Lasers Med Sci DOI 10.1007/s10103-015-1753-2


Effect of two lasers on the polymerization of composite resins: single vs combination Jung-Hoon Ro 1 & Sung-Ae Son 2 & Jeong-kil Park 2 & Gye-Rok Jeon 1 & Ching-Chang Ko 3 & Yong Hoon Kwon 4

Received: 26 January 2015 / Accepted: 2 April 2015 # Springer-Verlag London 2015

Abstract The selection of a light-curing unit for the curing composite resins is important to achieve best outcomes. The purpose of the present study was to test lasers of 457 and 473 nm alone or in combination under different light conditions with respect to the cure of composite resins. Four different composite resins were light cured using five different laser combinations (530 mW/cm2 457 nm only, 530 mW/cm2 473 nm only, 177 mW/cm2 457+ 177 mW/cm2 473 nm, 265 mW/cm2 457+265 mW/cm2 473 nm, and 354 mW/cm2 457+354 mW/cm2 473 nm). Microhardness and polymerization shrinkage were evaluated. A light-emitting diode (LED) unit was used for comparison purposes. On top surfaces, after aging for 24 h, microhardness achieved using the LED unit and the lasers with different conditions ranged 42.4–65.5 and 38.9–67.7 Hv, respectively, and on bottom surfaces, corresponding ranges were 25.2–56.1 and 18.5–55.7 Hv, respectively. Of the conditions used, 354 mW/cm 2 457 nm+354 mW/cm2 473 nm produced the highest bottom microhardness (33.8–55.6 Hv). On top and bottom surfaces, microhardness by the lowest total light intensity, 354 (177×2) mW/cm2, ranged 39.0–60.5 and 18.5–52.8 Hv, respectively.

* Yong Hoon Kwon [email protected] 1

Department of Biomedical Engineering, School of Medicine, Yangsan 626-770, Korea


Department of Conservative Dentistry, School of Dentistry, Pusan National University, Yangsan 626-870, Korea


Department of Orthodontics, School of Dentistry, University of North Carolina, Chapel Hill, NC 27599, USA


Department of Dental Materials, Biomedical Research Institute, School of Dentistry, Pusan National University, Yangsan 626-870, Korea

Generally, 530 mW/cm2 at 457 nm produced the lowest polymerization shrinkage. However, shrinkage values obtained using all five laser conditions were similar. The study shows the lasers of 457 and 473 nm are useful for curing composite resins alone or in combination at much lower light intensities than the LED unit. Keywords Lasers . Microhardness . Polymerization shrinkage . 457 nm . 473 nm

Introduction The polymerization of composite resins is a process that converts monomers to polymer using external light. A lightcuring unit (LCU) is a device which emits blue light for activating photoinitiator (camphorquinone, CQ). The activated CQ becomes free radicals in the presence of an amine accelerator and then initiates a chain reaction. The choice of a LCU for light curing is optional, but each LCU has different characteristics. Quartz-tungsten-halogen (QTH) LCUs have the widest spectral distribution of available LCUs, which ranges approximately 380–520 nm. Lightemitting diode (LED) LCUs have narrower spectral distributions than QTH LCUs. Usually, LED LCUs emit a continuum of wavelengths with intensities that peak near the absorption peak of CQ [1–3]. However, recently, dual-peak (polywave) LED LCUs for curing additional coinitiator-containing composite resins were developed [4–6]. These coinitiators include 2,2-dimethoxy[1,2]diphenyletanone (DMBZ); 1phenyl-1,2-propanedione (PPPD); and diphenyl(2,4,6trimethybenzoly)phosphinoxid (TPO) [7–10]. The argon laser has the narrowest emission band of commercial LCUs. However, despite this, the argon laser can polymerize composite resins to the same level as most LCUs [11, 12]. Such, an

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achievement by the argon laser could be possible even with light that only matches to the tail part of CQ absorption spectrum. However, high price and bulky size prevent the argon laser to be common as QTH or LED units. Problems associated with the QTH lamp are the generation of heat during operation and maintenance issues due to limited lamp lifetimes. In fact, in many cases, the temperature rise in composite resins due to LCU during light curing exceeds 5 °C that causes pulp necrosis [13]. On the other hand, LED units generate much higher light intensities, produce less heat, and are far more long-lived than QTH units. Diode-pumped solid state (DPSS) lasers are attractive light sources because they are cost-effective and offer more multi-wavelength/power options than many other lasers. Of the DPSS lasers, the 473-nm DPSS laser can be useful for curing composite resins because CQ has an absorption peak near 473 nm. Recent studies have demonstrated the potential of the 473-nm DPSS laser as a light source for the CQ-based composite resins [14–16]. Similarly, the 457-nm laser can also be a useful light source because 457 nm is closer to the CQ absorption peak (approximately 464 nm) than the emission peak of 473 nm and the argon laser (488 nm). However, the 457-nm laser has never been tested for the curing of composite resins. Accordingly, the purpose of the present study was to test the feasibility of the 457-nm laser for the cure of composite resins alone or in combination with the 473-nm laser under different light intensities. Microhardness and polymerization shrinkage were evaluated for various composite resins.

