Photomedicine and Laser Surgery Volume 33, Number 4, 2015 ª Mary Ann Liebert, Inc. Pp. 213–219 DOI: 10.1089/pho.2014.3849
Mechanical Properties and Polymerization Shrinkage of Composite Resins Light-Cured Using Two Different Lasers Tae-Wan Kim, DDS, MS,1 Jang-Hoon Lee, DDS, MS,1 Seung-Hwa Jeong, DDS, PhD,2 Ching-Chang Ko, DDS, PhD,3 Hyung-Il Kim, DDS, PhD,1 and Yong Hoon Kwon, PhD1
Abstract
Objective: The purpose of the present study was to investigate the usefulness of 457 and 473 nm lasers for the curing of composite resins during the restoration of damaged tooth cavity. Background data: Monochromaticity and coherence are attractive features of laser compared with most other light sources. Better polymerization of composite resins can be expected. Materials and methods: Eight composite resins were light cured using these two lasers and a light-emitting diode (LED) light-curing unit (LCU). To evaluate the degrees of polymerization achieved, polymerization shrinkage and flexural and compressive properties were measured and compared. Results: Polymerization shrinkage values by 457 and 473 nm laser, and LED ranged from 10.9 to 26.8, from 13.2 to 26.1, and from 11.5 to 26.3 lm, respectively. The values by 457 nm laser was significantly different from those by 473 and LED LCU ( p < 0.05). However, there was no statistical difference between values by 473 and LED LCU. Before immersion in distilled water, flexural strength (FS) and compressive modulus (CM) of the specimens were inconsistently influenced by LCUs. On the other hand, flexural modulus (FM) and compressive strength (CS) were not significantly different for the three LCUs ( p > 0.05). For the tested LCUs, no specific LCU could consistently achieve highest strength and modulus from the specimens tested. Conclusions: Two lasers (457 and 473 nm) can polymerize composite resins to the level that LED LCU can achieve despite inconsistent trends of polymerization shrinkage and flexural and compressive properties of the tested specimens.
Introduction
L
ight-curing composite resins are popular restorative materials in dentistry for the filling of decayed or damaged teeth, because of their convenience, favorable aesthetics, and excellent mechanical properties that are compatible with the host tooth. Restoration involves a curing process of dental restorative materials within a tooth cavity through a polymerization process by formed free radicals, and the interaction between restorative material and irradiating light is important. Because there are so many available resin products and light-curing units (LCUs), the degree (quality) of polymerization may change, depending upon the selection of resin product and LCU. Therefore, depending upon the purpose, function, and mechanical properties, selection and the resultant outcomes can be changed with a wide spectrum. Polymerization is a process that converts monomers to polymers via the activation of an initiator by chemical 1 2 3
agents (benzoyl peroxide) or external light. Initiation by external light is started through the activation (optical excitation) of a photoinitiator (camphorquinone [CQ]) and subsequently accelerated by amines. The excited CQ gets protons from the excited amine accelerator through proton transfer, and then both become radicals. The radical CQ and amine accelerators bind with other monomer molecules to be stabilized. This process occurs through the chain reaction and then finally forms a polymer network. CQ absorbs blue light in the range of 380–500 nm, which is why all LCUs emit blue light. To activate CQ, three different LCU types have been introduced. Quartz–tungsten–halogen (QTH) LCUs are still widely used, although their usage is declining because of the advent of light-emitting diode (LED) LCUs, and emit light in a broad range (370–520 nm) in the absorption band of CQ.1–3 This transition from QTH LCUs to LED LCUs is reflected by dental school training programs.4,5 LED LCUs are popular because of their high light intensities, portability,
Department of Dental Materials, School of Dentistry and Biomedical Research Institute, Pusan National University, Yangsan, Korea. Department of Preventive and Community Dentistry, School of Dentistry, Pusan National University, Yangsan, Korea. Department of Orthodontics, School of Dentistry, University of North Carolina, Chapel Hill, North Carolina.
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Table 1. Materials Tested in the Present Study Code PR SD ZX AB CX Z2 PF TF
Composition
Filler content vol%/wt%1/wt%2
Bis-EMA, TEGDMA, Barium glass, SiO2 Bis-EMA, TEGDMA, UDMA, Barium glass, SiO2 Bis-EMA, Bis-GMA, TEGDMA, UDMA, Non-aggregated silica, zirconia/silica, nanoclusters Bis-EMA, TEGDMA, Glass filler, amorphous silica Bis-GMA, TEGDMA, Barium glass, silica, SiO2 Bis-EMA, Bis-GMA, TEGDMA, UDMA, Zirconia/silica Bis-EMA, TEGDMA, Barium glass, silica fillers Bis-EMA, UDMA, TEGDMA, Barium glass, YbF3, SiO2
71/84/77.1 65/80/75.2 63.3/78.5/74.7 55/76/71.0 70/86/83.2 60/84.5/80.4 54.6/72.5/64.4 39/63/61.9
Manufacturer Kerr Orange, CA Coltene/Whaledent Cuyaho Falls, OH 3M ESPE St. Paul, MN Bisco Inc. Schaumburg, IL Kuraray, Tokyo, Japan 3M ESPE St. Paul, MN Kerr Orange, CA Ivoclar Vivadent, Schann, Liechtenstein
wt%1, nominal weight percent provided by the manufacturers; wt%2, weight percent determined by the ash method; PR, Premise Packable; SD, Synergy D6; ZX, Filtek Z350XT; AB, Aelite All Purpose Body; CX, Clearfil AP-X; Z2, Filtek Z250; PF, Premise Flow; TF, Tetric N Flow; Bis-EMA, ethoxylated bisphenol A glycidyl methacrylate; Bis-GMA, bisphenol A glycidyl methacrylate; TEGDMA, triethyleneglycol dimethacrylate; UDMA, urethane dimethacrylate.
and extended lifetimes, and because they generate less heat.6–8 Although their emission spectra (430–500 nm) are narrower than those of QTH LCUs, using LED LCUs can be efficient because their emission band is focused on the absorption peak of CQ. Furthermore, of the recently developed LEDs, dual-peak LEDs have an additional emission peak near 405 nm, which is required for the activation of a coinitiator, such as, 2,2-dimethoxy (1,2) diphenyletanone (DMBZ) or 1-phenyl-1,2-propanedione (PPD).9–11 The argon laser provides another light source for light curing.12–14 Unlike QTH and LED LCUs, the argon laser has an extremely narrow emission band. One of the argon laser emission peaks, a blue 488 nm, overlaps with the absorption peak of CQ near the tail part. However, despite its many excellent features, the argon laser is expensive and has not been well adopted. As a substitution, diode-pumped solid state (DPSS) lasers can be chosen. Initially, DPSS laser was introduced as a light source for flow cytometers.15 DPSS lasers are portable and relatively cheap, and provide several emission options. However, these lasers were not developed for dental purposes, and few studies other than those of 473 nm have addressed their feasibilities in dentistry.16,17 A DPSS laser of 473 nm is a good light source for light curing of CQ-containing composite resins because of their similar emission and absorption bands, respectively. Also, among the diode lasers introduced recently, a laser of 457 nm is attractive because it has never been tested in dentistry, and its wavelength similarly coincides with the absorption peak of CQ. The purpose of the present study was to test the feasibility of two lasers, that is, 457 and 473 nm lasers, as a light source for the curing of composite resins. Polymerization shrinkage and flexural and compressive properties were evaluated. The hypothesis to be tested is that lasers of 457 and 473 nm polymerize composite resins to the level that LED LCU can achieve.
