Effect of polymerization of bulk fill composite resins in different layer thicknesses and light irradiance on the degree of conversion, micro hardness and cytotoxicity of the composite

Purpose: The aim of this study is to evaluate the effects of polymerization of bulk fill composite resins in different layer thicknesses and different light power modes on the degree of conversion, micro hardness and cytotoxicity of composites. Materials and Methods: This study is an experimental study and two different bulk fill composite resins named Sonic Fill (SF) and Filtek Bulk Fill (FBF) were used in this study. A total of 180 composite samples with 5 mm diameter, 2, 4 and 6 mm thicknesses and polimerized with 2 different power modes (1000mW/cm2-20 sn and 3200 mW/cm2- 3sn) were prepared for the degree of conversion, micro hardness and cytotoxicity analaysis. The degree of conversion was measured with Fourier Transform Infrared Spectrometry, micro hardness measurement with the Vickers hardness tester from the lower and upper surfaces of the samples. Cytotoxicity was measured on days 1,7 and 21 using fibroblast cells (L929) with the WST-1 test. Statistical analysis of the data obtained was made with SPSS 22 program. Results: As the layer thickness of bulk fill composite resins increased, the degree of transformation in the lower surfaces of the composites were decreased significantly(p<0,05). There were no statistically significant difference except FBF 4 mm and 6 mm thickness samples. When the upper and lower surface micro hardness ratios of Bulk fill composite resins were examined, samples of both composites prepared in 2 mm thickness polymerized by both polymerization methods and 4 mm thickness samples polymerized in standard power mode have exceeded the acceptable threshold value of 0.80. According to the results of the WST-1 test; on the 7th day, the highest cell viability was seen in the 2 mm thick sample of Sonic Fill 2 composite resin polymerized in standard power mode. According to the results of WST-1 experiment; as the layer thickness of bulk fill composites increased, cell viability decreased. Conclusions: Although bulk fill composite resins provide ease of application and time savings, they may have insufficient physical and mechanical properties due to increased layer thickness and may show cytotoxic effects.