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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.

Background: This study aimed to reveal the effect of preheating on the surface microhardness and shear strength of composite materials used in the restoration of posterior teeth. Methods and Results: There were 3 composite materials under the study: Estelite Posterior, Harmonize and Filtek Z550. To make static and dynamic tests of them, 120 filling samples were prepared. Of those, 60 samples were for surface hardness measurements and 60 samples were used to evaluate the shear strength of composite materials. We formed 12 study groups with 10 filling samples in each. Samples made off Estelite Posterior, Harmonize™, and Filtek™ were designated with E, H, and F capital letters, respectively; the "VH" abbreviation indicated static Vickers hardness testing and "SS" was assigned for dynamic shear testing; mark (°) was used when preheating was applied. Filling samples were made of heated (up to 60°C) and room-temperature (23-25°C) composite materials. The filling samples of EVH, EºVH, HVH, HºVH, FVH, and FºVH groups were subjected to a surface microhardness test. The samples of ESS, EºSS, HSS, HºSS, FSS, and FºSS groups were subjected to shear-strength assessment of materials. The surface microhardness of filling samples was measured using a ПMT-3 Vickers hardness tester and the Vickers hardness number (VHN) was calculated. Dynamic tests were carried out using an UltraTester machine (Ultradent, Inc., USA) and shear test method until the shear-strength (SS) filling sample had completely failed. After analysis of the obtained results, it was found that preheating had enhanced the surface hardness and mechanical strength of the composite materials used in the study. However, the positive influence of preheating was significant only in the EVH-EºVH, ESS-EºSS, HSS-HºSS, and FSS-FºSS groups in 1.21, 1.09, 1.33, and 1.16 times, respectively. In the HVH-HºVH and FVH-FºVH groups, the identified differences were not of significance despite the improvement in mean values at 1.08 and 1.1 times. Conclusion: Preheating of light-curing resin-based composites is not equally effective for static and dynamic mechanical properties of materials for dental restoration. Preliminary laboratory tests could have helped before their clinical use.