Influence of the photoinitiator system and light photoactivation units on the degree of conversion of dental composites

The aim of this study was to observe the influence of two light polymerization units (LED or halogen light) on the degree of conversion (DC) of three dental composites with lighter shades and a different photoinitiator system. The top (T) and bottom (B) surfaces of 60 discs of composite resin (Filtek Supreme, Filtek Z250, Tetric Ceram Bleach) cured either by LED or by halogen lamp (HL) were studied using an FTRaman spectrometer. The degree of conversion (DC) was evaluated by following the changes in the intensity of the methacrylate C=C stretching mode at 1640 cm-1. The calculated DC ranged from 54.2% (B) to 73.4% (T) and from 60.2% (B) to 76.6% (T) for the LED and HL, respectively. LED and halogen devices were able to produce an adequate DC for all the resins tested. Descriptors: Composite Resins; Dental Equipment; Curing Lights, Dental. Introduction The crucial point to contemplate during restorative dentistry procedures with composite resins is to obtain satisfactory restorations with an adequate photopolymerization technique. This procedure requires sufficient light energy intensity and an adequate wavelength in order to activate the photoinitiator within these materials, which will react with the reducer agent to form free radicals and initiate the polymerization process. Camphorquinone (CQ) is the most common photoinitiator used in composites, and it presents maximum energy absorption at 468 nm within the electromagnetic spectrum close to the emission spectrum of the light-emitting diode (LED λ: 450-490 nm) light-curing unit (LCU).1 Initiators, such as phenylpropanedione (PPD; 1-phenyl-1,2-propanedione), which has an absorption spectrum within the ultraviolet spectrum (maximum wavelength ≈ 410 nm),1,2 or Lucirin TPO (2,4,6-trimethylbenzoyldiphenylphosphine oxide), which has an absorption spectrum characteristic of a shorter wavelength (ranging from 375 to 410  nm),1 have become necessary to solve inconveniences related to aesthetics concerning composite resins for bleached teeth.2,3 Lucirin TPO is now used in some composites because it is completely colorless after the light curing reaction, and its polymers are less yellowish than others in which only camphorquinone is used as a photoinitiaInfluence of the photoinitiator system and light photoactivation units on the degree of conversion of dental composites Braz Oral Res. 2010 Oct-Dec;24(4):475-81 476 tor. When a bleached tooth needs to be restored, the reduction of discoloration related to the photoinitiator is clinically significant in order to obtain and maintain color in aesthetic restorations.3 Moreover, an excellent polymerization condition cannot be obtained when using the LED LCU to photoactivate materials containing photoinitiators that absorb energy from another wavelength.4,5 On the other hand, the broad spectrum of the halogen LCU, extending up to the ultraviolet region (UV-A), can be an advantage to excite coinitiators that absorb shorter wavelengths.2 The degree of conversion (DC) of methacrylatebased composite resin is influenced not only by light intensity and wavelength, as emitted by the LCU used to excite the photoinitiator molecules,6 but also by the irradiation time and material composition.7,8 Moreover, some factors that interfere in light transmission, such as sample thickness, color, translucence, inorganic fillers of composites and the distance between the LCU light tip and the material surface, can also alter this property.9,10 Among the methods to determine the DC of composites, Fourier transform Raman spectroscopy (FT-Raman) has been widely used as a reliable method as it detects the C=C stretching vibrations directly before and after the curing of materials.11-13 The purpose of this study was to examine the influence of two light polymerization units on the DC of dental composites with lighter shades and a different photoinitiator system. It was hypothesized that the LED and halogen LCU have similar curing performances. Materials and Methods Halogen lamp (HL) and LED LCUs (Demetron/ Kerr, Danbury, CT, USA, Table 1) were used to prepare 60 samples of composite. The wavelengths of LCUs were measured by using a spectroradiometer (Model 77702 