ORIGINAL ARTICLE

Time reduction of light curing: Influence on conversion degree and microhardness of orthodontic composites Patrıcia Alves Ferreira Amato,a Renato Parsekian Martins,b Carlos Alberto dos Santos Cruz,c Marisa Veiga Capella,d and Lıdia Parsekian Martinse Ribeir~ao Preto and Araraquara, S~ao Paulo, Brazil

Introduction: The aim of this study was to assess the influence of curing time and power on the degree of conversion and surface microhardness of 3 orthodontic composites. Methods: One hundred eighty discs, 6 mm in diameter, were divided into 3 groups of 60 samples according to the composite used—Transbond XT (3M Unitek, Monrovia, Calif), Opal Bond MV (Ultradent, South Jordan, Utah), and Transbond Plus Color Change (3M Unitek)— and each group was further divided into 3 subgroups (n 5 20). Five samples were used to measure conversion, and 15 were used to measure microhardness. A light-emitting diode curing unit with multiwavelength emission of broad light was used for curing at 3 power levels (530, 760, and 1520 mW) and 3 times (8.5, 6, and 3 seconds), always totaling 4.56 joules. Five specimens from each subgroup were ground and mixed with potassium bromide to produce 8-mm tablets to be compared with 5 others made similarly with the respective noncured composite. These were placed into a spectrometer, and software was used for analysis. A microhardness tester was used to take Knoop hardness (KHN) measurements in 15 discs of each subgroup. The data were analyzed with 2 analysis of variance tests at 2 levels. Results: Differences were found in the conversion degree of the composites cured at different times and powers (P\0.01). The composites showed similar degrees of conversion when light cured at 8.5 seconds (80.7%) and 6 seconds (79.0%), but not at 3 seconds (75.0%). The conversion degrees of the composites were different, with group 3 (87.2%) higher than group 2 (83.5%), which was higher than group 1 (64.0%). Differences in microhardness were also found (P \0.01), with lower microhardness at 8.5 seconds (35.2 KHN), but no difference was observed between 6 seconds (41.6 KHN) and 3 seconds (42.8 KHN). Group 3 had the highest surface microhardness (35.9 KHN) compared with group 2 (33.7 KHN) and group 1 (30.0 KHN). Conclusions: Curing time can be reduced up to 6 seconds by increasing the power, with a slight decrease in the degree of conversion at 3 seconds; the decrease has a positive effect on the surface microhardness. (Am J Orthod Dentofacial Orthop 2014;146:40-6)

T a

he light-curing process of orthodontic composites begins when the initiators in the formula absorb the energy emitted by a light source. These

Private practice, Ribeir~ao Preto, S~ao Paulo, Brazil. Invited professor, Orthodontic Program, Araraquara School of Dentistry, S~ao Paulo State University, Araraquara, S~ao Paulo, Brazil; private practice, Araraquara, S~ao Paulo, Brazil. c Associate professor, Department of Dental Materials and Prosthodontics, Araraquara School of Dentistry, S~ao Paulo State University, Araraquara, S~ao Paulo, Brazil. d Assistant professor, Department of Physical Chemistry, Araraquara Institute of Chemistry, S~ao Paulo State University, Araraquara, S~ao Paulo, Brazil. e Chairman, Department of Orthodontics, Araraquara School of Dentistry, S~ao Paulo State University, Araraquara, S~ao Paulo, Brazil. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Address correspondence to: Renato Parsekian Martins, Rua Carlos Gomes 2158, Araraquara, S~ao Paulo, Brazil 14801340; e-mail, dr_renatopmartins@hotmail. com. Submitted, October 2013; revised and accepted, March 2014. 0889-5406/$36.00 Copyright Ó 2014 by the American Association of Orthodontists. http://dx.doi.org/10.1016/j.ajodo.2014.03.022 b

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initiators are chemical substances that react with the double bonds between the carbons in the monomers, converting them into polymers during the light-curing process of the composites.1,2 The degree of conversion of a composite determines the amount of carbon double bonds, present in the monomers, broken during the conversion process into polymers, a process called polymerization. Since it is established that the total amount of energy produced by a light source (from the light's source power and irradiation time)3 is related to the degree of conversion,4,5 product manufacturers have developed high-power light sources to reduce the irradiation time. This seemed to be advantageous, since the bonding procedures would require less chair time.3 However, the relationship between irradiation time and light source power on the physical and chemical properties of composites is not well established, because there is also some controversy in the literature.4,6-9 Although it had been demonstrated

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Table I. Groups tested according to the orthodontic

Table II. Composite, power, exposure time, and total

composites, with manufacturers and batch numbers

energy used in each subgroup

Group 1

Composite Transbond XT

2

Opal Bond MV

3

Transbond Plus Color Change

Manufacturer 3M Unitek, Monrovia, Calif Ultradent, South Jordan, Utah 3M Unitek, Monrovia, Calif

