Influence of increment thickness on microhardness and dentin bond strength of bulk fill resin composites

ObjectivesTo investigate the influence of increment thickness on Vickers microhardness (HV) and shear bond strength (SBS) to dentin of a conventional and four bulk fill resin composites.MethodsHV and SBS were determined on specimens of the conventional resin composite Filtek Supreme XTE (XTE) and the bulk fill resin composites SDR (SDR), Filtek Bulk Fill (FBF), x-tra fil (XFIL), and Tetric EvoCeram Bulk Fill (TEBF) after 24 h storage. HV was measured either as profiles at depths up to 6 mm or at the bottom of 2 mm/4 mm/6 mm thick resin composite specimens. SBS of 2 mm/4 mm/6 mm thick resin composite increments was measured to dentin surfaces of extracted human molars treated with the adhesive system OptiBond FL, and the failure mode was stereomicroscopically determined at 40× magnification. HV profiles and failure modes were descriptively analysed whereas HV at the bottom of resin composite specimens and SBS were statistically analysed with nonparametric ANOVA followed by Wilcoxon rank sum tests (α = 0.05).ResultsHV profiles (medians at 2 mm/4 mm/6 mm): XTE 105.6/88.8/38.3, SDR 34.0/35.5/36.9, FBF 36.4/38.7/37.1, XFIL 103.4/103.9/101.9, TEBF 63.5/59.7/51.9. HV at the bottom of resin composite specimens (medians at 2 mm/4 mm/6 mm): XTE (p < 0.0001) 105.5 > 85.5 > 31.1, SDR (p = 0.10) 25.8 = 21.9 = 26.0, FBF (p = 0.16) 26.6 = 25.3 = 28.9, XFIL (p = 0.18) 110.5 = 107.2 = 101.9, TEBF (p < 0.0001) 63.0 > 54.9 > 48.2. SBS (MPa, medians at 2 mm/4 mm/6 mm): XTE (p < 0.0001) 23.9 > 18.9 = 16.7, SDR (p = 0.26) 24.6 = 22.7 = 23.4, FBF (p = 0.11) 21.4 = 20.3 = 22.0, x-tra fil (p = 0.55) 27.0 = 24.0 = 23.6, TEBF (p = 0.11) 21.0 = 20.7 = 19.0. The predominant SBS failure mode was cohesive failure in dentin.SignificanceAt increasing increment thickness, HV and SBS decreased for the conventional resin composite but generally remained constant for the bulk fill resin composites.