| Abstract|| |
Aims: The purpose of this paper was to evaluate the influence of different light curing units on the conversion of four composite resins with different compositions (Durafill VS® - Heraeus-Kulzer, Tetric Ceram® - Ivoclar/Vivadent, Filtek™ Supreme XT - 3M ESPE™ e Aelite™ LS Packable - Bisco), using differential scanning calorimetry.
Materials and Methods: A stainless steel matrix was used to prepare 48 cylindrical composite test samples (n=6), measuring 3 mm in diameter and 1 mm in thickness. The samples were photoactivated using a halogen lamp (Optilux™ 500 - Demetron/Kerr) and three different generations of light-emitting diodes (LEDs) (LEC-470 I - MMOptics, Radii Plus - SDI and Ultra-Lume™ LED 5 - Ultradent). After removal of the matrix, each sample was weighed and hermetically sealed in an aluminum pan and analyzed. The amount of heat liberated by thermopolymerisation of residual monomers after photoactivation was measured in Joules/gram (J/g). The data were submitted to Analysis of Variance (ANOVA) test (P ≤ 0.002) and the Tukey test (P < 0.05).
Results: The Ultra-Lume™ LED 5 was superior on degree of conversion for all resins. The Radii Plus was equal to the Ultra-Lume™ LED 5, except for the resin Tetric Ceram® , were the Optilux™ 500 was superior. The LEC-470 I was inferior for the conversion of all resins.
Conclusion: The study proves the importance of the compatibility of the different photoinitiators in resin composites with the different light sources.
Keywords: Composite resins, conversion, differential scanning calorimetry, photoactivator units, photoinitiators
|How to cite this article:|
Silva EH, Albuquerque RC, Lanza LD, Vieira GC, Peixoto RT, Alvim HH, Yoshida MI. Influence of different light sources on the conversion of composite resins. Indian J Dent Res 2011;22:790-4
For more than 20 years light curing units (LCUs) using halogen lamps (quartz-tungsten-halogen - QTH) have been used to polymerize dental composite resin.  The equipment consists of a tungsten filament bulb, reflector, filter, cooling system and fiber optics to conduct light. The halogen lamp produces incandescent light, as the tungsten filament is heated atoms are excited, producing white light. Light should be restricted to the blue spectrum, which promotes photopolymerization.  However the bulb has an average useful life of about 100 h and the reflector and filter degrade with time. As a result, a reduction in efficiency of polymerization occurs with use, negatively affecting the physical properties of the polymerized composite resin. 
|How to cite this URL:|
Silva EH, Albuquerque RC, Lanza LD, Vieira GC, Peixoto RT, Alvim HH, Yoshida MI. Influence of different light sources on the conversion of composite resins. Indian J Dent Res [serial online] 2011 [cited 2019 Jul 22];22:790-4. Available from: http://www.ijdr.in/text.asp?2011/22/6/790/94670
In order to overcome the problems inherent to halogen LCUs, solid-state light-emitting diode (LED) technology has been developed for curing photoactivated dental materials.  LEDs use junctions of doped semiconductors (p-n junctions) for light generation instead of the heated filament of halogen lamps, reducing overheating of the appliance. LEDs convert electric energy directly into light by electroluminescence through bundles of semiconductors. In the case of gallium nitride (GaN) LEDs, the light emitted is blue, with the spectrum centered at 470 nm,  which renders unnecessary the use of filters. , In addition, they have an operational lifespan of thousands of hours without loss of efficiency. As the energy consumption of LEDs is much lower, it has been possible to develop wireless LCUs facilitating clinical use.
Individual LEDs have a relatively low light irradiance output. For this reason, the first generation of LCUs used multiple sets of LEDs.  Some LED LCUs with high-power light sources had irradiance equal to conventional halogen LCUs,  obtaining statistically similar values for Barcol hardness and compressive strength.