Materials and Methods Composite resins and LCUs
For the study, eight different dental composite resins [three nanocomposite resins: Premise Packable (PR), Synergy D6 (SD), and Filtek Z350XT (ZX); three microhybrid resins: Aelite All Purpose Body (AB), Clearfil AP-X (CX), and Filtek Z250 (Z2); two flowable resins: Premise Flow (PF) and Tetric N Flow (TF)] were used. Their details are listed in Table 1. For light curing, one LED (L.E. Demetron, Kerr, Danbury, CT) and two lasers (LVI Technology, Seoul, Korea) of 457 and 473 nm were used. The emission spectra of LCUs were measured using a photodiode array detector (M1420, EG&G PARC, Princeton, NJ) connected to a spectrometer (SpectroPro-500, Acton Research, Acton, MA) (Fig. 1). The output light intensity of LED was 900 mW/cm2 as measured using a built-in radiometer. The output power and spot size of the laser beam were 150 mW (PM3/FIELDMAX, Coherent, Portland, OR) and 6 mm, respectively, and its resultant light intensity was *530 mW/cm2. The original laser beam was expanded to 6 mm using a beam expander (Thorlabs Inc., Newton, NJ). Polymerization shrinkage testing
The polymerization shrinkages of specimens during and after curing were measured (n = 7 for each product) using a linometer (RB 404, R&B Inc., Daejon, Korea). A resin cylinder (diameter 4 mm, thickness 2 mm) was placed over an aluminum disc (the specimen stage of the measurement system) and its top surface was flattened by a glass slide. The end of the light guide of LED was placed in contact with the glass slide. Before light curing, the initial position of the aluminum disc was set to zero (in the case of the laser, the guided light using a mirror was irradiated over the specimen with a 90 degree angle without an aid of fiber
COMPOSITE RESINS LIGHT-CURED USING TWO DIFFERENT LASERS
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FIG. 1. Emission spectra of the light-curing units (LCUs) used, and the absorption spectrum of camphorquinone (CQ).
optics). The light was irradiated from LCU for 40 sec. As resin polymerized, it shrank and the base aluminum disc moved upward. The amount of disc displacement that occurred because of polymerization shrinkage was measured automatically for 130 sec using a noncontacting inductive sensor placed below the aluminum disc. The resolution and measurement range of the sensor were 0.1 and 100 lm, respectively. Three-point bending test
A three-point bending test was performed to determine the resin flexural properties [flexural strength (FS) and modulus (FM)]. A metal mold (25 · 2 · 2) was filled with resin, according to ISO 4049.18 Top and bottom resin surfaces were covered with glass slides to flatten surfaces. Specimens were irradiated for 40 sec by each LCU. The end of the light guide of LED was placed in contact with the glass slide. Because specimens were much wider (25 mm) than the tip or expanded laser beam size (7 or 6 mm), five overlapping exposures were performed per side. After curing, specimens were removed from molds and aged for 24 h in a 37C dry dark chamber. Other specimens were immersed in distilled water and stored in a 37C dry, dark chamber for 2 weeks. After aging (n = 7) or immersion (n = 7), specimens were loaded into a universal test machine (Instron 3345, Grove City, PA), which was operated at a crosshead speed of 1 mm/min. FS (rf in MPa) was obtained using the following formula rf ¼ 3DP=(2WH2 ) where D is the distance between supports (20 mm), P is the maximum failure load (N), W is the width (2 mm), and H is the height (2 mm) of the tested specimen. FM (E in GPa) was calculated using E ¼ (P=D) · [D3 =(4WH3 )] where P/D is the slope of the linear portion of the loaddisplacement curve.
Compression test
To measure compressive properties [compressive strength (CS) and compressive modulus (CM)], specimens were prepared using a hollow metal mold (3 mm in diameter and 6 mm high). The metal mold was composed of two identical hollow hemicylinders. After filling the mold, both the top and bottom surfaces were covered with glass slides to make the surface flat, and then irradiated for 5 sec (because light does not reach to the bottom surface, light curing would be better through the lateral surface after exposure). Subsequently, one part of the metal mold was removed by sliding. The exposed surface was light cured for 40 sec. The opposite side was also light cured for 40 sec again after removing the other part. After light curing, specimens were randomly removed from the mold and aged for 24 h in a 37C dry, dark chamber. Some other specimens were immersed in distilled water and kept in a 37C dry, dark chamber for 2 weeks. After aging (n = 7) or immersion (n = 7), the specimens were loaded to a universal test machine for compression tests at a crosshead speed of 1 mm/min. CS (rc in MPa) was obtained using the following formula rc ¼ P=A where P is the maximum failure load (N) and A is specimen cross-sectional area. CM (in GPa) was defined as the slope of the linear portion of the load-displacement curve. Statistical analysis
Test results were analyzed by two-way ANOVA for LCU and resin product. A post-hoc Tukey test was then followed for a multiple comparison. The student’s t test was used to analyze results after immersion in distilled water. Statistical significance was accepted for p values < 0.05. All the statistical analysis was performed by SPSS software (IBM SPSS, Version 21, New York, NY). Results
Table 2 and Fig. 2 show the polymerization shrinkage values and profiles of the specimens, respectively. Overall
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Table 2. Polymerization Shrinkage of the Specimens Cured Using Different LCUs
PR1 SD2 ZX3 AB4 CX3 Z23 PF5 TF5 p Value
457A
473B
LEDB
10.9 – 1.1 15.7 – 0.4 12.6 – 0.8 21.2 – 1.3 13.3 – 2.2 12.2 – 1.0 28.2 – 1.3 30.1 – 1.3 a = 0.002
13.2 – 0.4 16.5 – 0.5 14.2 – 0.4 23.6 – 2.0 14.4 – 1.2 15.0 – 0.1 27.3 – 2.4 25.2 – 0.5 b < 0.001
11.5 – 1.0 16.3 – 1.0 15.5 – 0.5 21.0 – 0.7 13.6 – 0.5 13.6 – 0.3 22.7 – 1.1 25.2 – 1.1 a · b = 0.013
Statistically significant difference in light-curing units (LCUs) is shown by superscript letters, and statistically significant difference in resin product is shown by superscript numbers. Items with the same letters or numbers are not significantly different ( p > 0.05). For p values, the letters a and b denote LCUs and resin products, respectively. PR, Premise Packable; SD, Synergy D6; ZX, Filtek Z350XT; AB, Aelite All Purpose Body; CX, Clearfil AP-X; Z2, Filtek Z250; PF, Premise Flow; TF, Tetric N Flow.
values ranged 10.0–26.8 lm depending on LCU and resin product. Regardless of condition, flowable resins had the highest shrinkages. Specimens light cured using the 473 nm laser had significantly different ( p < 0.05) shrinkage values, although differences between absolute values were not high. Shrinkages were found to be inverse linearly correlated with filler content regardless of LCU (R = 0.73–0.90, 0.83–0.95, 0.83–0.90 for vol%, nominal wt%, ash method wt%, respectively). In Fig. 2, there are profiles of only two products, but all the tested products have the same shrinkage profile trend. Table 3 shows the flexural properties (FS and FM) of specimens before and after immersion in distilled water for 2 weeks. Before immersion, overall FS and FM values ranged from 110.7 to 185.7 MPa and from 5.71 to 24.75 GPa, respectively. However, after immersion, they ranged from 81.9 to 164.8 MPa and from 3.61 to 21.85 GPa, respectively. The
influence of LCU type on the obtained data had no consistent trend. The linear correlation between FS and filler content was R = 0.01–0.63, and that of FM was R = 0.63– 0.95 depending upon LCU. Table 4 shows CS and CM values. Before immersion, CS and CM ranged from 277.8 to 408.2 MPa and from 3.25 to 5.37 GPa, respectively, depending upon LCU and resin brand. After immersion, they changed to 275.8–431.9 MPa and 2.89–5.19 GPa, respectively. CS values for different LCUs were not significantly different regardless of immersion ( p > 0.05). CM values were mostly lowered after immersion. CS dependence on filler content was R = 0.14–0.61, and that of CM was R = 0.44–0.98 depending upon LCU. Discussion
Polymerization involves an interaction between externally irradiated light and the host resin. Because the light curing of composite resins initiates polymerization by activating a photoinitiator (CQ), the characteristics of the light used are important. The three light sources that were tested have different spectral distributions. The two lasers have a very narrow bandwidth compared with LED light, and have emission peaks at 457 and 473 nm, respectively, which are close to the absorption peak of CQ. On the other hand, LED light has a much wider spectral distribution and degree of overlap with the absorption peak of CQ. Laser is also highly coherent compared with LED light. Such natures (monochromaticity and coherence) would be beneficial to the lasers even those whose light intensity is *40% lower than that of LED light. The two lasers differ only in terms of their peak wavelength. From the obtained data, the hypothesis tested has to be accepted, because lasers of 457 and 473 nm can polymerize composite resins to the level that LED LCU achieves. Polymerization shrinkage occurs as monomers convert to polymer network by changing the governing force between molecules from van der Waals to covalents; therefore, polymerization shrinkage is an inherent feature of dimethacrylatebased composite resins, and it produces many undesirable side effects.19–21 In the present study, wavelength was found to
FIG. 2. Polymerization shrinkage profiles of the specimens cured with different light-curing units (LCUs). All the tested resin products showed the same profile trend depending on the laser and LED LCU.
