Journal of Investigative and Clinical Dentistry (2015), 6, 53–58
ORIGINAL ARTICLE Dental Biomaterials
Effect of energy density and delay time on the degree of conversion and Knoop microhardness of a dual resin cement bora A. N. L. Lima, Giselle M. Marchi, Maria do Carmo A. J. Mainardi, Maria Cecılia C. Giorgi, De ucia M. Ambrosano, Luiz A. M. S. Paulillo & Fla vio H. B. Aguiar Gla Piracicaba Dental School, University of Campinas, Piracicaba, Brazil
Keywords degree of conversion, hardness, polymerization, resin cement, root canal. Correspondence Ms M. C. A. J. Mainardi, Piracicaba Dental School, University of Campinas, 901 Limeira Avenue, Piracicaba, S~ ao Paulo 13414-903, Brazil. Tel: +55-19-2106-5200 Email: [email protected] Received 30 January 2013; accepted 9 June 2013. doi: 10.1111/jicd.12075
Abstract Aim: In the present study, we evaluated the influence of the photo-curing delay time and energy density on the degree of conversion and the Knoop microhardness of a resin cement. Methods: Seventy-eight samples were assigned to 13 groups (n = 6), one of which received no light curing (control). The samples were made of a dualcured resin cement (RelyX ARC) with the aid of a Teflon matrix, submitted to one of the following energy densities (J/cm²): 7, 14, 20, and 28. Delay times were immediate (0), 1 min, or 2 min. After 24 h, the degree of conversion and microhardness were measured at three segments: cervical, medium, and apical. Data were submitted to three-way ANOVA and Tukey’s and Dunnett’s tests, the latest of which was used to compare the control to the experimental groups. Results: No interaction was observed between delay time and energy density regarding the degree of conversion. The cervical segment showed the highest values, while the apical showed the lowest. Microhardness values concerning the cervical segment in all groups were statistically different from that obtained for the control. Conclusion: A high-irradiance light-curing unit allows for a reduced irradiation exposure time with a short delay time, aimed at tooth restorations using a dual-cured resin cement.
Introduction Indirect tooth-colored restorations have been increasingly used for endodontically-treated teeth using fiber post.1 Dual- and self-cured resin cements are recommended for the cementation of intra-radicular posts. Light-cured resin cements are inappropriate for post-cementation because they provide incomplete polymerization in the apex of the canal.2 Dual-cured resin cements combine the desirable characteristics of photocuring and chemical curing resin,3 providing proper polymerization in either the presence or the absence of light.4 The self-curing component of the resin cement is useful because it favors its degree of conversion in environments where radiant ª 2014 Wiley Publishing Asia Pty Ltd
energy is unavailable.5 Both modes of activation present in dual-cured resin cements are complementary and independent.6 Furthermore, some dual resin cements are dependent on light activation, resulting in an inadequate degree of conversion if the cement is not light cured.7 High values of the degree of conversion might be an indicator of good physical and mechanical properties of resin-based materials. Poor monomer conversion of these materials might lead to low bond strength and high water sorption, compromising their physical properties.8 The immediate light curing of dual-cured resin cements might cause a reduction in the degree of conversion.9 The reason is that immediate light curing causes the rapid formation of cross-linked polymer chains, entrapping 53
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Resin cement and polymerization protocols
Seventy-eight samples simulating root canals (2 mm in diameter and 13 mm in length) were randomly assigned to 12 experimental groups (dual-cured resin plus lightemitting diode [LED] light curing) and a control (no light curing). All groups, including the control, contained six specimens each. The dual-cured resin cement (RelyX ARC, shade A1; 3M-ESPE, St Paul, MN, USA) was manipulated for 10 s, inserted into a Teflon matrix, and light cured immediately, 1 min, or 2 min after the insertion using a LED device (Valo; Ultradent Products, South Jordan, UT, USA). The specimens were exposed to one of the following energy densities (J/cm²): 7 (350 mW/cm2 and 20 s), 14 (700 mW/cm2 and 20 s), 20 (1000 mW/ cm2 and 20 s), and 28 (700 mW/cm2 and 40 s). The irradiance values were monitored by a radiometer (Model 100 Optilux Radiometer; Kerr, Middleton, WI, USA), and those lower than 1000 mW/cm2 were obtained with the aid of spacers previously prepared in a pilot study. Twenty-four hours after confection, a double-sided diamond disc (KG, Cotia, SP, Brazil) was used to section the specimens into 4-mm-long segments: cervical, medium, 54
Degree of conversion measurement The measures of degree of conversion were made by FTIR. The coronal face of each specimen was put above the attenuated total reflectance crystal (ATR), attached to the FTIR device, which is compound by diamond/zinc selenide crystals. Each specimen covered the total area of the crystal. With the aid of a pressure arm lock, the specimens were submitted to a force of 90 N, in order to ensure direct contact between the specimen and the crystal. The spectra of polymerized and non-polymerized resin cement (Figure 1) were obtained through the ATR using 16 scans at a resolution of 4/cm. The absorbance peaks at 1638/cm (aliphatic C=C bonds) and 1608/cm (aromatic C=C bonds) were used to calculate the percentage of the remaining double bonds of carbon in the resin cement after polymerization using the following formula16: (a)
Absorbance
Materials and methods
and apical. The specimens were submitted to Fourier transform infrared spectroscopy (FTIR; Spectrum 100; PerkinElmer, SP, Brazil) to measure the degree of conversion, and then to Knoop hardness test to measure the microhardness, both aimed at the top surface.