Number N182301 C013 N130353

that a higher degree of conversion can be achieved with a shorter curing time and a higher power,9 some studies have shown that reduction of the curing time does not increase the conversion degree when shorter curing times and higher powers are used.4,7,8 Such a discrepancy occurs because most studies have compared different light sources and different levels of energy, confusing the results. Another overlooked aspect regarding orthodontic composites is their surface microhardness. Because these composites remain in the oral cavity for a long time, they should possess good adhesiveness. It has been established that there is a positive correlation between hardness10 and mechanical resistance11 to inorganic load content, and composites with a high level of inorganic load seem to provide greater bond strength.12 Therefore, high surface hardness could be a secondary indicator of high inorganic load content and bond strength. Each composite, on the other hand, responds differently to the light-curing process, even when the same energy is used from different combinations of irradiation time and power.6 Since this happens because of different chemical compositions, which could influence the initial rate of curing, each composite should have its curing process evaluated individually.13 Therefore, the aim of this study was to analyze the influence of the light-curing times and power variations on the conversion degree and surface hardness of 3 orthodontic composites light cured using the same energy level and the same light source. MATERIAL AND METHODS

One hundred eighty composite discs (diameter, 6 mm; thickness, 1 mm) were prepared and divided into 3 groups of 60 samples according to the composite used: Transbond XT (3M Unitek, Monrovia, Calif; group 1), Opal Bond MV (Ultradent, South Jordan, Utah; group 2), and Transbond Plus Color Change (3M Unitek; group 3) (Table I). The discs were made by inserting the composites into a metallic mold, which was placed over a 10-mm-thick glass slide and polyester tape. The upper surface of the

Subgroup G1-1 G1-2 G1-3 G2-1 G2-2 G2-3 G3-1 G3-2 G3-3

Composite Transbond XT Transbond XT Transbond XT Opal Bond MV Opal Bond MV Opal Bond MV Transbond Plus CC Transbond Plus CC Transbond Plus CC

Power (mW) (P1) 530 (P2) 760 (P3) 1520 (P1) 530 (P2) 760 (P3) 1520 (P1) 530 (P2) 760 (P3) 1520

Time (s) T1: 8.5 T2: 6 T3: 3 T1: 8.5 T2: 6 T3: 3 T1: 8.5 T2: 6 T3: 3

Total energy (J) 4.50 4.56 4.56 4.50 4.56 4.56 4.50 4.56 4.56

G, Group; P1, standard; P2, high; P3, plasma emulation; CC, Color Change.

disc was also covered with polyester tape, and a 1-mm-thick glass slide was placed over it. Curing was performed with the light-curing unit tip in contact with the glass slide, standardizing the incidence of the light perpendicular to the surface of the disc. Each group (n 5 60) was further divided into 3 subgroups (n 5 20). Five samples were used for measurement of the conversion, and 15 were used for measurement of microhardness. A third-generation light-emitting diode curing unit with multiwavelength emission of broad light spectrum (VALO LED curing light, Ultradent) was used for curing the samples. This device operates at a wavelength range of 380 to 510 nm, with 2 emission peaks at 403.87 and 459.11 nm, and 3 power levels: standard, high, and plasma emulation. The 3 power levels were measured with a Fieldmaster laser power meter (Coherent, Santa Clara, Calif), and values of 530, 760, and 1520 mW were found, respectively. All discs were cured using a similar level of energy, but with variations in curing times and light-source power levels according to the following formula: energy 5 power 3 time (Table II). After the curing process, the discs were individually stored in dark vials until sample preparations for conversion degree and hardness evaluation. For analysis of the conversion degree, 5 composite discs from each group were ground into a fine powder with a mortar and pestle, and 5 mg of the resulting powder was mixed with 10 mg of potassium bromide. The mixture was placed into an 8-mm-diameter metallic mold (PerkinElmer, Waltham, Mass) and then compressed by a pressing machine operating at 10 tons (SKAY, S~ao Jose do Rio Preto, Brazil) for 60 seconds. These tablets were compared with 5 other ones made in a similar manner, except using the respective uncured composite.

