Effect of Two Light Curing Intensities on Depth of Cure of Bulk Fill Composite.

Effect of Two Light Curing Intensities on Depth of Cure of Bulk Fill Composite.

Running title:

Evaluation of depth of cure of bulk fill composite

Clinical relevance:

Evaluating the depth of cure of tooth colored materials is essential to assure the quality of restorative procedures.

Summary:

The purpose of the study was to evaluate the effect of different shades and light intensities on depth of cure of bulk fill composite resins. Two intensities, 1000 mW/cm² and 3200 mW/cm², of an LED light curing unit (Valo/Ultradent) were used to polymerize shades A1 (L), and A3 (D) of SonicFill (SF) and Filtek Supreme Ultra (FSU) and shades IVA (L) and IVB (D) of Tetric EvoCeram (TE). Brass molds (10mm diameter) of two thicknesses, 2mm and 4mm, were filled with composite and cured for 10 seconds from the top. Knoop surface microhardness was measured for the top and bottom surfaces using a Tukon 2100B testing machine after 24 hours storage in the dark (n=10). Three readings of each surface were taken and bottom/top microhardness ratio was recorded. Acceptable depth of cure ratio (DOC) was at least 80%. Statistical analysis was done using SPSS and microhardness data were analyzed using multiple t-test and p<0.004 after Bonferroni correction.

Results: When comparing high (3200 mW/cm²) and low (1000 mW/cm²) light cure intensities at 2 mm thickness, FS and SF had a DOC above 80% (Ranges B/T=0.82-0.96) for both light intensities; TE was slightly below 80% (0.78-0.79) at 1000 mW/cm² but 0.90-0.95 at 3200 mW/cm². At 4 mm thickness, all materials were at 0.65 or less at 1000 mW/cm² (0.13-0.65), but each material was significantly increased at 3200 mW/cm² (0.30-0.80). Only SF in the light shade had a DOC of 80% at 4mm with 3200m W/cm² and could be an acceptable bulk cure material. For 4mm samples, lighter shades of composite had significantly greater DOC than darker shades but the range of values was low (0.13-0.72), except for SF at 3200 mW/cm², which reached 0.80 depth of cure ratio.

Conclusion: Depth of cure was enhanced by decreasing incremental thickness, using lighter shades, and increasing intensity of LED light curing units. None of bulk-fill materials reached 80% depth of cure at 4mm except SonicFill shade A1.

Introduction

The aim of using restorative dental material is to restore form and function of missing or defective tooth structure. Although, conventional direct restorative materials such as amalgam have a long-term clinical success, resin composite has become more popular for use among dentists ^1, 2. Resin composite can be defined as a three dimensional combination of two or more chemicals and composed of three phases: the organic phase or the matrix; the interfacial phase or the coupling agent; the dispersed phase or the filler phase.³ The initiator and accelerators are added to the polymeric matrix to control the reaction and classify composite resins into self cure (chemical cure), light cure, and dual cure. Several pigmentations are added to the matrix as well to produce composites with multiple shades.^3, 4 The most popular photo-initiator in light-curing resin systems is camphorquinone which has a maximum absorption ability when the wavelength of the activating light reaches 470nm. ^5, 6

Several light cure systems are available including: halogen, plasma arc, laser, and light-emitting diode (LED). ⁷ LED units are widely used and have some advantages over halogen light curing units. This includes and is not limited to: light is emitted at a specific wavelength so no filters or ventilating fans are required, has a high intensity output, does not generate high temperatures, has unceasing efficiency, and a long bulb life.⁸ Lately, light-emitting diode (LED) systems have been more popular and later generations of the LED light units have a poly-wave spectrum, which matches the absorption spectrum of most photo-initiators.^9,10

The deeper uncured portion of the composite fillings can lead to some problems.  It  can be dissolved in a short time which may affect the marginal integrity of the filling and subsequently lead to recurrent decay. It can also leach the unreacted monomer and produce tissue irritation.^11, 12 Sufficient polymerization of composite can be affected by several factors, such as: adequate light intensity and wavelength, light tip distance, exposure time, resin composite type, shade, and thickness.^13-18

