Evaluation of hardness and degree of conversion of different bulk-fill materials cured with Quartz Tungsten Halogen and light emitting diode devices

Ashok Suman; Jagvinder Singh Mann; Sumeeta Sandhu; Sonal Maurya

doi:10.4103/sidj.sidj_2_18

ORIGINAL ARTICLE

Year : 2017 | Volume : 3 | Issue : 1 | Page : 4-10

Evaluation of hardness and degree of conversion of different bulk-fill materials cured with Quartz Tungsten Halogen and light emitting diode devices

Ashok Suman, Jagvinder Singh Mann, Sumeeta Sandhu, Sonal Maurya
Department of Conservative Dentistry and Endodontics, Government Dental College and Hospital, Patiala, Punjab, India

Date of Web Publication

16-Apr-2018

Correspondence Address:
Ashok Suman
Department of Conservative Dentistry and Endodontics, Government Dental College and Hospital, Patiala, Punjab
India

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/sidj.sidj_2_18

Abstract

Aims: The aim of this study is to evaluate the hardness and degree of conversion (DC) of different bulk-fill materials when cured with the Quartz Tungsten Halogen (QTH) and Light emitting diode (LED) devices using same energy density.
Settings and Design: This was in vitro study.
Materials and Methods: Three different bulk-fill materials, i.e., Filtek™ [3M ESPE], Tetric^® N-Ceram [Ivoclar Vivadent], and SDR (Smart Dentin Replacement) by Dentsply Caulk, were used for making 180 samples (60 samples each) in Teflon mold. Out of these 60 samples, samples of three heights were prepared, i.e., 2 mm, 4 mm, and 6 mm (20 each). All the sample molds were filled in single increment and were exposed to the QTH and LED using the same energy density. Then, these samples were tested for microhardness using the Vickers hardness (VH) testing machine (Mitutoyo, Japan) and DC using Fourier Transform Infrared Spectroscope (Cary 630 FTIR Agilent Technologies, Germany).
Statistical Analysis Used: The statistical analysis was performed on the collected data. The Kolmogorov–Smirnov and Shapiro–Wilk tests showed that the data are normally distributed. Since all the variables were continuous and statistically independent, independent t-test was applied to compare mean values of VH and DC of inter groups.
Results: The statistical analysis of readings revealed that there was no significant difference in the mean values of microhardness and % DC for both groups cured by QTH and LED.
Conclusions: It can be concluded that irrespective of the type of the light cure unit if the energy density applied is same then almost similar performance can be expected from the devices.

Keywords: Bulk-fill composites, curing lights, degree of conversion, microhardness, same energy density

How to cite this article:
Suman A, Mann JS, Sandhu S, Maurya S. Evaluation of hardness and degree of conversion of different bulk-fill materials cured with Quartz Tungsten Halogen and light emitting diode devices. Saint Int Dent J 2017;3:4-10

How to cite this URL:
Suman A, Mann JS, Sandhu S, Maurya S. Evaluation of hardness and degree of conversion of different bulk-fill materials cured with Quartz Tungsten Halogen and light emitting diode devices. Saint Int Dent J [serial online] 2017 [cited 2023 Jun 5];3:4-10. Available from: https://www.sidj.org/text.asp?2017/3/1/4/230202

The success of dental composites in restorative dentistry stems from their good esthetic properties and adequate durability. The durability of restoration is dependent on the strength, hence the amount and quality of polymerization of the monomer. The quality of polymerization has been one of the most studied parameters since the development of composite resins polymerized by light. Thus, there is a need for light sources that promote an appropriate conversion of monomers into polymers, and hence the restoration has appropriate physical, chemical, and mechanical properties.^[1]

With advancement in technology more efficient and durable resin-based materials have evolved.^[2] These materials are now being used in posterior teeth to restore the function and shape. Recently, many clinicians have shown a preference for such time saving restorative procedures involving posterior resin applications. This led to the evolution of a new category of resins composites, known as bulk-fill resin composite. They were developed to speed up the restoration process by enabling up to 4 mm or more thick increments to be cured in a single step, thereby skipping the time-consuming layering process.^[3] Bulk-fill can be defined as composites that can be properly cured in a single layer of (mostly) a 4 mm thickness.^[4] They are also intended to minimize the shrinkage and the resulting stress using the same exposure time and light intensity used for the regular composites.^[5]

The clinical performance of composite resins is directly related to the degree of monomer conversion after photopolymerization.^[2] More the monomer conversion better is the strength hence the durability of the restoration. Different curing modes can influence the degree of conversion (DC) of the monomer into polymer. Appropriate energy density is important for obtaining a high DC. Energy density is determined using the curing light intensity and exposure time.^[6],[7] Curing light intensity is influenced by the light sources needed to cure the composites.