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115.3 – 13.7 120.1 – 6.0 149.6 – 5.5 119.2 – 19.3 142.9 – 17.7 140.8 – 13.6 145.2 – 15.6 132.5 – 10.7 a < 0.001
110.7 – 12.1 142.6 – 11.5 173.4 – 11.5 128.2 – 25.0 159.8 – 21.0 174.6 – 19.7 120.3 – 14.4 121.4 – 4.1 b < 0.001
473AB 116.1 – 8.6 130.0 – 5.4 164.0 – 8.8 149.5 – 10.7 185.7 – 12.1 162.9 – 13.4 131.5 – 9.9 130.1 – 10.4 a · b < 0.001
LEDB PR1 SD1 ZX2 AB1 CX3 Z24 PF5 TF6 12.56 – 0.77 11.25 – 1.32 14.23 – 0.61 11.51 – 0.89 20.79 – 0.91 17.06 – 1.46 8.47 – 0.82 6.49 – 0.47 a = 0.065
457A 12.10 – 0.71 11.32 – 0.60 14.86 – 0.68 13.36 – 0.55 24.75 – 1.78 16.17 – 1.85 8.06 – 0.31 5.71 – 0.17 b < 0.001
473A
FM{
12.66 – 0.78 13.59 – 0.77 15.25 – 0.21 10.62 – 0.60 20.49 – 1.21 15.66 – 1.60 9.09 – 0.24 7.40 – 0.26 a · b < 0.001
LEDA PR1 SD12 ZX12 AB2 CX3 Z23 PF2 TF12 91.8 – 6.6 106.8 – 8.8 100.8 – 11.7 114.8 – 24.4 139.1 – 19.8 133.8 – 17.1 111.4 – 11.4 107.9 – 7.12 a = 0.016
457AB 81.9 – 11.3 85.6 – 9.7 114.0 – 25.0 100.0 – 4.9 131.9 – 14.1 130.1 – 16.1 106.7 – 8.26 101.2 – 9.7 b < 0.001
473A
FS{
95.7 – 8.7 108.6 – 16.8 90.6 – 9.5 115.6 – 10.4 164.8 – 9.8 135.2 – 11.8 107.0 – 6.7 99.0 – 10.3 a · b = 0.003
LEDB PR1 SD2 ZX3 AB2 CX4 Z25 PF6 TF7
457A 9.90 – 0.73 9.94 – 8.80 11.64 – 1.43 10.06 – 0.71 18.68 – 0.76 13.45 – 1.23 7.80 – 0.29 4.99 – 0.52 a = 0.015
DW-2 weeks
7.93 – 0.21 9.02 – 0.35 11.68 – 1.28 10.34 – 0.33 21.85 – 1.72 10.96 – 0.20 7.56 – 0.40 3.61 – 0.11 b < 0.001
473B
FMx
9.28 – 0.42 10.77 – 0.23 11.02 – 0.56 10.31 – 0.31 20.35 – 1.50 13.32 – 0.38 7.25 – 0.39 4.14 – 0.20 a · b < 0.001
LEDA
339.5 – 29.7 307.4 – 43.6 359.8 – 30.5 340.4 – 47.9 353.7 – 50.4 408.2 – 57.1 329.2 – 36.2 299.8 – 45.5 a = 0.086
334.8 – 30.9 319.4 – 50.3 315.0 – 46.8 347.9 – 61.0 357.3 – 50.5 329.5 – 56.9 296.0 – 47.5 337.9 – 54.4 b = 0.053
473A 352.7 – 14.8 305.9 – 31.5 338.4 – 19.4 277.8 – 39.7 328.7 – 42.9 317.3 – 41.5 351.8 – 25.8 295.5 – 24.1 a · b = 0.015
LEDA PR1 SD2 ZX3 AB12 CX4 Z23 PF5 TF5 4.20 – 0.15 4.27 – 0.16 4.63 – 0.10 4.20 – 0.20 5.14 – 0.61 4.80 – 0.11 3.59 – 0.11 3.35 – 0.14 a < 0.001
457A
LEDC
3.93 – 0.28 3.72 – 0.26 4.17 – 0.16 4.19 – 0.06 4.92 – 0.11 4.54 – 0.05 4.39 – 0.10 3.70 – 0.26 5.37 – 0.15 4.90 – 0.11 4.94 – 0.10 4.55 – 0.10 3.25 – 0.51 3.48 – 0.31 3.35 – 0.16 3.25 – 0.20 b < 0.001 a · b < 0.001
473B
CM{
PR12 SD1 ZX14 AB14 CX3 Z224 PF24 TF12
332.6 – 16.1 275.8 – 34.5 307.6 – 38.4 291.8 – 43.0 400.9 – 27.0 407.0 – 31.8 344.3 – 28.7 287.8 – 50.8 a = 0.585
457A
294.9 – 38.7 313.8 – 32.6 322.9 – 41.5 366.3 – 26.7 431.9 – 40.4 300.5 – 22.4 319.8 – 32.0 328.1 – 16.4 b < 0.001
473A
CS{
322.6 – 20.7 345.1 – 42.8 347.0 – 26.4 332.4 – 47.6 332.2 – 47.8 327.0 – 27.0 357.3 – 22.4 352.7 – 26.2 a · b < 0.001
LEDA PR1 SD2 ZX3 AB2 CX4 Z23 PF1 TF5
457A 3.91 – 0.17 4.11 – 0.13 4.29 – 0.16 3.96 – 0.12 5.19 – 0.15 4.57 – 0.15 3.28 – 0.23 2.89 – 0.22 a < 0.001
DW-2 weeks
LEDB 3.09 – 0.43 3.23 – 0.30 3.83 – 0.22 4.01 – 0.14 4.54 – 0.12 3.86 – 0.15 4.05 – 0.14 3.79 – 0.17 5.03 – 0.17 4.93 – 0.04 4.63 – 0.03 4.01 – 0.28 3.47 – 0.14 3.28 – 0.13 2.92 – 0.10 2.98 – 0.09 b < 0.001 a · b < 0.001
473A
CMx
Statistically significant difference in light-curing units (LCUs) is shown by superscript letters, and statistically significant difference in resin products is shown by superscript numbers. For the t test, statistical results are represented by symbols (e.g., * or {). Items with the same letters or numbers are not significantly different ( p > 0.05). For p values, the letters a and b denote light-curing unit and resin product, respectively. DW, distilled water; CS, compressive strength; CM, compressive modulus; PR, Premise Packable; SD, Synergy D6; ZX, Filtek Z350XT; AB, Aelite All Purpose Body; CX, Clearfil AP-X; Z2, Filtek Z250; PF, Premise Flow; TF, Tetric N Flow.
PR1 SD1 ZX1 AB1 CX1 Z21 PF1 TF1 p Value
457A
CS*
24 h
Table 4. Compressive Properties of the Specimens Prepared Using Different LCUs
Statistically significant difference in light-curing units (LCUs) is shown by superscript letters, and statistically significant difference in resin products is shown by superscript numbers. For the t test, statistical results are represented by symbols (e.g., * or {). Items with the same letters or numbers are not significantly different ( p > 0.05). For p values, the letters a and b denote LCUs and resin products, respectively. DW, distilled water; FS, flexural strength; FM, flexural modulus; PR, Premise Packable; SD, Synergy D6; ZX, Filtek Z350XT; AB, Aelite All Purpose Body; CX, Clearfil AP-X; Z2, Filtek Z250; PF, Premise Flow; TF, Tetric N Flow.