Bulk fill composite materials have been introduced to overcome some of the disadvantages associated with the incremental technique of placing composite such as incorporating voids and extending the time required for placement.¹⁹ Several studies evaluated mechanical properties of bulk fill composite and have shown that bulk fill composites have marginal integrity, creep and shrinkage stresses comparable to the composite cured in increments.^20-22 Also, bulk fill composite showed reduced cuspal deflection compared to composites placed incrementally.^20, 21, 23 However, in general, their mechanical properties are inferior when compared to conventional composite. ^24,25 ,26  Manufacturers have also introduced high power light curing units that are capable of curing composite resin with much shorter exposure times. No study has reported the effect of these higher intensity light units on the bulk fill composite of different shades. The aim of this study was to evaluate the effect of different composite shades and curing light intensities on depth of cure of bulk fill composite resins. The null hypothesis tested was that the depth of cure is not affected by the shade or thickness of the composite or the light intensity

Material and Methods

 

Three commercially available resin composites with different shades were used (Table 1). Two intensities, 1000 mW/cm² and 3200 mW/cm², of an LED light curing unit (Valo/Ultradent, South Jordan, UT) were used to polymerize the composite. Depth of cure was assessed at two composite thicknesses (2 mm and 4 mm) using bottom to top hardness ratios. Ten samples (n=10) of each combination of shade, light intensity and thickness were fabricated and cured for 10 seconds. Two brass molds were fabricated, 10 mm diameter, 2 and 4 mm deep (Fig. 1).  The molds were cleaned with isopropyl alcohol gauze (70%) to remove any residues after every use.  A releasing agent (Al-Cote, Dentsply International, York, Pennsylvania) was applied to ease the removal of samples and prevent adhesion to the metal base. A clear plastic strip was placed over the metal base to prevent the formation of the resin rich layer after curing. The molds were attached together and secured using two metal screws. The composite was then placed in each hole of the brass molds and the excess was removed using a metal spatula. Another clear strip was positioned on top of the samples and a clean 1 mm thick glass slide was placed with finger pressure to allow standardized sample thickness and curing tip distance. To complete the assembly, two large clips were attached to the molds to hold the glass slides in place. Each sample was light cured according to study design for 10 seconds. The glass slide was removed immediately after curing, the brass plates were unscrewed, and the samples were pushed out using finger pressure.

Both sides of the light cured samples were polished for 60 seconds in an elliptical pattern with 600-grit wet silicon carbide paper (MAGER Scientific, Dexter, MI) to remove the resin-rich layer. A rotary grinder-polisher (Micro star 2000, Inc., Ontario, Canada) was used at a rotation speed of 102 rpm to polish the samples. With moderate to heavy finger pressure, the samples were polished with 800 grit silicon carbide paper (MAGER Scientific, Dexter, MI) for 3 minutes to achieve a glossy surface. Each sample was rinsed with tap water and kept in a small envelope in the dark for 24 hours at room temperature prior to testing.

After the storage period, a Tukon 2100B-testing machine (Wilson Instrument, Northwood, MA) was prepared and calibrated for the microhardness testing. Each sample was positioned on the stand and stabilized by tightening the screw below the platform. The platform maintained parallelism with the mounting stage in all directions. A magnification of 20X was used to visualize the surface of the sample by moving the lens up and down until focus was achieved. The lenses were then rotated to the Knoop diamond indenting tool and a 200 g load was applied to the sample surface with a dwell time of 15 seconds. The indentation was subsequently measured using 20X magnification by positioning cursors on both ends of the diagonal and pressing the button on the eyepiece of the hardness tester. If the indentation was larger than cursor’s length, the lenses were rotated and the measurements were done using 10X magnification.

The length of the long diagonal and the Knoop Hardness number appearing on the monitor were then transcribed to an Excel sheet. All measurements were made within a 4mm radius from the center of the specimen. Three hardness measurements were made on both the top and the bottom surfaces of each specimen and averaged. Hardness values (Knoop Hardness numbers) were calculated for each filar reading using the following formula:

KHN= L ÷ I x 2Cp

L represents the load applied in kilograms, I is the length of the long diagonal of indentation (mm) and Cp is a constant to the projected area of the indentation (0.07028).

The top/bottom ratio of each sample was calculated and the acceptable depth of cure was set at 80%.

Statistical analysis

The results were analyzed by using SPSS 20.0 (IBM, Armonk, New York).  Microhardness data and bottom to top ratio were analyzed using multiple t-test to determine statistically significant differences. Because of multiple groups involved, Bonferroni correction was applied and p value was p<0.004.

