure resin composites. 11,12 These LCUs are susceptible to intensity output degradation with time as a result of the age of the bulb and its reflector, blistering and cracking of the filter, damage to the fiber-optic tips, because of repeated sterilization or heat generation. 12 To overcome these problems, boosted versions of QTH, PAC and LED LCUs that possess higher light intensity and shorter polymerization cycles than conventional LCUs have been developed. 13,14 These high intensity LCUs may provide higher values of degree of conversion, and better physical and mechanical properties for the polymerized resin composites. 15 Among these properties, testing microhardness is an efficient method to assess the relative degree of conversion of resins and, thus, the efficiency of the tested light curing sources. 16,17 Additionally, the hardness of a material is a relative measure of its resistance to indentation when a specific and constant load is applied. Thus, hardness might be described as a measure of the ability of a material to resist indentation or scratching. 16 An adequate polymerization of resin composites is crucial for the ultimate success and longevity of the restoration. 17 It depends not only on the irradiance of the curing light and irradiation time but also on the distance of the light tip from the tooth-restorative material. 18,19 Because the light intensity diminishes as the tip of the source light moves away from the resin composite's surface, the light-curing tip unit should be in direct contact with the restoration's surface. However, sometimes cavity design does not allow the polymerization within this distance. 20 On these grounds, the purpose of this investigation was to evaluate the effect of various light-tip distances (2 mm and 9 mm) on Vicker's microhardness (VHN) values of different resin composites cured with QTH, LED and PAC LCUs.
The null hypothesis tested was that the type and distance of the LCUs from the restoration would affect the VHN microhardness of the tested resin composites.

MATERIALS AND METHODS Specimen preparation
Five commercial light-cured resin composites were used in this study (Table 1). Shade A2 was chosen to minimize the effect of color on the photopolymerization process. 21 The experimental setup is given in Figure 1. Disc-shaped specimens, 2 mm in height and 6 mm in diameter, were prepared according to the manufacturers' instructions by packing the resin composites into circular polytetrafluoroethylene moulds. A polyethylene film was placed on the top and base of the resin composite materials. Additionally, a 1 mm thick glass slide was seated on the top of the mould to exclude excessive resin composite material and to eliminate possible air bubbles. Then the samples were irradiated from the top through the polyethylene films using a quartz tungsten halogen light (QTH, 1000 mW/cm 2 , Blue Swan Digital, Dentanet, Ankara, Turkey) for 20 s, a light-emitting diode (LED, 1200 mW/cm 2 , Elipar Freelight 2, 3M Espe, USA) for 20 s and a plasma arc (PAC, 2250±50 mW/cm 2 , PlasmaStar, SP-2000, Monitex, Taiwan) for 10 s at different irradiation distances (2 mm and 9 mm). Additionally, the power outputs of the LCUs at 2 mm and 9 mm were measured by a radiometer (Cure Rite, EFOS, New York NY, USA). The curing tip distances were controlled via the use of metal rings. After light polymerization, the specimens were stored dry in dark at 37ºC for 24 h before testing.
Vicker's microhardness test A total of 210 samples were polished under wet conditions with 220, 360, and 600 grit silicon carbide grinding paper (FEPA, Federation of European Producers of Abrasives, Paris, France) and placed on the platform of the tester with the surface being tested facing the diamond indenter (n=7/per group). The VHN test was performed on the cement layer with a microhardness tester (Shimadzu HMV; Shimadzu Corporation, Tokyo, Japan) with 200 g of load application for 15 seconds. Three indentations taken from each sample were not closer than 1 mm to the margin and were averaged to determine the hardness value of each sample. Vicker's hardness values were converted into microhardness values by the machine. All specimen preparations and VHN measurements were performed by the same operator in a darkened environment.

Statistical analysis
Statistical analysis was performed by Statistical Package for Social Sciences (SPSS) 11.5 soft-ware (SPSS Inc., Chicago, IL, United States). The Shapiro Wilk test showed that VHN values of the resin composites were not normally distributed. Data were expressed as median (25 th -75 th ) percentiles. The differences among the resin composites and LCU groups were evaluated by using Kruskal Wallis test, and the irradiation distances were compared by Mann Whitney U test. If the p value from Kruskal Wallis test statistics was statistically significant, multiple comparison tests were used to know which group differed from each other (P<.05). All possible subgroup analyses with Bonferroni Adjustment were applied to control Type I error.

rEsuLts
The median VHN values and (25 th -75 th ) percentiles for samples are shown in Table 2. The results of the VHN test indicated that significant differences were observed based on the type of resin composite (P<.05) and the LCU (P<.05). Moreover, irradiation distances had significant effects on microhardness values of resin composites (P<.05).