PR1 SD2 ZX3 AB2 CX3 Z23 PF2 TF12 p Value
457A
FS*
24 h
Table 3. Flexural Properties of the Specimens Prepared Using Different LCUs
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affect polymerization shrinkages because the resultant polymerization shrinkage values by 457 and 473 nm lasers were significantly different, despite their similarity in spectral width. Apparent differences between the lasers and LED LCU were the light intensities used (530 vs. 900 mW/cm2) and spectral width. However, the similar polymerization shrinkages of the resin products tested suggest that the lasers can achieve a similar degree of polymerization as to LED LCU, with much lower light intensity. The high linear correlation observed between polymerization shrinkage and filler content regardless of LCU could provide a useful means of estimating shrinkages. Because polymerization shrinkage is the result of monomers’ conversion to polymer, high filler content, that is low monomer content, favors less shrinkage. In the shrinkage after 40 sec of light curing, specimens showed slightly different shrinkage profiles depending upon the LCU. The specimens light cured using the lasers showed a smoother profile than those light cured using LED. The difference between profiles is the result of the difference of light delivery. During 40 sec light irradiation, in the case of LED, photons and heat from the light source are transmitted through the 5–10 cm long fiber optic. On the other hand, in this study, laser was delivered without fiber optic, and no heat was transmitted to the specimen. If the light guide is in contact with the top surface of the specimen, both shrinkage and expansion caused by polymerization and heat, respectively, occur simultaneously. In this situation, some of the polymerization shrinkage can be suppressed by the thermal expansion of the specimen. However, immediately after the termination of light irradiation, the suppression by the thermal expansion will be relieved because of the missing heat source. More apparent shrinkage profile in the LED-treated specimens after 40 sec light curing should be the result of the absence of thermal expansion caused by the heat. According to reports, the temperature rise on the composite resins reaches to the point that feels hotness, because of exothermic heat by polymerization and conducted heat from the LED unit through the fiber optic.22,23 A three-point bending test provides a means of determining the FS and FM values of specimens, which are measures of the ability of a material to resist extended external stress without fracture. In the present study, before immersion, specimens had FS values ranging from 115.3 to 145.2 MPa, from 110.7 to 174.6 MPa, and from 116.1 to 185.7 MPa for 457 nm, 473 nm, and LED LCU, respectively. For the same resins, FS values were not consistently different among LCUs. After immersion, the initial FS values decreased by 2.7–32.6%, 11.3–40.0%, and 11.3–44.8% for 457 nm, 473 nm, and LED LCU, respectively. FM values had no consistent dependence on LCU; however, the values were consistently reduced after immersion. The FM values obtained before immersion ranged from *6 to 25 GPa, and, therefore, appeared to be lower or compatible with the FM of dentine, which is 17–25 GPa.24–26 Inconsistent FS and FM values may be partly caused by the overlapping exposure of specimens, which was required because specimen sizes as required by ISO 4049 were much larger than the tip or spot sizes of the LCUs tested. During overlapping exposures, neighboring regions are weakly exposed to the diverging light, and it initiates incomplete
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polymerization. Such weakly polymerized regions are not fully polymerized by second exposure. In addition, the mold-filling procedure could produce inconsistencies in flexural properties. Because the mold was 25 mm long, several fillings were needed. During this process, uneven or loose regions can occur, and these can cause inconsistencies. The poor correlation between FS and filler content could indicate that filler content was not the principal determinant of FS. Generally, FS is known to depend more on material volume and internal defects, such as cracks or voids generated during the manufacturing or specimen preparation processes.27–29 CS and CM are measures of a material’s ability to resist sustained longitudinal heavy loads during mastication.30 Tested specimens showed higher CS values (*278– 408 MPa) than many other composite resins tested (based on > 70 tested resins, which have CS values of 100– 290 MPa).31 In the present study, CS values obtained using different LCUs were not statistically different, and found to be poorly correlated with filler content. However, CM showed a strong correlation with filler weight (as determined by nominal wt% or ash method wt%) in the range from 0.68 to 0.98, depending upon the LCU used. Because the modulus is related to material stiffness, materials with high filler contents might be expected to have high modulus.32 The tested resin products showed a strong linear correlation (0.86–0.97) between their FM and CM values regardless of LCU, which could be useful for estimation when the FM or CM is known. On the other hand, no such correlation was found between FS and CS values, which in the present study, before and after immersion, ranged from 3.61 to 24.75 GPa and from 2.89 to 5.37 GPa, respectively. Nevertheless, these values are lower than those of dentin (17–25 GPa and 11–19 GPa for FM and CM, respectively), which means that they would protect the underlying dentin from external heavy loads.24,33,34 The results of the present study raise the question of whether monochromatic or polychromatic light is better for the polymerization of composite resins. To answer this question, studies at much lower light intensities are needed to evaluate the advantages of coherent monochromatic light versus incoherent polychromatic light, with respect to the mechanical properties of composite resins. Because now LED is definitely efficient and effective as an LCU, studies, such as those pertaining to degree of conversion, temperature rise, or color stability, other than the present one, will also be further discussed in forthcoming articles to elucidate the feasibility of two lasers as a light source for light curing composite resins. Conclusions
In many cases, the polymerization shrinkage and flexural and compressive properties of the tested resin products showed inconsistent trends with respect to the LCU used. However, the similar mechanical properties from the tested resin products may suggest that the two lasers (457 and 473 nm) can cure composite resins to the level that LED LCU can achieve. Author Disclosure Statement
No competing financial interests exist.
COMPOSITE RESINS LIGHT-CURED USING TWO DIFFERENT LASERS References
1. Faria-E-Silva AL, Casselli H, Martins LR. Effect of curing unit and adhesive system on marginal adaptation of composite restorations. Gen Dent 2012;60:e408–e412. 2. Khosla M, Malhotra N, Mala K. An in vitro evaluation of shear bond strength of silorane and bis-GMA resin-based composite using different curing units. J Conserv Dent 2012;15:278–282. 3. Marchan SM, White D, Smith WA, Raman V, Coldero L, Dhuru V. Effect of reduced exposure times on the microhardness of nanocomposites polymerized by QTH and secondgeneration LED curing lights. Oper Dent 2011;36:98–103. 4. Lynch CD, Frazier KB, McConnell RJ, Blum IR, Wilson NH. Minimally invasive management of dental caries: contemporary teaching of posterior resin composites in U.S. and Canadian dental schools. J Am Dent Assoc 2011;142:612–620. 5. Lynch CD, Frazier KB, McConnell RJ, Blum IR, Wilson NH. State–of-the art techniques in operative dentistry: contemporary teaching of posterior composites in UK and Irish dental schools. Brit Dent J 2010;209:129–136. 6. Polydorou O, Ko¨nig A, Altenburger MJ, Wolkewitz M, Hellwig E, Ku¨mmerer K. Release of monomers from four different composite materials after halogen and LED curing. Am J Dent 2011;24:315–321. 7. Alomari QD, Barrieshi–Nusair K, Ali M. Effect of C-factor and LED curing mode on microleakage of class V resin composite restorations. Eur J Dent 2011;5:400–408. 8. Rencz A, Hickel R, Ilie N. Curing efficiency of modern LED units. Clin Oral Investig 2012;16:173–179. 9. Sim JS, Seol HJ, Park JK, Garcia–Godoy F, Kim HI, Kwon YH. Interaction of LED light with coinitiator-containing composite resins: effect of dual peaks. J Dent 2012;40:836–842. 10. Santini A, Miletic V, Swift MD, Bradley M. Degree of conversion and microhardness of TPO-containing resinbased composites cured by polywave and monowave LED units. J Dent 2012;40:577–584. 11. Schneider LF, Consani S, Sakaguchi RL, Ferracane JL. Alternative photoinitiator system reduces the rate of stress development without compromising the final properties of the dental composite. Dent Mater 2009;25:566–572. 12. Turbino ML, Belan LC, Soprano V, Rode KM, Lloret PR, Youssef MN. Argon ion laser curing depth effect on a composite resin. Lasers Med Sci 2011;26:421–425. 13. Mirsasaani SS, Atai MM, Hasani–Sadrabadi MM. Photopolymerization of a dental nanocomposite as restorative material using the argon laser. Lasers Med Sci 2011;26:553–561. 14. Tielemans M, Compere P, Geerts SO, et al. Comparison of microleakages of photo-cured composites using three different light sources: halogen lamp, LED and argon laser: an in vitro study. Lasers Med Sci 2009;24:1–5. 15. Habbersett RC, Naivar MA, Woods TA, Goddard GR, Graves SW. Evaluation of a green laser pointer for flow cytometry. Cytometry A 2007;71:809–817. 16. Kwon, YH, Jang CM, Shin DH, Seol HJ, Kim HI. The applicability of DPSS laser for light curing of composite resins. Lasers Med Sci 2008;23:407–414. 17. Heo YJ, Lee GH, Park JK, et al. Effect of energy density on low-shrinkage composite resins: diode-pumped solid state laser versus quartz-tungsten-halogen light-curing unit. Photomed Laser Surg 2013;31:28–35.
