RESEARCH ARTICLE
Effect of Photoinitiator Combinations on Hardness, Depth of Cure, and Color of Model Resin Composites VINÍCIUS ESTEVES SALGADO, DDS, MS*, MILA METRI BORBA, DDS, MS†, LARISSA MARIA CAVALCANTE, DDS, MS, PhD‡,§, RAFAEL RATTO DE MORAES, DDS, MS, PhD**, LUIS FELIPE SCHNEIDER, DDS, MS, PhD§,††
ABSTRACT Objective: This study aims to determine the influence of photoinitiators’ combinations on the hardness, depth of cure, and color of model resin composites. Materials and Methods: The composites were formulated by a mixture of BisGMA and triethyleneglycol dimethacrylate (60:40 mol), with barium–aluminum–silicate glass and silicon dioxide particles as inorganic fillers (60 wt%). Three photoinitiator types were tested: camphorquinone/amine (CQ), monoacylphosphine oxide (TPO), and bysacylphosphine oxide (BAPO). Six experimental groups were formed by differences in photoinitiator systems: CQ, TPO, BAPO, CQ+TPO, CQ+BAPO, and CQ+TPO+BAPO. Hardness was determined by Knoop indentation at the top and bottom surfaces (n = 5). Depth of cure was performed according to ISO 4049 scraping method (n = 5). Color was obtained by the CIELAB method (n = 10), 24 hours after curing (baseline), after 30 days storage in distilled water, and after 30 days storage in coffee solution. CIELAB color difference (ΔE*) was calculated for both periods. Data were submitted to analysis of variance, followed by Student–Newman–Keuls method (α = 0.05). Results: The photoinitiator system influenced hardness, where CQ presented the lowest top and bottom values. No statistical difference among groups was observed for the bottom/top hardness ratio. Regarding the depth of cure, the CQ and those formulated with CQ associations presented higher values than TPO and BAPO. Regarding color, BAPO and CQ+BAPO presented the highest ΔE* after 30 days in water immersion, whereas CQ+TPO and CQ+TPO+BAPO presented the lowest after 30 days in coffee immersion. Conclusion: The associations CQ+TPO and CQ+TPO+BAPO presented improved color stability and hardness when compared with CQ, and did not influence the depth of cure.
CLINICAL SIGNIFICANCE The combination of alternative photoinitiators with the traditional camphorquinone/amine system improved the color stability of the model resin composites and maintaining their mechanical properties. (J Esthet Restor Dent 27:S41–S48, 2015)
INTRODUCTION Since the introduction of visible light-activated resin composites, the most used photosensitizer in dental materials has been the camphorquinone (CQ). In spite
of its relative success as a photoinitiator, some disadvantages could be attributed to CQ. The molecular structure of CQ includes a chromatic group that makes the material photoactive but, in turn, it is responsible for an intense yellow hue that could affect the esthetic
*PhD student of Graduate Program in Dentistry, Federal University of Pelotas, Pelotas, RS, Brazil † Private Practitioner, Federal Fluminense University, Niterói, RJ, Brazil ‡ Professor of School of Dentistry, Salgado de Oliveira University, Niterói, RJ, Brazil § Professor of School of Dentistry, Federal Fluminense University, Niterói, RJ, Brazil **Professor of School of Dentistry, Federal University of Pelotas, Pelotas, RS, Brazil †† Professor, Nucleus for Dental Biomaterials Research, School of Dentistry, Veiga de Almeida University, Rio de Janeiro, RJ, Brazil
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result of dental restorations in some situations.1 CQ also requires a co-initiator, a tertiary amine, which leads to acceleration of the polymerization process but undergoes oxidation and reduces the materials’ color stability overtime.2 Additionally, amines are known to generate by-products during photoreaction, tending to cause discoloration from yellow to brown depending on their type and fraction present in the system.3 Considerations regarding the cytotoxicity potential of unreacted CQ and amines eluted from the material have also been the subject of research.4–6 For esthetic reasons, the use of photoinitiator alternatives to CQ has been suggested. Phosphine oxide derivatives, such as diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO) and phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide (BAPO), could minimize the yellow hue of CQ and improve the color stability.7 These molecules absorb light at shorter wavelengths than CQ and do not require a co-initiator to trigger the polymerization reaction.4,8 Beyond enabling less chromatic composites, these alternative photosensitizers could improve the optical and mechanical properties due to the higher cure efficiency, promoted by the increase in active free radicals in the polymerization process.9 The use of these alternative systems in composite formulations could additionally result in enhanced aging resistance, reducing the need for replacing restorations. Recently, it was observed that TPO-based materials presented lower “yellowing” and higher color stability than CQ-based and BAPO-based materials, regardless of amine presence7 or different filler sizes.10 In addition, BAPO-based materials did not result in any advantage regarding yellow hue or color stability compared with CQ-based materials.7,10 Moreover, it was already shown that TPO-based and BAPO-based materials might lead to higher C=C conversion than materials formulated with CQ.1,11 It is also known that materials formulated with phosphine-oxide derivatives might present lower depth of cure than CQ-based materials.8 However, there is no information in the literature concerning the combinations of these alternatives systems with CQ and the respective consequences on hardness, depth of cure, and color.
