Curing Depth and Degree of Conversion of Five Bulk-Fill Composite Resins Compared to a Conventional Composite

Background: Limited curing depth and its effect on the degree of conversion are among the challenges of working with light-cure composite resins. The use of bulk-fill composites is one strategy to overcome these limitations. Methods: Ever X Posterior (EXP), Filtek Bulk-Fill Posterior (FBP), Sonic Fill 2 (SF2), Tetric N-Ceram Bulk-Fill (TNB), and X-tra Fil (XF) bulk-fill and Filtek Z250 conventional composite were evaluated in this in vitro experimental study. Six samples for the assessment of microhardness and three samples for the evaluation of DC were fabricated of each composite. After light curing and polishing, the samples were incubated at 37'C for 24 hours. Microhardness was measured by a Vickers hardness tester three times and the mean value was calculated. DC of the top and bottom surfaces was determined using Fourier-Transform Infrared Spectroscopy (FTIR). Data were analyzed using one-way ANOVA and Tukey's test. Results: Microhardness and DC were significantly different among the groups (P<0.001). XF and Z250 equally showed the highest bottom-to-top surface microhardness ratio (0.97 +/- 0.01) and significantly higher DC in the top (P<0.001) and bottom (P<0.005) surfaces compared to other groups. TNB showed the lowest microhardness ratio (0.88 +/- 0.04) and DC (68.66 +/- 1.52 and 61.00 +/- 2.00); the difference in DC of the bottom surface was statistically significant (P<0.003). Conclusion: It appears that bulk-fill composites evaluated in this study are adequately polymerized at 4 mm depth. Their DC was optimal and within the range of conventional composites.