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18. ISO 4049: Dentistry-polymer-based filling, restorative and luting materials. Geneva:International Organization for Standardization, 2000. 19. Kemp–Scholte CM, Davidson CL. Marginal sealing of curing contraction gaps in class V composite resin restorations. J Dent Res 1988;67:841–845. 20. Lai JH, Johnson AE. Measuring polymerization shrinkage of photo-activated restorative materials by a water-filled dilatometer. Dent Mater 1993;9:139–143. 21. Davidson CL, Feilzer AJ. Polymerization shrinkage and polymerization shrinkage stress in polymer-based restoratives. J Dent 1997;25:435–440. 22. Knezevic´ A, Tarle Z, Meniga A, Sutalo J, Pichler G. Influence of light intensity from different curing units upon composite temperature rise. J Oral Rehabil 2005;32:362– 367. 23. Hofmann N, Markert T, Hugo B, Klaiber B. Effect of high intensity vs. soft-start halogen irradiation on light-cured resin-based composites. Part I. Temperature rise and polymerization shrinkage. Am J Dent 2003;16:421–430. 24. Kinney JH, Marshall SJ, Marshall GW. The mechanical properties of human dentin: a critical review and re-evaluation of the dental literature. Crit Rev Oral Biol Med 2003;14:13–29. 25. Xu HH, Smith DT, Jahanmir S, Romberg E, Kelly JR, Thompson VP, Rekow ED. Indentation damage and mechanical properties of human enamel and dentin. J Dent Res 1998;77:472–480. 26. Mahoney E, Holt A, Swain M, Kilpatrick N. The hardness and modulus of elasticity of primary molar teeth: an ultramicro-indentation study. J Dent 2000;28:589–594. 27. Zeng K, Ode´n A, Rowcliffe D. Flexure tests on dental ceramics. Int J Prosthodont 1996;9:434–439. 28. Della Bona A, Anusavice KJ, DeHoff PH. Weibull analysis and flexural strength of hot-pressed core and veneered ceramic structure. Dent Mater 2003;19:662–669. 29. Mecholsky JJ Jr. Fracture mechanics principles. Dent Mater 1995;11:111–112. 30. Ilie N, Hickel R. Investigations on mechanical behaviour of dental composites. Clin Oral Investig 2009;13:427–438. 31. Anusavice KJ. Phillips’ science of dental materials. St. Louis: W.B. Saunders Co., 2003. 32. Park JK, Lee GH, Kim JH, et al. Polymerization shrinkage, flexural and compression properties of low-shrinkage dental resin composites. Dent Mater J 2014;33:104–110. 33. Craig RG, Peyton FA. Elastic and mechanical properties of human dentin. J Dent Res 1958;37:710–718. 34. Watts DC, el Mowafy OM, Grant AA. Temperature-dependence of compressive properties of human dentin. J Dent Res 1987;66:29–32.
Address correspondence to: Yong Hoon Kwon Department of Dental Materials School of Dentistry and Biomedical Research Institute Pusan National University Yangsan 626-870 Korea E-mail: [email protected]
Mechanical Properties and Polymerization Shrinkage of Composite Resins Light-Cured Using Two Different Lasers Tae-Wan Kim, DDS, MS,1 Jang-Hoon Lee, DDS, MS,1 Seung-Hwa Jeong, DDS, PhD,2 Ching-Chang Ko, DDS, PhD,3 Hyung-Il Kim, DDS, PhD,1 and Yong Hoon Kwon, PhD1
Abstract
Objective: The purpose of the present study was to investigate the usefulness of 457 and 473 nm lasers for the curing of composite resins during the restoration of damaged tooth cavity. Background data: Monochromaticity and coherence are attractive features of laser compared with most other light sources. Better polymerization of composite resins can be expected. Materials and methods: Eight composite resins were light cured using these two lasers and a light-emitting diode (LED) light-curing unit (LCU). To evaluate the degrees of polymerization achieved, polymerization shrinkage and flexural and compressive properties were measured and compared. Results: Polymerization shrinkage values by 457 and 473 nm laser, and LED ranged from 10.9 to 26.8, from 13.2 to 26.1, and from 11.5 to 26.3 lm, respectively. The values by 457 nm laser was significantly different from those by 473 and LED LCU ( p < 0.05). However, there was no statistical difference between values by 473 and LED LCU. Before immersion in distilled water, flexural strength (FS) and compressive modulus (CM) of the specimens were inconsistently influenced by LCUs. On the other hand, flexural modulus (FM) and compressive strength (CS) were not significantly different for the three LCUs ( p > 0.05). For the tested LCUs, no specific LCU could consistently achieve highest strength and modulus from the specimens tested. Conclusions: Two lasers (457 and 473 nm) can polymerize composite resins to the level that LED LCU can achieve despite inconsistent trends of polymerization shrinkage and flexural and compressive properties of the tested specimens.
Introduction
L
ight-curing composite resins are popular restorative materials in dentistry for the filling of decayed or damaged teeth, because of their convenience, favorable aesthetics, and excellent mechanical properties that are compatible with the host tooth. Restoration involves a curing process of dental restorative materials within a tooth cavity through a polymerization process by formed free radicals, and the interaction between restorative material and irradiating light is important. Because there are so many available resin products and light-curing units (LCUs), the degree (quality) of polymerization may change, depending upon the selection of resin product and LCU. Therefore, depending upon the purpose, function, and mechanical properties, selection and the resultant outcomes can be changed with a wide spectrum. Polymerization is a process that converts monomers to polymers via the activation of an initiator by chemical 1 2 3
agents (benzoyl peroxide) or external light. Initiation by external light is started through the activation (optical excitation) of a photoinitiator (camphorquinone [CQ]) and subsequently accelerated by amines. The excited CQ gets protons from the excited amine accelerator through proton transfer, and then both become radicals. The radical CQ and amine accelerators bind with other monomer molecules to be stabilized. This process occurs through the chain reaction and then finally forms a polymer network. CQ absorbs blue light in the range of 380–500 nm, which is why all LCUs emit blue light. To activate CQ, three different LCU types have been introduced. Quartz–tungsten–halogen (QTH) LCUs are still widely used, although their usage is declining because of the advent of light-emitting diode (LED) LCUs, and emit light in a broad range (370–520 nm) in the absorption band of CQ.1–3 This transition from QTH LCUs to LED LCUs is reflected by dental school training programs.4,5 LED LCUs are popular because of their high light intensities, portability,
Department of Dental Materials, School of Dentistry and Biomedical Research Institute, Pusan National University, Yangsan, Korea. Department of Preventive and Community Dentistry, School of Dentistry, Pusan National University, Yangsan, Korea. Department of Orthodontics, School of Dentistry, University of North Carolina, Chapel Hill, North Carolina.
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Table 1. Materials Tested in the Present Study Code PR SD ZX AB CX Z2 PF TF
Composition
Filler content vol%/wt%1/wt%2
Bis-EMA, TEGDMA, Barium glass, SiO2 Bis-EMA, TEGDMA, UDMA, Barium glass, SiO2 Bis-EMA, Bis-GMA, TEGDMA, UDMA, Non-aggregated silica, zirconia/silica, nanoclusters Bis-EMA, TEGDMA, Glass filler, amorphous silica Bis-GMA, TEGDMA, Barium glass, silica, SiO2 Bis-EMA, Bis-GMA, TEGDMA, UDMA, Zirconia/silica Bis-EMA, TEGDMA, Barium glass, silica fillers Bis-EMA, UDMA, TEGDMA, Barium glass, YbF3, SiO2
71/84/77.1 65/80/75.2 63.3/78.5/74.7 55/76/71.0 70/86/83.2 60/84.5/80.4 54.6/72.5/64.4 39/63/61.9
Manufacturer Kerr Orange, CA Coltene/Whaledent Cuyaho Falls, OH 3M ESPE St. Paul, MN Bisco Inc. Schaumburg, IL Kuraray, Tokyo, Japan 3M ESPE St. Paul, MN Kerr Orange, CA Ivoclar Vivadent, Schann, Liechtenstein
wt%1, nominal weight percent provided by the manufacturers; wt%2, weight percent determined by the ash method; PR, Premise Packable; SD, Synergy D6; ZX, Filtek Z350XT; AB, Aelite All Purpose Body; CX, Clearfil AP-X; Z2, Filtek Z250; PF, Premise Flow; TF, Tetric N Flow; Bis-EMA, ethoxylated bisphenol A glycidyl methacrylate; Bis-GMA, bisphenol A glycidyl methacrylate; TEGDMA, triethyleneglycol dimethacrylate; UDMA, urethane dimethacrylate.