However, studies have also shown that LED LCUs with relatively low irradiances are available in the market, which may result in insufficiently cured composites and, therefore, inferior mechanical properties of restorations. ,, New LEDs, which are regarded as second-generation LEDs, have been introduced in the market and will be used in most LED LCUs currently available. These single LEDs consist of multiple emitters on the same substrate. The large area chip (die) is bonded directly to a substrate that can in turn be bonded directly to a large heat sink. The substrate provides a short thermal path and allows the flip chip configuration to dissipate heat quite rapidly. The large area die and special thermal management allow high power operation without thermal damage to the LED. The LED is mounted in a plastic lens filled with silicone gel, making a robust and reliable package. The LED is about 8 mm in diameter and can, therefore, be easily mounted in pen-like polymerization LCUs for dental applications.  The second generation of LEDs can generate irradiance above 500 milliwatts (mW) of power using a single LED, which can exceed 1,000 mW in the most modern equipment. ,
Despite the high power output of the second-generation LEDs, they have a narrow emission spectrum, similar to the first-generation LEDs. Their peak emission coincides with the maximum absorption of camphoroquinone (CQ), but does not reach the region of the UV-A as the QTH bulb does. This limits their use with other materials using different CQ photoinitiators.  Third-generation LED-LCUs use a combination of LEDs to produce a broader spectral output, and they are usually very powerful LEDs, similar to the second-generation examples, associated with other less powerful violet LEDs. This enables an emission spectrum comparable to the QTH bulbs, with irradiance similar to the second-generation LEDs. ,
Three main factors are necessary for correct photo-polymerization: Sufficient light intensity,  the correct wavelength of light and curing time. , The characteristics of composite curing are also dependent on the type and quantity of photo initiator present. Camphorquinone is the photo-sensitive agent used in the majority of commercial products, present in concentrations ranging from 0.17 to 1.03% w/w of the resin phase.  The increase in the quantity of camphorquinone in composites resins maximizes the conversion of monomers, increasing the mechanical and biological properties of these materials.  However, camphorquinone has an intense yellow color which interferes with lighter colors.  Identification and quantification of camphorquinone in different colored resins was undertaken and demonstrated variations in the quantities and types of photoinitiators in their composition. ,,
To avoid these problems new photoinitiators have been tested, among them PPD (1-phenyl,2-propanodione). This photoinitiator has been shown to be more efficient than camphorquinone, as well as being less dependent on the presence of tertiary amine for the generation of free radicals. Other advantages of PPD are its clear yellow color, and its liquid state at room temperature, facilitating the incorporation of greater quantities of other components. Other photoinitiators studied are derivatives of bisacylphosphine oxide (BAPO) and monoacylphosphine oxide (MAPO or Lucirin TPO). As a result of lighter color, these photoinitiators absorb light at a lower wavelength, nearer to ultraviolet, than camphorquinone. This wavelength is perfectly covered by halogen light units, but is not reached by using blue LEDs, due to its narrow emission spectrum. Thus resins containing these photoinitiators would not be adequately polymerized by this equipment, resulting in a low level of polymerization and poor mechanical properties. 
A higher conversion level provides the restorative material with better mechanical properties, such as: Wear, hardness, compressive and shear strength. , However, insufficient polymerization of the composite increases the release of its components, which are deemed mainly responsible for the high prevalence of dermal allergy caused by polymeric materials. Thus, better polymerization of the material would certainly improve its biocompatibility, since this property is associated mainly with the nature and amount of the components released. 