Wavelength
(b)
Absorbance
unreacted monomers and free radicals into the polymeric network, and thus jeopardizing the chemical curing. Also, the curing reaction that occurs at the same time by selfand light polymerization of the cement might speed its vitrification process,10 making it unnecessary to prolong the light-curing time, especially if a delay occurs between manipulating the resin luting agent and the light activation. The polymerization of dual-cured resin cements can also be evaluated by the microhardness test.11 Microhardness is one of the most important properties of dental materials12 and might be used to predict the degree of conversion; the higher the microhardness, the higher the degree of conversion is expected to be. However, this correlation might be contradictory,13 as polymers are dependent on different polymerization modes, which might affect the hardness, but not the degree of conversion of a resin cement.14 Dual-cured resin cements might result in different polymer structures that vary in cross-linked density, affecting their ultimate mechanical properties.15 Therefore, the aim of the present study was to evaluate the influence of delayed photoactivation times and energy density (varying irradiance and light-activation time) on the degree of conversion and microhardness of a dualcured resin cement considering three root segments: cervical, medium, and apical. The null hypothesis is that the degree of conversion and microhardness might not be affected by these factors.
Wavelength
Figure 1. Representative Fourier transform infrared spectroscopy of the non-polymerized resin cement (a) and the polymerized resin cement (b).
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Resin cement and polymerization protocols
DC ð%Þ ¼ 100 ð1 Rpolymerized =Runpolymerized Þ R = peak at 1.638/cm/peak at 1.608/cm, where DC is the degree of conversion. Microhardness measurement Every specimen was embedded in polystyrene resin block and then sandpapered (#600 and #1200) in a polishing machine (Arotec; Cotia, SP, Brazil). The top surface microhardness was measured using the Knoop hardness test (Shimadzu, Japan), set at 20 g for 10 s. Three indentations were made for each specimen. The largest diagonal of the lozenge-shaped indentation in each specimen was measured to obtain the Knoop hardness test values. Data were submitted to three-way ANOVA and Tukey’s test. Dunnett’s test was used for comparison between the experimental groups and the control. The correlation test was used to correlate the degree of conversion and the Knoop hardness test values.
Results Degree of conversion analysis Table 1 shows the degree of conversion values obtained for all the groups. Significant differences were found among the three root segments studied (P < 0.0001); however, no difference was found among the energy densities (P = 0.1698) and delay times (P = 0.3132) assessed. The cervical segment showed the highest degree of conversion values, statistically different from those found for the others segments (Tukey’s test). The apical revealed the lowest values considering all experimental conditions. For the apical segment, Dunnett’s test
showed no statistically-significant difference among the groups when compared to the control. Significant differences were found for the cervical segment when each group was compared to the control, except for the energy density of 7 J/cm² at 0 and 1 min. For the medium segment, no statistical difference was found when the groups were compared to the control, considering all the variables tested, except for the density of 14 J/cm² at 0 min and of 20 J/cm² at 1 min. Knoop hardness test analysis Table 2 shows the Knoop hardness test values obtained for all groups. ANOVA showed triple interaction toward all the variables tested (P < 0.001). Regarding the immediate (0) time, no statistical difference was observed for the Knoop hardness test values among the three segments at all densities, except for the apical segment at the density of 14 J/cm². Significant differences were found between the apical and the other segments, at densities of 7 and 14 J/cm² at 1 min. When compared to the cervical segment, the apical segment revealed the lowest Knoop hardness test value for all densities at 2 min, except for 28 J/ cm². The mean values obtained for the cervical and apical segments at the densities of 20 and 28 J/cm², respectively for the 2-min delay time were significantly higher than those at 0 and 1 min. For the cervical segment, Dunnett’s test showed significant differences when the groups were compared to the control. For the densities of 7 J/cm² at 1 min, 14 J/cm² at 0 min, and 28 J/cm² at 1 and 2 min, the Knoop hardness test value found for the medium segment was statistically different from that (same segment) observed for the control. The Knoop hardness test values concerning the apical segment were statistically different from that found for the control for all groups and the
Table 1. Degree of conversion results Energy density (J/cm²) and standard deviation Time (min)
Segment
7
0
Cervical Medium Apical Cervical Medium Apical Cervical Medium Apical Cervical Medium Apical
78.41 77.71 73.81 78.74 77.93 76.42 81.55 79.98 76.36 74.36 74.74 75.00
1
2
Control
14 (8.87)a (3.82)b (9.43)c (3.18)a (2.26)b (3.44)c (6.47)a,† (6.68)b (3.64)c (5.85) (9.03) (5.91)
80.07 80.20 75.41 80.85 77.87 72.83 80.65 79.16 74.34
20 (2.35)a,† (2.85)b,‡ (4.63)c (2.41)a,† (2.88)b (9.91)c (2.13)a,† (2.47)b (2.21)c
81.64 79.74 74.92 81.55 81.72 76.60 84.63 79.12 77.01
28 (1.04)a,† (2.09)b (2.42)c (1.68)a,† (3.92)b,‡ (1.62)c (5.82)a,† (3.34)b (2.15)c
80.79 78.72 75.78 80.36 79.34 76.00 81.06 79.60 76.78
(1.07)a,† (1.63)b (0.90)c (1.32)a,† (1.66)b (1.72)c (1.23)a,† (1.24)b (0.88)c
Different lowercase letters differ statistically. †Difference between groups and control concerning the same segment; ‡differs from control concerning the medium segment.