and extended lifetimes, and because they generate less heat.6–8 Although their emission spectra (430–500 nm) are narrower than those of QTH LCUs, using LED LCUs can be efficient because their emission band is focused on the absorption peak of CQ. Furthermore, of the recently developed LEDs, dual-peak LEDs have an additional emission peak near 405 nm, which is required for the activation of a coinitiator, such as, 2,2-dimethoxy (1,2) diphenyletanone (DMBZ) or 1-phenyl-1,2-propanedione (PPD).9–11 The argon laser provides another light source for light curing.12–14 Unlike QTH and LED LCUs, the argon laser has an extremely narrow emission band. One of the argon laser emission peaks, a blue 488 nm, overlaps with the absorption peak of CQ near the tail part. However, despite its many excellent features, the argon laser is expensive and has not been well adopted. As a substitution, diode-pumped solid state (DPSS) lasers can be chosen. Initially, DPSS laser was introduced as a light source for flow cytometers.15 DPSS lasers are portable and relatively cheap, and provide several emission options. However, these lasers were not developed for dental purposes, and few studies other than those of 473 nm have addressed their feasibilities in dentistry.16,17 A DPSS laser of 473 nm is a good light source for light curing of CQ-containing composite resins because of their similar emission and absorption bands, respectively. Also, among the diode lasers introduced recently, a laser of 457 nm is attractive because it has never been tested in dentistry, and its wavelength similarly coincides with the absorption peak of CQ. The purpose of the present study was to test the feasibility of two lasers, that is, 457 and 473 nm lasers, as a light source for the curing of composite resins. Polymerization shrinkage and flexural and compressive properties were evaluated. The hypothesis to be tested is that lasers of 457 and 473 nm polymerize composite resins to the level that LED LCU can achieve.
Materials and Methods Composite resins and LCUs
For the study, eight different dental composite resins [three nanocomposite resins: Premise Packable (PR), Synergy D6 (SD), and Filtek Z350XT (ZX); three microhybrid resins: Aelite All Purpose Body (AB), Clearfil AP-X (CX), and Filtek Z250 (Z2); two flowable resins: Premise Flow (PF) and Tetric N Flow (TF)] were used. Their details are listed in Table 1. For light curing, one LED (L.E. Demetron, Kerr, Danbury, CT) and two lasers (LVI Technology, Seoul, Korea) of 457 and 473 nm were used. The emission spectra of LCUs were measured using a photodiode array detector (M1420, EG&G PARC, Princeton, NJ) connected to a spectrometer (SpectroPro-500, Acton Research, Acton, MA) (Fig. 1). The output light intensity of LED was 900 mW/cm2 as measured using a built-in radiometer. The output power and spot size of the laser beam were 150 mW (PM3/FIELDMAX, Coherent, Portland, OR) and 6 mm, respectively, and its resultant light intensity was *530 mW/cm2. The original laser beam was expanded to 6 mm using a beam expander (Thorlabs Inc., Newton, NJ). Polymerization shrinkage testing
The polymerization shrinkages of specimens during and after curing were measured (n = 7 for each product) using a linometer (RB 404, R&B Inc., Daejon, Korea). A resin cylinder (diameter 4 mm, thickness 2 mm) was placed over an aluminum disc (the specimen stage of the measurement system) and its top surface was flattened by a glass slide. The end of the light guide of LED was placed in contact with the glass slide. Before light curing, the initial position of the aluminum disc was set to zero (in the case of the laser, the guided light using a mirror was irradiated over the specimen with a 90 degree angle without an aid of fiber
COMPOSITE RESINS LIGHT-CURED USING TWO DIFFERENT LASERS
215
FIG. 1. Emission spectra of the light-curing units (LCUs) used, and the absorption spectrum of camphorquinone (CQ).
optics). The light was irradiated from LCU for 40 sec. As resin polymerized, it shrank and the base aluminum disc moved upward. The amount of disc displacement that occurred because of polymerization shrinkage was measured automatically for 130 sec using a noncontacting inductive sensor placed below the aluminum disc. The resolution and measurement range of the sensor were 0.1 and 100 lm, respectively. Three-point bending test
A three-point bending test was performed to determine the resin flexural properties [flexural strength (FS) and modulus (FM)]. A metal mold (25 · 2 · 2) was filled with resin, according to ISO 4049.18 Top and bottom resin surfaces were covered with glass slides to flatten surfaces. Specimens were irradiated for 40 sec by each LCU. The end of the light guide of LED was placed in contact with the glass slide. Because specimens were much wider (25 mm) than the tip or expanded laser beam size (7 or 6 mm), five overlapping exposures were performed per side. After curing, specimens were removed from molds and aged for 24 h in a 37C dry dark chamber. Other specimens were immersed in distilled water and stored in a 37C dry, dark chamber for 2 weeks. After aging (n = 7) or immersion (n = 7), specimens were loaded into a universal test machine (Instron 3345, Grove City, PA), which was operated at a crosshead speed of 1 mm/min. FS (rf in MPa) was obtained using the following formula rf ¼ 3DP=(2WH2 ) where D is the distance between supports (20 mm), P is the maximum failure load (N), W is the width (2 mm), and H is the height (2 mm) of the tested specimen. FM (E in GPa) was calculated using E ¼ (P=D) · [D3 =(4WH3 )] where P/D is the slope of the linear portion of the loaddisplacement curve.
Compression test
To measure compressive properties [compressive strength (CS) and compressive modulus (CM)], specimens were prepared using a hollow metal mold (3 mm in diameter and 6 mm high). The metal mold was composed of two identical hollow hemicylinders. After filling the mold, both the top and bottom surfaces were covered with glass slides to make the surface flat, and then irradiated for 5 sec (because light does not reach to the bottom surface, light curing would be better through the lateral surface after exposure). Subsequently, one part of the metal mold was removed by sliding. The exposed surface was light cured for 40 sec. The opposite side was also light cured for 40 sec again after removing the other part. After light curing, specimens were randomly removed from the mold and aged for 24 h in a 37C dry, dark chamber. Some other specimens were immersed in distilled water and kept in a 37C dry, dark chamber for 2 weeks. After aging (n = 7) or immersion (n = 7), the specimens were loaded to a universal test machine for compression tests at a crosshead speed of 1 mm/min. CS (rc in MPa) was obtained using the following formula rc ¼ P=A where P is the maximum failure load (N) and A is specimen cross-sectional area. CM (in GPa) was defined as the slope of the linear portion of the load-displacement curve. Statistical analysis
Test results were analyzed by two-way ANOVA for LCU and resin product. A post-hoc Tukey test was then followed for a multiple comparison. The student’s t test was used to analyze results after immersion in distilled water. Statistical significance was accepted for p values < 0.05. All the statistical analysis was performed by SPSS software (IBM SPSS, Version 21, New York, NY). Results
Table 2 and Fig. 2 show the polymerization shrinkage values and profiles of the specimens, respectively. Overall
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Table 2. Polymerization Shrinkage of the Specimens Cured Using Different LCUs
PR1 SD2 ZX3 AB4 CX3 Z23 PF5 TF5 p Value
457A
473B
LEDB
10.9 – 1.1 15.7 – 0.4 12.6 – 0.8 21.2 – 1.3 13.3 – 2.2 12.2 – 1.0 28.2 – 1.3 30.1 – 1.3 a = 0.002
13.2 – 0.4 16.5 – 0.5 14.2 – 0.4 23.6 – 2.0 14.4 – 1.2 15.0 – 0.1 27.3 – 2.4 25.2 – 0.5 b < 0.001
11.5 – 1.0 16.3 – 1.0 15.5 – 0.5 21.0 – 0.7 13.6 – 0.5 13.6 – 0.3 22.7 – 1.1 25.2 – 1.1 a · b = 0.013
Statistically significant difference in light-curing units (LCUs) is shown by superscript letters, and statistically significant difference in resin product is shown by superscript numbers. Items with the same letters or numbers are not significantly different ( p > 0.05). For p values, the letters a and b denote LCUs and resin products, respectively. PR, Premise Packable; SD, Synergy D6; ZX, Filtek Z350XT; AB, Aelite All Purpose Body; CX, Clearfil AP-X; Z2, Filtek Z250; PF, Premise Flow; TF, Tetric N Flow.