Oriel Instruments, Danbury, CT, USA), and the power densities (light intensity) were also measured, using a specific radiometer for each unit (Optilux Radiometer or L.E.D. Radiometer Demetron/Kerr, Danbury, CT, USA). Table 2 Technical specifications of the composite materials tested in the study. Resin Manufacturer Shade Organic matrix Photoinitiator Inorganic matrix Filtek Supreme (FS) 3M/ESPE Dental Products, St Paul, MN, USA WB Bis-GMA Bis-EMA6 UDMA TEGDMA Camphorquinone (468 nm) Silica (20 nm) Zirconia and silica (0.6 1.4 μm) 59.5% in volume. Filtek Z250 (FZ) 3M/ESPE Dental Products, St Paul, MN, USA B 0.5 Bis-GMA Bis-EMA6 UDMA TEGDMA Camphorquinone (468 nm) Zirconia and silica (0.01 3.5 μm) 60.0% in volume. Tetric Ceram Bleach (TCB) Ivoclar/Vivadent AG, Schaan, Liechtenstein L Bis-GMA UDMA TEGDMA Lucirin TPO3 (375-410 nm) + camphorquinone (468 nm) Barium glass, ytterbium trifluoride, Ba-Al-fluorosilicate glass, silica highly dispersed, mixed oxide and prepolymers (0.04 1 μm) 60.0% in volume Equipment L. E. Demetron 1 Opitlux 401 Manufacturer Demetron/Kerr, Danbury, CT, USA Demetron/Kerr, Danbury, CT, USA Light source LED Halogen Power density (mW/cm2) 900 700 Energy density (J/cm2) 36 28 Wavelength (nm) 430-490 390-530 Curing time 40 s 40 s Technique Conventional continuous Conventional continuous Table 1 Technical specifications of the light-curing units used in the study. Porto ICCM, Soares LES, Martin AA, Cavalli V, Liporoni PCS Braz Oral Res. 2010 Oct-Dec;24(4):475-81 477 Three commercially available composites (Table 2), chosen in accordance with their different combinations of photoinitiators, were tested: Filtek Supreme (FS) (3M/ESPE Dental Products, St Paul, MN, USA), Filtek Z250 (FZ) (3M/ESPE Dental Products, St Paul, MN, USA) and Tetric Ceram Bleach (TCB) (Ivoclar/Vivadent AG, Shaan, Liechtenstein). Sixty samples were produced using a fabricated stainless steel mold in a ring form, with a 7 mm internal diameter and a height of 2 mm, cured at the top surface for 40 s by each light-curing unit, according to the groups: LED curing (GI: FS; GII: FZ; GIII: TCB) and halogen curing (GIV: FS; GV: FZ; GVI: TCB). A lateral mark with graphite indicated the irradiated surface. Raman spectra were recorded after 24 h of storage in distilled water at 37°C. The top (T) and bottom (B) surfaces were analyzed by FT-Raman spectroscopy in order to evaluate the DC. Spectra of the cured and uncured resins were recorded by an FT-Raman Spectrometer (RFS 100/S, Bruker Inc., Karlsruhe, Germany). To excite the spectra, the defocused λ1064.1 nm line of an Nd:YAG laser source was used, and the spectrum resolution was set to 4 cm-1.11,12 The spectra of FS and FZ resin specimens were obtained with the maximum laser power output of 200 mW and 100 scans. The spectra of TCB resin specimens were obtained with the maximum laser power output of 25 mW and 1000 scans. The FT-Raman spectra were analyzed by selecting a spectrum region from 1590 to 1660 cm-1. The Raman vibrational stretching modes in 1610 and 1640  cm-1 were fitted with Lorentzian shapes to obtain the height of the peaks using the Microcal Origin software (Microcal Software Inc., Northampton, MA, USA). The DC was calculated from the ratio between the peaks of the aliphatic C=C bond (1640  cm-1) to the aromatic C=C bond (1610  cm-1) obtained from the cured and uncured specimens by the following equation: The Kruskal-Wallis test, used under the condition of nonhomogeneity of variances (Bartlett’s p  <  0.05), and Dunn’s Multiple Comparisons Test were used to test the effects of the interaction between LCUs and composites on the DC. A pairedsamples t-test was applied to both the top and bottom surfaces of the same sample. All statistical analyses were performed at a 95% significance level using the SAS System 9.1 software (SAS Institute Inc, Cary, NC, USA). Results The Raman spectra of the cured and uncured samples of each tested resin, in the spectral region of 1550 cm-1 to 1650 cm-1, are presented in Figures 1-3. The peak of stretching mode in 1640 cm-1, which represents the vibrational mode of the double C=C bonds of the dimethacrylate aliphatic group, DC (%) = 100 × [1 – (Rcured / Runcured)] where R = peak height at 1640 cm-1 / peak height at 1610 cm-1.11,12 In te ns ity (a .u .) Raman Shift (cm-1) 166