There are several light-curing units available including the Quartz Tungsten Halogen (QTH) light-curing units, plasma-arc lights and lights utilizing light-emitting diodes (LEDs). However, the optimal light-curing unit for curing of composites has not yet been determined. QTH units were introduced in 1980. They have been a common source of blue light for curing restorative composites. More recently, LED curing units have become commercially available that feature narrow spectral ranges which are highly efficient. The two lights used in this study were QTH (Dentsply) and LED (Satelec aceteon).

The assessment of the amount of polymerization that has taken place can be done either through direct methods or through indirect methods. Techniques such as Fourier transform infrared (FTIR) spectroscopy, RAMAN, electron paramagnetic resonance, nuclear magnetic resonance, differential scanning calorimetry 1, and differential thermal analysis have been used to determine the DC. Among these, FTIR is the most frequently used technique, which has been employed in this study.

The surface microhardness has been used to evaluate the efficiency of light cure unit and to evaluate the extent of polymerization indirectly.^[8] It has been observed that microhardness of composites decreased with increasing the depth of composite.^[9] The study by Coffey et al.^[10] showed that the top surfaces of resin composite materials show higher microhardness values compared to the bottom surfaces. According to ISO 4049:2000, to achieve the acceptable degree of polymerization composite resins have to meet the requirement of greater than or equal to 80% bottom/top ratio microhardness at 2 mm depth.^{[2],[11],[12]} There are several types of surface microhardness tests, Barcol, Brinnel, Rockwell, Shore, Knoop, and Vickers. In this study, Vickers hardness (VH) test has been employed to measure the surface microhardness.

Materials and Methods

A total of 180 samples of three different bulk-fill materials were prepared [Figure 1], 60 samples of each kind were used. Bulk-fill materials used were as follows:

Figure 1: Bulk-fill materials and instruments used in the study

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Filtek™, 3M ESPE, St. Paul, MN, USA
Tetric ^® N-Ceram, Ivoclar Vivadent, Schaan, Liechtenstein
Smart Dentin Replacement (SDR), Dentsply Caulk, Milford, DE, USA.

Out of 60 samples of each type of composite A, B, and C [Figure 2], [Figure 3], [Figure 4]:

Figure 2: Samples of Filtek Ceram prepared in Teflon mold of sizes 2 mm, 4 mm, and 6 mm (top surface marked with black marker)

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Figure 3: Samples of Tetric N Ceram prepared in Teflon mold of sizes 2 mm, 4 mm, and 6 mm (top surface marked with red marker)

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Figure 4: Samples of Smart Dentin Replacement prepared in Teflon mold of sizes 2 mm, 4 mm, and 6 mm (top surface marked with blue marker)

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Twenty samples were of 2 mm height
Twenty samples were of 4 mm height
Twenty samples were of 6 mm height.

From these, 10 samples each of height 2 mm, height 4 mm, and height 6 mm were separated.

Thus, it divided 60 samples of each type of composites A, B, and C into two sets of 30 samples. In this manner, 180 samples were divided into two parts of 90 samples each. One set of 90 samples were cured with the QTH device, and the other set of 90 samples were cured with LED device.

Experimental groups

The specimens were grouped as follows:

Specimens cured with the Quartz Tungsten Halogen device (90 samples)

Group I: Ten cylindrical samples of composite material A, B, and C, each with the 4 mm diameter and height 2 mm
Group II: Ten cylindrical samples of composite material A, B, and C, each with the 4 mm diameter and height 4 mm
Group III: Ten cylindrical samples of composite material A, B, and C, each with the 4 mm diameter and height 6 mm.

Specimens cured with the light-emitting diode device (90 samples)

Group IV: Ten cylindrical samples of composite material A, B, and C, each with the 4 mm diameter and height 2 mm
Group V: Ten cylindrical samples of composite material A, B, and C, each with the 4 mm diameter and height 4 mm
Group VI: Ten cylindrical samples of composite material A, B, and C, each with the 4 mm diameter and height 6 mm.

According to the manufacturer's instructions of each material used in this study, the energy density required for adequate curing ranged from 11 to 20 J/cm ². For this study, the energy density used was 14 J/cm ².

Energy density is determined using the curing light intensity and exposure time as follows:^[6],[7]

Energy density = light intensity (mW/cm ²) × time(s).

For the purpose of generating the same energy density, the irradiation time was set accordingly as shown in [Table 1].

Table 1: Calculation of same energy densities for the light sources used in the study

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Preparation of specimens

The specimen samples were prepared by placing the material in the cylindrical Teflon mold. The Teflon molds were prepared from the Teflon tube of internal diameter of 4 mm and external diameter of 6 mm, by cutting the tube at three different heights, namely, 2 mm, 4 mm, and 6 mm.

