|Year : 2015 | Volume
| Issue : 3 | Page : 109-116
Effect of different curing modes on the degree of conversion and the microhardness of different composite restorations
Reem Ali Ajaj1, Mohammed Khalil Yousef2, Abeer Ibrahim Abo El Naga2
1 Department of Operative Dentistry, Biomaterials Division, King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia
2 Department of Operative Dentistry, Operative Division, King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia
|Date of Web Publication||28-Aug-2015|
Reem Ali Ajaj
Faculty of Dentistry, King Abdulaziz University, PO Box: 80200, Jeddah - 21589
Kingdom of Saudi Arabia
Source of Support: None, Conflict of Interest: None
Introduction: This study aims to evaluate the effects of different curing units and modes on the degree of conversion (DC) and microhardness (MH) of two different resin composites [ESTELITE ∑ QUICK (EQ), and Z350 XT (Z3)]. Materials and Methods: One hundred (100) discs of each tested material were made and divided into two subgroups (n = 50) according to the discs' dimensions: 5 mm diameter × 2 mm thickness, and 2 mm diameter × 2 mm thickness. Each subgroup was further subdivided into the following five classes (n = 10): I) cured with halogen light curing-unit; II) cured with light-emitting diode (LED) unit; III) cured with argon laser; IV) cured with halogen light-curing unit for 5 s, 10 s rest followed by 20 s curing; and V) cured with halogen light-curing unit for 10 s, then 10 s rest, followed by 10 s curing. The first subgroup was tested for MH using the Vickers Microhardness tester and the second subgroup was tested for DC using Fourier transform infrared spectroscopy (FTIR). Data were statistically analyzed using two-way analysis of variance (ANOVA) and Tukey's post hoc test P < 0.05. Results: Specimens in class IV showed the highest mean DC and MH, followed by class III, then class II. Class I showed significantly lower mean values for both DC and MH. On the other hand, Z3 showed statistically significantly higher mean DC and MH than EQ. Conclusion: Although the two tested composites did not perform similarly under the test conditions, curing with halogen unit for 5 s, then 10 s rest, followed by 10 s curing improved the DC and the MH of both the tested materials.
Keywords: Degree of conversion (DC), light-curing mode, microhardness (MH), resin composite
|How to cite this article:|
Ajaj RA, Yousef MK, Abo El Naga AI. Effect of different curing modes on the degree of conversion and the microhardness of different composite restorations. Dent Hypotheses 2015;6:109-16
|How to cite this URL:|
Ajaj RA, Yousef MK, Abo El Naga AI. Effect of different curing modes on the degree of conversion and the microhardness of different composite restorations. Dent Hypotheses [serial online] 2015 [cited 2020 May 29];6:109-16. Available from: http://www.dentalhypotheses.com/text.asp?2015/6/3/109/163815
| Introduction|| |
Resin composite is a widely used final restorative material that requires light-activated polymerization reaction for its setting. Adequate polymerization is crucial for the longevity of the resin composite restorations. , The quality of light and the curing mode are important factors that affect polymerization and thus the clinical performance of the resin composite restoration.  Camphorquinone is the photoinitiator present in most of the resin composites and is activated by blue light in the wavelength range of 400-525 nm. ,,,
The tungsten halogen lamp and the light-emitting diode (LED) are the common light-activation modes used in dental clinics. Argon laser is another method for light activation of resin composite materials. The halogen lamp emits a wide spectrum of light with a spectral output between 400 nm and 500 nm and needs a filter to produce blue light. ,, Halogen-curing units can suffer from fluctuations in the line voltage, damage to the fiber optic bundle, and overheating of the bulb, and the condition of the bulb filter can compromise its efficiency of performance; thus the polymerization of resin composites and their mechanical properties. , The LED provides a narrow spectrum of wavelength between 470 nm and 490 nm, which is close to the camphorquinone excitation wavelength, plus it will need less polymerizing energy. , LED produces blue light without requiring a filter and generates less heat and less degradation over time compared to halogen light-curing units.  An argon laser curing unit generates energy by exciting ions in the argon-filled resonating chamber. , It produces a non-continuous variety of wavelengths that are within the absorption range required by camphorquinone: 454 nm, 458 nm, 466 nm, 472 nm, 488 nm, and 497 nm. ,, It also generates less heat due to less infrared output. ,,, However, these units are high in cost and have a limited lifespan, which limits their use by many practitioners. ,
Different curing modes can influence the hardness of the resin composite. , Appropriate energy density is important for obtaining a high degree of conversion (DC). ,, Energy density is determined by the curing light intensity and exposure time.  High power density activates a large number of photoinitiator molecules at the same time, which will generate more inner stresses. , In the pulse-delay curing mode, there is a short period between the initial low-energy exposure and the final high-energy cure.