Introduction
The crucial point to contemplate during restorative dentistry procedures with composite resins is to obtain satisfactory restorations with an adequate photopolymerization technique.This procedure requires sufficient light energy intensity and an adequate wavelength in order to activate the photoinitiator within these materials, which will react with the reducer agent to form free radicals and initiate the polymerization process.
Camphorquinone (CQ) is the most common photoinitiator used in composites, and it presents maximum energy absorption at 468 nm within the electromagnetic spectrum close to the emission spectrum of the light-emitting diode (LED -λ: 450-490 nm) light-curing unit (LCU). 1 Initiators, such as phenylpropanedione (PPD; 1-phenyl-1,2-propanedione), which has an absorption spectrum within the ultraviolet spectrum (maximum wavelength ≈ 410 nm), 1,2 or Lucirin  TPO (2,4,6-trimethylbenzoyldiphenylphosphine oxide), which has an absorption spectrum characteristic of a shorter wavelength (ranging from 375 to 410 nm), 1 have become necessary to solve inconveniences related to aesthetics concerning composite resins for bleached teeth. 2,3ucirin  TPO is now used in some composites because it is completely colorless after the light curing reaction, and its polymers are less yellowish than others in which only camphorquinone is used as a photoinitia-tor.When a bleached tooth needs to be restored, the reduction of discoloration related to the photoinitiator is clinically significant in order to obtain and maintain color in aesthetic restorations. 3Moreover, an excellent polymerization condition cannot be obtained when using the LED LCU to photoactivate materials containing photoinitiators that absorb energy from another wavelength. 4,5On the other hand, the broad spectrum of the halogen LCU, extending up to the ultraviolet region (UV-A), can be an advantage to excite coinitiators that absorb shorter wavelengths. 2he degree of conversion (DC) of methacrylatebased composite resin is influenced not only by light intensity and wavelength, as emitted by the LCU used to excite the photoinitiator molecules, 6 but also by the irradiation time and material composition. 7,8Moreover, some factors that interfere in light transmission, such as sample thickness, color, translucence, inorganic fillers of composites and the distance between the LCU light tip and the material surface, can also alter this property. 9,10][13] The purpose of this study was to examine the influence of two light polymerization units on the DC of dental composites with lighter shades and a different photoinitiator system.It was hypothesized that the LED and halogen LCU have similar curing performances.

