The effect of curing time by conventional quartz tungsten halogens and new light-emitting diodes light curing units on degree of conversion and microhardness of a nanohybrid resin composite.

BACKGROUND: Little is known about the relationship between the minimal light-curing time required for proper polymerization on various quartz-tungsten-halogen (QTH) and light-emitting diode (LED) light-curing units that have different light intensities. AIM: To evaluate the effects of curing time by QTH and LED light-curing units on the degree of conversion (DoC) and surface microhardness of a nanohybrid resin composite. SETTING AND
DESIGN: Experimental design.
MATERIALS AND METHODS: One hundred and twenty cylindrical specimens (4.0 mm in diameter, 2.0 mm thick) of shade A2 resin composite were prepared and polymerized with either QTHs or LEDs for 20 and 40 s. The DoC and the top and bottom surface microhardness were recorded. STATISTICAL ANALYSIS USED: Two-way analysis of variance, Tukey's test, and the t-test (α = 0.05) were used.
RESULTS: Surface microhardness and DoC values were affected by light intensity and curing time (P < 0.05). In terms of microhardness and DoC, LED groups gave significantly more values than QTH groups (P < 0.05).
CONCLUSION: Curing time affected surface microhardness and DoC values of a nanohybrid resin composite in both conventional QTH and new LED light-curing units.

INTRODUCTION

Demand for esthetics in dentistry has presently increased. One of the esthetic restorative materials used is light-cured resin composites which have been widely applied in clinical dentistry. This has also resulted in a rapid increase in the development of the number of light-curing units to polymerize light-activated resin composites.

Quartz–tungsten–halogen (QTH) light-curing units are the most commonly employed light-activation units in dentistry. Minimal intensity (400 mW/cm2) in the proper spectral distribution is necessary for complete polymerization of light-cured resin composite of 2 mm in depth.[1] The use of QTH curing units to polymerize resin composite has several drawbacks despite their popularity. The halogen bulbs have a limited effective lifetime of about 40–100 h. In addition, the reflector and filter degrade over time due to high operating temperatures and the large quantity of heat produced during the curing cycles.[1] To overcome the several drawbacks of QTH curing light units, blue light-emitting diodes (LEDs) have been developed for polymerization of light-activated resin composite. LED curing units feature very narrow spectral ranges of around 470 nm and a bandwidth of about 20 nm.[2] They have a lifetime of more than 10,000 h and undergo little degradation of light output overtime. They use junctions of doped semiconductors (p-n junctions) to generate light and require no filters to produce light. LED curing units are also resistant to shock and vibration. Their relatively low-power consumption makes them suitable for portable use. Previous studies have demonstrated good performance of LED light-curing units in terms of adequate depth of cure, flexural strength, and surface microhardness.[345]

The degree of conversion (DoC) is an important factor that affects clinical performance of resin composite restorations.[67] It can be correlated with the composition of monomers and oligomers used in the material, which is the number of ethylene double carbon bonds converting into single bonds, and it provides the DoC (in percentage) for a resin composite. Several methods have been used to determine the DoC for a resin composite. Fourier-transform infrared spectroscopy (FTIR) has been widely used as a reliable method for examining the DoC. It detects the C=C stretching vibrations directly before and after curing of materials.[8] FTIR spectra of both uncured and cured samples were analyzed using an accessory of the reflectance diffusion. However, to measure the DoC for bulk resin composite by FTIR, the procedure is time-consuming as the polymerized specimens need to be pulverized. Therefore, one of the most frequently used indirect methods for verifying the degree of resin composite polymerization is the microhardness test,[9] which indicates the strength under compressive loading.

The curing times, correct wavelength of the light source, and material compositions strongly influence the DoC.[710] Previous studies have reported that high-intensity light provides higher values for the DoC[31112] and demonstrated that LED curing units provide deeper depth of polymerization than QTH lamps.[3] However, little is known about the relationship between the minimal light-curing time required for proper polymerization on various QTH and LED light-curing units that have different light intensities. Therefore, the aim of this study was to evaluate the DoC and the hardness of resin composite polymerized by various QTH and LED curing units at different polymerized times. The null hypothesis was that there would be no effect of various light sources or intensities on the DoC and microhardness of a resin composite at different polymerized times used.

MATERIALS AND METHODS

Specimen preparations

One hundred and twenty cylindrical specimens of one brand of resin composite [Premise shade A2, Kerr Corp., Orange, USA: Table 1] were prepared in polytetramethylfluoroethylene ring molds, 4.0 mm in internal diameter and 2 mm in thickness. To minimize the effects of colorants on the light penetration, the vita shade A2 of resin composite was selected.[13] The mold cavity was then filled with resin composite in a single increment on a glass plate and covered with a mylar matrix strip and a second glass plate was placed over the mylar strip. A static load of approximately 500 g was applied to this plate for 30 s to extrude excess resin composite and to obtain a flat surface that would facilitate hardness testing.