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Therefore, the objective of this study was to determine the effects of TPO and BAPO combinations with CQ on hardness, depth of cure, and color stability of experimental composites. The research hypotheses of this study were: (1) The association of TPO or BAPO with CQ would improve the composite surface hardness (2) The association of TPO or BAPO with CQ would reduce the depth of cure (3) The association of TPO with CQ would improve the color stability of the composite, while the association of BAPO with CQ would not
MATERIALS AND METHODS Study Design and Composites Formulation Experimental composites were prepared with a resin matrix formulated with 2,2 bis[4-2(2-hydroxy-3methacroyloxypropoxy)phenyl] propane (Bis-GMA, Esstech Inc., Essington, PA, USA) and triethyleneglycol dimethacrylate (TEGDMA, Esstech Inc.) at a 60:40 molar ratio. The following photoinitiator and co-initiator components (Sigma-Aldrich, St. Louis, MO, USA) were added: camphorquinone associated with the tertiary amine ethyl 4-dimethylaminobenzoate (EDMAB), diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide. The molecular structure of these components is shown in Figure 1. To avoid premature curing, 2,6-di-tert-butyl-4-methyl-phenol (Sigma-Aldrich) was added as radical scavenger. In total, six experimental groups were considered: CQ, TPO, BAPO, and the combinations CQ+TPO, CQ+BAPO, and CQ+TPO+BAPO, as shown in Table 1. Inorganic fillers were added at a total 60 wt%, which was comprised by 20% of 2 μm barium–aluminium– silicate (Esstech Inc.); 20% of 0.7 μm barium– aluminum–silicate (Esstech Inc.), and 60% of 14 nm silicon dioxide (Evonik Industries AG, Hanau-Wolfgang, Germany). All components were measured with 0.0001 g accuracy in an analytical balance (Shimadzu AX 200, Shimadzu Corporation, Tokyo, Japan), and
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FIGURE 1. Photoinitiators and co-initiator used in the study.
TABLE 1. Formulation of experimental groups Groups
Photoinitiator system
Other components
CQ
Camphorquinone (1.0 mol %) + ethyl 4-dimethylaminobenzoate (1.0 mol %)
TPO
Monoacylphosphine oxide (1.0 mol %)
Organic Matrix (40 wt%) BisGMA (60 mol%) TEGDMA (40 mol%)
BAPO
Bysacylphosphine oxide (1.0 mol %)
CQ/TPO
Camphorquinone (0.5 mol %) + monoacylphosphine oxide (0.5 mol %) + ethyl 4-dimethylaminobenzoate (1.0 mol %)
CQ/BAPO
Camphorquinone (0.5 mol %) + bysacylphosphine oxide (0.5 mol %) + ethyl 4-dimethylaminobenzoate (1.0 mol %)
CQ/TPO/BAPO
Camphorquinone (0.33 mol %) + monoacylphosphine oxide (0.33 mol %) + bysacylphosphine oxide (0.33 mol %) + ethyl 4-dimethylaminobenzoate (1.0 mol %)
were mechanically mixed (DAC 150 Speed Mixer, Flacktek, Landrum, SC, USA) for 1 minute to produce a homogeneous paste. For light-curing, a quartz–tungsten–halogen curing unit (Optilux 501, Kerr, Orange, CA, USA) with 600 mW/cm2 of irradiance was used.
Hardness For the hardness analysis, the specimens were made in a cylindrical metal mold of 8-mm inner diameter and 2-mm thickness. The top surface was covered with a
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Inorganic Content (60 wt%) 20% of 2 μm barium– aluminium–silicate 20% of 0.7 μm barium–aluminium– silicate 60% of 14 nm silicon dioxide
Mylar strip and made flat by pressing down with a glass slab. For each group, five specimens were light-cured for 40 seconds from the top surface. After 24 hours dry–storage in the dark, the specimens were submitted to the Knoop hardness test (Micromet 5104—Buehler, Japan) under a load of 25 g for 15 seconds. Readings were taken at five locations at both top and bottom composite surfaces. The Knoop hardness number (KHN, kgf/mm2) for each surface was calculated as the average of the five readings. It was also calculated the ratio of bottom/top hardness values .The closer to one the ratio value is, the more efficient the curing process.
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Depth of Cure For depth of cure analysis, following International Organization for Standardization 4049 standard recommendations,12 each model composite was inserted into a cylindrical Teflon mold of 10-mm inner diameter and 10-mm thickness. The top surface was covered with a Mylar strip and made flat by pressing down with a glass slab. Five cylinder-shaped specimens were light-cured for 40 seconds and removed from the mold. The uncured material from the bottom surface was scraped using a plastic spatula. The thickness of the cured composite remnant was measured three times with a digital caliper (ABS Digimatic Caliper, Mitutoyo, Tokyo, Japan), averaged, and divided by two.
Color The specimens were made in a cylindrical metal mold of 8-mm inner diameter and 2-mm thickness. The top surface was covered with a Mylar strip and made flat by pressing down with a glass slab. For each group, 10 specimens were light-cured for 40 seconds. Color measurement was expressed by CIELAB parameters (CIE L*, CIE a*, and CIE b*), where CIE L* is the lightness, with 100 for white and 0 for black, and the axes CIE a* and CIE b* are the red-green and yellow-blue color coordinates, respectively. A positive CIE a* or CIE b* value represents a red or yellow shade, and a negative CIE a* or CIE b* represents green or blue, respectively. The specular component included (SCI) mode13 was used over a zero calibrating box (CIE L* = 0.0, CIE a* = 0.0, and CIE b* = 0.0) and white background (CIE L* = 93.2, CIE a* = -0.3, and CIE b* = 1.6) using a spectrophotometer (CM-2600d, Konica Minolta, Tokyo, Japan) with 6-mm diameter aperture. Illuminating and viewing configurations complied with CIE 10° observer geometry and D65 illuminant. Color measurements were obtained at three different periods: 24 hours after polymerization and dry storage at 37°C (baseline), after 30 days of storage in distilled water at 37°C, and after 30 days of storage in distilled water + 30 days of storage in coffee solution at 37°C. The coffee solution was made with 3.6 g of coffee powder dissolved in 300 mL of boiling distilled water.
