In vitro assesment of bulk-fill composite resins in terms of conversion degree and microhardness

The aim of this in vitro study was to evaluate bulk fill composite resins in terms of microhardness and conversion degree. In the study, 5 bulk fill composite resins [Filtek Bulk Fill Posterior (FB), 3M ESPE; Tetric EvoCeram Bulk Fill (TBF), Ivoclar Vivadent; SonicFill 2 Bulk Fill (SF), Kerr Co.; SDR Bulk Fill (SDR), Dentsply and Venus Bulk Fill (VB), Heraeus Kulzer] and 1 conventional composite resin [Filtek Ultimate (FU), 3M ESPE] were used. Teflon molds with a diameter of 6 mm and 1, 2, 3, 4 or 5 mm in height were prepared and composite resins were placed in these molds to prepare 30 samples (n = 6) for each composite resin in total 180 samples. Then, the bottom and top surface microhardness [Vickers hardness (HV)] values were determined. The conversion degrees (DC) were calculated by dividing the top surface to the bottom surface hardness value of the samples. DC and HV values were statistically evaluated using the Kruskal Wallis test and Dunn Sidak correction (p = 0.05). The highest top surface HV value at all thicknesses was measured on the SF group at 3 mm (116.52 ± 1.32 N / mm2), and the lowest bottom HV value was measured on the SDR group at 5 mm (37.78 ± 1.59 N / mm2) (p<0.05). The highest bottom surface HV value was measured on the SF group at 1 mm (109.41 ± 1.41 N/mm2), and the lowest bottom HV value was measured on the SDR group at 5 mm (33.47 ± 1.26 N/mm2) (p<0.05). DC % decreased with increasing thickness in all materials and VB group showed the highest DC % (97.07 %) at 1 mm thickness, FU showed the lowest DC (47.36 %) at 5 mm thickness (p<0.05). In this study, DC % were above the threshold value of 80 % in all the composite resin groups at 1, 2 and 3 mm thicknesses (p<0.05). In all groups, as thickness of the composite increased, the bottom surface HV values decreased and it was found that composite resin type affected HV and DC values.