Materials and methods Specimens and light-curing conditions For this study, four different composite resins all of A3 shade were used [Aelite All-Purpose Body (AP), Filtek P90 (P9), Premise Packable (PP), and Filtek Z350 Flow (ZF)]. Table 1 shows their compositional details. For light curing, two lasers of 457 and 473 nm (LVI Technology, Seoul, Korea) and one

Table 1 Materials tested in the present study

LED LCU [L.E. Demetron (DE), Kerr, Danbury, CT, USA] were used. The output light intensity of LED unit was approximately 900 mW/cm2 as measured using a built-in radiometer. The spot size of the 457 and 473 nm lasers was made to 6 mm using a beam expander. Laser output powers were measured using a power meter (PM3/FIELDMAX, Coherent, Portland, OR, USA). The emission spectra of the lasers and LED unit and the absorption spectrum of CQ (Fig. 1) were measured using a photodiode array detector (M1420, EG&G PARC, Princeton, NJ) connected to a spectrometer (SpectroPro-500, Acton Research, Acton, MA, USA). For curing specimens with the lasers, five different curing conditions were chosen [II, 457 nm (150 mW; 530 mW/cm2); III, 473 nm (150 mW; 530 mW/cm2); IV, 457 nm (50 mW; 177 mW/cm2)+473 nm (50 mW; 177 mW/cm2); V, 457 nm (75 mW; 265 mW/cm2)+ 473 nm (75 mW; 265 mW/cm2); VI, 457 nm (100 mW; 354 mW/cm2)+473 nm (100 mW; 354 mW/cm2), details are shown in Table 2]. Using a beam combiner (Edmund Optics, Barrington, NJ, USA), the two laser beams were combined to irradiate the same spot by making one beam heading perpendicular to the other beam. While combining two beams, ND filters were placed ahead of each laser to control output powers from 50 (177 mW/cm2) to 100 mW (354 mW/cm2). The output power of the unfiltered lasers was both 150 mW (530 mW/cm2). The LED unit was used as a control (condition I). Microhardness test To measure the surface microhardness (Hv) of specimens, each resin was filled into a metal mold (4×2×3 mm) and light cured for 40 s using LCU. Cured specimens were removed from the mold and aged for 24 h at 37 °C in a dry, dark chamber. Half of the aged specimens were randomly selected, and microhardness of top (z=0) and bottom (z=3 mm) surfaces was measured using a Vickers hardness tester (MVKH1, Akashi, Tokyo, Japan) by evaluating the sizes of microindentations (n=12 for each test condition). To make microindentations, a 200 gf load and a 10-s dwell time were



Filler content vol%/wt%a/wt%b



BisEMA, TEGDMA glass filler, amorphous silica Silorane silanized quartz, yttrium fluoride BisEMA, TEGDMA barium glass, SiO2 BisGMA, TEGDMA zirconia/silica cluster fillers


Bisco Inc., Schaumburg, IL, USA

55/76/75.6 71/84/77.1 55/65/61.0

3M ESPE, St. Paul, MN, USA Kerr, Orange, CA, USA 3M ESPE, St. Paul, MN, USA


AP Aelite All-Purpose Body, P9 Filtek P90, PP Premise Packable, ZF Filtek Z350 Flow a

wt%, weight percent provided by the manufacturers


wt%, weight percent determined by the ash method

Lasers Med Sci Fig. 1 Emission spectrum of LCUs with absorption spectrum of CQ

used. The remaining specimens were then immersed in distilled water for 2 weeks; after that, microhardness of top and bottom surfaces was measured. Cross-link density test To assess cross-link density, specimens were prepared as described above and aged for 24 h in a 37 °C dry, dark chamber. Half of the aged specimens were then subjected to microhardness testing as described above. The microhardness of the remaining specimens was measured after immersing them in ethanol for 24 h. Polymerization shrinkages Polymerization shrinkage (μm) of specimens during and after light curing was measured (n=7 for each product) using a

Table 2 Light-curing conditions with LCU(s)

linometer (RB 404, R&B Inc., Daejeon, Korea). A resin of cylindrical shape (diameter 4 mm, thickness 2 mm) was placed over an aluminum disc (the specimen stage of the measurement system), and its top surface was covered using a glass slide. The end of the light guide (DE) was placed in contact with the glass slide (in the case of the laser, specimens were irradiated vertically). Before light curing, the initial position of the aluminum disc was set to zero (in the case of laser, specimens were irradiated with light at 90° to the specimen surface). Light was irradiated for 40 s. As polymerization progressed, specimens shrank away to the light source, and the aluminum disc under the resin moved accordingly. The amount of disc displacement that occurred was measured automatically for 130 s using a non-contacting inductive sensor placed below the aluminum disc. The resolution and measurement range of the shrinkage sensor were 0.1 and 100 μm, respectively.





DE 457 nm (150 mW) 473 nm (150 mW) 457 nm (50 mW)+473 nm (50 mW)

Light intensity: 900 mW/cm2 Light intensity: 530 mW/cm2


457 nm (75 mW)+473 nm (75 mW)


457 nm (100 mW)+473 nm (100 mW)

Combining of two 50-mW laser beams (2×177=354 mW/cm2) Combining of two 75-mW laser beams (2×265=530 mW/cm2) Combining of two 100-mW laser beams (2×354=708 mW/cm2)

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Statistical analysis Microhardness, cross-link density, and polymerization shrinkage results were analyzed by two-way ANOVA with post-hoc Tukey’s test for multiple comparisons. Statistical significance was accepted for p values

Effect of two lasers on the polymerization of composite resins: single vs combination.

The selection of a light-curing unit for the curing composite resins is important to achieve best outcomes. The purpose of the present study was to te...
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