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After 10 minutes of stirring, the solution was filtered through a filter paper.14 The CIELAB color difference (ΔE*) was calculated by the formula: 12
ΔE * = ⎡⎣( ΔL*)2 + ( Δa*)2 + ( Δb*)2 ⎤⎦
where ΔL*, Δa* and Δb* are the mathematical differences between CIE L*, CIE a*, and CIE b* of the different evaluated periods (30 days of water storage with baseline and 30 days of coffee storage with baseline).
Statistical Analyses Statistical analyses were conducted using SigmaPlot 13.0 software (Systat Software, San Jose, CA, USA). Data for hardness at top, hardness at bottom, bottom/top hardness ratio, depth of cure, and CIELAB individual parameters at baseline were analyzed by one-way analysis of variance (ANOVA). The ΔE* was analyzed using two-way ANOVA, with photoinitiator system and storage condition as factors. CIE b* and ΔE* data were heteroscedastic and thus were transformed to ranks before subjected to statistical analysis. All pairwise multiple comparisons procedures were performed using the Student–Newman–Keuls’ method at a preset alpha of 0.05.
RESULTS Table 2 shows the hardness, cure efficiency, and depth of cure results. For the hardness at top surfaces, CQ+TPO+BAPO, TPO, BAPO, and CQ+BAPO showed the highest values (p < 0.001), followed by CQ+TPO (p < 0.001), while CQ presented the lowest value (p < 0.001). For hardness at bottom surfaces, CQ presented the lowest value (p = 0.02), and there were no significant differences among the other groups. Regarding the cure efficiency indirectly evaluated by the bottom/top hardness ratio, there were no statistical differences among the tested groups (p = 0.903). For the depth of cure, CQ+TPO presented significantly higher values than BAPO and TPO, but it was statistically equivalent to CQ, CQ+BAPO, and CQ+TPO+BAPO
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TABLE 2. Hardness analysis (n = 5): hardness at top surface, at bottom surface (Knoop hardness number, kgf/mm2), and the bottom/top ratio. Depth of cure (n = 5) analysis by International Organization for Standardization 4049 scraping method.Values are means ± standard deviations CQ
TPO
BAPO
CQ/TPO
CQ/BAPO
CQ/TPO/ BAPO
Top hardness
26.3 ± 0.7 c
33.5 ± 2.4 a
32.9 ± 1.7 a
30.6 ± 1.7 b
32.0 ± 2.9 a,b
35.0 ± 1.7 a
Bottom hardness
18.8 ± 1.2 b
24.4 ± 1.9 a
25.6 ± 2.9 a
22.9 ± 3.8 a
24.5 ± 2.3 a
25.4 ± 1.5 a
Bottom/top ratio
0.7 ± 0.1 a
0.8 ± 0.2 a
0.8 ± 0.2 a
0.8 ± 0.1 a
0.8 ± 0.2 a
0.7 ± 0.1 a
Depth of cure
3.7 ± 0.1 a,b
3.2 ± 0.1 c
3.6 ± 0.1 b
3.8 ± 0.1 a
3.7 ± 0.1 a,b
3.7 ± 0.1 a,b
Lowercase letters in each row indicate differences among groups (α = 0.05). Comparisons between the different methodologies were not carried out.
TABLE 3. Color measurement (n = 10).Values are means ± standard deviations: The CIELAB parameters at baseline (24 hours after curing) CQ
TPO
BAPO
CQ/TPO
CQ/BAPO
CQ/TPO/ BAPO
CIE L*
59.8 ± 0.8 a
59.5 ± 0.5 a
58.8 ± 1.1 a
59.7 ± 1.1 a
59.5 ± 0.8 a
59.8 ± 0.5 a
CIE a*
−1.2 ± 0.1 c
−0.8 ± 0.1 a
−1.1 ± 0.2 c
−1.0 ± 0.1 b
−1.1 ± 0.1 b
−1.1 ± 0.1 b
CIE b*
2.4 ± 0.5 b
−0.5 ± 0.4 e
3.4 ± 0.9 a
1.9 ± 0.7 c
1.9 ± 0.4 c
0.8 ± 0.5 d
Lowercase letters in each row indicate differences among groups (α = 0.05).
groups. TPO presented the lowest values (p < 0.001), followed by BAPO (p < 0.001).
show significant differences among themselves. After 30 days of water storage and 30 days in coffee, BAPO presented the highest values, followed by CQ+BAPO, TPO, and CQ, which were statistically equivalent, and then by CQ+TPO, and CQ+TPO+BAPO. The storage in coffee solution increased the ΔE* values for materials formulated with CQ, TPO, and BAPO, but decreased for those formulated with CQ+TPO, CQ+BAPO, and CQ+TPO+BAPO. Statistical difference was only observed in CQ (p = 0.023), TPO (p = 0.031), and CQ+TPO (p = 0.019) groups.
Table 3 shows the results for the CIELAB parameters at baseline. There was no significant difference in CIE L* values among groups (p = 0.115). Regarding CIE a*, TPO presented the highest values (p < 0.001), followed by CQ+TPO, CQ+BAPO, and CQ+TPO+BAPO, which were statistically equivalent, and BAPO and CQ, which presented the lowest values. The CIE b* values showed that BAPO presented the highest values (p = 0.001), followed by CQ, CQ+BAPO, and CQ+TPO+BAPO, which were statistically equivalent, CQ+TPO and then by TPO, which presented the lowest values.
DISCUSSION
Figure 2 presents the results for ΔE*. There was statistically significant interaction between the factors “photoinitiator system” and “storage condition” (p = 0.004). It was observed that after 30 days of water storage, BAPO and CQ+BAPO presented significantly higher values than the other groups, which did not
The first hypothesis of this study was accepted since the combinations of TPO and BAPO with CQ improved the hardness of model composites. Hardness is associated with mechanical strength, rigidity, and softening resistance. KHN correlates well with the degree of monomer conversion,15 which plays an
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FIGURE 2. Results for ΔE* after 30 days of distilled water storage (WS) and 30 days of water storage + 30 days of coffee solution storage (CS), compared with baseline. Bars are means + standard deviations (n = 10). Distinct letters indicate statistical differences between storage conditions, for each group. Statistically similar groups in each storage condition are indicated by connected dots above bars.