Fourier transform infrared spectroscopy (FTIR) is a direct method used to determine the degree of DC by utilizing molecular vibrations to quantify the ration of monomer conversion into polymers by assessing specific band positions to compare the unpolymerized aliphatic C=C stretching band at 1640 cm -1 to the aromatic C=C stretching band at 1610 cm -1 . ,,, A hardness test is an indirect method used to assess the quality of polymerization of the light-activated resin composite. ,
Several studies showed that, regardless of the curing unit or technique, using different types of resin composite would give different results for microhardness (MH) and DC. , Other studies reported that regardless of the type of resin composite used, the use of different types of light-curing units would give different results for MH and DC. ,,
The aim of this study was to evaluate and compare the effects of different curing units and curing modes by means of measuring the DC and MH of two different resin composites [ESTELITE ∑ QUICK (EQ), Tokuyama Dental, Tokyo, Japan, and Z350 XT (Z3) 3M/ESPE, St. Paul, MN, USA] and to find out if a different resin composite material would respond differently under the same curing unit and mode.
The independent variables we tested are:
The dependent variables we measured are:
- Curing unit type
- Curing mode
- Material type
| Materials and Methods|| |
Protocol of this study was approved by local ethical committee. Two specially fabricated split cylindrical Teflon molds were used for making the disc specimens of the two tested restorative materials. The first mold was of 5 mm internal diameter and 2 mm thickness, while the other mold was of 2 mm internal diameter and 2 mm thickness.
Two different restorative materials (EQ and Z3) were tested in this study. The manufacturers, manufacturers' instructions, and detailed composition are presented in [Table 1]. Shade A2 (universal color) was used for preparing the samples from the tested materials. One hundred discs (100) of each tested material were made.
|Table 1: Manufacturers, manufacturers' instructions, and compositions of the used restorative materials |
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To standardize the technique and ensure proper curing of the resin composite samples, two 1-mm increments were inserted. Discs examined for MH were fabricated by carefully inserting an increment of tested restorative material using a nitride-plated resin-composite instrument (Aesculap, Melsungen, Germany) into a circumferential Teflon mold positioned onto a 0.051-mm thick transparent polyester film strip (Mylar, DuPont, Wilmington, DE, USA) over a glass slide. The first increment was light-cured for 20 s using the assigned light-curing unit. The second increment was inserted and then another 0.051-mm thick transparent polyester filmstrip was applied on top of the Teflon mold filled with the tested material. An additional glass slide was placed over the previously positioned polyester filmstrip, and a 1-kg weight was applied for 1 min to extrude the excess material and to obtain a uniformly smooth specimen surface. Afterward, the weight was removed and the second increment of tested restorative material was light-cured for 20 s through the polyester filmstrip. The output light intensity was continuously monitored with a radiometer (SDS Demetron, Orange, CA, USA) to ensure a constant value of 600 mW/cm 2 . A notch on the side to be examined for MH marked the top surface of the disc against which the load was applied.
