Impact of Different LED Units, Tip Distance and Time of Light Exposure on Microhardness of Resin-Based Composite for Posterior Teeth

Background: The aim of this in vitro study was to reveal in a time-dependent manner the effect of received irradiance produced by different light-emitting-diode (LED) lamps on the surface microhardness of composite material used in the restoration of posterior teeth. Methods and Results: A single nanofilled resin composite, Estelite Posterior (Toquyama Dental Corp, Japan) shade PA3, was used to perform this in vitro study. Ninety cylindrical filling samples were made with the help of plastic ring molds and plates to obtain flat surfaces. The mean diameter and height of prepared specimens were 5.5±0.05mm and 2.5±0.1mm, respectively. For the polymerization of the composite, two different types of light-curing units (LCU) were selected for this study: Valo X (Ultradent Products, Inc., USA) operated in a “standard” mode, and Bluephase 20i (Ivoclar Vivadent, Liechtenstein) ran in a “high” mode. Photoactivation was done with the light tips fixed parallel to the surface of the filling material and above it at a distance level of 1 mm, 4 mm, and 8 mm. The radiant power (mW) of each LCU was registered in a PL-MW2000 Optical power meter (Beijing Perfectlight Technology Co., Ltd., China). A total of 18 groups, with five filling samples in each, were formed and processed. Upon the completion of light treatment, by the ISO-4049 protocol, each specimen was immersed in water and stored at 37°C for 24 hours in a lightproof container. After this period, samples were washed and tested for the hardness of top surfaces using a “ПMT-3” Vickers hardness tester. The means of light intensities of two LCUs in the study were significantly different at 4 mm and 8 mm tip distances in 1.26 and 2 times, respectively. Analysis of hardness values did not reveal significant differences in the light-curing efficacy of two LED lamps. The differences in VHN values were obvious among the 10- and 40-second time exposure groups when the photoactivation was carried out at an 8mm tip distance. Conclusion: The effective polymerization of composite restorations with poor accessibility is still a matter of time, not the power of LCU.