In vitro comparison of mechanical properties and degree of cure of bulk fill composites

The aim of our study was to measure and compare degree of conversion (DC) as well as micro- (indentation modulus, E; Vickers hardness, HV) and macromechanical properties (flexural strength, σ; flexural modulus, E (flexural)) of two recently launched bulk fill resin-based composites (RBCs): Surefil® SDR™ flow (SF) and Venus® bulk fill (VB).

DC (n = 6) was investigated by Fourier transform infrared spectroscopy (FTIR) in clinical relevant filling depths (0.1, 2, and 4 mm; 6 mm bulk, 6 mm incremental) and irradiation times (10, 20, 40 s). Micro- (n = 6) and macromechanical (n = 20) properties were measured by an automatic microhardness indenter and a three-point bending test device after storing the specimens in distilled water for 24 h at 37°C. Furthermore, on the 6-mm bulk samples, the depth of cure was determined. A field emission scanning electron microscope was used to assess filler size. Results were evaluated using one-way analysis of variance, Tukey's honest significance test post hoc test, a multivariate analysis (α = 0.05) and an independent t test. Weibull analysis was used to assess σ.

VB showed, in all depth, significant higher DC (VB, 62.4-67.4 %; SF, 57.1-61.9 %), but significant lower macro- (VB, E (flexural) = 3.6 GPa; σ = 122.7 MPa; SF, E (flexural) = 5.0 GPa; σ = 131.8 MPa) and micromechanical properties (VB, E = 7.3-8.8 GPa, HV = 40.7-46.5 N/mm²; SF, E = 10.6-12.2 GPa, HV = 55.1-61.1 N/mm²). Both RBCs showed high reliability (VB, m = 21.6; SF, m = 26.7) and a depth of cure of at least 6 mm at all polymerization times. The factor "RBC" showed the strongest influence on the measured properties (η (2) = 0.35-0.80) followed by "measuring depth" (η (2) = 0.10-0.46) and "polymerization time" (η (2) = 0.03-0.12).

Significant differences between both RBCs were found for DC, E, σ, and E (flexural) at all irradiation times and measuring depths.

Curing the RBCs in 4-mm bulks for 20 s can be recommended.