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

Objectives: Flowable bulk-fill resin bonded composites (RBCs) are supposed to show improved abrasion resistance and fracture toughness in comparison to flowable conventional RBCs.

Materials and methods: Specimens of eight flowable RBCs (5× flowable conventional, 3× 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 (α =0.05).

Results: Relative fracture toughness varied between 0.64 and 1.34 (1.00 = 1.69 MPam½) and relative wear rates between 1.24 and 0.55 (1.00 = 134 μm). 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 °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.

Conclusions: Flowable bulk-fill RBCs showed no improved abrasion resistance and fracture toughness in comparison to flowable conventional RBCs.

Clinical relevance: 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.