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Table 2. Results of Knoop hardness test Energy density (J/cm²) and standard deviation Time (min) 0
1
2
Control
Segment Cervical Medium Apical Cervical Medium Apical Cervical Medium Apical Cervical Medium Apical
7 45.35 44.78 42.06 48.76 48.94 42.48 45.94 45.85 39.61 39.51 42.10 39.48
14 A,a,
(1.56) † (1.87)A,a (2.01)A,a (3.32)A,a,† (1.68)A,a,† (1.46)A,b (1.74)B,a,† (1.34)A,a (0.89)B,b (1.22) (0.31) (0.50)
20
46.19 46.76 41.31 48.65 44.10 43.21 49.53 44.37 40.04
A,a,
(2.13) † (3.25)A,a,† (1.77)A,b (1.87)A,a,† (2.41)A,a,b (1.93)A,b,† (0.94)B,a,† (3.42)A,b (1.96)B,b
28
46.56 44.30 42.35 44.59 44.57 41.81 55.81 45.25 42.59
A,a,
(1.77) † (1.33)A,a (1.88)A,a (2.49)A,a,† (2.53)A,a (0.66)A,a (2.75)A,a,†‡ (2.27)A,b (1.25)B,b
46.42 43.20 44.09 47.36 47.07 42.94 49.75 46.44 48.47
(1.49)A,a,† (1.71)A,a (2.26)A,a,† (2.91)A,a,† (1.80)A,a,† (1.90)A,a,† (2.76)B,a,† (2.24)A,a,† (4.43)A,a,†§
Different lowercase letters in the same column and different uppercase letters in the same row indicate statistical difference. †Difference between groups and control concerning the same segment; ‡difference among groups concerning the cervical segment; §difference among groups concerning the apical segment.
following densities and delay times: 14 J/cm² at 1 min and 28 J/cm² at 0, 1, and 2 min.
Figure 2 shows the correlation between the degree of conversion and the Knoop hardness test. The correlation test showed a slight correlation between these two properties (P < 0.0001, r = 0.346).
60
Microhardness
Correlation between the degree of conversion and Knoop hardness test
70
50 40 30 20 10 0
Discussion As recommended by its manufacturer, the RelyX ARC cement should undergo a 40-s photoactivation time. Like other dual resin cements, the RelyX combines physical and chemical activation.6 Its chemical activation starts as its manipulation is initiated, and physical activation occurs during photoactivation.17 However, it is questionable whether the delay time for light curing might affect the polymerization patterns, and consequently, the degree of conversion and Knoop hardness test values of dualcured resin cements, as non-reactive monomers and free radicals might be entrapped on the matrix formed by the chemical activation. Because physical and chemical reactions are known to occur at the same time during light curing, it is also questionable whether it is necessary to prolong the light-curing time until 40 s. Therefore, the present study aimed to evaluate the degree of conversion and Knoop hardness test values of a dual-cured resin cement photoactivated at different energy densities (different irradiances and light-curing time) and delay times. In the present study, the null hypothesis concerning the degree of conversion was partially accepted, as it was 56
0
20
40
60
80
100
Degree of Conversion Figure 2. Correlation between degree of conversion and Knoop hardness test. P < 0.0001, r = 0.346.
found to vary significantly among the segments tested, but not among energy densities and delay times in each group. The degree of conversion highest values observed for the cervical segment, as well as the lowest found for the apical segment, at all experimental conditions could be explained as follows: light fails to reach some root depths because of the distance between the tip of the light source and the light-curing target, as well as the ability of the cement to absorb and scatter light.4 The dual-cured resin cements have a complementary chemical- and lightcuring mechanism, and an adequate degree of conversion is crucial for overall clinical success of the final restoration.18 It is known that light transmittance in resin-based materials decreases in logarithmic proportion with increasing depth.19 Based on the results obtained for the degree of conversion in the present study, only the selfcuring process of the cement seems to have occurred, as ª 2014 Wiley Publishing Asia Pty Ltd
M.C.A.J. Mainardi et al.
no statistical difference concerning the apical segment was found between the groups and the control. A previous study investigating the degree of conversion of RelyX ARC cement found statistical differences among the segments tested, with the lowest values obtained for the apical segment.7 Statistical differences were also found among different segments (push-out test), with the apical segment showing the lowest values.20 The lowest energy density (7 J/cm²) tested yielded results similar to those obtained with the other densities (14, 20, and 28 J/cm²), suggesting that the chemical activation of resin cements might be crucial in its full polymerization. No significant difference was found for the degree of conversion among the delay times tested, a result that is in accordance with that reported in a previous study, investigating the kinetic and degree of conversion of three dualcured resin cements, including the RelyX ARC, showing similar degree of conversion values at two different delay times (0 and 5 min). This cement presents a high percentage of photoactivators,4 which makes it highly dependent on light curing to reach an effective degree of conversion. The self-curing mode was ineffective in compensating the absence of light during the delay periods.21 The cervical segment revealed degree of conversion values similar to those found for the same segment in the control at the density of 7 J/cm² and at delay times of 0 and 1 min. This could be explained by the low dose of energy used, indicating that such energy density and delay times were inefficient in improving the degree of conversion during light curing. However, the degree of conversion values for the cervical segment were statistically different from those obtained for the control for the 2-min delay time. Self-polymerization had already started at the 2-min delay time, a condition that might have contributed to an increase in the ultimate