To evaluate the effect of time on the Vickers microhardness (VH) at the top and bottom surfaces of six conventional resin-based composites (RBCs) up to twelve weeks after light curing.
Five specimens of Filtek Supreme Ultra, Herculite Ultra, Mosaic Ultra, Tetric EvoCeram, TPH Spectra HV, and Venus Pearl were packed into opaque molds that were 2.3 mm in diameter and 2.5 mm deep. The uncured RBC specimens were covered by a polyester strip and photo-cured with an Elipar DeepCure-S light-curing unit (LCU) according to the manufacturer’s instructions. After irradiation, the polyester strip was removed, and the Vickers microhardness was measured immediately at top and bottom surfaces. The hardness measurements were repeated after 30 min, 1 h, 2 h, 4 h, 24 h, 1 week, 4 weeks, and 12 weeks. In between, the specimens were stored in dry and dark conditions at 37 °C. Two-way ANOVA ( α = 0.05) followed by Tukey–Kramer post hoc multiple comparison tests were used to determine where statistically significant differences existed.
The micro-hardness values at the top surface always exceeded those at the bottom surface. A significant logarithmic increase of the micro-hardness due to post-irradiation curing took place between 30 min and 24 h ( p < 0.05). There was no significant increase in the VH after 24 h. Depending on the RBC, compared to the immediate values the hardness 24 h post-irradiation had increased by 11–27% at the top surface and by 21–58% at the bottom.
Even after 12 weeks, the bottom hardness values never reached the top microhardness values. The results of studies that wait 24 h or longer before measuring the properties of RBC specimens will be significantly enhanced by the impact of post-irradiation curing. Especially within the first 4 h, the time when specimens are measured is critical information and should be reported.
One of the concerns when light-curing resin-based composites (RBCs) is the possibility that inadequate light-curing will adversely affect their clinical performance and may also have a negative effect on their biocompatibility . Conversely, delivering too much energy may cause an unacceptable temperature rise in either the tooth or in the soft tissues . One method to monitor how well the RBC has been photo-cured is to measure the surface micro-hardness either at the top and bottom or down the sides of the RBC because it has been shown that the degree of monomer conversion (DC) correlates qualitatively with the hardness of an RBC . The change in hardness of the RBC has been used to either obtain information about the inferred degree of conversion or the influence of media, such as bleaching agents and beverages on the mechanical performance of the RBC. Although this step takes time, many studies polish the specimens after light curing before the specimens are tested , while others do not polish the specimens . The post-irradiation time in seconds before the specimens were tested for these investigations typically ranged from ‘immediately’ after photo-curing to 30 or more days later . Even though it was reported in the 1980s that even small time differences in time can have large effects on early post-irradiation curing, the impact of the precise time between light exposure and when the measurements were made is often overlooked. For example, although the ISO 4049 test states that the depth of cure should be measured immediately after light curing , but the actual time interval is rarely reported in seconds post-irradiation.
Since it has already been reported that time has an effect on the amount of post-irradiation-curing , the impact of time on the post-irradiation curing of several contemporary RBCs samples is required to better understand the potential impact of the post-irradiation time on the results and conclusions of future research. Therefore, six contemporary and commonly used RBCs were photo-cured. Then, the micro-hardness was measured at post-irradiation times that ranged from several seconds to 12 weeks. The hypotheses of this investigation were:
The micro-hardness values measured at the top and bottom surface will reach the same values 3 months after light-curing.
The post-irradiation curing processes will result in a logarithmic time-dependent increase in the micro-hardness of the RBC.
Materials and methods
Materials and sample preparation
Six conventional RBCs were investigated after photocuring for their manufacturer recommended exposure times of 10 or 20 s, Table 1 . For each RBC, five specimens were packed into opaque plastic molds with an inner diameter of 2.3 mm and a depth of 2.5 mm. These dimensions were specifically chosen to fulfill the research hypotheses. To ensure a uniform smooth surface and to minimize the effects of oxygen inhibition, a polyester strip was placed both on top and bottom surfaces of the uncured RBCs that were then flattened and compressed with a flat glass slab. This technique produced smooth flat surfaces that did not require polishing before hardness testing. The glass slab was removed, and the specimens were photocured at room temperature using an Elipar DeepCure-S (3M, St. Paul, MN, USA) light-curing unit (LCU) positioned 2 mm above the top surface. After irradiation, the polyester strips were removed, and micro-hardness measurements were performed using a micro-hardness tester (HM 123, Mitutoyo, Kawasaki, Kanagawa, Japan) that applied 300 g load for 8 s. The RBC specimens were tested 36 s after the end of the light exposure. This was the average time it took from the end of light curing to correctly position the specimen in the hardness tester. The specimens were then retested after 30 min, 1 h, 2 h, 4 h, 24 h, 7 days, 28 days, and 3 months. In between these measurements, the samples were stored in dry and dark conditions at 37 °C. Sample preparation was randomized.