Table 1

Resin composite materials used in this study

Trade name	Manufacturer	Composition		Average particle size (mm)
		Matrix	Filler
Premise	Kerr Corp., Orange, CA, USA	Bis-EMA, UDMA, TEGDMA	Prepolymerized filler, barium glass	Prepolymerized filler, barium glass filler 0.4

Bis-EMA: Ethoxylated bisphenol A dimethacrylate, UDMA: Urethane dimethacrylate, TEGDMA: Triethyleneglycol dimethacrylate

Subsequently, the glass plate was removed from the top of the mold. The curing-light tip was then placed in contact with the top surface of the specimens. Three QTH light-curing units: Elipar 2500 (EL2500) (3M ESPE, Grafenau, Germany); Sprectrum 800 (SP) (Dentsply DeTrey GmbH, Konstanz, Germany); and Demetron LC (DELC) (SDS Kerr, Danbury, CT, USA), and three LED light-curing units: Elipar S10 (ELS10) (3M ESPE, Grafenau, Germany); BlueShot (BS) (Shofu, Kyoto, Japan); and Demi (DELED) (SDS Kerr, Danbury, USA) were used in this present study. All LED light-curing units were used in the continuous mode. The intensity of all the curing light units was checked with a radiometer (Cure Rite model 8000, EFOS Inc., Mississauga, Canada) prior to use to ensure consistency in intensity output from the light source. The light-curing times used were 20 and 40 s for QTH and LED curing unit groups.

After polymerization, the mylar strip on the top and the glass plate on the bottom of the mold were removed. Subsequently, the specimen was removed from a ring mold. Ten specimens were assigned to each of the twelve groups as shown in Table 1.

Surface hardness measurement

Five specimens (n = 5) of each group were tested using a Vickers hardness testing device (Micromet II, Buehler Ltd., Lake Bluff, USA). Hardness measurement was taken under a 100 g load for 10 s. Five hardness indentations, of 2 mm in thickness, were made on the top surface (near the light source) and five hardness indentations were made on the bottom surface (away from the light source) of each specimen. The mean of the five hardness measurements on each specimen was recorded as the hardness value of that surface of the specimen (top and bottom). The hardness ratio was also calculated by dividing hardness values of the top surface by hardness values of the bottom surface for each thickness and curing time. This value should be >0.8.[13]

Determination of the degree of conversion

Five specimens (n = 5) of each group were tested using an FTIR spectrometer (model EQUINO × 55, Bruker Optics Inc., Billerica, USA). Uncured paste of resin composite was smeared onto a potassium bromide disc, and the absorbance peaks before curing were obtained by the transmission mode of FTIR. The polymerized specimen was pulverized into fine powder with a hard tissue-grinding machine (model MA590, Marconi, Piracicaba, Brazil) immediately after curing. The absorbance peaks were then recorded using the diffuse-reflection mode of FTIR. The percentage of unreacted carbon-carbon double bonds (%C=C) was examined from the ratio of absorbance intensities of aliphatic C=C (peak at 1635 cm−1) against an internal standard before and after curing of the specimen and the aromatic C-C (peak at 1614 cm−1). The DoC was calculated by subtracting the C=C% from 100% according to the following formula:[8]

Statistical analysis

A two-way analysis of variance and Tukey's honestly significant difference test were applied to test the effect of the light-curing units (or intensities) and polymerized times on the DoC and hardness of the resin composite. One sample t-test was used to test these effects on the surface hardness on the top and bottom of the resin composite (α = 0.05).

RESULTS

The intensities of QTH and LED light-curing units, mean surface hardness, hardness ratio, and DoC are shown in Table 2. LED groups had more intensity than QTH groups. In terms of hardness ratio, no significant differences were found among QTH groups or among LED groups. LED groups have significantly more hardness ratio values than QTH groups (P < 0.05). In terms of DoC, LED groups also have significantly more DoC values than QTH groups (P < 0.05).

Table 2

Mean hardness, degree of conversion, and standard deviations of resin composite at different polymerized times, using quartz–tungsten–halogen and light-emitting diode light-curing units

