How light attenuation and filler content affect the microhardness and polymerization shrinkage and translucency of bulk-fill composites?

The purpose of this study was to investigate the microhardness, polymerization shrinkage, and translucency of bulk-fill composites (BFCs) which have different light attenuation properties and filler contents. Five BFCs [Filtek Bulk Fill (FB), SureFil SDR (SS), Venus Bulk Fill (VB), SonicFill (SF), and Tetric N-Ceram Bulk Fill (TB)] and two resin-based composites (RBCs) [Tetric N-Ceram (TN) and Filtek Z350XT Flowable (ZF)] were chosen. Numbers of transmitted photons, refractive index, microhardness at different thicknesses, polymerization shrinkage, and translucency parameter (TP) were evaluated. Attenuation coefficients (ACs) based on measured photons ranged from -0.88 to -1.90. BFCs, except SF, had lower AC values than TN or ZF, and BFCs, except TN, had smaller refractive index decreases between top and bottom surfaces. Regardless of an exponential decrease in photon counts, microhardness decreased linearly as specimen thickens. Moreover, microhardness of BFCs showed smaller top-to-bottom decreases (11.5-48.8 %) than TN or ZF (57.3 and 71.5 %, respectively). BFCs with low filler contents showed lower microhardness and higher polymerization shrinkage than those of high filler contents. Also, BFCs had significantly higher (p < 0.001) TP values than TN or ZF. BFCs attenuated light less than RBCs. However, some BFCs had much lower top surface microhardness and higher polymerization shrinkage than tested RBCs. Despite the convenience of bulk filling, careful selection of BFC is advised because some BFCs have worse microhardness and polymerization shrinkage than RBCs due to their lower filler contents.