|Resin-based composites||Short name||Manufacturer||Shade||Lot #||v F (%)||t irrad (s)|
|Filtek Supreme Ultra||FSU||3M||A2 enamel shade||N778771||63||10|
|Herculite Ultra Nanohybrid Composite Restorative||HU||Kerr||A2 dentin||6115957||60||10|
|Mosaic Universal Composite||MU||Ultradent||EN||BDX35||68||20|
|Tetric EvoCeram||TEC||Ivoclar Vivadent||A2||U54029||53–55||10|
|TPH Spectra Universal Composite Restorative HV||TPH||Dentsply Sirona||A2||150903||60–62||10|
Characterization of the light-curing unit (LCU)
The total light output from the LCU and the irradiance received by the specimen were measured using a 6-inch integrating sphere (Labsphere, North Sutton, NH, USA) connected to a fiber-optic spectrometer (USB 4000, Ocean Insight: Largo, Fl, USA). While the average radiant exitance (tip irradiance) was calculated by dividing the radiant power emitted by the LCU into a fixed 9 mm aperture into the sphere, the irradiance received by the 2.3 mm diameter specimen was measured by placing the mold at the entrance of the sphere. The LCU tip was brought in contact with the surface of the mold and centered over the mold. Then, the LCU tip was moved 2 mm from the mold surface. The light that entered the sphere through the 2.3 mm diameter specimen mold, and thus the irradiance received by the specimens that were 2-mm away from the light tip, was calculated by dividing the radiant power received through the mold by the area of the mold. In addition, the light output of the LCU was checked with the dental radiometer integrated into the Elipar DeepCure-S charging station each day before starting and after finishing light-curing activities to verify that the light output had not degraded during the study.
Five repeats of photo-cured samples of each RBC were checked by a two-way ANOVA with a significance value of α = 0.05, followed by Tukey–Kramer post hoc multiple comparison tests to determine if there were differences in the microhardness data collected over time and between the top and bottom surfaces. Given the sample size of 5, an adjusted R 2 -value was used with OriginPro (OriginLab, Northampton, MA, USA).
Characterization of the LCU
The Elipar DeepCure-S LCU delivered a radiant power of 59.0 mW to the 2.3 mm diameter specimens, this delivered an irradiance of 1419 mW/cm 2 and a radiant exposure of 14.2 J/cm 2 to FSU, HU, TEC, TPH or 28.4 J/cm 2 to MU and VP because the exposure time was doubled for these RBCs. The emission spectrum from the LCU showed a single blue light peak at 449 nm with a full width at half maximum value of 18.5 nm ( Fig. 1 ).
Effects of post-irradiation time on micro-hardness
The micro-hardness values at the top surface were always significantly greater than those at the bottom surface for all RBCs, and all the values showed significant increases ( p < 0.05) after photo-curing ( Table 2 ). Of note, the bottom surfaces of four RBCs ( Table 2 , highlighted in yellow) reached the initial micro-hardness values measured at the top surfaces after a post-irradiation time of 3 months. However, the top surface hardness values had also increased during this time. The micro-hardness at the bottom surface of two RBCs, HU and VP ( Table 2 , highlighted in red) still differed by at least 10 HV after a post-irradiation time of 3 months from the starting micro-hardness values at the top surface. Of note, HU appeared to be insufficiently cured at the bottom of 2.5 mm because it has a low micro-hardness at the bottom surface starting with 14 HV and only achieved a maximum value of 23 HV. Interestingly, for VP there was little increase of micro-hardness at the bottom surface with time.
|Mean Vickers Micro-hardness at the top surface|
|RBC||v F %||VH 0h||SD 0h||VH 24h||SD 24h||DVH increase 24h %||VH 2016h||SD 2016h||DVH increase 2016h VH %|
|Mosaic Ultra||68||66||2.9||74||2||11||75||1||8.8 13|
|Filtek Supreme Ultra||63||69||1.6||83||2.3||19||83||6.8||13.6 20|
|TPH Spectra||61||52||2.1||65||3.5||25||65||3.5||13.0 25|
|Herculite Ultra||60||49||2||60||2.8||24||60||2.4||11.9 25|
|Venus Pearl||58||69||1.5||78||1.9||12||81||4.5||11.6 17|
|Tetric EvoCeram||54||41||1.8||51||1||27||55||2.4||14.5 36|
|Mean Vickers Micro-hardness at the bottom surface|
|RBC||v F %||VH 0h||SD 0h||VH 24h||SD 24h||DVH increase 24h %||VH 2016h||SD 2016h||DVH increase 2016h VH %|
|Mosaic Ultra||68||55||1.6||66||1||21||66||2.2||11.4 21|
|Filtek Supreme Ultra||63||48||4.7||62||2.3||31||70||3.3||22.5 47|
|TPH Spectra||61||30||9.8||48||2.4||58||50||7||19.9 63|
|Herculite Ultra||60||14||1||17||2.4||24||23||2.6||8.9 64|
|Venus Pearl||58||44||4.3||55||5.8||26||58||3.8||14.8 34|
|Tetric EvoCeram||54||24||1.7||34||2.7||44||38||1.9||14.0 60|
The micro-hardness values among these commercial RBCs differ from one to another due to their composition, but there was no direct correlation with the filler contents of the RBCs. Instead, the increase in microhardness was RBC and time-dependent. The increase at 24 h post-irradiation ranged from 11 to 27% at the top surface and 21 to 58% at the bottom ( Table 2 ). At 3 months, the increases ranged from 13 to 36% at the top surface and 21 to 64% at the bottom ( Table 2 ). In general, most of the post-irradiation curing took place within 24 h after photo-curing ( Table 2 and Figs. 2 and 3 ). The RBCs tested showed an increase in their micro-hardness VH( t ) with a logarithmic dependency on the post-irradiation time within the first 24 h ( Table 3 ) that can be described by the equation:
with time t , slope m , and intercept VH 0 . The evaluated values for slope m and intercept HV 0 , together with the adjusted R 2 are reported in Table 3 . The micro-hardness values at the bottom depended logarithmically on the post-irradiation time between 0.006 (36 s) and 24 h, except for Herculite Ultra. For the micro-hardness values at the top surface, the logarithmic behavior was only observed for the interval from 0.5 to 24 h, Table 3 . Except for the Mosaic Ultra RBC, the slope’s ‘ m ’ value at the top surface always exceeded those measured at the bottom surface ( Figs. 4 and 5 ).