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hardness is associated with a reduced irradiance as light travels through the material, as well as to lower quantity of photons that reaches the deep composite area.20 Light achieves the top surface with minimum attenuation, consequently more efficiently reaching and exciting the photoinitiator molecules, in comparison with the bottom surface. However, there was no statistical difference among groups regarding the bottom/top hardness ratio.
important role in determining the success of restorations, since lower conversion corresponds to inferior mechanical properties16 and increased leaching of toxic substances from the composite.17 As observed in Table 2, TPO and BAPO presented higher hardness values at top and bottom surfaces than CQ. Similar results were reported in a previous study.10 This could be related to the distinct reactivity of each photoinitiator system. The combination CQ+TPO presented significantly lower hardness values at top than the TPO group, but significantly higher values than the CQ group. The combination of CQ+BAPO was not significantly different from the BAPO group but showed higher values than the CQ group. This could be related to lower cure efficiency of the binary system of CQ/amine8 and by competitiveness between the photoinitiator molecules in the propagation and formation of a polymer network, as well the characteristics of the polymer formed. In polymerization terms, CQ produces only one active free radical, whereas TPO produces two and BAPO produces four. Theoretically, the greater the free-radical level, the better the cure efficiency.18 TPO and BAPO are more efficient photoinitiators than CQ, presenting respectively two and four potentially active radicals for each molecule.19 Thus, more active centers for radical formation and chain extension are formed, also favoring cross-linking, which makes the polymer less susceptible to softening.
The second hypothesis of this study was rejected, as the association of TPO with CQ did not reduce the depth of cure. As observed in Table 2, TPO presented the lowest depth of cure results followed by BAPO. This could be explained by the maximum absorption of TPO, is at shorter light wavelengths, theoretically imposing limitations for in-depth cure. Similar results were already reported.8,21 It was expected that the combination of CQ with TPO would present lower depth of cure in comparison with the CQ-based group, but the opposite was observed. This could be explained by a synergistic effect that occurs when two or more photoinitiators are used together.22 A synergistic effect could also explain the higher depth of cure of CQ+TPO and CQ+BAPO in comparison to TPO and BAPO used alone. The cure efficiency of a material can be related directly to the degree of C=C conversion,7 polymerization rate and/or depth of cure analysis,8 or indirectly to the surface hardness.10
In this study, the hardness values at the top were higher than at the bottom for all groups. The lower bottom
The third hypothesis of this study was accepted. The association of TPO with CQ increased the color
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stability (lower ΔE*) of the model composites, whereas the association of BAPO with CQ decreased the color stability (higher ΔE*). As demonstrated in Figure 2, the ΔE* values ranged between 2.50 (CQ+TPO+BAPO) and 4.32 (BAPO) for water storage and between 2.47 (CQ+TPO+BAPO) and 4.72 (BAPO) for water and coffee storage. A more color-stable material was obtained when the three photoinitiator molecules were associated. Although not measured, this could be related to the number of cross-links in each group and to the polymeric network formed. The higher the color stability, the lesser the changes promoted by hydrolysis, leading to reduced leaching of chemical components. Regarding color measurement at baseline, BAPO followed by CQ presented the highest yellowing degree (CIE b*) and TPO the lowest. This is in accordance with previous studies.7,10,23 From an esthetic standpoint, the alternative photoinitiator should reduce the yellowing hue of the CQ/amine system. Although BAPO exhibited higher cure efficiency than CQ, no improvement in the yellowing hue (lower CIE b*) was observed when it was used alone, unlike the TPO group. This result could be related to the fact that TPO exhibits a much higher molar extinction coefficient than BAPO,19 meaning a higher probability of molecule consumption of TPO in comparison with CQ and BAPO. Moreover, the combinations CQ+TPO, followed by CQ+BAPO and CQ+TPO+BAPO showed lower CIE b* in comparison with the CQ group. From a clinical perspective, ΔE* values lower than 1.0 are considered as unperceivable by the human eye. As well, those between 1.0 and 3.3 are perceivable by skilled operators, but are still in a clinically acceptable range.24 ΔE* values above 3.3 are perceivable by untrained observers and, for this reason, are considered not clinically acceptable.25 The color of resin composites can be influenced by extrinsic factors, such of absorption of pigments of some solutions, such as coffee. If the staining gets through the superficial composite layers, it will not be possible to be removed by polishing. Color can also be modified by intrinsic factors due to chemical reactions such as the leaching of unreacted monomers by hydrolysis reaction26 and
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photoinitiator components that were not consumed during light exposure.7 In order to achieve successful esthetics, resin composites should maintain the color over time of clinical service. As reported by Baldissera et al., most restorations failures in anterior teeth occur because of esthetic reasons, mainly by mismatch in color and translucency.27 In summary, the combination of alternative photoinitiators with CQ presented encouraging results, from both esthetic and mechanical viewpoints due to improving their color stability and maintaining the mechanical properties of the model resin composites.
CONCLUSIONS Within the limitations of this study, it can be concluded that TPO and BAPO combinations with CQ improved composite hardness but did not improve depth of cure. Combinations of TPO and BAPO with CQ decreased the yellow degree of model composites, but increased color stability was only observed in associations with the presence of TPO.
DISCLOSURE The authors do not have any financial interest in the companies whose materials are included in the article.
