Effectiveness of resin composite polymerization when cured at different depths and with different curing lights
By Fernanda Regina Voltarelli, DDS, MS
Claudia Batitucci Dos Santos-Daroz, DDS, MS
Marcelo Correa Alves, MS
Alessandra Rezende Peris, DDS, MS, PhD
Giselle Maria Marchi, DDS, MS, PhD
Featured in General Dentistry, July/August 2009
Pg. 314-319
Posted on Thursday, July 02, 2009
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This study evaluated how photocuring devices affected the microhardness of composite resin cylinders. For this study, 120 specimens of composite were fabricated and allocated randomly into 12 groups (n = 10), according to the light source (quartz-tungsten-halogen (QTH), LED, argon laser, plasma arc) and the height of the specimen (2.0, 4.0, or 6.0 mm). Twenty-four hours after the specimens were fabricated, the Knoop microhardness test was performed on bottom and top surfaces.
Statistical analysis showed significant interaction among light sources, between light sources and specimen heights, and between the surfaces. Compared to the QTH specimens, the argon laser and plasma arc specimens showed reduced polymerization on the top surface, while the plasma arc specimens showed reduced polymerization on the bottom surface. The 4.0 mm samples demonstrated higher Knoop microhardness than the 2.0 mm and 6.0 mm samples, especially when argon laser and plasma arc curing lights were used. The microhardness was always higher on the top surface than on the bottom surface. No photocuring unit was able to properly polymerize the bottom surface as completely as the top surface.
Received: December 4, 2008
Accepted: February 20, 2009
Until recently, the photocuring unit used most frequently was a quartz lamp with a tungsten filament in a halogen environment (QTH); however, more recently introduced visible light curing (VLC) sources have offered certain advantages compared to QTH lamps.1,2 QTH lamps use incandescence to produce a very broad spectrum of light; as a result, filters are required to restrict the emitted light to the blue region of the spectrum, which allows for the polymerization of resin composites.3 In addition, the technology of QTH lamps is less expensive compared to other VLC sources.4 However, halogen bulbs, reflectors, and filters can degrade over time due to the large amount of heat produced during the operating cycles. This degradation reduces curing effectiveness over time and increases the risk of premature restoration failure.3
LED curing lights produce a blue light when subjected to an electrical current; the light emitted is a narrow band with a peak emission of 470 nm.1 Battery-powered LEDs have low power requirements and thus produce a minimal amount of heat during curing.1 The newer generation of LEDs offer a higher power density than previous LED lights, allowing them to polymerize some resins as well as or better than some QTH lamps.1,5 However, as their light spectrum is narrow, these photocuring units can be used only on materials that utilize camphoroquinone as the photoinitiator system.6
Argon laser curing units produce usable energy by exciting ions in an argon-filled resonating chamber.6 These units emit a distinctly different light compared to QTH or LED lights. The argon laser delivers energy in a variety of wavelengths (454, 458, 466, 472, 488, and 497 nm) that fall within the absorption requirements for camphoroquinone.7 The specific wavelengths travel in parallel waves that are in phase spatially and temporally.8 Because the argon laser generates little infrared output, it does not produce much heat.1 Although argon laser curing units maintain light intensity regardless of distance, these VLC sources are expensive and currently are not popular.1,9
Plasma arc curing (PAC) systems have a higher power intensity than conventional QTH lamps, which makes it possible to save time during procedures that require exposing patients to light multiple times.1,10 Because PAC systems offer lower wavelengths of light at higher intensity, they are capable of polymerizing composite resins with photoinitiator systems other than camphoroquinone.10 However, PAC systems are more expensive than QTH or LED lights; in addition, silver precipitation on the lamp causes the power output of PAC lights to decline over time.1,6
A composite resin’s degree of conversion can indicate many of the composite restoration’s physical and mechanical properties.11 Microhardness is considered an adequate indicator of the degree of conversion/polymerization of composite resin.11 Polymerization depends in part on the effectiveness of the radiation source, including spectral distribution, intensity, exposure time, and alignment of the light-tip guide.12
As it passes through the composite material, light is absorbed and scattered, decreasing in intensity and reducing its effectiveness as the depth of cure (DOC) increases.11 This decrease in light intensity results in what is commonly referred to as the depth of cure problem.7 Because the light is most intense on the top surface of a photoinitiated composite, poly-merization proceeds quickly and is more complete because virtually all camphoroquinone ions are excited. Farther below the surface, light attenuation results in fewer excited photoinitiator molecules and the probability of a collision with an amine decreases dramatically.7 As a result, the hardness of the top surface of a restoration is a poor predictor of the resin’s hardness at the bottom of the restoration.13 The hardness of the bottom surface of a clinically relevant thickness of composite should be measured to determine the curing abilities of different lights.13,14
This study sought to evaluate how the four different photocuring units (QTH, LED, argon laser, and PAC) affected the polymerization of resin composite cylinders, simulating cavities of different depths (2.0, 4.0, and 6.0 mm).