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The light-cured and uncured tablets for each group were placed into a spectrometer (Spectrum 2000; PerkinElmer) for spectrum acquisition. The measurements were made in absorbance mode at intervals from 4000 to 400 cm1 and operating at 32 scans with a resolution of 4 cm1. Specific software was used for analysis of the spectra (Spectrum, version 5.3.1 for Windows; PerkinElmer). The degree of conversion was calculated as a percentage through the analysis of the obtained spectra through the decrease of the intensity of the vinyl double bonds (C5C) in relation to the intensity of the bond of aromatic ring bonds (C-C), which was used as the internal sample standard because it does not change after a light-curing reaction. The formula used is the following:  a DC ð%Þ5 1  bc 3 100

Table III. Two-way ANOVA comparing exposure time,

composite, and interaction between both factors for the variable degree of conversion

Time (s) T1: 8.5 T2: 6 T3: 3 Composite G1 G2 G3 Time 3 composite

Degree of conversion (%)

SD

80.7A 79.0A 75.0B

11.2 10.8 10.08

64.0C 83.5B 87.2A

0.66 1.45 0.51

P \0.001

\0.001

0.424

Averages and standard deviations of the degree of conversion profiles for times and composites are shown and compared with the Tukey post hoc test (different letters indicate different groups within factors). G, Group.

d

where DC is degree of conversion, a is the peak height of the C5C bond at 1637 cm1 for the light-cured resin, b is the peak height of the C-C bond at 1610 cm1 for the light-cured resin, c is the peak height of the C5C bond at 1637 cm1 for the uncured resin, and d is the peak height of the C-C bond at 1610 cm1 for the uncured resin. For analysis of the Knoop hardness (KHN), a microhardness tester (Micromet S103; Buehler, Lake Bluff, Ill) was used on 15 discs of each subgroup at a load of 30 gf for 15 seconds. Eight measurements were taken on the surface opposed to the light application (bottom surface) for each disc, and their respective mean values were calculated. The values obtained were normally distributed, and 2 separate 2-way analysis of variance (ANOVA) tests were performed (P 5 0.05) using SPSS software (version 16.0; SPSS, Chicago, Ill). One was used to assess the influences of curing times and composites on the conversion degree, and the other was used to assess the same effects on the surface microhardness. The post hoc Tukey test was applied for comparison between the groups when a significant difference was found. RESULTS

A difference was found in the conversion degree of the composites cured at different times but with the same level of energy (P \0.001) (Table III). A similar degree of conversion was found when the composites were cured at 8.5 seconds (80.7%) and 6 seconds (79.0%), but it was lower at 3 seconds (75.0%) (Table III, Fig 1). A difference in the conversion degree of the composites was also found (P \0.001) (Table III). Group 3 showed the

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Fig 1. Degree of conversion of the composites cured at different times.

highest value of conversion (87.2%), followed by group 2 (83.5%) and group 1 (64.0%) (Table III). No interaction was found between curing times and composites for conversion degree (Table III). A difference was found in the surface microhardness of the composites cured for different times but with the same level of energy (P \0.001) (Table IV). The composites showed lower values of microhardness when cured at 8.5 seconds (35.2 KHN), but no difference was found between 6 seconds (41.6 KHN) and 3 seconds (42.8 KHN) (Table IV, Fig 2). Microhardness values were also

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Table IV. Two-way ANOVA comparing exposure time,

composite, and interaction between both factors for the variable microhardness Time (s) T1: 8.5 T2: 6 T3: 3 Composite G1 G2 G3 Time 3 composite

Microhardness (KHN)

SD

35.21A 41.60B 42.78B

9.75 10.64 13.06

30.0C 35.7B 53.6A

4.39 5.92 6.95

P \0.001

\0.001

0.005

Averages and standard deviations of the microhardness profiles for times and composites are shown and compared with the Tukey post hoc test (different letters indicate different groups within factors). G, Group.

different among the composites tested (P \0.001) (Table IV). Group 3 showed a higher surface microhardness (53.6 KHN) compared with group 2 (35.7 KHN) and group 1 (30.0 KHN) (Table IV). A significant interaction was found between composites and curing times on microhardness (P \0.001) (Table IV). DISCUSSION

A lower degree of conversion was observed in composites cured over a shorter period of time and with higher power and the same level of energy. Even though there may be, theoretically, a relationship between energy, curing time, and power, such a relationship can be compromised at short curing times. This can occur because of the expected decrease in monomer mobility during the curing reaction as the polymeric network is formed,14 thus causing nonreacted monomers to be trapped in it.8 Even though the literature has demonstrated that a shorter time can influence negatively the conversion degree of composites, a significant decrease of conversion was only found on the shortest curing time (3 seconds) associated with the highest power (1520 mW).4,8,15 The difference, however, was small compared with 8.5 and 6 seconds, and the degree of conversion was high (75%). It can be concluded that the theoretical relationship cited above is true up to a point, since the conversion degree can be compromised by a specific light-curing speed. Therefore, although new technologies promise efficient light curing in a short time, clinical studies are necessary to assess which conversion degree should be considered acceptable for orthodontic composites so that the relationship between time and power can be properly calculated.

Fig 2. Knoop hardness of the orthodontic composites cured at different times.

