Hardness is one of the basic properties of dental materials, specially composite resins which is relevant to their polymerization. The aim of this study was to evaluate the effect of light curing distance and the color of clear Mylar strips on surface hardness of Silorane-based (SCR) and Methacrylate-based composite resins (MCR).
Materials and methods:
40 samples of MCRs (Filtek Z250) and SCRs (Filtek P90) were prepared in size of 5 mm×2 mm (80 samples in total). The samples divided into 8 groups (10 samples in each one) based on the color of clear Mylar strips (white or blue) and distance from light curing source (0 mm or 2 mm). All the samples cured for 40 second and stored in incubator for 24 hours in 37°C temperature. Surface hardness test was done by Vickers test machine and the collected data were analyzed by one-way ANOVA and paired T-test by using SPSS software version 13 at significant level of 0.05.
MCRs cured with blue Mylar strips from 0 mm distance had the highest (114.5 kg/mm2) and SCRs cured with white Mylar strips from 2 mm distance had the lowest (42.2 kg/mm2) mean of surface hardness. Also, the results of comparison among SCRs and MCRs showed significant differences among all groups (all P values <0.01).
The hardness decreased as the distance increased and the blue Mylar strips provided higher hardness than clear ones. Also, Filtek Z250 showed higher hardness compared to Filtek P90.
The use of composite resins has opened a new approach in dentistry as it can fulfill the expectations and requirements in the oral environment with proper function and special esthetics .
In order to modify the properties of composites, structural changes were executed. Silorane-based composite resins (SCR) introduced alongside with conventional Methacrylate-based composite resins (MCR) as a new composite resin to decrease the polymerization shrinkage .
Surface hardness is one of the important characteristics of composite resins which can affect the clinical success rate of restorations. Nowadays various types of composite resins are available based on different fillers, which provides different hardness of composite resins too . The distance of composite resins from the light curing source is another factor that influences the hardness. It is proved that the hardness is higher in lower curing distances [4-8].
In one study, Caldes et al. evaluated the influence of curing distance on the knoop Hardness. They examined three different distances (0, 6 and 12 mm) and the result showed that the hardness of the resin composite decreased as the light tip distance increased .
In another study, de Araújo et al. investigated the effect of light curing method, composite resin shade and depth of curing on their microhardness. They concluded that light curing method including variations of time, depth of curing and the composite resin shade influenced their microhardness .
It is assumed that shade is an important factor that can affect the mechanical properties of light-curing polymerization  and several studies stated that darker shades made higher hardness [1, 12]. Also, the resemblance in color between the intermediate materials (Mylar strips) and curing light may affect the polymerization procedure, thus some manufacturers produce the Mylar strips in blue.
As there were no previous study based on effect of Mylar strip’s color on polymerization, the aim of this study was to evaluate the effect of light curing distance and the color of clear Mylar strips on surface hardness of two different composite resins (SCR and MCR).
MATERIALS AND METHODOLOGY
In this analytical-observational in vitro study, a hard polymeric disk-shaped mold (washer) in the size of 5 mm×2 mm was placed on a slab for preparing composite resin disks. Then 40 samples of Methacrylate-based (Filtek Z250, 3M ESPE dental product, USA) and Silorane-based composite resins (Filtek P90, 3M ESPE dental product, USA) were packed in the mold (80 samples in total). The samples divided into 8 groups (10 samples in each one), based on the color of clear Mylar strips and distance from light curing source, in the following procedure:
Group M1: MCRs cured from 0mm distance with using clear blue Mylar strips (HaweBlue Striproll, Switzerland).
Group M2: MCRs cured from 2 mm distance with using clear blue Mylar strips.
Group M3: MCRs cured from 0mm distance with using clear white Mylar strips (100 universal Strips, Alfred Becht Gmbh,D Offenburg, Germany).
Group M4: MCRs cured from 2 mm distance with using clear white Mylar strips.
Group S1: SCRs cured from 0 mm distance with using clear blue Mylar strips.
Group S2: SCRs cured from 2 mm distance with using clear blue Mylar strips.
Group S3: SCRs cured from 0 mm distance with using clear white Mylar strips.
Group S4: SCRs cured from 2 mm distance with using clear white Mylar strips.
Also, another mold was placed on the samples for providing 2mm distance in mentioned designed groups. All the samples were cured by LED light curing unit (Litex 695C, Dentamerica, Taiwan) with the same time (40 seconds) and output (checked 1200 mw/cm2) for all the groups. All the samples stored in incubator for 24 hours in 37°C temperature. Finally the samples were subjected to surface hardness test by Vickers test machine (Fretz, Germany) based on indentation method. The test was prepared 3 times for each sample with 10×10-3 kg/mm2 in 10 seconds.
The collected data were analyzed by one-way ANOVA and paired T-test (for comparing groups, 2 by 2) with using SPSS software version 13 at significant level of 0.05.