values ranged 10.0–26.8 lm depending on LCU and resin product. Regardless of condition, flowable resins had the highest shrinkages. Specimens light cured using the 473 nm laser had significantly different ( p < 0.05) shrinkage values, although differences between absolute values were not high. Shrinkages were found to be inverse linearly correlated with filler content regardless of LCU (R = 0.73–0.90, 0.83–0.95, 0.83–0.90 for vol%, nominal wt%, ash method wt%, respectively). In Fig. 2, there are profiles of only two products, but all the tested products have the same shrinkage profile trend. Table 3 shows the flexural properties (FS and FM) of specimens before and after immersion in distilled water for 2 weeks. Before immersion, overall FS and FM values ranged from 110.7 to 185.7 MPa and from 5.71 to 24.75 GPa, respectively. However, after immersion, they ranged from 81.9 to 164.8 MPa and from 3.61 to 21.85 GPa, respectively. The
influence of LCU type on the obtained data had no consistent trend. The linear correlation between FS and filler content was R = 0.01–0.63, and that of FM was R = 0.63– 0.95 depending upon LCU. Table 4 shows CS and CM values. Before immersion, CS and CM ranged from 277.8 to 408.2 MPa and from 3.25 to 5.37 GPa, respectively, depending upon LCU and resin brand. After immersion, they changed to 275.8–431.9 MPa and 2.89–5.19 GPa, respectively. CS values for different LCUs were not significantly different regardless of immersion ( p > 0.05). CM values were mostly lowered after immersion. CS dependence on filler content was R = 0.14–0.61, and that of CM was R = 0.44–0.98 depending upon LCU. Discussion
Polymerization involves an interaction between externally irradiated light and the host resin. Because the light curing of composite resins initiates polymerization by activating a photoinitiator (CQ), the characteristics of the light used are important. The three light sources that were tested have different spectral distributions. The two lasers have a very narrow bandwidth compared with LED light, and have emission peaks at 457 and 473 nm, respectively, which are close to the absorption peak of CQ. On the other hand, LED light has a much wider spectral distribution and degree of overlap with the absorption peak of CQ. Laser is also highly coherent compared with LED light. Such natures (monochromaticity and coherence) would be beneficial to the lasers even those whose light intensity is *40% lower than that of LED light. The two lasers differ only in terms of their peak wavelength. From the obtained data, the hypothesis tested has to be accepted, because lasers of 457 and 473 nm can polymerize composite resins to the level that LED LCU achieves. Polymerization shrinkage occurs as monomers convert to polymer network by changing the governing force between molecules from van der Waals to covalents; therefore, polymerization shrinkage is an inherent feature of dimethacrylatebased composite resins, and it produces many undesirable side effects.19–21 In the present study, wavelength was found to
FIG. 2. Polymerization shrinkage profiles of the specimens cured with different light-curing units (LCUs). All the tested resin products showed the same profile trend depending on the laser and LED LCU.
217
115.3 – 13.7 120.1 – 6.0 149.6 – 5.5 119.2 – 19.3 142.9 – 17.7 140.8 – 13.6 145.2 – 15.6 132.5 – 10.7 a < 0.001
110.7 – 12.1 142.6 – 11.5 173.4 – 11.5 128.2 – 25.0 159.8 – 21.0 174.6 – 19.7 120.3 – 14.4 121.4 – 4.1 b < 0.001
473AB 116.1 – 8.6 130.0 – 5.4 164.0 – 8.8 149.5 – 10.7 185.7 – 12.1 162.9 – 13.4 131.5 – 9.9 130.1 – 10.4 a · b < 0.001
LEDB PR1 SD1 ZX2 AB1 CX3 Z24 PF5 TF6 12.56 – 0.77 11.25 – 1.32 14.23 – 0.61 11.51 – 0.89 20.79 – 0.91 17.06 – 1.46 8.47 – 0.82 6.49 – 0.47 a = 0.065
457A 12.10 – 0.71 11.32 – 0.60 14.86 – 0.68 13.36 – 0.55 24.75 – 1.78 16.17 – 1.85 8.06 – 0.31 5.71 – 0.17 b < 0.001
473A
FM{
12.66 – 0.78 13.59 – 0.77 15.25 – 0.21 10.62 – 0.60 20.49 – 1.21 15.66 – 1.60 9.09 – 0.24 7.40 – 0.26 a · b < 0.001
LEDA PR1 SD12 ZX12 AB2 CX3 Z23 PF2 TF12 91.8 – 6.6 106.8 – 8.8 100.8 – 11.7 114.8 – 24.4 139.1 – 19.8 133.8 – 17.1 111.4 – 11.4 107.9 – 7.12 a = 0.016
457AB 81.9 – 11.3 85.6 – 9.7 114.0 – 25.0 100.0 – 4.9 131.9 – 14.1 130.1 – 16.1 106.7 – 8.26 101.2 – 9.7 b < 0.001
473A
FS{
95.7 – 8.7 108.6 – 16.8 90.6 – 9.5 115.6 – 10.4 164.8 – 9.8 135.2 – 11.8 107.0 – 6.7 99.0 – 10.3 a · b = 0.003
LEDB PR1 SD2 ZX3 AB2 CX4 Z25 PF6 TF7
457A 9.90 – 0.73 9.94 – 8.80 11.64 – 1.43 10.06 – 0.71 18.68 – 0.76 13.45 – 1.23 7.80 – 0.29 4.99 – 0.52 a = 0.015
DW-2 weeks
7.93 – 0.21 9.02 – 0.35 11.68 – 1.28 10.34 – 0.33 21.85 – 1.72 10.96 – 0.20 7.56 – 0.40 3.61 – 0.11 b < 0.001
473B
FMx
9.28 – 0.42 10.77 – 0.23 11.02 – 0.56 10.31 – 0.31 20.35 – 1.50 13.32 – 0.38 7.25 – 0.39 4.14 – 0.20 a · b < 0.001
LEDA
339.5 – 29.7 307.4 – 43.6 359.8 – 30.5 340.4 – 47.9 353.7 – 50.4 408.2 – 57.1 329.2 – 36.2 299.8 – 45.5 a = 0.086
334.8 – 30.9 319.4 – 50.3 315.0 – 46.8 347.9 – 61.0 357.3 – 50.5 329.5 – 56.9 296.0 – 47.5 337.9 – 54.4 b = 0.053
473A 352.7 – 14.8 305.9 – 31.5 338.4 – 19.4 277.8 – 39.7 328.7 – 42.9 317.3 – 41.5 351.8 – 25.8 295.5 – 24.1 a · b = 0.015
LEDA PR1 SD2 ZX3 AB12 CX4 Z23 PF5 TF5 4.20 – 0.15 4.27 – 0.16 4.63 – 0.10 4.20 – 0.20 5.14 – 0.61 4.80 – 0.11 3.59 – 0.11 3.35 – 0.14 a < 0.001
457A
LEDC
3.93 – 0.28 3.72 – 0.26 4.17 – 0.16 4.19 – 0.06 4.92 – 0.11 4.54 – 0.05 4.39 – 0.10 3.70 – 0.26 5.37 – 0.15 4.90 – 0.11 4.94 – 0.10 4.55 – 0.10 3.25 – 0.51 3.48 – 0.31 3.35 – 0.16 3.25 – 0.20 b < 0.001 a · b < 0.001
473B
CM{
PR12 SD1 ZX14 AB14 CX3 Z224 PF24 TF12
332.6 – 16.1 275.8 – 34.5 307.6 – 38.4 291.8 – 43.0 400.9 – 27.0 407.0 – 31.8 344.3 – 28.7 287.8 – 50.8 a = 0.585
457A
294.9 – 38.7 313.8 – 32.6 322.9 – 41.5 366.3 – 26.7 431.9 – 40.4 300.5 – 22.4 319.8 – 32.0 328.1 – 16.4 b < 0.001
473A
CS{
322.6 – 20.7 345.1 – 42.8 347.0 – 26.4 332.4 – 47.6 332.2 – 47.8 327.0 – 27.0 357.3 – 22.4 352.7 – 26.2 a · b < 0.001
LEDA PR1 SD2 ZX3 AB2 CX4 Z23 PF1 TF5
457A 3.91 – 0.17 4.11 – 0.13 4.29 – 0.16 3.96 – 0.12 5.19 – 0.15 4.57 – 0.15 3.28 – 0.23 2.89 – 0.22 a < 0.001
DW-2 weeks
LEDB 3.09 – 0.43 3.23 – 0.30 3.83 – 0.22 4.01 – 0.14 4.54 – 0.12 3.86 – 0.15 4.05 – 0.14 3.79 – 0.17 5.03 – 0.17 4.93 – 0.04 4.63 – 0.03 4.01 – 0.28 3.47 – 0.14 3.28 – 0.13 2.92 – 0.10 2.98 – 0.09 b < 0.001 a · b < 0.001
473A
CMx
Statistically significant difference in light-curing units (LCUs) is shown by superscript letters, and statistically significant difference in resin products is shown by superscript numbers. For the t test, statistical results are represented by symbols (e.g., * or {). Items with the same letters or numbers are not significantly different ( p > 0.05). For p values, the letters a and b denote light-curing unit and resin product, respectively. DW, distilled water; CS, compressive strength; CM, compressive modulus; PR, Premise Packable; SD, Synergy D6; ZX, Filtek Z350XT; AB, Aelite All Purpose Body; CX, Clearfil AP-X; Z2, Filtek Z250; PF, Premise Flow; TF, Tetric N Flow.
