Effect of 457 nm light on the polymerization of dental composite resins

INTRODUCTION

The use of light-curing dimethacrylate-based dental composite resins is popular and has become routine for the restoration of damaged or decayed teeth, because of their excellent aesthetic qualities and because their mechanical properties are compatible with host teeth [Ferracane, 2011]. In addition, these resins are user-friendly because of straightforward, rapid, controlled initiation of polymerization process.

Two ingredients are essential for external light-composite resin interactions, that is, photons and the photoinitiator [Leprince et al., 2013; Cramer et al., 2011]. Photons supplied externally by a light-curing unit (LCU) initiate the polymerization process, whereas the photoinitiator, which is excited by photons to produce free radicals, is a constituent of the resin matrix. The free radicals so formed then convert monomers into a polymer network. Camphorquinone (CQ) is the photointiator used in all light-curing dental composite resins and has a light absorption band approximately at 380-510 nm. Since light is maximally absorbed through the complementary relation, blue light matches maximally with yellow CQ, which is why all LCUs emit blue light.

Quartz-tungsten-halogen (QTH)-lamp based LCUs have the widest spectral distribution [Kumar et al., 2012; Mousavinasab and Meyersm, 2011; 조호성 등, 2010], and their emission band (at 370-520 nm) matches the absorption band of CQ. However, light-emitting diode (LED) LUCs have much narrower emission band (approximately 425-500 nm) [Poggio et al., 2012; Polydorou et al., 2011; 이응주 등, 2005], which is more focused on the absorption peak of CQ. On the other hand, the light emitted by lasers is highly narrow (monochromatic), coherent, and directional, and of the available lasers, the argon laser had been widely studied in dentistry in conjunction with dental composite resins due to its spectral match with CQ in the higher wavelength region of its absorption band (488 nm) [Mirsasaani et al., 2011; Tielemans et al., 2009; 박주석 등, 1995]. However, despite its excellent ability to polymerize dental composite resins, the argon laser has not been widely accepted as a LCU because of its cost. Under these circumstances, the diode-pumped solid state (DPSS) laser can provide an alternative. Initially, this type of laser was used as a light source for flow cytometers [Habbersett et al., 2007; Telford et al., 2005]. However, recent advancements in laser diode technology have resulted in the availability of high power DPSS and diode lasers of various wavelengths. The ability of a laser emitting at 473 nm has been examined for the curing of dental composite resins [Kwon et al., 2008; Park et al., 2010; Yun et al., 2012]. According to these studies, this DPSS laser is able to polymerize various dental composite resins to the levels achieved by other LCUs, such as, QTH and LED LCUs. This encourages the consideration that a diode laser of 457 nm, which lies in the absorption band of CQ, might be useful for the polymerization of dental dental composite resins. Due to the recent appearance, the test of this laser in dentistry is limited [권 등, 2017; 박, 2017]. Accordingly, the aim of this study was to evaluate the degree of polymerization of dental composite resins using the 457 nm diode laser as a light source. Through the study, microhardness and degree of conversion (DC) were examined and compared.

MATERIALS AND METHODS

1. Dental composite resins and LCUs

For the study six different dental composite resins with A3 shade [two nanocomposite resins: Premise Packable (PR) and Filtek Z350XT (ZX); two microhybrid resins: Aelite LS Posterior (AL) and Tetric EvoCeram (TE); two flowable resins: Tetric N Flow (TF) and Esthet X Flow] were used. Their details are listed in Table 1.

Table 1.

Materials tested in the present study

For light curing, a LED (L.E.Demetron, Kerr, Danbury, CT, USA) and a diode laser (LVI Technology, Seoul, Korea) of 457 nm were used (Figure 1). The output light intensity of LED was 900 mW/cm² according to its built-in radiometer, and the output power and spot size of the laser beam were approximately 150 mW (PM3/FIELDMAX, Coherent, Portland, OR, USA) and 6 mm, respectively. The resultant light intensity was approximately 530 mW/cm².

Figure 1.

Emission spectrum of the used LCUs

RESULTS

Table 2 shows the DC achieved by two LCUs. For the specimens, flowable resin EF had the highest DC regardless of the LCU used, and AL had the second highest. The DC achieved using two LCUs were not significantly different on top (54.4-65.9% for the laser; 55.2-67.1% for LED) and bottom surfaces (35.1-53.8% for the laser; 45.4-53.1% for LED).