Results:

The bottom to top hardness ratio was evaluated for three commercially available resin composite materials, Filtek Supreme Ultra, SonicFill and Tetric EvoCeram. Ten samples (n=10) of each combination of light intensity, shade and thickness of each material were made and tested. Table 3 and Fig 6 show the mean and standard deviation of depth of cure ratio between light and dark shades of composite materials tested at both light intensities and thicknesses. At 2mm thickness and an intensity of 1000mW/cm², there was a significant difference in depth of cure between light and dark shades of Filtek Supreme Ultra (P < 0.001), although both were greater than 80% cured. On the other hand, SonicFill (P= 0.048) and Tetric EvoCeram (P=0.316) showed no significant difference at the same depth and intensity. At 2mm thickness and 3200mW/cm² there was no significant difference between the light and dark shades of SonicFill (P = 0.682) and Filtek Supreme Ultra (P = 0.004). However, a significant difference was found between dark and light shades of Tetric EvoCeram (P < 0.001). At 4mm thickness and both intensities, all materials showed a significant difference between dark and light shades (P < 0.001), and all were less than 80% cured.

Table 4 and figure 7 show mean and standard deviation of depth of cure between two intensities of light, 1000mW/cm² and 3200mW/cm², at the two thicknesses and the two shades of composites. Filtek Supreme Ultra showed a statistically significant difference in depth of cure when two light intensities were compared at all shade and thickness (P < 0.001). SonicFill showed a significant difference between the two light intensities only at 4 mm thickness (P < 0.001). Tetric EvoCeram showed significant difference between the two intensities except for the 4 mm dark shade (P < 0.001), all of which were less than 80% cured. Table 5 and Figure 8 show the mean and standard deviation for the depth of cure of the materials tested between 2 mm and 4 mm thickness samples at two different intensities and shades. All materials showed statistically significant difference in depth of cure ratio between 2 mm and 4 mm thicknesses when tested at both intensities and shades (P < 0.001). None of the bulk fill materials (SonicFill and Tetric EvoCeram) reach the 80% depth of cure ratio at 4 mm thickness regardless of the intensity and shade used except for SonicFill shade A1 at 3200 mW/cm².

Discussion:

In this study, the depth of cure was measured by bottom to top Knoop Hardness ratio. There was a statistically significant difference in depth of cure of bulk fill materials and it was affected by the shade, thickness and light intensity, thus, the null hypothesis was rejected. The manufacturers’ claim that the bulk fill materials used can cure for more than 4 mm thickness in 10 seconds and a light intensity of 1000 mW/cm² or more. The results of this study showed that none of the bulk fill materials cured to 4mm except SonicFill shade A1 at a high light intensity (3200mW/cm²).

Several methods have been introduced to measure depth of cure of resin composite.^27-29 Degree of Conversion analysis compares the percentage of carbon double bonds present in the composite before and after polymerization. Optical microscopy technique is done by determining the demarcation between cured and uncured resin with an explorer under a microscope. Scraping method requires a 10 mm sample which is cured and the soft material of the composite cylinder is removed after the polymerization. The remaining thickness is divided by two to calculate the depth of cure. Hardness testing can be done by determining the hardness at different depths and the depth of cure ratio is calculated by dividing the bottom to top hardness. Modifications of these testing methods were developed.^29, 30 Several studies compared different testing methods and showed that the degree of conversion and hardness testing correlated well together while the scraping method and the optical microscopy correlated well together but tend to over estimate the depth of cure. ^27, 28, 31

Hardness testing was used for this study after 24 hours storage to allow complete polymerization of the samples. The depth of cure judged to be adequate when it reached 80% or more. The variation in depth of cure values between different materials tested may be due to the variation in filler translucency, light scattering on the fillers, or the photoinitiator system used by the manufacturers.

Shade effect:

This study showed that darker shades of the Filtek Supreme Ultra and Sonic Fill cured less when compared to the lighter shades of the same material. The depth of cure pattern in relation to different shades of Tetric EvoCeram was not clear. Although the shading system is different for Tetric EvoCeram than the Vita shading system, both shades showed different performance when compared at similar light intensity and depth. Several studies compared the depth of cure between lighter and darker shades.^10, 29, 32 Most of these studies showed an increased depth of cure of the lighter shades compared to the darker shades which is in agreement with most of our results. Shortall, in 2005, showed that light shades of composite such as B1, cured less than the darker shades due to their higher opacity.¹⁸ That may explain the variation in curing behavior with Tetric EvoCeram used in our study, as it has three shades, IVA, IVB and IVW that have different opacities which may have affected the penetration of the light energy.

