Effect of different curing protocols on the mechanical properties of low-viscosity bulk-fill composites.

Objectives: This study aimed to assess the impact of curing conditions-exposure time, mode, energy density and exposure distance-on the efficiency of curing flowable bulk-fill resin-based composites (RBCs) at simulated clinical relevant filling depths.
Materials and Methods: Four flowable bulk-fill RBCs were investigated by assessing in 200 μm steps the variation in micro-mechanical properties (Vickers hardness (HV) and indentation modulus (E)) within simulated 6-mm-deep fillings (n = 5), considering 16 different curing modes/conditions. The exposure duration was 5, 20 and 40 s in the standard power mode; 3, 4 and 8 s in the high power mode; and 3 and 6 s in the plasma mode. Besides, the curing unit was placed at 0 and 7 mm distance away from the specimen's surface. Measurements were performed after 24 h of storage in distilled water at 37 °C. The depth of cure (DOC) was calculated as the 80 % hardness drop-off. The curing unit's irradiance at exposure distances up to 10 mm was monitored for all irradiation modes in 1 mm steps by means of a laboratory-grade spectrometer.
Results: Results were compared using one- and multiple way ANOVA and Tukey's honest significant difference (HSD) post hoc test (α = 0.05). A multivariate analysis (general linear model) assessed the effect strength of the parameters material, energy density reaching the specimen's surface (2.63 to 47.03 J/cm(2)), exposure distance and curing mode on HV, E and DOC. The effect of the parameter material was significant and strong on all measured properties (p < 0.05, partial eta-squared (η P (2)) = 0.683 for E, 0.724 for HV and 0.199 for DOC). Energy density exerted in all materials the strongest influence on the measured properties, while the influence of distance was strong on DOC and low or even not significant on HV and E.
Conclusions: The susceptibility to variation in irradiance under the simulated clinical conditions was material dependent, while lower and upper energy density limits for curing the materials were defined.
Clinical Relevance: Materials react differently to the supplied irradiance. An exposure time of 20 s at moderate irradiance is recommended for all materials. The highest mechanical properties were reached not at the specimens' surface, but in deeper layers (0.4 to 3.1 mm).