Group and intensity (mW/cm²)	Time (s)	Mean hardness (kg/mm²)±SD		Hardness ratio	Degree of conversion, (%)±SD
		Top	Bottom
QTH
DELC	20	61.63±3.72a	51.51±3.63*,a	0.83±0.05a	67.06±0.04a
402.00±7.21	40	70.70±3.76b	64.97±2.89*,b	0.91±0.11†,b	69.09±0.01‡,b
SP	20	61.88±3.43a	51.03±3.23*,a	0.82±0.04a	67.12±0.01a
450.67±4.04	40	71.89±4.02b	64.51±3.03*,b	0.89±0.05†,b	69.10±0.12‡,b
EL2500	20	61.31±3.76a	52.73±2.67*,a	0.83±0.07a	67.16±0.01a
555.33±11.02	40	71.11±2.48b	64.68±2.18*,b	0.90±0.05†,b	69.14±0.02‡,b
LED
DELED	20	71.54±3.88b	64.04±3.98b	0.89±0.14b	69.15±0.01b
076.33±24.09	40	84.69±3.62c	78.07±3.48c	0.92±0.14†,c	71.08±0.04‡,c
ELS10	20	71.76±3.31b	64.06±3.66b	0.89±0.03b	69.11±0.01b
1417.00±14.93	40	83.91±3.87c	77.73±3.34c	0.92±0.02†,c	72.10±0.06‡,c
BS	20	71.61±3.90b	63.69±3.9*,b	0.88±0.03b	69.03±0.07b
1911.67±4.72	40	84.93±3.09c	78.50±3.48*,c	0.92±0.09†,c	71.13±0.04‡,c

*Indicates significant difference (in row) between top and bottom surface hardness of each time according to the t-test (P<0.05), †Indicates significant difference (in column) between 20 and 40 s of hardness ratio for each QTH and LED light-curing units according to one sample t-test (P<0.05), ‡Indicates significant difference (in column) between 20 and 40 s of the degree of conversion for each QTH and LED light-curing units according to one sample t-test (P<0.05), a,b,cIndicates significant difference (in column) among different times and light-curing units of each hardness and degree of conversion according to Tukey’s HSD test (P<0.05). QTH: Quartz tungsten halogen, DELC: Demetron LC, DELED: Demetron LED, ELS10: Elipar S10, SD: Standard deviation, SP: Sprectrum, BS: BlueShot, LED: Light-emitting diode, HSD: Honestly significant difference

DISCUSSION

The results of this present study support rejection of the null hypothesis because there would be an effect of various light sources or intensities on the DoC and hardness of resin composite at different polymerized times. The need for an adequate polymerization of the resin composite results in good physical and mechanical properties of the materials created for clinicians concerning the selection of the appropriate light-curing unit.

One of the most frequently used indirect methods for verifying the degree of resin composite polymerization is the microhardness test[91415] that indicates the strength under compressive loading. In the present study, the results presented that the polymerized time used affected hardness values and DoC values. In fact, as light passed through the bulk of a resin composite, its intensity is greatly decreased due to the absorption and scattering of light by filler particles and the resin matrix. This decreasing results in a gradation of cure such that it decreases from the top surface inward. This then accounted for the difference between top surface hardness and bottom surface hardness of all specimens cured with each light source[16] which was different to results when using the FTIR technique. To measure the DoC of bulk resin composite by FTIR, polymerized specimens need to be pulverized into fine powder. So accounting the DoC of specimens were the average value of bulk resin composite, which found a significant difference among the evaluated groups in this present study. At the equal time used, new LED curing units provide deeper depth of polymerization than conventional QTH. However, there was no statistical difference in irradiation time between conventional QTH irradiation time at 40 s and new LED light-curing units at 20 s. This result is caused from new LED curing units performed high intensity of light more than conventional QTH, associated with some recent studies which found that high-intensity light provides higher values for the DoC.[31112]

For all light-curing units in the present study, microhardness values were high at the top surface, which can be attributed to the relationship between irradiation distance and effectiveness of polymerization. In particular, the hardness values of the bottom surface should be close to the hardness values of the top surface, resulting in a hardness ratio >0.8.[13] In the present study, effective hardness ratios were achieved with most light-curing unit groups, which is in agreement with previous studies.[311121718] A plausible reason for this outcome could be related to the light intensities and duration of the curing time used. A greater intensity of light energy and lengthy duration are sufficient to excite the camphorquinone in the resin composite material. As shown in the present study, increasing the duration of irradiation time provided significantly more polymerization than a short irradiation time for the same thickness, agreeing with the results of previous studies.[192021] These results might be applied in a clinical situation.

It must be noted that there are some limitations in the present study. The specimen surfaces were flat, whereas, clinically, resin composite restorations have irregular shapes with convex and concave surfaces. Specimens used for extraoral evaluations are usually monochromatic, uniformly translucent, and untextured while intraoral restorations contrast with all of the above. These limitations may considerably affect these results. More in vivo studies are needed to assess the effects of different light intensities of light-curing units, time and thickness on microhardness, and degrees of conversion.

CONCLUSION

Within the limitations of this study, the following conclusions could be drawn: curing time affected surface microhardness and DoC values of a nanohybrid resin composite in both of conventional QTHs and new LEDs light-curing units.

Financial support and sponsorship

The study was funded by the Faculty of Dentistry research fund, Prince of Songkla University.

Conflicts of interest

There are no conflicts of interest.