Do high-viscosity bulk-fill composites polymerize sufficiently at deep layers?

Introduction
High-viscosity bulk-fill composites may polymerize insufficiently at deep layers after a short curing time. This in-vitro study aimed to estimate and compare the degree of conversion (DC) and micro-hardness (MH), following 20 and 40 seconds photopolymerization of three high-viscosity bulk-fill composites at 2-4 mm depth.

Material and methods
Three new brands of bulk-fill dental composites were selected for this study, including Beautifil-Bulk (Shofu Inc., Kyoto, Japan), Tetric N-Ceram Bulk Fill (Ivoclar Vivadent AG, Schaan, Liechtenstein), and Opus Bulk Fill (FGM, Joinville, SC, Brazil). For each composite, six disc samples located at a 2-4 mm depth were prepared via photopolymerization for 20 (n = 3) or 40 (n = 3) seconds. Each sample was 2 mm thick and 8 mm wide. Following one-hour post-cure, DC and MH assessments were applied using Fourier transform infrared spectroscopy (FTIR) and Vickers MH devices. Data were analyzed using parametric ANOVA and LSD tests.

Results
Values of DC and MH were higher following 40 seconds compared with 20 seconds photopolymerization. However, none of the composites reached the minimum accepted DC (≥ 55%). For both curing times, Tetric N-Ceram showed the highest DC% (42%, 51%), while Beautifil-Bulk had the highest MH values for top (58.2 ± 0.5, 60.0 ± 1.5) and bottom (47.5 ± 3.1, 53.5 ± 1.0) surfaces.

Conclusions
At a 2-4 mm depth and one-hour post-cure, bulk-fill composites showed insufficient DC and low MH values when photopolymerized for 20 or 40 seconds. The extension of curing time to 40 seconds improved the DC and MH values.