degree of conversion. High Knoop hardness test values might be related not only to high degree of conversion values, but also to the high cross-linked density in the polymeric matrix, the chemical composition (amount of filler), the matrix composition, the shade, and the translucency of the material.22 Rapid light curing promotes multiple growth centers, providing a larger number of covalent bonds among different chains. A slow rate of polymerization, such as the chemical curing in dual-cure resin cements, results in fewer growth centers, forming a more linear polymeric structure and decreasing the Knoop hardness test values. Thus, polymers with similar degrees of conversion can have different cross-linked densities, and therefore different Knoop hardness test values.23 These findings might account for the results obtained in the present study, in which the degree of conversion was found to decrease as the depth of the root increased; such ª 2014 Wiley Publishing Asia Pty Ltd
Resin cement and polymerization protocols
a decrease had a slight correlation with the decrease in the Knoop hardness test values (Table 2) under the same experimental conditions: delay time and energy density. With regard to the Knoop hardness test values, the null hypothesis was rejected because it was influenced by the segments, energy density, and delay times. In relation to the immediate delay time (0), no significant difference was observed among the segments and the energy densities in almost all situations (Table 2). At the energy density of 20 J/cm², the high power density of 1000 mW/cm², when associated with a reduced time (20 s) of photoactivation, was efficient in deep light curing, resulting in a high number of growth centers by activating the camphorquinone and producing more free radicals in the deepest region of the cement. The same can be said for the energy density of 28 J/cm² with a high power density of 700 mW/cm², which is associated with 40 s of photoactivation. The 7 J/cm² density at immediate delay time resulted in a desired Knoop hardness test value on the cervical segment, different from that in the control, but statistically similar to those in the deeper segments (i.e. medium and apical). Different results were reported for RelyX ARC, with its top surface showing the highest Knoop hardness test value and the bottom surface the lowest, statistically similar to that obtained for the control.11 For the delay times of 1 and 2 min, the apical segment revealed Knoop hardness test values lower than those found for the other segments, except for the densities of 28 J/cm² and 20 J/cm² and the 1-min delay time. For the 1-min delay time in the apical segment, the results were statistically different from those found for the control, suggesting that the polymeric structure formed by chemical polymerization prior to light curing was essential to create a matrix with higher cross-linked densities, resulting in higher Knoop hardness values. Even under a low dose of energy, dual-cured resin cements might have a greater number of free radicals that are formed by self-curing and become entrapped in the matrix. Although these radicals caused no increase in the degree of conversion in the present study, they were observed to improve the cross-linked densities resulting from the double links of the methacrylate groups.15 With regard to the 2-min delay time with an energy density of 20 J/cm², the cervical segment showed Knoop hardness values greater than those obtained for the other energy densities (7, 14, and 28 J/cm²). This might be due to the high intensity of light (1000 mW/cm²), resulting in a number of photons that active the photoinitiator (camphorquinone) to form the free radicals, and consequently, the growth centers that are crucial for the development of a cross-linked polymer. Based on the data obtained in the present study, our conclusions are that the depth of root canals can influence the 57
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degree of conversion and Knoop hardness values of dualcured resin cements; the delay time and energy density might also affect the Knoop hardness values. A high-irradiance curing unit associated with a delay time of 0 or 1 min could be used to reduce the light-curing time.
References 1 Ho YC, Lai YL, Chou IC, Yang SF, Lee SY. Effects of light attenuation by fibre posts on polymerization of a dual-cured resin cement and microleakage of post-restored teeth. J Dent 2011; 39: 309–15. 2 Radovic I, Corciolani G, Magni E et al. Light transmission through fiber post: the effect on adhesion, elastic modulus and hardness of dual-cured resin cement. Dent Mater 2009; 25: 837–44. 3 Ozyesil AG, Usumez A, Gunduz B. The efficiency of different light sources to polymerize composite beneath a simulated ceramic restoration. J Prosthet Dent 2004; 91: 151–7. 4 Faria-e-Silva A, Boaro L, Braga R, Piva E, Arias V, Martins L. Effect of immediate or delayed light activation on curing kinectics and shrinkage stress of dual-cure resin cements. Oper Dent 2011; 36: 196–04. 5 Acquaviva PA, Cerutti F, Adami G et al. Degree of conversion of three composite materials employed in the adhesive cementation of indirect restorations: a micro-Raman analysis. J Dent 2009; 37: 610–5. 6 Pedreira APRV, Pegoraro LF, de G oes MF, Pegoraro TA, Carvalho RM. Microhardness of resin cements in the intraradicular environment: effect of water storage and softening treatment. Dent Mater 2009; 25: 868–76. 7 Faria-e-Silva A, Arias VG, Soares LE, Martin AA, Martins LRM. Influence of fiber-post translucency on the degree of conversion of a dual-cured resin cement. J Endod 2009; 33: 303–5. 8 Bueno ALN, Arrais CAG, Jorge ACT, Reis AF, Amaral CM. Light-activation through indirect ceramic restorations: does the overexposure compensate for
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Acknowledgment This study was supported by CAPES, Brazil.