Materials and methods
Specimen fabrication
Metallic cylindrical matrices (5 mm in diameter) were used to fabricate 120 specimens of composite resin Filtek Z250, color A2 (3M ESPE). The specimens were divided into 12 groups (n = 10) according to the light source used (QTH, LED, argon laser, and PAC) and the height of the samples (2.0, 4.0, or 6.0 mm).
The composite resin specimens were fabricated using horizontal increments of approximately 2.0 mm. The specimens were photocured with the tip of the curing light contacting the top surface of the polyacetate matrix using one of four light sources: a QTH lamp (Demetron Optilux 501, Kerr Dental); an LED lamp (Elipar Freelight 2, 3M ESPE), an argon laser (AccuCure 3000, LaserMed), or a PAC system (Apollo 95E Elite, Dental/Medical Diagnostic Systems).
Power density calculation
Because power density influences the kinetics of the resin composite’s polymerization reaction, the real power density was calculated for each light source within the range of maximum camphoroquinone absorption (450–490 nm).15 The spectrum emitted by each light source was analyzed in a spectrometer (USB 2000, Dental/Medical Diagnostic Systems). At that point, a power meter (Nova Ophir Power, Ophir Optronics Inc.) was used to check the power from each light source. This value was divided by the area of the light tip for the corresponding unit, making it possible to determine the total light intensity emitted. Origin 6.1 software (OriginLab Corp.) was used to determine the correlation between the total intensity emitted by the VLC sources and the spectrum recorded previously through the construction of a graph in which the X-axis represented the wavelength (nm) and the Y-axis represented the light intensity (mW/cm2). This graph made it possible to select a specific range and verify the light intensity of a particular wavelength through integral calculation of the area.
Photoactivation was performed for Filtek Z250 per the manufacturer’s recommendation. Increments of Filtek Z250 (2.0 mm thick) were placed under halogen lamp units with a photo emission time of 20 seconds and an energy density of 6.04 J/cm2. Applying the calculation formula for energy density (ED = mW/cm2 x seconds/1,000) and using the values of light intensity emitted by each curing light in the range of absorption for camphorquinone, it was possible to approximate the energy density of 6.04 J/cm2 by adjusting the photo emission times. Based on this calculation of energy density, it was determined that QTH lights had a photo emission time of 20 seconds, compared to 30 seconds for argon lasers, 10 seconds for LEDs, and 6 seconds for PAC lights.
Knoop microhardness test
After 24 hours of fabrication, Knoop hardness was measured at five different points on the bottom and top surfaces of the specimens (Fig. 1) using a microhardness tester (Future Tech FM-1E, YMT International) under a load of 25 g per 20 seconds.
For each surface, an average value was calculated for the five points and transformed into a Knoop hardness number (KHN) using the formula KHN = 14,230 x F/d2, where “14,230” is a constant, “F” is the force in grams (25 g in all instances), and “d” refers to the size of the horizontal indentation (measured in µm).