PR1 SD1 ZX1 AB1 CX1 Z21 PF1 TF1 p Value
457A
CS*
24 h
Table 4. Compressive Properties of the Specimens Prepared Using Different LCUs
Statistically significant difference in light-curing units (LCUs) is shown by superscript letters, and statistically significant difference in resin products is shown by superscript numbers. For the t test, statistical results are represented by symbols (e.g., * or {). Items with the same letters or numbers are not significantly different ( p > 0.05). For p values, the letters a and b denote LCUs and resin products, respectively. DW, distilled water; FS, flexural strength; FM, flexural modulus; PR, Premise Packable; SD, Synergy D6; ZX, Filtek Z350XT; AB, Aelite All Purpose Body; CX, Clearfil AP-X; Z2, Filtek Z250; PF, Premise Flow; TF, Tetric N Flow.
PR1 SD2 ZX3 AB2 CX3 Z23 PF2 TF12 p Value
457A
FS*
24 h
Table 3. Flexural Properties of the Specimens Prepared Using Different LCUs
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affect polymerization shrinkages because the resultant polymerization shrinkage values by 457 and 473 nm lasers were significantly different, despite their similarity in spectral width. Apparent differences between the lasers and LED LCU were the light intensities used (530 vs. 900 mW/cm2) and spectral width. However, the similar polymerization shrinkages of the resin products tested suggest that the lasers can achieve a similar degree of polymerization as to LED LCU, with much lower light intensity. The high linear correlation observed between polymerization shrinkage and filler content regardless of LCU could provide a useful means of estimating shrinkages. Because polymerization shrinkage is the result of monomers’ conversion to polymer, high filler content, that is low monomer content, favors less shrinkage. In the shrinkage after 40 sec of light curing, specimens showed slightly different shrinkage profiles depending upon the LCU. The specimens light cured using the lasers showed a smoother profile than those light cured using LED. The difference between profiles is the result of the difference of light delivery. During 40 sec light irradiation, in the case of LED, photons and heat from the light source are transmitted through the 5–10 cm long fiber optic. On the other hand, in this study, laser was delivered without fiber optic, and no heat was transmitted to the specimen. If the light guide is in contact with the top surface of the specimen, both shrinkage and expansion caused by polymerization and heat, respectively, occur simultaneously. In this situation, some of the polymerization shrinkage can be suppressed by the thermal expansion of the specimen. However, immediately after the termination of light irradiation, the suppression by the thermal expansion will be relieved because of the missing heat source. More apparent shrinkage profile in the LED-treated specimens after 40 sec light curing should be the result of the absence of thermal expansion caused by the heat. According to reports, the temperature rise on the composite resins reaches to the point that feels hotness, because of exothermic heat by polymerization and conducted heat from the LED unit through the fiber optic.22,23 A three-point bending test provides a means of determining the FS and FM values of specimens, which are measures of the ability of a material to resist extended external stress without fracture. In the present study, before immersion, specimens had FS values ranging from 115.3 to 145.2 MPa, from 110.7 to 174.6 MPa, and from 116.1 to 185.7 MPa for 457 nm, 473 nm, and LED LCU, respectively. For the same resins, FS values were not consistently different among LCUs. After immersion, the initial FS values decreased by 2.7–32.6%, 11.3–40.0%, and 11.3–44.8% for 457 nm, 473 nm, and LED LCU, respectively. FM values had no consistent dependence on LCU; however, the values were consistently reduced after immersion. The FM values obtained before immersion ranged from *6 to 25 GPa, and, therefore, appeared to be lower or compatible with the FM of dentine, which is 17–25 GPa.24–26 Inconsistent FS and FM values may be partly caused by the overlapping exposure of specimens, which was required because specimen sizes as required by ISO 4049 were much larger than the tip or spot sizes of the LCUs tested. During overlapping exposures, neighboring regions are weakly exposed to the diverging light, and it initiates incomplete
KIM ET AL.
polymerization. Such weakly polymerized regions are not fully polymerized by second exposure. In addition, the mold-filling procedure could produce inconsistencies in flexural properties. Because the mold was 25 mm long, several fillings were needed. During this process, uneven or loose regions can occur, and these can cause inconsistencies. The poor correlation between FS and filler content could indicate that filler content was not the principal determinant of FS. Generally, FS is known to depend more on material volume and internal defects, such as cracks or voids generated during the manufacturing or specimen preparation processes.27–29 CS and CM are measures of a material’s ability to resist sustained longitudinal heavy loads during mastication.30 Tested specimens showed higher CS values (*278– 408 MPa) than many other composite resins tested (based on > 70 tested resins, which have CS values of 100– 290 MPa).31 In the present study, CS values obtained using different LCUs were not statistically different, and found to be poorly correlated with filler content. However, CM showed a strong correlation with filler weight (as determined by nominal wt% or ash method wt%) in the range from 0.68 to 0.98, depending upon the LCU used. Because the modulus is related to material stiffness, materials with high filler contents might be expected to have high modulus.32 The tested resin products showed a strong linear correlation (0.86–0.97) between their FM and CM values regardless of LCU, which could be useful for estimation when the FM or CM is known. On the other hand, no such correlation was found between FS and CS values, which in the present study, before and after immersion, ranged from 3.61 to 24.75 GPa and from 2.89 to 5.37 GPa, respectively. Nevertheless, these values are lower than those of dentin (17–25 GPa and 11–19 GPa for FM and CM, respectively), which means that they would protect the underlying dentin from external heavy loads.24,33,34 The results of the present study raise the question of whether monochromatic or polychromatic light is better for the polymerization of composite resins. To answer this question, studies at much lower light intensities are needed to evaluate the advantages of coherent monochromatic light versus incoherent polychromatic light, with respect to the mechanical properties of composite resins. Because now LED is definitely efficient and effective as an LCU, studies, such as those pertaining to degree of conversion, temperature rise, or color stability, other than the present one, will also be further discussed in forthcoming articles to elucidate the feasibility of two lasers as a light source for light curing composite resins. Conclusions
In many cases, the polymerization shrinkage and flexural and compressive properties of the tested resin products showed inconsistent trends with respect to the LCU used. However, the similar mechanical properties from the tested resin products may suggest that the two lasers (457 and 473 nm) can cure composite resins to the level that LED LCU can achieve. Author Disclosure Statement
No competing financial interests exist.
COMPOSITE RESINS LIGHT-CURED USING TWO DIFFERENT LASERS References
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Address correspondence to: Yong Hoon Kwon Department of Dental Materials School of Dentistry and Biomedical Research Institute Pusan National University Yangsan 626-870 Korea E-mail: [email protected]