Materials and Methods
Halogen lamp (HL) and LED LCUs (Demetron/ Kerr, Danbury, CT, USA, Table 1) were used to prepare 60 samples of composite.The wavelengths of LCUs were measured by using a spectroradiometer (Model 77702 -Oriel Instruments, Danbury, CT, USA), and the power densities (light intensity) were also measured, using a specific radiometer for each unit (Optilux Radiometer or L.E.D. Radiometer -Demetron/Kerr, Danbury, CT, USA).Three commercially available composites (Table 2), chosen in accordance with their different combinations of photoinitiators, were tested: Filtek  Supreme (FS) (3M/ESPE Dental Products, St Paul, MN, USA), Filtek  Z250 (FZ) (3M/ESPE Dental Products, St Paul, MN, USA) and Tetric  Ceram Bleach (TCB) (Ivoclar/Vivadent AG, Shaan, Liechtenstein).Sixty samples were produced using a fabricated stainless steel mold in a ring form, with a 7 mm internal diameter and a height of 2 mm, cured at the top surface for 40 s by each light-curing unit, according to the groups: LED curing (GI: FS; GII: FZ; GIII: TCB) and halogen curing (GIV: FS; GV: FZ; GVI: TCB).A lateral mark with graphite indicated the irradiated surface.
Raman spectra were recorded after 24 h of storage in distilled water at 37°C.The top (T) and bottom (B) surfaces were analyzed by FT-Raman spectroscopy in order to evaluate the DC.Spectra of the cured and uncured resins were recorded by an FT-Raman Spectrometer (RFS 100/S, Bruker Inc., Karlsruhe, Germany).To excite the spectra, the defocused λ1064.1 nm line of an Nd:YAG laser source was used, and the spectrum resolution was set to 4 cm -1 . 11,12he spectra of FS and FZ resin specimens were obtained with the maximum laser power output of 200 mW and 100 scans.The spectra of TCB resin specimens were obtained with the maximum laser power output of 25 mW and 1000 scans.
The FT-Raman spectra were analyzed by selecting a spectrum region from 1590 to 1660 cm -1 .The Raman vibrational stretching modes in 1610 and 1640 cm -1 were fitted with Lorentzian shapes to obtain the height of the peaks using the Microcal Origin  software (Microcal Software Inc., Northampton, MA, USA).The DC was calculated from the ratio between the peaks of the aliphatic C=C bond (1640 cm -1 ) to the aromatic C=C bond (1610 cm -1 ) obtained from the cured and uncured specimens by the following equation: The Kruskal-Wallis test, used under the condition of nonhomogeneity of variances (Bartlett's p < 0.05), and Dunn's Multiple Comparisons Test were used to test the effects of the interaction between LCUs and composites on the DC.A pairedsamples t-test was applied to both the top and bottom surfaces of the same sample.All statistical analyses were performed at a 95% significance level using the SAS System 9.1 software (SAS Institute Inc, Cary, NC, USA).

Results
The Raman spectra of the cured and uncured samples of each tested resin, in the spectral region of 1550 cm -1 to 1650 cm -1 , are presented in Figures 1-3 was compared to the peak of stretching mode in 1610 cm -1 , which corresponds to the vibrational mode of double bonds C=C of the benzene aromatic group, and this was used as an internal standard because it remains unaltered during polymerization.Figures 1-3 also show the changes in the remaining carbon-carbon double bonds (C=C) in the cured resin, whose characteristic peak reaches 1640 cm -1 and presents reduced intensity upon polymer formation.The calculated DC (by the equation) ranged from 54.2 ± 10.0% to 76.6 ± 8.0% for the bottom and top surfaces, respectively.The DC values calculated are shown in Table 3.The DC was statistically higher in the top than in the bottom surface for all groups.
Statistical comparisons between composites treated by the same LCU intensity showed that the DC in the top surface was higher in the GIII and GVI specimens than in the GI and GIV specimens (p < 0.05).In the bottom surface, the DC was statistically higher in the GII specimens than in GI and GIII specimens (p < 0.01).
Statistical comparisons between LCUs while considering the same composites showed no statistically significant differences at the top surface (p > 0.05) among the tested groups.At the bottom surface and considering the same composite, the results showed that the FZ resin presented a significant difference when cured by the LED compared to FZ cured by halogen light (GII x GV) (p = 0.0173).

