Polymer and Mechanical Characteristics of Bulk-fill, Low-shrinkage and Hybrid Composites

Objectives: To compare degree of conversion (DC), cross-link density (CLD), microhardness (VHN) and flexural strength (FS) of sculptable bulk-fill, low-shrinkage, nanohybrid and microhybrid composites.
Methods: Bulk-fill composites Filtek Bulk Posterior Restorative (Filtek Bulk, 3M ESPE), Tetric EvoCeram Bulk Fill (TEC Bulk, Ivoclar Vivadent), EverX Posterior (EverX, GC) and Beautifil Bulk Restorative (Beautifil, Schofu), low-shrinkage Kalore (GC), nanohybrid Tetric EvoCeram (TEC, Ivoclar Vivadent) and microhybrid Filtek Z250 (Z250, 3M ESPE) were prepared (N=5/group) in standardized molds, 4mm or 2mm deep for DC and VHN and 2x2x25mm for FS. DC was determined using micro-Raman spectroscopy, VHN using Vickers microhardness tester and FS using a universal testing machine. Following polishing, surface uncured C=C bonds were quantified by repeating DC 48h post-immersion in ethanol. CLD was determined using the alcohol softening method. Data were statistically analyzed using analyses of variance and paired t-tests (α=0.05).
Results: Initial DC of bulk-fill composites (range: 56-70%) was comparable to hybrid control composites and higher than low-shrinkage Kalore (mean±SD: 50±2%; p<0.05). DC increased by up to 6% following ethanol immersion. Initial microhardness was similar for Filtek Bulk, EverX and Beautifil (72-90 VHN, p>0.05) but significantly higher than TEC Bulk and Kalore (55-66 VHN, p<0.05)). Initial VHN of TEC decreased by 55% after ethanol softening, followed by EverX and Kalore (by about 40%). FS of Filtek Bulk and EverX (110-140 MPa) was significantly higher than that of all other tested composites (70-100 MPa, p<0.05) except Z250 (117±6 MPa, p>0.05).
Conclusions: DC of bulk-fill composites was similar to hybrid composites. Uncured, potentially leachable, material remains on composite surface even after removing the oxygen-inhibition layer by polishing. EverX and Kalore were characterized by more linear polymers than Filtek Bulk and Beautifil. Mechanical properties of bulk-fill and low-shrinkage composites were similar or better than nanohybrid TEC but similar or inferior to microhybrid Z250.