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Reprint requests: Vinícius E. Salgado, DDS, MS, PhD student, Rua Gonçalves Chaves, 457, Centro, Pelotas, RS, Brazil; Tel.: + 55 53 32256741; email: [email protected]
DOI 10.1111/jerd.12146
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Effect of Photoinitiator Combinations on Hardness, Depth of Cure, and Color of Model Resin Composites VINÍCIUS ESTEVES SALGADO, DDS, MS*, MILA METRI BORBA, DDS, MS†, LARISSA MARIA CAVALCANTE, DDS, MS, PhD‡,§, RAFAEL RATTO DE MORAES, DDS, MS, PhD**, LUIS FELIPE SCHNEIDER, DDS, MS, PhD§,††
ABSTRACT Objective: This study aims to determine the influence of photoinitiators’ combinations on the hardness, depth of cure, and color of model resin composites. Materials and Methods: The composites were formulated by a mixture of BisGMA and triethyleneglycol dimethacrylate (60:40 mol), with barium–aluminum–silicate glass and silicon dioxide particles as inorganic fillers (60 wt%). Three photoinitiator types were tested: camphorquinone/amine (CQ), monoacylphosphine oxide (TPO), and bysacylphosphine oxide (BAPO). Six experimental groups were formed by differences in photoinitiator systems: CQ, TPO, BAPO, CQ+TPO, CQ+BAPO, and CQ+TPO+BAPO. Hardness was determined by Knoop indentation at the top and bottom surfaces (n = 5). Depth of cure was performed according to ISO 4049 scraping method (n = 5). Color was obtained by the CIELAB method (n = 10), 24 hours after curing (baseline), after 30 days storage in distilled water, and after 30 days storage in coffee solution. CIELAB color difference (ΔE*) was calculated for both periods. Data were submitted to analysis of variance, followed by Student–Newman–Keuls method (α = 0.05). Results: The photoinitiator system influenced hardness, where CQ presented the lowest top and bottom values. No statistical difference among groups was observed for the bottom/top hardness ratio. Regarding the depth of cure, the CQ and those formulated with CQ associations presented higher values than TPO and BAPO. Regarding color, BAPO and CQ+BAPO presented the highest ΔE* after 30 days in water immersion, whereas CQ+TPO and CQ+TPO+BAPO presented the lowest after 30 days in coffee immersion. Conclusion: The associations CQ+TPO and CQ+TPO+BAPO presented improved color stability and hardness when compared with CQ, and did not influence the depth of cure.
CLINICAL SIGNIFICANCE The combination of alternative photoinitiators with the traditional camphorquinone/amine system improved the color stability of the model resin composites and maintaining their mechanical properties. (J Esthet Restor Dent 27:S41–S48, 2015)
INTRODUCTION Since the introduction of visible light-activated resin composites, the most used photosensitizer in dental materials has been the camphorquinone (CQ). In spite
of its relative success as a photoinitiator, some disadvantages could be attributed to CQ. The molecular structure of CQ includes a chromatic group that makes the material photoactive but, in turn, it is responsible for an intense yellow hue that could affect the esthetic
*PhD student of Graduate Program in Dentistry, Federal University of Pelotas, Pelotas, RS, Brazil † Private Practitioner, Federal Fluminense University, Niterói, RJ, Brazil ‡ Professor of School of Dentistry, Salgado de Oliveira University, Niterói, RJ, Brazil § Professor of School of Dentistry, Federal Fluminense University, Niterói, RJ, Brazil **Professor of School of Dentistry, Federal University of Pelotas, Pelotas, RS, Brazil †† Professor, Nucleus for Dental Biomaterials Research, School of Dentistry, Veiga de Almeida University, Rio de Janeiro, RJ, Brazil
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result of dental restorations in some situations.1 CQ also requires a co-initiator, a tertiary amine, which leads to acceleration of the polymerization process but undergoes oxidation and reduces the materials’ color stability overtime.2 Additionally, amines are known to generate by-products during photoreaction, tending to cause discoloration from yellow to brown depending on their type and fraction present in the system.3 Considerations regarding the cytotoxicity potential of unreacted CQ and amines eluted from the material have also been the subject of research.4–6 For esthetic reasons, the use of photoinitiator alternatives to CQ has been suggested. Phosphine oxide derivatives, such as diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO) and phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide (BAPO), could minimize the yellow hue of CQ and improve the color stability.7 These molecules absorb light at shorter wavelengths than CQ and do not require a co-initiator to trigger the polymerization reaction.4,8 Beyond enabling less chromatic composites, these alternative photosensitizers could improve the optical and mechanical properties due to the higher cure efficiency, promoted by the increase in active free radicals in the polymerization process.9 The use of these alternative systems in composite formulations could additionally result in enhanced aging resistance, reducing the need for replacing restorations. Recently, it was observed that TPO-based materials presented lower “yellowing” and higher color stability than CQ-based and BAPO-based materials, regardless of amine presence7 or different filler sizes.10 In addition, BAPO-based materials did not result in any advantage regarding yellow hue or color stability compared with CQ-based materials.7,10 Moreover, it was already shown that TPO-based and BAPO-based materials might lead to higher C=C conversion than materials formulated with CQ.1,11 It is also known that materials formulated with phosphine-oxide derivatives might present lower depth of cure than CQ-based materials.8 However, there is no information in the literature concerning the combinations of these alternatives systems with CQ and the respective consequences on hardness, depth of cure, and color.