Discussion
The tested composite resins in this study contain CQ as a principal photoinitiator or in combination with Lucirin TPO3.5][16] In this study, no significant increase in the DC values was found when the energy density levels exceeded 27 J/cm².Due to the complex mechanism of the polymerization reaction, the DC of Bis-GMA-based resin composites reported is between 45% and 85%. 3,5,12o date, the minimum DC for a clinically satisfactory restoration has not been precisely established.Nevertheless, a negative correlation of in vivo abrasive wear depth with DC has been found for values in the range of 55%-65%.This suggests that, at least for occlusal restorative layers, DC values below 55% may be cause for concern. 12,17he DC values measured at the top surface of the samples cured with a halogen LCU for 40 s reached mean values of 71.6% (FS), 70.1% (FZ) and 76.6% (TCB).These results are in agreement with those of previous studies that used similar spectroscopic methods, variables and composites as those used in this study. 3,4,18,19The samples cured with the LED presented DC mean values at the top surface of 68.1% (FS), 71.5% (FZ) and 73.4% (TCB).In a similar study by Calheiros et al. 15 , the Tetric  Ceram resin also presented average DC values higher than those presented by Filtek  Z250 resin.The mean DC values at the top surface of all the resins tested were above the minimum considered adequate for polymerization (55%) 17 and are in agreement with those of the work of Price et al., 5 in which the LED LCU cured resin samples, even those containing other coinitiators.
Significant differences among composites cured by the same LCU were observed for top and bot-tom surfaces.At the top surface, the TCB composite cured by the LED showed a higher DC than FS cured by the same LCU (p < 0.05).This fact was also found between TCB and FZ resin when considering the halogen LCU (p < 0.05).
Those results could be due to some differences in resin composition.The DC seems strongly dependent on monomer composition and the amount and type of photoinitiator present. 2,4,16,20,21It is also related to double C=C bonds that react during polymerization.Consequently, the differences in the values obtained in this study could be related to a number of factors, such as the photoinitiator/light source, the presence of more flexible monomer molecules or even the proportion of inorganic load within the composite resin. 16FS and FZ resins present identical organic matrix compositions, and part of the TEGDMA was replaced by UDMA and BisEMA. 6hese materials present fewer double bonds per unit of weight and diminished conversion of these bonds during polymerization.TCB resin presents a different reactivity to light, due to the presence of organically modified silicate organized in layers.
The higher DC values observed in this study could be due to differences in the composition of the organic and inorganic portions of the composites tested.The load particles interfere with polymerization, promoting differences in light absorption and dispersion, 9,10 but they do not spectrally interfere with the bands used to determine the percentage of conversion of dental composites. 21][24] A high TEGDMA ratio in the Bis-GMA/TEG-DMA mixture favors DC values, 18,20 which can be 20% higher when compared to the UDMA/TEG-DMA systems. 25In FS and FZ resins, all the monomers presented a high molecular weight but different mobility, which resulted in an adequate DC value. 20owever, on the bottom, LED curing produced a lower DC for TCB compared to FZ and FS (p < 0.01).A previous observation revealed differences in the reactivity and activation properties for the halogen LCU between Tetric  Ceram and Z100 resins. 25Therefore, low values in the bottom surface of TCB resin could be due to the fact that the wavelength of light necessary to excite the coinitiator of this resin is strongly reduced by the sample due to the high degree of absorption that occurs as it penetrates the composite. 26t is important to understand how much the inhomogeneous surface of the sample influences DC measurements and that differences in the spectra of the cured and uncured resin are probably due to surface heterogeneity and polymerization effects.Variations in the size of load particles could be responsible for the heterogeneity of resins with the same organic matrix and could cause interference in the Raman scattering in that area, resulting in large differences in the DC values. 27hen composition is considered, FS and FZ resins present similar percentages of inorganic matrix, but the particle size is different.The former contains 20 nm silica nanoparticles and 0.6-1.4µm zirconia-silica nanoclusters, whereas the latter contains zirconia-silica particles varying from 0.01-3.5 µm.Smaller particles spread light and make polymerization more difficult. 9,10Using the same irradiation time, it is possible that the lower DC values found for the FS resin are due to the interaction between the light and the small particle size of the inorganic load or even the incorporation of coinitiators, which absorb light of shorter wavelengths.
This study has shown that the LED and halogen LCUs tested produced an adequate degree of conversion values on resins with lighter shades and that the different combination of photoinitiator systems produces similar DC in the composites.The hypothesis was accepted because the different composites tested reached an adequate degree of conversion regardless of the LCU used.

Conclusion
LED and halogen LCUs influenced the DC at both the top and bottom surfaces of the composites tested.However, the resins reached an adequate DC after photoactivation with LED and halogen devices, thus exhibiting similar curing performances for the LCU.

Table 2 -
Technical specifications of the composite materials tested in the study.

Table 1 -
Technical specifications of the light-curing units used in the study.

Table 3 -
Mean and standard deviation (SD) of the degree of conversion (%) measured at the top and bottom surfaces from the composites of the experimental groups (n = 10).Asterisk denotes statistically significant difference (P < 0.05) between top and bottom DC %.Values in the columns with the same superscript letters are not statistically different when considering the same LCU (α = 0.05).A paired-samples t-test was used to test significances between top and bottom surfaces.