the attenuation in light intensity during resin cement polymerization? J Appl Oral Sci 2011; 19: 22–7. Pereira SG, Fulg^encio R, Nunes TG, Toledano M, Osorio R, Carvalho RM. Effect of curing protocol on the polymerization of dual-cured resin cements. Dent Mater 2011; 26: 710–8. Moraes RR, Faria-e-Silva AL, Ogliari FA, Correr-Sobrinho L, Demarco FF, Piva E. Impact of immediate and delayed light activation on self-polymerization of dual-cured dental resin luting agents. Acta Biomater 2009; 5: 2095–100. Sinhoreti MAC, Maneta IP, Tango RN, Iriyama NT, Consani RLX, Correr-Sobrinho L. Effect of light-curing methods on resin cement Knoop hardness at different depths. Braz Dent J 2007; 18: 305–8. Cekic-Nagas I, Ergun G. Effect of different light curing methods on mechanical and physical properties of resin-cements polymerized through ceramic discs. J Appl Oral Sci 2011; 19: 403–12. Marchan SM, White D, Smith WA, Raman V, Coldero L, Dhuru V. Effect of reduced exposure times on the microhardness of nanocomposites polymerized by QTH and secondgeneration LED curing lights. Oper Dent 2011; 36: 98–103. Aguiar FHB, Braceiro ATB, Ambrosano GMB, Lovadino JR. Hardness and diametral tensile strength of a hibridy composite resin polymerized with different modes and immersed in ethanol or distilled water media. Dent Mater 2005; 21: 1098–103. Meng X, Yoshida K, Atsuta M. Influence of ceramic thickness on mechanical properties and a polymer
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structure of dual-cured resin luting agents. Dent Mater 2008; 24: 594–9. Noronha-Filho JD, Brand~ao NL, Poskus LT, Guimar~aes JGA, da Silva EM. A critical analysis of the degree of conversion of resin-based luting agents. J Appl Dent 2008; 18: 442–6. Pick B, Gonzaga CC, Junior WS, Kawano Y, Braga RR, Cardoso PEC. Influence of curing light attenuation caused by aesthetic indirect restorative materials on resin cement polymerization. Eur J Dent 2010; 4: 314–23. Tezvergil-Mutluay A, Lassila LVJ, Vallittu PK. Degree of conversion of dual-cured luting agents lightpolymerized through various materials. Acta Odontol Scand 2007; 65: 201–5. Leprince JG, Leveque P, Nysten B, Galle B, Devaux J, Leloup G. New insight into the “depth of cure” of dimethacrylate-based dental composites. Dent Mater 2012; 28: 512–20. Zorba YO, Erdemir A, Turkyilmaz A, Eldeniz AU. Effects of different curing units and luting agents on push-out bond strength of translucency posts. J Endod 2010; 36: 1521–5. Aguiar TR, Francescantonio MD, Arrais CAG, Ambrosano GMB, Davanzo C, Giannini M. Influence of curing mode and time on degree of conversion of one conventional and two self-adhesive resin cements. Oper Dent 2010; 35: 295–9. Vignolo V, Fuentes MV, Garrido MA, Rodrıguez J, Ceballos L. Microhardness of different resin shades inside the root canal. Med Oral Patol Oral Cir Bucal 2012; 17: 859–64. Soh MS, Yap AUJ. Influence of curing modes on crosslink density in polymer structures. J Dent 2004; 32: 321–6.
ª 2014 Wiley Publishing Asia Pty Ltd
ORIGINAL ARTICLE Dental Biomaterials
Effect of energy density and delay time on the degree of conversion and Knoop microhardness of a dual resin cement bora A. N. L. Lima, Giselle M. Marchi, Maria do Carmo A. J. Mainardi, Maria Cecılia C. Giorgi, De ucia M. Ambrosano, Luiz A. M. S. Paulillo & Fla vio H. B. Aguiar Gla Piracicaba Dental School, University of Campinas, Piracicaba, Brazil
Keywords degree of conversion, hardness, polymerization, resin cement, root canal. Correspondence Ms M. C. A. J. Mainardi, Piracicaba Dental School, University of Campinas, 901 Limeira Avenue, Piracicaba, S~ ao Paulo 13414-903, Brazil. Tel: +55-19-2106-5200 Email: [email protected] Received 30 January 2013; accepted 9 June 2013. doi: 10.1111/jicd.12075
Abstract Aim: In the present study, we evaluated the influence of the photo-curing delay time and energy density on the degree of conversion and the Knoop microhardness of a resin cement. Methods: Seventy-eight samples were assigned to 13 groups (n = 6), one of which received no light curing (control). The samples were made of a dualcured resin cement (RelyX ARC) with the aid of a Teflon matrix, submitted to one of the following energy densities (J/cm²): 7, 14, 20, and 28. Delay times were immediate (0), 1 min, or 2 min. After 24 h, the degree of conversion and microhardness were measured at three segments: cervical, medium, and apical. Data were submitted to three-way ANOVA and Tukey’s and Dunnett’s tests, the latest of which was used to compare the control to the experimental groups. Results: No interaction was observed between delay time and energy density regarding the degree of conversion. The cervical segment showed the highest values, while the apical showed the lowest. Microhardness values concerning the cervical segment in all groups were statistically different from that obtained for the control. Conclusion: A high-irradiance light-curing unit allows for a reduced irradiation exposure time with a short delay time, aimed at tooth restorations using a dual-cured resin cement.
Introduction Indirect tooth-colored restorations have been increasingly used for endodontically-treated teeth using fiber post.1 Dual- and self-cured resin cements are recommended for the cementation of intra-radicular posts. Light-cured resin cements are inappropriate for post-cementation because they provide incomplete polymerization in the apex of the canal.2 Dual-cured resin cements combine the desirable characteristics of photocuring and chemical curing resin,3 providing proper polymerization in either the presence or the absence of light.4 The self-curing component of the resin cement is useful because it favors its degree of conversion in environments where radiant ª 2014 Wiley Publishing Asia Pty Ltd
energy is unavailable.5 Both modes of activation present in dual-cured resin cements are complementary and independent.6 Furthermore, some dual resin cements are dependent on light activation, resulting in an inadequate degree of conversion if the cement is not light cured.7 High values of the degree of conversion might be an indicator of good physical and mechanical properties of resin-based materials. Poor monomer conversion of these materials might lead to low bond strength and high water sorption, compromising their physical properties.8 The immediate light curing of dual-cured resin cements might cause a reduction in the degree of conversion.9 The reason is that immediate light curing causes the rapid formation of cross-linked polymer chains, entrapping 53
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Resin cement and polymerization protocols
Seventy-eight samples simulating root canals (2 mm in diameter and 13 mm in length) were randomly assigned to 12 experimental groups (dual-cured resin plus lightemitting diode [LED] light curing) and a control (no light curing). All groups, including the control, contained six specimens each. The dual-cured resin cement (RelyX ARC, shade A1; 3M-ESPE, St Paul, MN, USA) was manipulated for 10 s, inserted into a Teflon matrix, and light cured immediately, 1 min, or 2 min after the insertion using a LED device (Valo; Ultradent Products, South Jordan, UT, USA). The specimens were exposed to one of the following energy densities (J/cm²): 7 (350 mW/cm2 and 20 s), 14 (700 mW/cm2 and 20 s), 20 (1000 mW/ cm2 and 20 s), and 28 (700 mW/cm2 and 40 s). The irradiance values were monitored by a radiometer (Model 100 Optilux Radiometer; Kerr, Middleton, WI, USA), and those lower than 1000 mW/cm2 were obtained with the aid of spacers previously prepared in a pilot study. Twenty-four hours after confection, a double-sided diamond disc (KG, Cotia, SP, Brazil) was used to section the specimens into 4-mm-long segments: cervical, medium, 54
Degree of conversion measurement The measures of degree of conversion were made by FTIR. The coronal face of each specimen was put above the attenuated total reflectance crystal (ATR), attached to the FTIR device, which is compound by diamond/zinc selenide crystals. Each specimen covered the total area of the crystal. With the aid of a pressure arm lock, the specimens were submitted to a force of 90 N, in order to ensure direct contact between the specimen and the crystal. The spectra of polymerized and non-polymerized resin cement (Figure 1) were obtained through the ATR using 16 scans at a resolution of 4/cm. The absorbance peaks at 1638/cm (aliphatic C=C bonds) and 1608/cm (aromatic C=C bonds) were used to calculate the percentage of the remaining double bonds of carbon in the resin cement after polymerization using the following formula16: (a)
Absorbance
Materials and methods
and apical. The specimens were submitted to Fourier transform infrared spectroscopy (FTIR; Spectrum 100; PerkinElmer, SP, Brazil) to measure the degree of conversion, and then to Knoop hardness test to measure the microhardness, both aimed at the top surface.