Manufacturers do not declare all the chemical compounds in their products, including the type of photoinitiator. This information is absolutely relevant for the clinician to evaluate the compatibility of the composite resin used with the LCU used, as well as to be able to prevent possible patient allergic reaction.  Thus, the purpose of this paper was to evaluate the influence of different LCUs on the conversion of four composite resins with different compositions.
| Materials and Methods|| |
For this study four composite resins of different composition [Table 1] and four different light sources were selected: A halogen lamp (Optilux™ 500 - Demetron/Kerr) and three different generations of LEDs (LEC-470 I - MMOptics, Radii Plus - SDI, and Ultra-Lume™ LED 5 - ULTRADENT) [Table 2].
|Table 2: Types, cross-sectional areas of light transmission tips, intensities, and manufacturers of the units used in this study|
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A spectrometer was used (Spectrometer Ocean Optics-São Carlos-USP, SP-Brazil) to measure the emission spectrum of each LCU, in order to evaluate if the zone of emission included the wavelength absorbed by photoinitiators of each resin used in the study. The power output was measured using a Fieldmaster powermeter (Coherent Commercial Products Division, model no. FM, set no. WX65, part no. 33-0506, United States) and the power density values were calculated using the formula
I = P/A
Where P is power in milliwatts and A is the area of the light tip in square centimeters.
The thermogravimetric (TG) analysis was performed to determine the ideal range of temperature without degradation of composite resins for use in differential scanning calorimetry (DSC) and their thermal stability. Three cylindrical test samples (n=3) of each composite resin, measuring 3 mm in diameter and 1 mm in depth, were cured by the LCU Optilux TM 500 for 20 seconds in a stainless steel matrix and then immediately removed and tested in a thermogravimetric analyzer (TG) (METTLER TG 50 - Instrument AG, Greifensee - Switzerland). The TG curves were obtained by heating at a rate of 10°C/min in a dynamic air atmosphere from 25-750°C. The TG test also obtained the percentage of filler loading of each composite resin tested. The TG test also obtained the percentage of filler loading of each composite resin tested. The results of the TG test are shown in [Table 3]
The DSC was used to determine the amount of residual monomers after photopolymerization. Twenty-four samples of each composite resin were prepared of the same form as used for TG analysis and divided into four groups (n=6). Since each LCU had a fiber optic light source of different diameter, an individual matrix was made for each LCU tested, to allow better adaptation and less light loss. After insertion into the matrix, the resins were photoactivated for 20 s, according to the manufacturers' instructions. After removal from the matrix each test sample was weighed in an electronic analytic balance (SHIMADZU LIBROR AEG-SM - SHIMADZU CORP.), and then sealed in an aluminum pan and analyzed using a DSC-50 (SHIMADZU CORPORATION, Kyoto-Japan). Stepwise heating at a rate of 10°C/min, from room temperature to 250°C, promoted thermal polymerization of residual monomer. Since this reaction is exothermic, the quantity of heat liberated by the sample is proportional to the quantity of unreacted monomer. The results were shown as Joules per gram of composite resin (J/g). Thus, the higher the heat released, lower the efficiency of polymerization. The data were submitted to ANOVA test (P ≤0.002) and Tukey's test (P<0.05) [Table 4] for comparison of averages and standard deviations in J/g between experimental groups.
|Table 4: Comparison of averages and standard deviations of experimental groups. Averages (J/g) followed by distinct letters (column), show differences between them using Tukey test (P<0.05%)|
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| Results|| |
The TG test demonstrated that the resins tested retained different quantities of inorganic load, according to [Table 4]. This test was undertaken to obtain the best heating ramp for the DSC.
With respect to the light source, the Ultra-Lume™ LED 5 was superior for all resins. The Radii Plus was only inferior to the Ultra-Lume™ LED 5 with the Tetric Ceram® resin. The Optilux™ 500 was shown to be superior with Tetric Ceram® whereas, the LEC-470 I was inferior in the conversion of all resins, as shown in [Table 4].
