Comparison of flowable bulk-fill and flowable resin-based composites: an in vitro analysis

Flowable bulk-fill resin bonded composites (RBCs) are supposed to show improved abrasion resistance and fracture toughness in comparison to flowable conventional RBCs. Specimens of eight flowable RBCs (5x flowable conventional, 3x flowable bulk-fill) were fabricated for testing relative fracture toughness (SENB), relative three-body wear, the Vickers hardness, glass transition Tg (differential scanning calorimetry; DSC) and filler mass fraction (thermal gravimetric analysis; TGA). A laboratory veneering composite was used as a reference. Fracture toughness and wear values were related to this reference. Scanning electron microscope images were evaluated for fraktographical and microstructural investigations. Statistical analyses were performed using one-way Anova, the Bonferroni post hoc test and the Pearson correlation test (alpha =0.05). Relative fracture toughness varied between 0.64 and 1.34 (1.00 = 1.69 MPamA1/2) and relative wear rates between 1.24 and 0.55 (1.00 = 134 mu m). The Vickers hardness ranged between 14.4 and 57.2 HV. TGA showed filler fractions between 55 and 77 wt.%. Tg values varied between -67.8 and -40.9 A degrees C. None of the tests identified clear differences between flowable bulk-fills or conventional flowable RBCs. The Pearson correlation coefficient (cc) showed significant correlations (cc > 0.583; p < 0.001) between relative fracture toughness and hardness or filler content. There was a significant correlation (cc = 0.757; p = 0.005) between relative wear and glass transition temperature or between filler fraction and the Vickers hardness (cc > 0.702; p < 0.001). For all filler sizes breakdown was found, where clusters and pre-polymerized particles were partly disintegrated. Flowable bulk-fill RBCs showed no improved abrasion resistance and fracture toughness in comparison to flowable conventional RBCs. Differences in the properties were higher between the individual materials than between the material groups. Therefore the appropriate material selection may be essential for a clinical success.