The selected 2 mm height is the established thickness of the composite utilized in incremental technique. The selected 4 mm layer is recommended by the manufacturers of the bulk fill composites, to be cured in one step.^[13] Although the manufacturer of Filtek claim up to the height of 5 mm. The selected 6 mm height served as extreme bulk-filling procedure, although not recommended by any of the manufacturers.^[14]

The heights were measured using the digital Vernier caliper scale. Teflon mold was positioned on transparent Mylar strip placed over a glass slide. Then, the mold was filled from bottom to top in a single increment with a Teflon coated instrument and then another Mylar strip was placed on the top of the Teflon mold filled with the tested material. An additional glass slide was placed over that, and 500 load was applied on the top surface to compact the material, to prevent bubble formation, to remove the excess material, and to obtain a smooth specimen surface.^[6] Then, the glass slide was removed, and the specimen was cured with curing light through the Mylar strip. The output of light intensity was continuously monitored with the help of radiometer (3H) to ensure a constant value of light intensity. After light curing, the specimens were stored in an opaque light proof container at room temperature for 24 h. An upper time limit of 24 h post exposure was chosen for measurements, as according to data, microhardness has been reported to be optimized by this time.^[10] Furthermore, 24 h storage allows polymer post cure effects which leads to increased double bond conversion. Dry storage is especially important before DC measurements using FTIR because any liquid storage medium could facilitate monomer elution resulting in artificially high DC values.^[14]

Set up of Vickers hardness machine

VH measurement is one of the most important parameters to compare the hardness of restorative materials and is defined as the resistance to indenter penetration or standing on the surface. It is a mechanical property that should always be taken into account especially when the restorations are to be placed in areas of high masticatory forces.^[1]

Vickers microhardness measurements were performed on both top and the bottom surface of the specimen with a microhardness tester (Mitutoyo, Japan) following the dry storage for 24 h in the light-proof box. A total of 300 g of the load was used for 15 s for each specimen to make indentations. Three readings for each specimen surface were carried out for the top as well as bottom surfaces and were independently averaged and reported in VH number by using the formula

VH = 1.854F/d ²

Where F is force (load) in kilogram-force and d is average diameter of indentation in mm.

Fourier transform infrared spectroscopy

Evaluation of the DC was done using a Fourier Transform Infrared Spectroscopy (Cary 630 FTIR, Agilent Technologies, Germany). For this technique, after 24 h of photoactivation, the specimens were pulverized into a fine powder. Five milligrams of ground powder was thoroughly mixed with 50 μg of KBr (potassium bromide) powder salt, and this mixture was placed into a pelleting device to obtain a pellet, which is then placed in the spectroscope. The FTIR spectra for both cured and uncured specimens were obtained. The measurements were recorded in absorbance mode operating under the following conditions: 32 scans, 2 cm ⁻¹ resolution, and a 600–3800 cm ⁻¹ wavelength. The percentage of unreacted carbon–carbon double bonds was determined from the ratio of the absorbance intensities of aliphatic C = C (peak at 1634 cm ⁻¹) against an internal standard before and after the curing of the specimen: aromatic C-C (peak at 1608 cm ⁻¹). The peak heights at different wavenumbers were calculated with the help of inbuilt software, and DC was calculated using following formula:

Evaluating criteria

Intergroup

Reading for VH were taken for three different bulk-fill RBCs at 2 mm, 4 mm, and 6 mm cured by QTH and LED at the:

Top surface
Bottom surface
% bottom/top ratio.

Intergroup

Readings for % DC were taken for three different bulk-fill RBCs at 2 mm, 4 mm, and 6 mm cured by the QTH and LED.

Results

The mean values for VH of various experimental groups with sample heights of 2mm, 4mm and 6mm respectively were obtained as shown in [Table 2], [Table 3], [Table 4]. VH was recorded for top surface and the bottom surface. Also the bottom/top % was calculated. Comparison of materials cured by QTH was done with materials cured with LED. Similarly the [Table 5], [Table 6], [Table 7] shows comparison of mean values of DC of various experimental groups used in the study. On comparing the mean values of both parameters (cured with QTH and LED) used in the study overall difference observed was statistically insignificant.