Our results showed that in general, composite materials showed statistically significant difference in depth of cure between high and low intensities. Several studies show that higher light intensities produce a greater depth of cure than lower intensities at the same shade and thickness.^13-15, 17 SonicFill showed some difference at the 2mm thicknesses at both intensities but did not reach a statistically significant difference. Darker shade of Tetric EvoCeram at 4mm showed no difference between the high and low intensities. This result does not have a clinical significance since both intensities did not cure Tetric EvoCeram to the clinically acceptable 80% ratio.

Several studies have been done with hardness testing using molds with one thickness and then the depth of cure was measured at different thicknesses after splitting each sample.^27, 28 Other studies used molds at set depths and the hardness measurements were taken at the center or near the center of each surface of the sample. ^17, 33 In this study, two brass molds with set thicknesses (2mm, 4mm) were fabricated to determine the depth of cure at the recommended 2mm increment for the regular composite and 4mm increment as claimed for the bulk fill composites. Our results showed that depth of cure is affected by the thickness of the sample at a set shade and light intensity. This is in agreement with other studies which showed that by increasing the thickness of the sample, insufficient light energy is delivered to the bottom of the sample.^16, 29, 31

Several studies have examined the depth of cure of bulk fill composites. Garoushi et al in 2013 showed that SonicFill and Tetric EvoCeram did not reach the curing depths claimed by the manufacturer.³⁴ However, other studies showed that SonicFill and Tetric EvoCeram cured to or more than the recommended depths when a 20 second exposure time was used.^22, 35 Our study showed that none of the materials tested reached the 80% depth of cure at 4 mm depth for both intensities and shades except the light shade of SonicFill. Tetric EvoCeram reached the 80% depth of cure ratio only at 2mm and 3200 mW/cm² and barely reached the acceptable ratio at 1000 mW/cm². This may possibly be explained by the new photoinitiator used (Ivocerin, a dibenzoyl germaniumcompound), which is reported to have a wider range of absorption spectrum, and is more sensitive to light than other photoinitiators. On the other hand, the manufacturer of the Valo light claims that the poly wavelength light can match the absorption spectrum of most of the photoinitiators. The variation in the results of Tetric EvoCeram at 2 and 4 mm may be due to a mismatch between the peak of the light wavelength and the absorption spectrum of the Ivocerin photoinitiator.³⁶

In general, bulk fill composites tested did not match the acceptable depth of cure that was claimed by the manufacturer. The composite material should be examined thoroughly for its effectiveness and ability to polymerize using available light curing units. More studies are recommended to examine the physical properties of these materials to determine the effect of under curing on material properties. Also, high intensities light units should be evaluated for their biological effect.

Conclusions:

Within the limitation of this study:

• Depth of cure was diminished by increasing incremental thickness.
•  Lighter shades of composites had greater depth of cure for all materials at 4 mm except for Tetric EvoCeram at 1000 mW/cm² in which the dark shade was greater.
• At 2 mm thickness, shade had no significant effect on depth of cure except for Filtek Supreme Ultra at 1000 mW/cm² and for Tetric EvoCeram at 3200mW/cm² in which the lighter shades had greater depth of cure.
• Increasing intensity from 1000 mW/cm² to 3200 mW/cm² improved the depth of cure at 2mm except for Sonicfill and at 4 mm except for Tetric EvoCeram.
•  None of the bulk-fill materials reached 80% depth of cure at 4 mm except SonicFill shade A1.

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Table 1: Composite materials used in the study.

Material	Filler wt.  %	Manufacture	Shade
Tetric EvoCeram	83%	Ivoclar Vivadent  (Amherst, NY)	IVA, IVB
SonicFill	80%	Kerr Manufacturer  (Orange, CA)	A1, A3
Filtek Supreme Ultra  (Control)	78.5%	3M/ESPE  (St. Paul, MN)	A1, A3

Table 2: Light curing unit and intensities used in the study.

Light cure unit	Intensities used	Manufacture
Valo	1000 mW/cm2  3200 mW/cm2	Ultradent  (South Jordan, UT)

Figure 1: Schematic view of the molds used in the study.