European Journal of Dentistry
The statistical ranking for VHN median values among resin composites was obtained as follows: Aelite Aesthetic Enamel ~ Simile < Filtek Z250 < Clearfil AP-X ~ Grandio caps (P<.05). Clearfil AP-X cured with LED at a distance of 2 mm yielded the highest median VHN value (119 VHN) whereas Simile cured with QTH LCU at a distance of 9 mm presented the lowest value overall (61.1 VHN) ( Table 2). In addition, QTH LCU presented similar VHN values with PAC LCU and lower values than LED LCU (P<.05). Additionally, in all groups except Filtek Z250 and Grandio Caps cured with PAC, and Aelite Aesthetic enamel cured with QTH, the VHN values of resin composites decreased with the increase in irradiation distance ( Table 2).

dIscussIon
In the present study, the hardening of resin composites was investigated to ensure the efficacy of different LCUs. Previous studies showed hardness as a good indicator of conversion of double bonds 22,23 and was therefore used in the present study as an indirect measurement of conversion. It was also reported that hardness was useful in determining the development of the mechanical properties of resin composites during their polymerization reaction, and that there was a direct correlation between degree of conversion and hardness development during polymerization, as a consequence of the increase in stiffness and strength of the material. [24][25][26][27] Conflicting results are often indicated in the literature when the effects of different LCUs on resin composites are reported. [28][29][30] This might be explained by the differences between irradiation protocols used, especially regarding the intensities. 31 Previous studies reported that LED LCU cured resin composites as well or better than some QTH LCUs. 17 Cekic-Nagas, Egilmez, Ergun and the results were explained by a formula that indicated the total energy that reached the resin composite. 21 With reference to that formula, in the present study, when the PAC LCU delivered 2200 mW/cm 2 and was used for 10 s, the resin composite might have received 22 J/cm 2 . Moreover, when both the QTH LCU which delivered 1000 mW/cm 2 and the LED LCU which delivered 1200 mW/cm 2 were used for 20 s, the resin composites might have received 20 J/cm 2 and 24 J/cm 2 , respectively (assuming that the same wavelengths are delivered). This could provide an explanation as to why no major differences in microhardness values of resin composites cured with different LCUs were seen. The present experimental results support the hypothesis that the type of LCU and the LCU tip distance from the restoration affect the VHN microhardness of resin composites. To represent clinical distances, the composites were irradiated at 2 mm and 9 mm away from the LCU. A previous study by Thomé et al 20 reported that adequate polymerization demands light intensities greater than 250-300 mW/cm 2 , increment thickness of 2 mm or less and a distance no greater than 6 mm between the LCU and the resin composite's surface. However, if there is an inability to place an LCU tip near the restorative material, this might reduce intensity and provide a lower degree of polymerization. 36 In addition, Caldas et al 37 evaluated the effect of irradiation distance (0, 6 and 12 mm) on hardness of resin composites in another study and found that hardness values of the resin composites decreased when the light tip distance increased. Similarly, in the current study, the VHN values of resin composites irradiated with LCU tip at 9 mm distance were lower than the values cured with LCU tip at 2 mm distance (except Filtek Z250 and Grandio Caps cured with PAC, and Aelite Aesthetic enamel cured with QTH) .
As for the comparison of microhardness values of the resin composites in the present study, Grandio caps and Clearfil AP-X exhibited higher VHN than Filtek Z250, Aelite Aesthetic Enamel and Simile. Despite being nanofilled composites, Aelite Aesthetic Enamel and Simile did not achieve the values that Grandio did. While Grandio has higher filler load (87%) than Aelite Aesthetic Enamel and Simile (73% and 75%, respectively), these resin composites (Aelite Aesthetic enamel and Simile) have similar nanofiller size with (40 nm and 5-20 nm, respectively) Grandio (20-60 nm). This might indicate that the size of the fillers may not be the determinative factor in hardness property when there is an important difference in filler loads. Therefore, differences in the organic matrix and the filler load (Table 1) might be responsible for the following ranking (Aelite Aesthetic Enamel ~ Simile < Filtek Z250 < Clearfil AP-X ~ Grandio caps). Similarly, a previous study by Mota et al 38 investigated the knoop microhardness of five nanofilled composites and correlated the higher knoop microhardness test values of Grandio and Supreme with their filler contents by weight. Besides, an investigation by Xu 39 which evaluated the effect of fillers on composite properties showed a strong positive correlation between the weight filler content and the microhardness values of composites.
In conclusion, from a clinical standpoint, polymerization of resin composites is important in order to obtain sufficient surface hardness. Moreover, the results of this experiment must be brought into relation with conversion data. Further in vitro tests and long-term clinical trials are needed to investigate the effect of microhardness and degree of conversion on the longevity of resin composites.

concLusIons
Within the limitations of this in-vitro study, the following conclusions could be drawn: • High-power LED LCUs might be considered more effective than QTH and PAC LCUs for polymerization of the resin-based materials.
• Resin composites showed different VHN values, depending on their composition, filler types and polymerization method.
• The VHN values of almost all resin composites decreased with the increased irradiation distance, except Filtek Z250 and Grandio Caps cured with PAC, and Aelite Aesthetic enamel cured with QTH.