Differences were found in the conversion degree among the composites tested. This was an expected finding, since this property is related to their chemical composition.6,8,13,16-21 The concentration, chemical structure,16,18 and viscosity of the monomers,13,19 including composition20 and concentration of the initiators21 in the organic matrix, have a direct influence on the curing kinetics. Using the same level of energy, Transbond Plus Color Change had a conversion degree of 87.2%, whereas Opal Bond MV and Transbond XT had, respectively, 83.5% and 64%. Even having the lowest degree of conversion, Transbond XT had a conversion degree within the range of 55% and 75%, which is usually observed in restorative composites under conventional irradiation conditions (halogen light for 40 seconds at room temperature).22,23 The highest degree of conversion observed in the Transbond Plus Color Change might have occurred because of its pink color before conversion, since the chromatic agents of its composition absorb more light compared with the other 2 white composites. Among the tested composites, only Transbond XT had been previously evaluated, showing conversion degrees from 39% to 83%, but since the methods and curing protocols were different from ours, it was not possible to compare the results.8,9,17,24-26 Four of these studies used halogen lights, whose light spectrum emission is different, and also total energy used was not reported.8,17,25,26 Although 2 other studies used light-emitting diode units, one9 used a different curing method and conversion analysis from our study, while the other24 used curing times, powers, and levels of energy different

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from those in our study. From the clinical point of view, composites with a high degree of conversion are related to better physical and chemical properties,27,28 whereas low degrees of conversion are related to adverse biologic reactions, since composites can release bisphenol-A, a precursor of BisGMA, which exhibits cytotoxicity that can be harmful to patients having lengthy treatments.29-31 Therefore, the orthodontist should choose a composite with a high degree of conversion depending on the light-curing time and power that are intended to be used. The composites showed lower microhardness when cured with less power and longer curing times. Lower energy density favors a primary cyclization reaction, which occurs during the curing process when an opened double bond reacts with a radical intramolecularly in its own propagation chain. Cyclization does not contribute to the polymeric network structure, since a microgel forms, thus making the polymer heterogeneous.14 Even promoting a higher local conversion, the cyclization does not decrease the system mobility, and this can cause a reduction in the reticulation density, decreasing consequently the mechanical resistance of the polymer.32,33 The only study comparing the hardness of orthodontic composites reported varied results, showing that surface hardness can be increased or remain insensitive to the reduction in curing time with the same energy.34 This might be due to the differences in composition and the level of inorganic load content. Our findings indicated that as the time decreased, the degree of conversion trended one way, but the hardness trended the other way. This was not expected, since within a given resin system they are usually correlated.27,35,36 These unexpected results have been shared by a study on restorative composites and possibly happened because of the higher power densities used for curing.37 The fast curing process could have decreased the mobility of the composite during the pregel state, increasing contraction stress, which is correlated to hardness37; moreover, the fast curing on the higher power group might also have caused an increased cross-link density on the composite,38-40 increasing its mechanical properties.41 It has been shown that even when similar degrees of cure are found, the polymer networks could show different cross-link density and, consequently, different mechanical properties.38,39,42 There was a difference in the microhardness of the composites, since this property is also related to their chemical composition, mainly to type and concentration of monomers in both the organic matrix and the inorganic load content of the composites.11,43 With the same energy, Transbond Plus Color Change had a microhardness of 53.6 KHN, which is higher than Opal

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Bond MV (35.7 KHN) and Transbond XT (30 KHN). Only Transbond XT has been evaluated in the current literature, with values of hardness lower24 and higher44 than those found in this study. Those 2 studies had different methodologies than ours, and they did not control the total energy or compare different types of composites. There was a significant interaction between curing time and power on the microhardness of the composites; this caused a different behavior depending on the curingtime reduction. Transbond Plus Color Change showed higher microhardness as time decreased and power increased. Whereas the other 2 composites showed similar behaviors when time was decreased from 8.5 and 6 seconds, there was no change in microhardness between 6 and 3 seconds. This differences are related to their composition, since the molecular characteristics of the monomers in the organic matrix determine their mobility and kinetic parameters,45,46 which in turn influence the mechanical performance of the material.47 CONCLUSIONS

Of the composites and in the conditions tested, we concluded the following. 1.

2.

Curing time can be reduced to 6 seconds by using 760 mW of power, reaching a satisfactory degree of cure. A further decrease in time, to 3 seconds, with increased power (1520 mW) will cause a slight decrease in the degree of conversion and an increase in the hardness of the composite.

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American Journal of Orthodontics and Dentofacial Orthopedics

Time reduction of light curing: Influence on conversion degree and microhardness of orthodontic composites.

The aim of this study was to assess the influence of curing time and power on the degree of conversion and surface microhardness of 3 orthodontic comp...
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