Influence of light energy density, composite type, composite thickness, and postcuring phase on degree of conversion of bulk-fill composites

Context: Achieving a high degree of conversion (DC) is one of the major concerns during photopolymerization of bulk-fill composites. Aims: To evaluate the effect of light energy densities (11.2 J/cm² and 20 J/cm²) on the DC and variation of DC in the 24-h postcuring of four bulk-fill composites: SDR, Venus Bulk Fill, MI FIL, and Tetric N-Ceram Bulk Fill at simulated clinically relevant filling depths. Settings and Design: This was an in vitro comparative study. Subjects and Methods: A total of twenty samples were prepared using a teflon mold. VALO curing light was used with two light intensity modes of 1000 mW/cm² for curing time of 20 s and 1400 mW/cm² for curing time of 8 s. The energy density was calculated as follows: energy density (J/cm²) is the light intensity (mW/cm²) applied during a certain time (s) divided by 1000. The DC was measured at two time intervals: immediately postcure and after 24-h storage in artificial saliva using an Fourier-transform infrared spectroscopy equipped with attenuated total reflectance accessory. Statistical Analysis Used: ANOVA and Bonferroni test at P < 0.05. Results: High energy density (20 J/cm²) leads to higher DC. Thickness, type of composites, and postcuring phase strongly influence the DC. DC values of the top surface for all the bulk-fill materials investigated were found significantly greater (P < 0.005) than those of their bottom surface. Among composites, SDR showed highest DC. DC strongly increased after 24-h postcure by 32% on top surface and 76% on bottom surface. Conclusions: Energy density more than 20 J/cm², derived by increasing curing time and low power density, helps obtain a high DC of bulk-fill composites for adequate clinical performance.