Wavelength
(b)
Absorbance
unreacted monomers and free radicals into the polymeric network, and thus jeopardizing the chemical curing. Also, the curing reaction that occurs at the same time by selfand light polymerization of the cement might speed its vitrification process,10 making it unnecessary to prolong the light-curing time, especially if a delay occurs between manipulating the resin luting agent and the light activation. The polymerization of dual-cured resin cements can also be evaluated by the microhardness test.11 Microhardness is one of the most important properties of dental materials12 and might be used to predict the degree of conversion; the higher the microhardness, the higher the degree of conversion is expected to be. However, this correlation might be contradictory,13 as polymers are dependent on different polymerization modes, which might affect the hardness, but not the degree of conversion of a resin cement.14 Dual-cured resin cements might result in different polymer structures that vary in cross-linked density, affecting their ultimate mechanical properties.15 Therefore, the aim of the present study was to evaluate the influence of delayed photoactivation times and energy density (varying irradiance and light-activation time) on the degree of conversion and microhardness of a dualcured resin cement considering three root segments: cervical, medium, and apical. The null hypothesis is that the degree of conversion and microhardness might not be affected by these factors.
Wavelength
Figure 1. Representative Fourier transform infrared spectroscopy of the non-polymerized resin cement (a) and the polymerized resin cement (b).
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M.C.A.J. Mainardi et al.
Resin cement and polymerization protocols
DC ð%Þ ¼ 100 ð1 Rpolymerized =Runpolymerized Þ R = peak at 1.638/cm/peak at 1.608/cm, where DC is the degree of conversion. Microhardness measurement Every specimen was embedded in polystyrene resin block and then sandpapered (#600 and #1200) in a polishing machine (Arotec; Cotia, SP, Brazil). The top surface microhardness was measured using the Knoop hardness test (Shimadzu, Japan), set at 20 g for 10 s. Three indentations were made for each specimen. The largest diagonal of the lozenge-shaped indentation in each specimen was measured to obtain the Knoop hardness test values. Data were submitted to three-way ANOVA and Tukey’s test. Dunnett’s test was used for comparison between the experimental groups and the control. The correlation test was used to correlate the degree of conversion and the Knoop hardness test values.
Results Degree of conversion analysis Table 1 shows the degree of conversion values obtained for all the groups. Significant differences were found among the three root segments studied (P < 0.0001); however, no difference was found among the energy densities (P = 0.1698) and delay times (P = 0.3132) assessed. The cervical segment showed the highest degree of conversion values, statistically different from those found for the others segments (Tukey’s test). The apical revealed the lowest values considering all experimental conditions. For the apical segment, Dunnett’s test
showed no statistically-significant difference among the groups when compared to the control. Significant differences were found for the cervical segment when each group was compared to the control, except for the energy density of 7 J/cm² at 0 and 1 min. For the medium segment, no statistical difference was found when the groups were compared to the control, considering all the variables tested, except for the density of 14 J/cm² at 0 min and of 20 J/cm² at 1 min. Knoop hardness test analysis Table 2 shows the Knoop hardness test values obtained for all groups. ANOVA showed triple interaction toward all the variables tested (P < 0.001). Regarding the immediate (0) time, no statistical difference was observed for the Knoop hardness test values among the three segments at all densities, except for the apical segment at the density of 14 J/cm². Significant differences were found between the apical and the other segments, at densities of 7 and 14 J/cm² at 1 min. When compared to the cervical segment, the apical segment revealed the lowest Knoop hardness test value for all densities at 2 min, except for 28 J/ cm². The mean values obtained for the cervical and apical segments at the densities of 20 and 28 J/cm², respectively for the 2-min delay time were significantly higher than those at 0 and 1 min. For the cervical segment, Dunnett’s test showed significant differences when the groups were compared to the control. For the densities of 7 J/cm² at 1 min, 14 J/cm² at 0 min, and 28 J/cm² at 1 and 2 min, the Knoop hardness test value found for the medium segment was statistically different from that (same segment) observed for the control. The Knoop hardness test values concerning the apical segment were statistically different from that found for the control for all groups and the
Table 1. Degree of conversion results Energy density (J/cm²) and standard deviation Time (min)
Segment
7
0
Cervical Medium Apical Cervical Medium Apical Cervical Medium Apical Cervical Medium Apical
78.41 77.71 73.81 78.74 77.93 76.42 81.55 79.98 76.36 74.36 74.74 75.00
1
2
Control
14 (8.87)a (3.82)b (9.43)c (3.18)a (2.26)b (3.44)c (6.47)a,† (6.68)b (3.64)c (5.85) (9.03) (5.91)
80.07 80.20 75.41 80.85 77.87 72.83 80.65 79.16 74.34
20 (2.35)a,† (2.85)b,‡ (4.63)c (2.41)a,† (2.88)b (9.91)c (2.13)a,† (2.47)b (2.21)c
81.64 79.74 74.92 81.55 81.72 76.60 84.63 79.12 77.01
28 (1.04)a,† (2.09)b (2.42)c (1.68)a,† (3.92)b,‡ (1.62)c (5.82)a,† (3.34)b (2.15)c
80.79 78.72 75.78 80.36 79.34 76.00 81.06 79.60 76.78
(1.07)a,† (1.63)b (0.90)c (1.32)a,† (1.66)b (1.72)c (1.23)a,† (1.24)b (0.88)c
Different lowercase letters differ statistically. †Difference between groups and control concerning the same segment; ‡differs from control concerning the medium segment.