Table 2: The comparisons of mean values of Vickers hardness of Group I A, I B, and I C with IV A, IV B, and IV C respectively, standard deviation, and P values of three bulk-fill composite materials cured with light-emitting diode and Quartz Tungsten Halogen at the height of 2 mm

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Table 3: The comparisons of mean values of Vickers hardness of Group II A, II B, and II C with V A, V B, and V C respectively, standard deviation, and P values of three bulk-fill composite materials cured with light-emitting diode and Quartz Tungsten Halogen at the height of 4 mm

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Table 4: The comparisons of mean values of Vickers hardness of Group III A, III B, and III C with VI A, VI B, and VI C respectively, standard deviation, and P values of three bulk-fill composite materials cured with light-emitting diode and Quartz Tungsten Halogen at the height of 6 mm

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Table 5: The comparisons of mean values of percentage degree of conversion of Group I A, I B, and I C with IV A, IV B, and IV C, respectively, standard deviation, and P values of three bulk-fill composite materials cured with light emitting diode and Quartz Tungsten Halogen at the height of 2 mm

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Table 6: The comparisons of mean values of percentage degree of conversion of Group II A, II B, and II C with V A, V B, and V C, respectively, standard deviation, and P values of three bulk-fill composite materials cured with light-emitting diode and Quartz Tungsten Halogen at the height of 4 mm

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Table 7: The comparisons of mean values of percentage degree of conversion of Group III A, III B, and III C with VI A, VI B, and VI C, respectively, standard deviation, and P values of three bulk-fill composite materials cured with light emitting diode and Quartz Tungsten Halogen at the height of 6 mm

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Discussion

Overall on comparison of the VH for different materials at three different heights of 2, 4, and 6 mm along with bottom/top percentage, it was observed that the mean value of VH at various levels was more for LED as compared to the QTH. It can be seen in following graphs [Graph 1],[Graph 2],[Graph 3]. However, the difference was not statistically significant. This study has shown that when different composite materials are irradiated with the same energy density using two different light-curing units, there is no significant difference observed in surface hardness values obtained. In concurrence with the present study, Halvorson et al.^[15] showed that regardless of the light-curing mode, providing similar energy densities will result in similar microhardness. A similar conclusion was also depicted by Gomes et al.^[16] However, a study by Esmaeili et al.^[17] has concluded that different light cure units (QTH and LED) with same energy density do not result necessarily the same hardness number in composites.

The results obtained in the present study % DC ranged between 58.25% and 64.65% for 2 mm and 4 mm for both QTH and LED, these results lie within the acceptable range of 55%–75% as mentioned by Galvão et al.^[1] However, at 6 mm level, they ranged between 33% and 39% indicating the polymerization level below the acceptable range of 55%–75% as shown in [Graph 4]. The results of this study are in coherence with the study conducted by Halvorson et al.^[15] also a study by Nomoto et al.^[18] who had concluded that when similar energy densities are supplied to the composite resins, similar DC and depth of polymerization will be obtained regardless of the curing mode. Soares et al.^[19] in their study reported that for occlusal restorative layers DC values should be at least 55% which conceded with the results of the present study.

According to Anderson et al.^[20] LED produced higher % DC as compared to QTH. However, Lima et al.^[21] differed in the manner that in their study the QTH lamp promoted better values on the DC with the nanofilled composite resin than the LED lamp. However, in both the above studies, unlike the present study, same energy density had not been maintained for both the light-curing units. Ozturk et al.^[22] indicated that the QTH having higher total energy showed higher % DC value than LED having lower light energy. Therefore, the amount of total energy the composite receives is also a significant determinant for DC. The analysis of the above-mentioned results showed that the height of 4 mm should not be exceeded for the bulk fill materials whether cured by the QTH or by LED. At 6 mm height, the % DC for all the materials individually was insufficient for restorative procedures.

Conclusions

It was concluded that:

Among the three bulk-fill composites, Filtek showed the highest surface hardness value followed by Tetric N Ceram and SDR
Bulk-fill resin composites used in this study showed bottom/top hardness ratio of almost 80% both with QTH and LED light-curing units in 2 mm and 4 mm thick specimens. However in 6 mm specimens, bottom/top hardness ratio for both LED and QTH was 39% ± 2%, indicating insufficient hardness for the restorative purpose
The bulk-fill resin composites used in this study showed % DC within the range of 58%–65% at 2 mm and 4 mm heights for both QTH and LED light-curing units. However, at 6 mm height, the % DC ranged between 33% and 39% indicating polymerization level below the acceptable range of 55%–75%. Therefore, the acceptable increment for the bulk-fill resin composite should be limited to 4 mm and not extend to 6 mm
The energy density employed in this study was 14 J/cm ² for both light-curing units. When QTH and LED were irradiated with the same energy density all the three bulk-fill resin composites showed almost similar values for microhardness at the top surface, bottom surface, bottom/top percentage ratios and % DC at three levels of 2 mm, 4 mm, and 6 mm height. It can be concluded that irrespective of the type of the light cure unit if the energy density applied is same then almost similar performance can be expected from the devices.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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