Table 2.

Degree of conversion (DC; %) of the tested specimens

The microhardness of specimens is shown in Table 3. On top surface, microhardness by the laser and LED were 28.5-83.6 Hv and 19.1-82.4 Hv, respectively. On bottom surface, for the laser and LED treated specimens, microhardness were 22.5-65.4 Hv and 16.8-74.4 Hv, respectively. Difference between top and bottom surfaces was 18.0-65.9% and 8.3-42.7% for the laser and LED, respectively. The microhardness values of two LCUs were not significantly different on top and bottom surfaces (p>0.05).

Table 3.

Microhardness (Hv) of the tested specimens

DISCUSSION

Lasers provide highly monochromatic, coherent, and directional light as compared with other light sources. Due to these exceptional features, lasers have been used in many fields as a light source. In dentistry, the 488 nm argon laser is used for the light curing of camphorquinone-based dental materials [Mirsasaani et al., 2011; Tielemans et al., 2009; 박주석 등, 1995], and is commonly used for the restoration of decayed or damaged teeth in conjunction with dental composite resins. However, an argon laser represents a substantial investment, and thus, other types of LCUs, such as, QTH and LED, are more popular in dental practices. DPSS lasers produce light by pumping a solid medium (crystal) with a powerful laser diode, and then doubles the frequency of the light produced. Initially, a 532 nm DPSS laser was used as a light source for flow cytometers [Habbersett et al., 2007]. Recently, the feasibility of a 473 nm laser was examined for the light curing of dental composite resins [Kwon et al., 2008; Park et al., 2010; Yun et al., 2012]. The advantages of the 473 nm DPSS laser as compared with the argon laser are its portability and lower price. Among the lasers, the 457 nm laser was only recently introduced and has not been previously examined in dentistry.

Polymerization is a complex process that converts monomers to a polymer network via the activation of a photoinitiator (CQ) in the presence of an accelerator (a tertiary amine). The activation of CQ is initiated by the blue light from LCUs. However, since the bandwidth of LED and laser blue light is quite different, the resultant degree of polymerization can be influenced by the difference of emission bands.

DC provides a means of quantifying monomer conversion to polymer [Rueggeberg et al., 1990]. In the present study, the DC values of the specimens produced by two LCUs at different light intensity (530 mW/cm² vs 900 mW/cm² for the 457 nm laser and LED, respectively) were similar (on top surface: 54.4-67.7% for the laser, 55.2-67.1% for LED; on bottom surface: 35.1-53.8% for the laser, 45.4-53.1% for LED), although AL had a much lower DC value on its bottom surface when cured using the laser. This achievement of a similar DC by the laser at a lower light intensity suggests that monochromatic light can more effectively achieve polymerization.

Microhardness can be used to quantify the degree of polymerization, and the Vickers hardness test evaluates hardness by making a microindentation in the specimen surface. The dimension (size) of microindentation can be affected by many factors, such as, filler content, cross-link density, and monomer content [Ferracane et al., 1985; Peutzfeldt and Asmussen, 2004]. The similar microhardness achieved (on top and bottom surfaces) using two LCUs in the present study suggests that the 457 nm laser provides an excellent means of curing dental composite resins. However, lower microhardness was observed for some specimens (PR, Z3, and AL) treated with the laser on the bottom surface. This could have been due to their higher filler contents (fillers block light transmission to the bottom surface) or its lower light intensity. The highest microhardness decrease was found in AL. Based on our preliminary data, it was attributed to an exceptionally high level of light attenuation than the other specimens

In the present study, DC and microhardness were poorly correlated (R<0.26), showing that higher DC values do not automatically mean high microhardness [Lima et al., 2012; Calheiros et al., 2008; Bouschlicher et al., 2004]. Thus, since cross-link density, filler distribution, and condensation by shrinkage, are not fully reflected to the DC values, it can be less sensitive in estimating the degree of polymerization than by microhardness.

CONCLUSION

Within the limitations of the present study, the following conclusions could be reached:

• 1. Dental composite resins cured using the 457 nm laser were found to be similar to the same resins cured with LED in terms of microhardness and DC.
• 2. Accordingly, it is believed that the 457 nm laser has substantial potential as a light source for the light-curing of dental composite resins.

Acknowledgments

* 이 논문은 부산대학교 기본연구지원사업(2년)에 의하여 연구되었음.

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