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Therefore, the objective of this study was to determine the effects of TPO and BAPO combinations with CQ on hardness, depth of cure, and color stability of experimental composites. The research hypotheses of this study were: (1) The association of TPO or BAPO with CQ would improve the composite surface hardness (2) The association of TPO or BAPO with CQ would reduce the depth of cure (3) The association of TPO with CQ would improve the color stability of the composite, while the association of BAPO with CQ would not
MATERIALS AND METHODS Study Design and Composites Formulation Experimental composites were prepared with a resin matrix formulated with 2,2 bis[4-2(2-hydroxy-3methacroyloxypropoxy)phenyl] propane (Bis-GMA, Esstech Inc., Essington, PA, USA) and triethyleneglycol dimethacrylate (TEGDMA, Esstech Inc.) at a 60:40 molar ratio. The following photoinitiator and co-initiator components (Sigma-Aldrich, St. Louis, MO, USA) were added: camphorquinone associated with the tertiary amine ethyl 4-dimethylaminobenzoate (EDMAB), diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide. The molecular structure of these components is shown in Figure 1. To avoid premature curing, 2,6-di-tert-butyl-4-methyl-phenol (Sigma-Aldrich) was added as radical scavenger. In total, six experimental groups were considered: CQ, TPO, BAPO, and the combinations CQ+TPO, CQ+BAPO, and CQ+TPO+BAPO, as shown in Table 1. Inorganic fillers were added at a total 60 wt%, which was comprised by 20% of 2 μm barium–aluminium– silicate (Esstech Inc.); 20% of 0.7 μm barium– aluminum–silicate (Esstech Inc.), and 60% of 14 nm silicon dioxide (Evonik Industries AG, Hanau-Wolfgang, Germany). All components were measured with 0.0001 g accuracy in an analytical balance (Shimadzu AX 200, Shimadzu Corporation, Tokyo, Japan), and
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FIGURE 1. Photoinitiators and co-initiator used in the study.
TABLE 1. Formulation of experimental groups Groups
Photoinitiator system
Other components
CQ
Camphorquinone (1.0 mol %) + ethyl 4-dimethylaminobenzoate (1.0 mol %)
TPO
Monoacylphosphine oxide (1.0 mol %)
Organic Matrix (40 wt%) BisGMA (60 mol%) TEGDMA (40 mol%)
BAPO
Bysacylphosphine oxide (1.0 mol %)
CQ/TPO
Camphorquinone (0.5 mol %) + monoacylphosphine oxide (0.5 mol %) + ethyl 4-dimethylaminobenzoate (1.0 mol %)
CQ/BAPO
Camphorquinone (0.5 mol %) + bysacylphosphine oxide (0.5 mol %) + ethyl 4-dimethylaminobenzoate (1.0 mol %)
CQ/TPO/BAPO
Camphorquinone (0.33 mol %) + monoacylphosphine oxide (0.33 mol %) + bysacylphosphine oxide (0.33 mol %) + ethyl 4-dimethylaminobenzoate (1.0 mol %)
were mechanically mixed (DAC 150 Speed Mixer, Flacktek, Landrum, SC, USA) for 1 minute to produce a homogeneous paste. For light-curing, a quartz–tungsten–halogen curing unit (Optilux 501, Kerr, Orange, CA, USA) with 600 mW/cm2 of irradiance was used.
Hardness For the hardness analysis, the specimens were made in a cylindrical metal mold of 8-mm inner diameter and 2-mm thickness. The top surface was covered with a
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Inorganic Content (60 wt%) 20% of 2 μm barium– aluminium–silicate 20% of 0.7 μm barium–aluminium– silicate 60% of 14 nm silicon dioxide
Mylar strip and made flat by pressing down with a glass slab. For each group, five specimens were light-cured for 40 seconds from the top surface. After 24 hours dry–storage in the dark, the specimens were submitted to the Knoop hardness test (Micromet 5104—Buehler, Japan) under a load of 25 g for 15 seconds. Readings were taken at five locations at both top and bottom composite surfaces. The Knoop hardness number (KHN, kgf/mm2) for each surface was calculated as the average of the five readings. It was also calculated the ratio of bottom/top hardness values .The closer to one the ratio value is, the more efficient the curing process.
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Depth of Cure For depth of cure analysis, following International Organization for Standardization 4049 standard recommendations,12 each model composite was inserted into a cylindrical Teflon mold of 10-mm inner diameter and 10-mm thickness. The top surface was covered with a Mylar strip and made flat by pressing down with a glass slab. Five cylinder-shaped specimens were light-cured for 40 seconds and removed from the mold. The uncured material from the bottom surface was scraped using a plastic spatula. The thickness of the cured composite remnant was measured three times with a digital caliper (ABS Digimatic Caliper, Mitutoyo, Tokyo, Japan), averaged, and divided by two.
Color The specimens were made in a cylindrical metal mold of 8-mm inner diameter and 2-mm thickness. The top surface was covered with a Mylar strip and made flat by pressing down with a glass slab. For each group, 10 specimens were light-cured for 40 seconds. Color measurement was expressed by CIELAB parameters (CIE L*, CIE a*, and CIE b*), where CIE L* is the lightness, with 100 for white and 0 for black, and the axes CIE a* and CIE b* are the red-green and yellow-blue color coordinates, respectively. A positive CIE a* or CIE b* value represents a red or yellow shade, and a negative CIE a* or CIE b* represents green or blue, respectively. The specular component included (SCI) mode13 was used over a zero calibrating box (CIE L* = 0.0, CIE a* = 0.0, and CIE b* = 0.0) and white background (CIE L* = 93.2, CIE a* = -0.3, and CIE b* = 1.6) using a spectrophotometer (CM-2600d, Konica Minolta, Tokyo, Japan) with 6-mm diameter aperture. Illuminating and viewing configurations complied with CIE 10° observer geometry and D65 illuminant. Color measurements were obtained at three different periods: 24 hours after polymerization and dry storage at 37°C (baseline), after 30 days of storage in distilled water at 37°C, and after 30 days of storage in distilled water + 30 days of storage in coffee solution at 37°C. The coffee solution was made with 3.6 g of coffee powder dissolved in 300 mL of boiling distilled water.
