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

Abstract

Objectives: 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).

Materials and methods: 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.

Results: 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).

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

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