One hundred (100) discs of each tested material were made and subdivided into the following two subgroups (n = 50) according to the discs' dimensions:
Each subgroup was further subdivided into 5 classes (n = 10) according to the curing technique:
- 5mm diameter × 2 mm thickness
- 2mm diameter × 2 mm thickness
The specimens of the first subgroup were tested for MH, while the specimens of the second subgroup were tested for the DC. Testing was done immediately after curing, with no storage period.
- Cured with halogen light-curing unit (PRO-DEN systems, Feasterville, USA)
- Cured with LED unit (Apollo95E, Westlake Village, CA, USA)
- Cured with argon laser of 488 nm (AccuCure 3000, LaserMed, West Jordan, UT, USA) with 5 mm beam size and 10 s exposure
- Cured with halogen light-curing unit for 5 s, then 10 s rest, followed by 20 s continuous curing
- Cured with halogen light-curing unit for 10 s, then 10 s rest, followed by 10 s continuous curing
The specimens in the subgroup assigned to evaluate the surface MH were examined using the Digital Display Vickers Microhardness Tester (Model HVS-50, Laizhou Huayin Testing Instrument Co., Ltd., Laizhou in Shandong Province, People's Republic of China) with a Vickers diamond indenter and a 20× objective lens. A load of 1000 g was applied to the surface of the specimens for 10 s. Three indentations were made on the surface of each specimen. These indentations were equally placed over a circle and not closer than 1 mm to the adjacent indentations or to the margin of the specimens. The diagonal lengths of the indentations were measured by the built-in scaled microscope and the Vickers values were converted into MH values.
The specimens in the subgroup that were assigned to evaluate the DC were examined using FTIR over a 4000-400 cm -1 range. DC was measured in a real-time profile (20 min, with 2 spectra/s) using an FTIR spectrometer with an attenuated total reflectance (ATR) accessory (Nexus, Thermo Nicolet, Madison, WI, USA). The nonpolymerized composite paste was applied directly on the diamond ATR crystal in a white Teflon mold 2 mm in height and 2 mm in diameter. To simulate a cavity, the mold was filled in two consecutive increments, each 1 mm high, cured by applying the assigned curing unit on each increment surface from the top of the 2 mm-high mold. DC was measured on the bottom of the samples (n = 10) and was calculated by assessing the variation in peak height ratio of the absorbance intensities of methacrylate carbon double bond (peak at 1,634 cm−1 ) and those of an internal standard (aromatic carbon double bond, peak at 1,608 cm−1 ) during polymerization, in relation to the uncured material. The following equation was used for the calculation:
Data were presented as mean and standard deviation (SD) values. Data were calculated and statistically analyzed using IBM × SPSS × Statistics Version 20 (Statistical Package for Scientific Studies) (SPSS Inc., Chicago, IL, USA). Two-way analysis of variance (ANOVA) was used in testing significance for the effect of the material, the curing modes, and their interactions on DC and MH. Tukey's post hoc test was used for pairwise comparison between the mean values when the ANOVA test was significant. The significance level was set at P < 0.05. Statistical analysis was performed with SPSS × Version 20 for Windows.
| Results|| |
The two-way ANOVA results, presented in [Table 2], showed that the material, the curing mode, and the interaction between the two variables had a statistically significant effect on the mean MH results (P value <0.001).
|Table 2: Results of the two-way ANOVA for the effect of the material, the curing mode, and their interactions on MH |
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Regarding the effect of the type of material on the MH, as seen in the results presented in [Table 3], Z 350 showed a statistically significantly higher mean MH than did EQ.
|Table 3: The mean and SD MH values compared between the materials regardless of curing mode |
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The highest mean of MH was found with class IV (halogen light for 5 s, then 20 s: 81.1), followed by class III (laser: 73.8), then class II (LED: 70.9). Class I (halogen) showed significantly lower mean values (68.4). Class V (halogen light for 10 s, then 10 s) showed the statistically significant lowest mean MH (62.2) [Table 4].