Table 1 represents the mean surface hardness of the groups based on curing distance and color of clear Mylar strips. As the results showed, the group M1 had the highest (114.5 kg/mm2) and group S4 had the lowest (42.2 kg/mm2) mean of surface hardness. Also, the results of one-way ANOVA demonstrated that the mean surface hardness showed significant differences among all the groups(p value<0.05).
The mean surface hardness (Kg/mm2) in different groups of study.
Two by two comparison by paired T-test between different groups.
M1 and M2
S1 and S2
M1 and S1
M1 and M3
S1 and S3
M2 and S2
M1 and M4
S1 and S4
M3 and S3
M2 and M3
S2 and S3
M4 and S4
M2 and M4
S2 and S4
M3 and M4
S3 and S4
Table 2 represents the comparisons of mean surface hardness among all the groups based on paired T-test. The comparison among MCRs clarified that group M1 had significant differences with groups M2, M3 and M4 (all P values<0.05). The mean surface hardness was higher in M3 than M4 despite of no significant differences (P value=0.2). The comparison among SCRs showed significant differences among all the groups (all P values<0.05) except between S1 and S3 (P value>0.05). Also, the results of comparison among SCRs and MCRs showed significant differences among all groups (all P values<0.01).
Based on the results, curing from 0mm distance (M1, S1 and S3) made higher surface hardness than curing from 2mm distance (M2, S2 and S4) (all P values<0.05), specially in SCRs, which indicates that distance have negative impact on the final surface hardness.
In 2009, Aguiar et al. conducted a study about the effect of curing distance and layer thickness on composite resins’ microhardness. They concluded that the microhardness decreased as the curing distance and layer thickness increased . In another study, Thome et al. observed the impact of curing distance (in 0, 6 and 12 mm), shade and filler size on final hardness of composite resins. Their final result showed that curing distance and composite resin’s color had significant effect on final microhardness . The result of present study confirms the findings of two mentioned studies. Also, many other studies reported the negative impact of curing distance too [5, 9, 13].
Based on physics, when a monochromatic light passes through a clear object with the same color, the light intensity increases which cause higher polymerization and hardness in composite resins. The blue Mylar strips (M1) had significant effect in comparison with white ones (M3) in 0mm distance (P value=0.02), but no significant difference was found when the distance was 2 mm (group M2 and M4) (P value>0.05). The results were reverse in SCRs meaning that, significant effect was found between groups S2 and S4 (2 mm distance) and the blue Mylar strips made higher surface hardness (P value<0.01). One study stated that the highest degree of conversion and Knoop hardness happened at 2 mm depth in SCRs . The other reason might be due to structure and polymerization characteristics of these two types of composite resins but no other studies have been surveyed the effect of clear Mylar strips’ color on polymerization since now.
As Table 2 illustrates, higher hardness was found in MCR groups (M1, M2,M3 and M4) when compared to similar groups in SCRs (S1, S2, S3 and S4) (all P values<0.01). This might be due to different polymerizations of these composite resins. It seems that cross link reaction in MCRs results in higher rates of polymerization and lower amounts of free monomers than ring opening reaction in SCRs. Also, Camphorquinone of initiators in MCRs, absorb higher ranges of LED light (maximum 486 nm) which is produced by LED light source. Adding diphenyl iodonium or triphenyl sulfonium salts to amin system, which is used in MCRs, not only reduces the transferring of photon-wasting electrons and recombination reactions, but also provides a recycling path for consumed photosensitizer and increases the rates of polymerization effectively [15-18]. In another view, SCRs are polymerized by cationic polymerization systems, which is different from the radical polymerization of MCRs . Maybe using LED light source, which contains multilayer of triphenylene, can provide harder and deeper curing in SCRs due to deep blue light(in range of 436-456 nm) [15, 20].
In one study, Bechtold et al. evaluated Knoop hardness and polymerization depth in Filtek P60 composite (MCR) and Filtek P90 composite (SCR) by using three different light curing techniques (Occlusal; transdental; transdental+ occlusal) in class II restorations. They claimed that SCRs showed lower hardness than MCRs significantly .
In another study, Kusgoz et al. compared the depth of curing, hardness, degree of conversion and cervical sealing ability in SCRs and MCRs. Their results revealed that the degree of conversion, hardness and curing depth in SCRs were significantly lower than MCRs . Besides the results of present study, two other studies [23, 24] confirmed this fact too.
With considering limitations of invitro studies, it can be concluded that:
The hardness of both types of composite decreased as the distance increased.
Blue Mylar strips provided higher hardness than white ones, depending on the composite type and light source distances.
The methacrylate-base composite resin (Filtek Z250) showed higher hardness than Silorane-based composite resin (Filtek P90).
CONFLICT OF INTEREST
The authors confirm that this article content has no conflict of interest.
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Restorative Dentistry Department, Torabinejad Dental Research Center and Dental Materials Research Center, School of Dentistry, Isfahan University of Medical Sciences and Islamic Azad University (Khorasgan branch), Isfahan, Iran
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