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After 10 minutes of stirring, the solution was filtered through a filter paper.14 The CIELAB color difference (ΔE*) was calculated by the formula: 12
ΔE * = ⎡⎣( ΔL*)2 + ( Δa*)2 + ( Δb*)2 ⎤⎦
where ΔL*, Δa* and Δb* are the mathematical differences between CIE L*, CIE a*, and CIE b* of the different evaluated periods (30 days of water storage with baseline and 30 days of coffee storage with baseline).
Statistical Analyses Statistical analyses were conducted using SigmaPlot 13.0 software (Systat Software, San Jose, CA, USA). Data for hardness at top, hardness at bottom, bottom/top hardness ratio, depth of cure, and CIELAB individual parameters at baseline were analyzed by one-way analysis of variance (ANOVA). The ΔE* was analyzed using two-way ANOVA, with photoinitiator system and storage condition as factors. CIE b* and ΔE* data were heteroscedastic and thus were transformed to ranks before subjected to statistical analysis. All pairwise multiple comparisons procedures were performed using the Student–Newman–Keuls’ method at a preset alpha of 0.05.
RESULTS Table 2 shows the hardness, cure efficiency, and depth of cure results. For the hardness at top surfaces, CQ+TPO+BAPO, TPO, BAPO, and CQ+BAPO showed the highest values (p < 0.001), followed by CQ+TPO (p < 0.001), while CQ presented the lowest value (p < 0.001). For hardness at bottom surfaces, CQ presented the lowest value (p = 0.02), and there were no significant differences among the other groups. Regarding the cure efficiency indirectly evaluated by the bottom/top hardness ratio, there were no statistical differences among the tested groups (p = 0.903). For the depth of cure, CQ+TPO presented significantly higher values than BAPO and TPO, but it was statistically equivalent to CQ, CQ+BAPO, and CQ+TPO+BAPO
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TABLE 2. Hardness analysis (n = 5): hardness at top surface, at bottom surface (Knoop hardness number, kgf/mm2), and the bottom/top ratio. Depth of cure (n = 5) analysis by International Organization for Standardization 4049 scraping method.Values are means ± standard deviations CQ
TPO
BAPO
CQ/TPO
CQ/BAPO
CQ/TPO/ BAPO
Top hardness
26.3 ± 0.7 c
33.5 ± 2.4 a
32.9 ± 1.7 a
30.6 ± 1.7 b
32.0 ± 2.9 a,b
35.0 ± 1.7 a
Bottom hardness
18.8 ± 1.2 b
24.4 ± 1.9 a
25.6 ± 2.9 a
22.9 ± 3.8 a
24.5 ± 2.3 a
25.4 ± 1.5 a
Bottom/top ratio
0.7 ± 0.1 a
0.8 ± 0.2 a
0.8 ± 0.2 a
0.8 ± 0.1 a
0.8 ± 0.2 a
0.7 ± 0.1 a
Depth of cure
3.7 ± 0.1 a,b
3.2 ± 0.1 c
3.6 ± 0.1 b
3.8 ± 0.1 a
3.7 ± 0.1 a,b
3.7 ± 0.1 a,b
Lowercase letters in each row indicate differences among groups (α = 0.05). Comparisons between the different methodologies were not carried out.
TABLE 3. Color measurement (n = 10).Values are means ± standard deviations: The CIELAB parameters at baseline (24 hours after curing) CQ
TPO
BAPO
CQ/TPO
CQ/BAPO
CQ/TPO/ BAPO
CIE L*
59.8 ± 0.8 a
59.5 ± 0.5 a
58.8 ± 1.1 a
59.7 ± 1.1 a
59.5 ± 0.8 a
59.8 ± 0.5 a
CIE a*
−1.2 ± 0.1 c
−0.8 ± 0.1 a
−1.1 ± 0.2 c
−1.0 ± 0.1 b
−1.1 ± 0.1 b
−1.1 ± 0.1 b
CIE b*
2.4 ± 0.5 b
−0.5 ± 0.4 e
3.4 ± 0.9 a
1.9 ± 0.7 c
1.9 ± 0.4 c
0.8 ± 0.5 d
Lowercase letters in each row indicate differences among groups (α = 0.05).
groups. TPO presented the lowest values (p < 0.001), followed by BAPO (p < 0.001).
show significant differences among themselves. After 30 days of water storage and 30 days in coffee, BAPO presented the highest values, followed by CQ+BAPO, TPO, and CQ, which were statistically equivalent, and then by CQ+TPO, and CQ+TPO+BAPO. The storage in coffee solution increased the ΔE* values for materials formulated with CQ, TPO, and BAPO, but decreased for those formulated with CQ+TPO, CQ+BAPO, and CQ+TPO+BAPO. Statistical difference was only observed in CQ (p = 0.023), TPO (p = 0.031), and CQ+TPO (p = 0.019) groups.
Table 3 shows the results for the CIELAB parameters at baseline. There was no significant difference in CIE L* values among groups (p = 0.115). Regarding CIE a*, TPO presented the highest values (p < 0.001), followed by CQ+TPO, CQ+BAPO, and CQ+TPO+BAPO, which were statistically equivalent, and BAPO and CQ, which presented the lowest values. The CIE b* values showed that BAPO presented the highest values (p = 0.001), followed by CQ, CQ+BAPO, and CQ+TPO+BAPO, which were statistically equivalent, CQ+TPO and then by TPO, which presented the lowest values.
DISCUSSION
Figure 2 presents the results for ΔE*. There was statistically significant interaction between the factors “photoinitiator system” and “storage condition” (p = 0.004). It was observed that after 30 days of water storage, BAPO and CQ+BAPO presented significantly higher values than the other groups, which did not
The first hypothesis of this study was accepted since the combinations of TPO and BAPO with CQ improved the hardness of model composites. Hardness is associated with mechanical strength, rigidity, and softening resistance. KHN correlates well with the degree of monomer conversion,15 which plays an
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FIGURE 2. Results for ΔE* after 30 days of distilled water storage (WS) and 30 days of water storage + 30 days of coffee solution storage (CS), compared with baseline. Bars are means + standard deviations (n = 10). Distinct letters indicate statistical differences between storage conditions, for each group. Statistically similar groups in each storage condition are indicated by connected dots above bars.