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Table 2. Results of Knoop hardness test Energy density (J/cm²) and standard deviation Time (min) 0
1
2
Control
Segment Cervical Medium Apical Cervical Medium Apical Cervical Medium Apical Cervical Medium Apical
7 45.35 44.78 42.06 48.76 48.94 42.48 45.94 45.85 39.61 39.51 42.10 39.48
14 A,a,
(1.56) † (1.87)A,a (2.01)A,a (3.32)A,a,† (1.68)A,a,† (1.46)A,b (1.74)B,a,† (1.34)A,a (0.89)B,b (1.22) (0.31) (0.50)
20
46.19 46.76 41.31 48.65 44.10 43.21 49.53 44.37 40.04
A,a,
(2.13) † (3.25)A,a,† (1.77)A,b (1.87)A,a,† (2.41)A,a,b (1.93)A,b,† (0.94)B,a,† (3.42)A,b (1.96)B,b
28
46.56 44.30 42.35 44.59 44.57 41.81 55.81 45.25 42.59
A,a,
(1.77) † (1.33)A,a (1.88)A,a (2.49)A,a,† (2.53)A,a (0.66)A,a (2.75)A,a,†‡ (2.27)A,b (1.25)B,b
46.42 43.20 44.09 47.36 47.07 42.94 49.75 46.44 48.47
(1.49)A,a,† (1.71)A,a (2.26)A,a,† (2.91)A,a,† (1.80)A,a,† (1.90)A,a,† (2.76)B,a,† (2.24)A,a,† (4.43)A,a,†§
Different lowercase letters in the same column and different uppercase letters in the same row indicate statistical difference. †Difference between groups and control concerning the same segment; ‡difference among groups concerning the cervical segment; §difference among groups concerning the apical segment.
following densities and delay times: 14 J/cm² at 1 min and 28 J/cm² at 0, 1, and 2 min.
Figure 2 shows the correlation between the degree of conversion and the Knoop hardness test. The correlation test showed a slight correlation between these two properties (P < 0.0001, r = 0.346).
60
Microhardness
Correlation between the degree of conversion and Knoop hardness test
70
50 40 30 20 10 0
Discussion As recommended by its manufacturer, the RelyX ARC cement should undergo a 40-s photoactivation time. Like other dual resin cements, the RelyX combines physical and chemical activation.6 Its chemical activation starts as its manipulation is initiated, and physical activation occurs during photoactivation.17 However, it is questionable whether the delay time for light curing might affect the polymerization patterns, and consequently, the degree of conversion and Knoop hardness test values of dualcured resin cements, as non-reactive monomers and free radicals might be entrapped on the matrix formed by the chemical activation. Because physical and chemical reactions are known to occur at the same time during light curing, it is also questionable whether it is necessary to prolong the light-curing time until 40 s. Therefore, the present study aimed to evaluate the degree of conversion and Knoop hardness test values of a dual-cured resin cement photoactivated at different energy densities (different irradiances and light-curing time) and delay times. In the present study, the null hypothesis concerning the degree of conversion was partially accepted, as it was 56
0
20
40
60
80
100
Degree of Conversion Figure 2. Correlation between degree of conversion and Knoop hardness test. P < 0.0001, r = 0.346.
found to vary significantly among the segments tested, but not among energy densities and delay times in each group. The degree of conversion highest values observed for the cervical segment, as well as the lowest found for the apical segment, at all experimental conditions could be explained as follows: light fails to reach some root depths because of the distance between the tip of the light source and the light-curing target, as well as the ability of the cement to absorb and scatter light.4 The dual-cured resin cements have a complementary chemical- and lightcuring mechanism, and an adequate degree of conversion is crucial for overall clinical success of the final restoration.18 It is known that light transmittance in resin-based materials decreases in logarithmic proportion with increasing depth.19 Based on the results obtained for the degree of conversion in the present study, only the selfcuring process of the cement seems to have occurred, as ª 2014 Wiley Publishing Asia Pty Ltd
M.C.A.J. Mainardi et al.