Results
Because the light source used was the only variable when photoactivating the top surface of the specimens, it is the only factor that could influence microhardness; as a result, the statistical analysis utilized one-way ANOVA. To verify the significant statistical effect, Tukey’s test was performed (p < 0.05) so that multiple comparisons could be made among the groups. When different light sources on the top surface of specimens were compared (see Table 1), no statistically significant differences were observed for the QTH and LED lights in terms of KHN values. The same was true for the LED and argon laser lights; however, the QTH light demonstrated significantly higher hardness values than the argon laser. The PAC system produced the lowest microhardness values.
When measuring the microhardness of the bottom surface of the specimens, the different light sources and the height of the specimen must be considered; for these two factors, the statistical analysis test was the variance analysis. After checking the significant statistical effect, Tukey’s test was applied (p < 0.05) for multiple comparison among the groups. There were no statistically significant differences among the QTH and LED samples when the different heights were compared (see Table 2); however, the 2.0 mm argon laser and PAC samples showed significantly lower KHN values than the 4.0 mm samples, while the 6.0 mm samples were not statistically different from either the 2.0 or 4.0 mm samples. QTH, LED, and argon laser samples displayed similar values regardless of specimen size, while the PAC samples displayed significantly lower KHN values throughout.
The non-parametric test was used to verify the difference in KHN values between the bottom and top surfaces. The symmetry of the data was evaluated initially by using the Shapiro-Wilk test. When the symmetry of the data was observed, Student’s t-test was utilized. When the normality was not observed, the Wilcoxon Signed Rank test was utilized. In all cases, Knoop microhardness values were significantly higher on the top surface than on the bottom surface, regardless of specimen size or light source utilized (see Table 3).
Discussion
Polymerization is an important step in the restoration of composite resins. The photoactivation techniques and light sources available today have advantages and disadvantages in terms of the restoration’s final properties and performance.16 The extension of the polymerization reaction of composite materials (frequently referred to as the degree of conversion) is very important because it indicates many of the composite restoration’s physical properties.11 According to Vargas et al, microhardness is an appropriate indicator of a composite resin’s polymerization.11 A correlation has been reported between Knoop hardness and infrared spectroscopy.17
In the present study, the QTH light demonstrated significantly higher hardness values on the top surface compared to the argon laser. The PAC system showed the lowest microhardness value, despite the light’s higher intensity, possibly because it had a photo emission time of six seconds. Despite supplying the same energy density as the other light sources, it is possible that the photo emission time for the PAC system may have been insufficient, thus affecting both the extension of crossed connections (that is, the link between two linear polymeric chains that strengthens the final material) and the final size of the polymeric chain.
During the initial steps of polymerization, the polymer tends to form in a linear manner. Significant connections occur only in the final stages of polymerization.18 High-intensity lights used for a short time (such as PAC lights) allow many centers of polymeric growth to form; however, the short exposure time does not allow the centers of polymeric growth to expand.19 According to Soh et al, the degree of polymerization and the crossed connections can affect the material’s physical properties significantly.14 Inadequate polymerization produces an inferior polymer.
The differences observed between the QTH light and the argon laser can be explained by the argon laser’s low light intensity (278 mW/cm2). Even when the photo emission time is prolonged in comparison to the other VLC sources, this low light intensity may have affected the degree of conversion of the restorative material. This lower intensity results in a slower polymerization of the material, which is associated with few centers of polymeric growth, resulting in a more linear structure and fewer crossed connections.19 According to Ferracane et al, the microhardness of restorative composites depends on the extension of polymerization for the resinous matrix.20 Additional polymerization is expected to increase both the number of crossed connections and the hardness of the material.
When the bottom of each specimen was analyzed, the PAC system demonstrated the lowest KHN value. Among the PAC and argon laser groups, the 2.0 mm samples demonstrated significantly lower microhardness values than the 4.0 mm samples, while the 6.0 mm samples demonstrated microhardness values similar to those of both the 2.0 mm and 4.0 mm samples. This result differs from previous studies that reported a lower degree of polymerization as the depth of the sample increased.16,21,22 This difference can be explained by the different methods used in the present study.