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hardness is associated with a reduced irradiance as light travels through the material, as well as to lower quantity of photons that reaches the deep composite area.20 Light achieves the top surface with minimum attenuation, consequently more efficiently reaching and exciting the photoinitiator molecules, in comparison with the bottom surface. However, there was no statistical difference among groups regarding the bottom/top hardness ratio.
important role in determining the success of restorations, since lower conversion corresponds to inferior mechanical properties16 and increased leaching of toxic substances from the composite.17 As observed in Table 2, TPO and BAPO presented higher hardness values at top and bottom surfaces than CQ. Similar results were reported in a previous study.10 This could be related to the distinct reactivity of each photoinitiator system. The combination CQ+TPO presented significantly lower hardness values at top than the TPO group, but significantly higher values than the CQ group. The combination of CQ+BAPO was not significantly different from the BAPO group but showed higher values than the CQ group. This could be related to lower cure efficiency of the binary system of CQ/amine8 and by competitiveness between the photoinitiator molecules in the propagation and formation of a polymer network, as well the characteristics of the polymer formed. In polymerization terms, CQ produces only one active free radical, whereas TPO produces two and BAPO produces four. Theoretically, the greater the free-radical level, the better the cure efficiency.18 TPO and BAPO are more efficient photoinitiators than CQ, presenting respectively two and four potentially active radicals for each molecule.19 Thus, more active centers for radical formation and chain extension are formed, also favoring cross-linking, which makes the polymer less susceptible to softening.
The second hypothesis of this study was rejected, as the association of TPO with CQ did not reduce the depth of cure. As observed in Table 2, TPO presented the lowest depth of cure results followed by BAPO. This could be explained by the maximum absorption of TPO, is at shorter light wavelengths, theoretically imposing limitations for in-depth cure. Similar results were already reported.8,21 It was expected that the combination of CQ with TPO would present lower depth of cure in comparison with the CQ-based group, but the opposite was observed. This could be explained by a synergistic effect that occurs when two or more photoinitiators are used together.22 A synergistic effect could also explain the higher depth of cure of CQ+TPO and CQ+BAPO in comparison to TPO and BAPO used alone. The cure efficiency of a material can be related directly to the degree of C=C conversion,7 polymerization rate and/or depth of cure analysis,8 or indirectly to the surface hardness.10
In this study, the hardness values at the top were higher than at the bottom for all groups. The lower bottom
The third hypothesis of this study was accepted. The association of TPO with CQ increased the color
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stability (lower ΔE*) of the model composites, whereas the association of BAPO with CQ decreased the color stability (higher ΔE*). As demonstrated in Figure 2, the ΔE* values ranged between 2.50 (CQ+TPO+BAPO) and 4.32 (BAPO) for water storage and between 2.47 (CQ+TPO+BAPO) and 4.72 (BAPO) for water and coffee storage. A more color-stable material was obtained when the three photoinitiator molecules were associated. Although not measured, this could be related to the number of cross-links in each group and to the polymeric network formed. The higher the color stability, the lesser the changes promoted by hydrolysis, leading to reduced leaching of chemical components. Regarding color measurement at baseline, BAPO followed by CQ presented the highest yellowing degree (CIE b*) and TPO the lowest. This is in accordance with previous studies.7,10,23 From an esthetic standpoint, the alternative photoinitiator should reduce the yellowing hue of the CQ/amine system. Although BAPO exhibited higher cure efficiency than CQ, no improvement in the yellowing hue (lower CIE b*) was observed when it was used alone, unlike the TPO group. This result could be related to the fact that TPO exhibits a much higher molar extinction coefficient than BAPO,19 meaning a higher probability of molecule consumption of TPO in comparison with CQ and BAPO. Moreover, the combinations CQ+TPO, followed by CQ+BAPO and CQ+TPO+BAPO showed lower CIE b* in comparison with the CQ group. From a clinical perspective, ΔE* values lower than 1.0 are considered as unperceivable by the human eye. As well, those between 1.0 and 3.3 are perceivable by skilled operators, but are still in a clinically acceptable range.24 ΔE* values above 3.3 are perceivable by untrained observers and, for this reason, are considered not clinically acceptable.25 The color of resin composites can be influenced by extrinsic factors, such of absorption of pigments of some solutions, such as coffee. If the staining gets through the superficial composite layers, it will not be possible to be removed by polishing. Color can also be modified by intrinsic factors due to chemical reactions such as the leaching of unreacted monomers by hydrolysis reaction26 and
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photoinitiator components that were not consumed during light exposure.7 In order to achieve successful esthetics, resin composites should maintain the color over time of clinical service. As reported by Baldissera et al., most restorations failures in anterior teeth occur because of esthetic reasons, mainly by mismatch in color and translucency.27 In summary, the combination of alternative photoinitiators with CQ presented encouraging results, from both esthetic and mechanical viewpoints due to improving their color stability and maintaining the mechanical properties of the model resin composites.
CONCLUSIONS Within the limitations of this study, it can be concluded that TPO and BAPO combinations with CQ improved composite hardness but did not improve depth of cure. Combinations of TPO and BAPO with CQ decreased the yellow degree of model composites, but increased color stability was only observed in associations with the presence of TPO.
DISCLOSURE The authors do not have any financial interest in the companies whose materials are included in the article.
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Reprint requests: Vinícius E. Salgado, DDS, MS, PhD student, Rua Gonçalves Chaves, 457, Centro, Pelotas, RS, Brazil; Tel.: + 55 53 32256741; email: [email protected]
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