In Vitro Comparison of Mechanical Properties and Degree of Cure of a Self-adhesive and Four Novel Flowable Composites

Purpose: To compare a self-adhesive with two low-shrinkage and two regular flowable resin-based composites (RBCs) in terms of degree of conversion (DC) as well as micromechanical (indentation modulus [E]; Vickers hardness [HV]) and macromechanical properties (flexural strength [σ], modulus of elasticity [Eflexural]). Materials and Methods: DC was investigated by an ATR-FTIR spectrometer at clinically relevant filling depths (0.1 mm, 2 mm, 4 mm, 6 mm bulk, 6 mm incremental) and irradiation times (10 s, 20 s, 40 s). Micro- and macromechanical properties were measured with an automatic microhardness indenter and a three-point bending test device after curing the specimens for 20 s and storing them in distilled water for 24 h at 37°C. Fillers were visualized using a field emission scanning electron microscope (FE-SEM). Results were evaluated using one-way ANOVA, Tukey's HSD post-hoc test, Pearson's correlation and a multivariate analysis (α = 0.05). A Weibull analysis was used to assess σ. Results: N'Durance Dimer flow (65.66%) reached the highest DC (at 2 mm depth, 20 s irradiation). In terms of macromechanical properties, EcuSphere-Flow (129.82 MPa) for σ and Synergy D6 Flow (3.74 GPa) for Eflexural reached the highest values. The highest micromechanical properties were measured for the self-adhesive RBC (Vertise Flow; E = 10.81 GPa, HV = 60.20 N/mm2). Reliability was highly influenced by filler weight (η2 = 0.77) and volume (η2 = 0.99) proportion. Conclusion: In the present study, the self-adhesive RBC showed the highest reliability, highest DC (together with one of the low-shrinkage RBCs), and highest micromechanical properties as well as good macromechanical properties. Moreover, a curing time of 40 s and an incremental thickness not exceeding 2 mm appeared to be necessary for such results. Keywords: self-adhesive flowable composites, degree of cure, mechanical properties