no statistical difference concerning the apical segment was found between the groups and the control. A previous study investigating the degree of conversion of RelyX ARC cement found statistical differences among the segments tested, with the lowest values obtained for the apical segment.7 Statistical differences were also found among different segments (push-out test), with the apical segment showing the lowest values.20 The lowest energy density (7 J/cm²) tested yielded results similar to those obtained with the other densities (14, 20, and 28 J/cm²), suggesting that the chemical activation of resin cements might be crucial in its full polymerization. No significant difference was found for the degree of conversion among the delay times tested, a result that is in accordance with that reported in a previous study, investigating the kinetic and degree of conversion of three dualcured resin cements, including the RelyX ARC, showing similar degree of conversion values at two different delay times (0 and 5 min). This cement presents a high percentage of photoactivators,4 which makes it highly dependent on light curing to reach an effective degree of conversion. The self-curing mode was ineffective in compensating the absence of light during the delay periods.21 The cervical segment revealed degree of conversion values similar to those found for the same segment in the control at the density of 7 J/cm² and at delay times of 0 and 1 min. This could be explained by the low dose of energy used, indicating that such energy density and delay times were inefficient in improving the degree of conversion during light curing. However, the degree of conversion values for the cervical segment were statistically different from those obtained for the control for the 2-min delay time. Self-polymerization had already started at the 2-min delay time, a condition that might have contributed to an increase in the ultimate degree of conversion. High Knoop hardness test values might be related not only to high degree of conversion values, but also to the high cross-linked density in the polymeric matrix, the chemical composition (amount of filler), the matrix composition, the shade, and the translucency of the material.22 Rapid light curing promotes multiple growth centers, providing a larger number of covalent bonds among different chains. A slow rate of polymerization, such as the chemical curing in dual-cure resin cements, results in fewer growth centers, forming a more linear polymeric structure and decreasing the Knoop hardness test values. Thus, polymers with similar degrees of conversion can have different cross-linked densities, and therefore different Knoop hardness test values.23 These findings might account for the results obtained in the present study, in which the degree of conversion was found to decrease as the depth of the root increased; such ª 2014 Wiley Publishing Asia Pty Ltd
Resin cement and polymerization protocols
a decrease had a slight correlation with the decrease in the Knoop hardness test values (Table 2) under the same experimental conditions: delay time and energy density. With regard to the Knoop hardness test values, the null hypothesis was rejected because it was influenced by the segments, energy density, and delay times. In relation to the immediate delay time (0), no significant difference was observed among the segments and the energy densities in almost all situations (Table 2). At the energy density of 20 J/cm², the high power density of 1000 mW/cm², when associated with a reduced time (20 s) of photoactivation, was efficient in deep light curing, resulting in a high number of growth centers by activating the camphorquinone and producing more free radicals in the deepest region of the cement. The same can be said for the energy density of 28 J/cm² with a high power density of 700 mW/cm², which is associated with 40 s of photoactivation. The 7 J/cm² density at immediate delay time resulted in a desired Knoop hardness test value on the cervical segment, different from that in the control, but statistically similar to those in the deeper segments (i.e. medium and apical). Different results were reported for RelyX ARC, with its top surface showing the highest Knoop hardness test value and the bottom surface the lowest, statistically similar to that obtained for the control.11 For the delay times of 1 and 2 min, the apical segment revealed Knoop hardness test values lower than those found for the other segments, except for the densities of 28 J/cm² and 20 J/cm² and the 1-min delay time. For the 1-min delay time in the apical segment, the results were statistically different from those found for the control, suggesting that the polymeric structure formed by chemical polymerization prior to light curing was essential to create a matrix with higher cross-linked densities, resulting in higher Knoop hardness values. Even under a low dose of energy, dual-cured resin cements might have a greater number of free radicals that are formed by self-curing and become entrapped in the matrix. Although these radicals caused no increase in the degree of conversion in the present study, they were observed to improve the cross-linked densities resulting from the double links of the methacrylate groups.15 With regard to the 2-min delay time with an energy density of 20 J/cm², the cervical segment showed Knoop hardness values greater than those obtained for the other energy densities (7, 14, and 28 J/cm²). This might be due to the high intensity of light (1000 mW/cm²), resulting in a number of photons that active the photoinitiator (camphorquinone) to form the free radicals, and consequently, the growth centers that are crucial for the development of a cross-linked polymer. Based on the data obtained in the present study, our conclusions are that the depth of root canals can influence the 57
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Resin cement and polymerization protocols
degree of conversion and Knoop hardness values of dualcured resin cements; the delay time and energy density might also affect the Knoop hardness values. A high-irradiance curing unit associated with a delay time of 0 or 1 min could be used to reduce the light-curing time.
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Acknowledgment This study was supported by CAPES, Brazil.
the attenuation in light intensity during resin cement polymerization? J Appl Oral Sci 2011; 19: 22–7. Pereira SG, Fulg^encio R, Nunes TG, Toledano M, Osorio R, Carvalho RM. Effect of curing protocol on the polymerization of dual-cured resin cements. Dent Mater 2011; 26: 710–8. Moraes RR, Faria-e-Silva AL, Ogliari FA, Correr-Sobrinho L, Demarco FF, Piva E. Impact of immediate and delayed light activation on self-polymerization of dual-cured dental resin luting agents. Acta Biomater 2009; 5: 2095–100. Sinhoreti MAC, Maneta IP, Tango RN, Iriyama NT, Consani RLX, Correr-Sobrinho L. Effect of light-curing methods on resin cement Knoop hardness at different depths. Braz Dent J 2007; 18: 305–8. Cekic-Nagas I, Ergun G. Effect of different light curing methods on mechanical and physical properties of resin-cements polymerized through ceramic discs. J Appl Oral Sci 2011; 19: 403–12. Marchan SM, White D, Smith WA, Raman V, Coldero L, Dhuru V. Effect of reduced exposure times on the microhardness of nanocomposites polymerized by QTH and secondgeneration LED curing lights. Oper Dent 2011; 36: 98–103. Aguiar FHB, Braceiro ATB, Ambrosano GMB, Lovadino JR. Hardness and diametral tensile strength of a hibridy composite resin polymerized with different modes and immersed in ethanol or distilled water media. Dent Mater 2005; 21: 1098–103. Meng X, Yoshida K, Atsuta M. Influence of ceramic thickness on mechanical properties and a polymer
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