Previous studies have evaluated the DOC from different light sources using specimens that were fabricated using composite material placed in a single increment. In two of these studies, the DOC was measured by using a needle that penetrated to the polymerized part of the material.21,22 Tsai et al evaluated the depth of polymerization from different light sources by fabricating their specimens through a single increment, removing the non-polymerized portion of each sample and measuring the remaining height.16
By contrast, the present study used the incremental technique to fabricate specimens. Following the manufacturer’s instructions, increments of approximately 2.0 mm were used.23 The 4.0 mm specimens used two horizontal increments while the 6.0 mm specimens used three horizontal increments, all of which were photocured with the tip of the photoactivator unit contacting the top surface of the matrix. The results obtained at the base of the 4.0 mm specimens can be explained by the fact that they were exposed to the light source twice; that is, these samples received twice as much total energy density as the 2.0 mm samples.
According to Yap and Seneviratne, the polymerization process depends on both the total energy density received by the restoration and the intensity of the light.17 The energy density in the range of camphoroquinone absorption used in this study (6.04 J/cm2) might not have been sufficient to produce high-quality polymeric chains, even though this density was determined in conjunction with the manufacturer’s instructions for the restorative material.
According to Vandewalle et al, the ideal total energy density for adequate polymerization of a restoration made from Filtek Z250 would be approximately 24 J/cm2.24 Based on that study, the ideal total energy density from the QTH lamp would be approximately 12 J/cm2 for camphoroquinone absorption.24 The critical depth of 6.0 mm specimens explains the absence of a statistically significant difference between the various sizes of specimens. Although the 6.0 mm specimens received more total energy density than the smaller samples, they could not be fully polymerized to obtain better results, as it was difficult for light to reach such depths.
Based on these results, it can be concluded that the total energy density a restoration receives after multiple exposures is crucial to achieving adequate polymerization of the bottom surface. As energy density supplied to the restoration increases, so does the possibility that light will reach the bottom surface, resulting in more complete polymerization. Conversely, as the depth of a cavity increases, polymerization is less efficient, as less light reaches the bottom due to the absorption and dissipation resulting from the larger amount of restorative material and the greater distance from the light source.
The KHN values on the top surface have always shown statistically significant differences (and been significantly higher) compared to the bottom surfaces (see Table 3), due to the attenuation of light intensity within the depth of the material and the distance between the polymerizing tip and the composite.25 As this distance increases—whether due to the clinical difficulty of positioning the polymerizing tip or by an increase in the depth of the cavity—the degree of polymerization on the bottom of the restoring material decreases significantly.
Conclusion
Based on the results of this study, photocuring with a PAC system had a negative effect on KHN values for both the top and bottom surfaces of specimens, regardless of the specimens’ height. For all specimens, at all heights, KHN values were statistically higher on the top surface than on the bottom surface.
This study showed that conventional QTH units still provide adequate polymerization of composite resins and the use of costly equipment that requires difficult maintenance is not necessary. However, when the photocuring effectiveness of different light sources is evaluated, other factors should be considered, including the total energy density supplied to the restorative material and the depth of the cavity that is to be restored. Additional in vitro studies are required to fully understand the properties and applications of new materials and technologies.
Acknowledgements
This study was supported by FAPESP (03/12592-8).
Author information
Drs. Voltarelli and Batitucci are PhD graduate students in restorative dentistry, Department of Restorative Dentistry, Piracicaba School of Dentistry, Areiao,
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Manufacturers
Dental/Medical Diagnostic Systems,
Kerr Dental,
LaserMed,
Ophir Optronics Inc.,
OriginLab Corp.,
YMT International,
3M ESPE,
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General Dentistry, July/August 2009 , Volume 57 , Issue 2
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