|Table 4: The mean and SD MH values compared between using different curing modes regardless of the material |
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The values comparing the MH results for the different variables (material and curing mode) are presented in [Table 5]. Z 350, cured with halogen for 5 s and then 20 s, showed the highest statistically significant mean for MH. There was no statistically significant difference between Z 350 cured with laser and EQ cured with halogen for 5 s then 20 s; both showed lower mean MH. There was no statistically significant difference between Z 350 cured with halogen, Z 350 cured with LED, ESTELITE cured with LED, and ESTELITE cured with laser; all showed lower mean MH. There was no statistically significant difference between Z 350 cured with halogen (10 s, then 10 s) and ESTELITE cured with halogen; both showed lower mean values. ESTELITE cured with halogen 10 s, then 10 s showed the statistically significant lowest mean for MH.
|Table 5: The mean and SD MH values comparing between different variables' interactions |
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The two-way ANOVA results showed that material, the curing mode, and the interaction between the two variables had a statistically significant effect on mean DC (P value <0.001) as presented in [Table 6].
|Table 6: Results of the two-way ANOVA for the effect of the material, the curing mode, and their interactions on the DC |
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The results for the effect of using different material types on the DC are presented in [Table 7]. Z 350 showed a statistically significantly higher mean for the DC compared to ESTELITE.
|Table 7: The mean and SD DC values compared between the materials regardless of the curing mode |
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The results for the effect of curing mode on the DC are presented in [Table 8]. The statistically significant highest mean for the DC was found with class IV (halogen light for 5 s, then 20 s; 86.1), followed by class III (laser; 73.5), then class II (LED; 69.2). Class I (halogen) showed significantly lower mean values (65.8). Class V (halogen light for 10 s, then 10 s; 61.6) showed the statistically significant lowest mean for the DC.
|Table 8: The mean and SD DC values compared between the results for curing modes regardless of the material |
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The values comparing the DC results for the different variables (material and curing mode) are presented in [Table 9]. There was no statistically significant difference between Z 350 cured with halogen light for 5 s, then 20 s, and ESTELITE cured with halogen light for 5 s, then 20s; both showed the statistically significant highest mean for the DC. This highest value was followed by the results for Z 350 cured with laser. There was no statistically significant difference between Z 350 cured with LED and ESTELITE cured with laser; both showed lower mean DC values. There was also no statistically significant difference between Z 350 cured with halogen and ESTELITE cured with LED; both showed lower mean DC values. This was followed by ESTELITE cured with halogen. There was no statistically significant difference between Z 350 cured with halogen light for 10 s, then 10 s and ESTELITE cured with halogen light for 10 s, then 10 s; both showed the statistically significant lowest mean for the DC.
|Table 9: The mean and SD DC values compared between different variables' interactions |
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| Discussion|| |
The results of the MH and DC tests with different materials regardless of the curing mode showed that there is a statistically significant difference between the two materials. This might be attributed to compositional factors, such as the difference in filler particle size, shape, and dispersion that may have direct effects by themselves or can affect the light distribution and thus polymerization of the material. Z 350 was found to have higher mean values for both MH and DC compared to ESTELITE ∑ QUICK. The filler particles of ESTELITE ∑ QUICK are monodispersing and spherical, with 0.2 mm diameter size compared to the filler in Z 350 composite, which is a combination of non-agglomerated/non-aggregated 20 nm silica and 4-11 nm zirconia filler, and agglomerated/aggregated zirconia/silica cluster filler with the average cluster particle size range is 0.6-10 mm [Table 1]. The higher cluster particle size of Z 350 (0.6-10 mm) might have contributed to the increase in the surface MH of this material. Our results are in agreement with the published reports of Albino et al. and Ribeiro et al. Albino et al. tested two resin composites (one nanofilled and another hybrid) cured with different light sources (halogen and LED), and the mean MH values for resin composite materials with different particle size were found to be different regardless of the curing unit used or the material opacity, although they did not report this finding.  Ribeiro et al. evaluated the DC of four resin composites (one nanofilled and three microhybrid resins) after light curing with second- and third-generation LED lights; they found that nanofilled resin had the lowest DC values and that there were differences in the DC values for different microhybrid resins. 