In relation to the composite resins, Tetric Ceram® , which uses a combination of CQ and Lucirin TPO as photoinitiators, produced the best results with wide-spectrum light sources (Optilux 500 and Ultra Lume 5). The other resins which used only CQ photoinitiator, produced the best results with high-intensity light sources (Radii and Ultra-Lume 5), since the photoinitiator is compatible with all light sources. The worst results were obtained with LEC 470i for all composite resins tested, due to its low power density.
| Discussion|| |
The degree of conversion is a measure currently employed in research, since it permits quantitative characterization of the transformation of monomers into polymer. Different methodologies may be used to analyze the degree of conversion of resins. Differential scanning calorimetry (DSC) allows the measurement of the conversion of metacrylic groups through the exothermic reaction of polymerization. ,
DSC is a technique that may be applied for the evaluation of incomplete polymerization, where the sample after photoactivation is submitted to a regime of temperature and residual activity is observed by analysis of graphs, which indicates that the sample was not fully polymerized. 
Polymerization efficiency of LCUs is totally dependent on the characteristics of the light produced. Determination of the spectrum of light emitted from the units evaluated in this study, demonstrates that the conventional LCU based on the halogen lamp (Optilux™ 500) operates in a wavelength range of 400-500 nm, and the LED LCUs (LEC-470 I and Radii Plus) operate in a wavelength range of 450-490 nm. The LED (Ultra-Lume™ LED 5) besides a central LED, common to the two lamps cited above, also contains four peripheral LEDs composed of semiconductors capable of emitting violet/blue violet light that operates in a wave length range of 380-440 nm. All units were found to produce a spectral range of absorption, from 450-490 nm.  However, analysis of the spectral emission shows that LEDs have a homogenous emission concentrated in a narrow band very close to the peak of absorption of CQ, that is 470 nm,  as seen in [Graph 1]. Only wavelengths that are immediately absorbed by the photoinitiator contribute to photoactivation. Thus practically all the light emitted by the LEDs is within the spectrum of maximum absorption of CQ, as opposed to the conventional units based on halogen lamps that produce light outside the spectrum of interest. Furthermore, since this light is not useful for photoactivation, the use of filters is necessary to restrict these wavelengths as well as a system of ventilation, due to heat production. These components degrade over time, reducing the efficiency of these units. ,
Since the spectral emission of LEDs (LEC-470 I and Radii Plus) is narrow and concentrated around 470 nm, they can only photoactivate materials that use camphorquinone as photoinitiator. The Ultra-lume™ LED 5 was the most efficient, since it has a greater spectral emission due to different LEDs present, which is a limitation of the first two units.  The LED LEC-470 I unit, besides operating in a narrow spectral range, also has less light intensity or irradiance, explaining the inferior position of this unit in the conversion of all resins used in this study.  The photoinitiator Lucirin TPO has a maximum absorption for activation in the region of ultraviolet (UVA), near 380 nm, however in the visible region, absorption diminishes drastically, reaching zero above 450 nm.  The greatest effective photoactivation in this study was obtained with the resin composite Tetric Ceram® , which according to the manufacturer contains camphorquinone photoinitiators (CQ) and Lucirin TPO, combined with the Ultra-Lume™ LED 5 unit, which provided the spectral range of emitted light (380-440 nm/450-490 nm). The unit based on a halogen lamp was more efficient than others,  when combined with a composite resin containing Lucirin TPO.
The results obtained in the quality of resin polymerization of the composite Aelite™ LS Packable, proves that LED units (LEC-470 I), although producing less irradiation than conventional halogen units (Optilux™ 500), may reach a greater depth of polymerization due to penetration of light. 
It must be kept in mind that the efficiency of photoactivator light units depends on the characteristics of the composite resins used. It is correct to affirm that the clinical properties of composites are highly influenced by the quality of the photoactivator light units. However, the type of material used has a great effect on the effectiveness of polymerization, including the composition of the resin matrix, the content of inorganic filler  as well as the type of photoactivator.  A protocol of polymerization is necessary, where not only the characteristics of the light units, but also the photoactive materials present in composites, are well-defined by the manufacturers, so that adequate polymerization is achieved.
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Hugo H Alvim
Department of Restorative Dentistry, School of Dentistry, Belo Horizonte
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2], [Table 3], [Table 4]