The results of the MH and DC tests using different curing modes regardless of the materials showed that there are statistically significant differences between all classes of different curing modes. The values of MH and DC indicate that the type of curing light and the technique or mode of curing is an important factor to consider during resin composite restoration. The pulse-delay technique showed the highest effectiveness only when used for 5 s followed by 10 s delay and then 20 s curing, and they showed the lowest values when used for 10 s followed by 10 s delay and then another 10 s light curing for the same curing machine. The initial short light-curing period (5 s) may allow more stress relief within the resin upon initial polymerization compared to the initial light-curing period (10 s). In contrast to our findings, it was hypothesized that the low power density may generate a small number of free radicals, which would produce a more linear polymeric structure with low crosslinking that negatively affects the materials' physical and chemical properties. , Another explanation of our finding was that the total light-curing period in class IV was 25 s compared to that in the class V group being 20 s, which might have caused the material in class IV to have more full potential polymerization than class V. Some studies showed that regardless of the light-curing mode, providing similar energy densities will result in similar DC, depth of polymerization, , and MH.  In our study, we continuously monitored the output light intensity using a radiometer to make sure that we were using a constant value of 600 mW/cm 2 , but we got statistically significant results of MH and DC for different curing lights and modes. This might be attributed to the different amounts of total energy delivered when using different curing times. For the continuous mode of cure using halogen and LED, they were used according to the manufacturers' instructions with 20 s exposure time; for the argon laser, it was only 10 s; and for the pulse-delay mode, it was either a total of 25 s for class IV or 20 s for class IV. Yet the results do not correspond to the length of exposure for some curing lights. Classes I, II, and IV all had 20 s of exposure time using the same light intensity, but showed different MH and DC results. We suggest that the light-curing unit and the mode together with the light density are all factors that can affect the polymerization of resin and its subsequent MH and DC results. Many studies have supported the effect of light density on the polymerization of resin composites. Correr et al. found that increasing energy densities for LED and xenon plasma arc increased the MH of two different resins, but that there was no significant difference when the halogen light-curing unit was used with different intensities.  Felix et al. reported that using high-power LED and halogen curing light can allow us to reduce the light exposure time up to 50% without reduction in MH results when compared to the recommended exposure time operating at the medium power setting, which can reduce operative time for resin restoration.  Other studies were in agreement with our finding that using different types of light-curing unit would have different effects on the polymerization of resin materials. In a study by Arrais et al., the DCs of adhesives light-cured with LED and halogen light for 10 s were measured and it was found that the DC values were lower for samples cured with LED light when compared to those cured with halogen light.  Carvalho et al. compared the amount of residual monomer in resin composites cured with LED and halogen units, and reported that using LED results in less residual monomer compared to halogen light even for half the light-curing time.  The same conclusion was reached by Cerveira et al. when they tested the DC and MH of orthodontic resin and found that using LED resulted in a 50% reduction in the light-curing time needed for halogen light, as recommended for its use. 
Both the type of material and the curing mode were found to be factors that have a statistically significant effect on the MH and DC of the resin composite. A similar finding was reported by Ceballos et al. when they measured the depth of cure using the scraping technique and the MH using the calibrated Vickers indenter: They concluded that the type of light-curing unit affected the curing effectiveness, and that the duration of exposure, the composite brand, and its thickness also had great effects. 
| Conclusion|| |
Curing unit type, curing mode, and resin composite material were all variables that were found in our study to have different effects on the values of MH and DC. The highest values for both MH and DC were found when using Z 350 resin composite material with a halogen curing unit for 5 s, followed by a rest period of 10 s, and then the final curing for 20 s.
Financial support and sponsorship
King Abdulaziz University.
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8], [Table 9]