Impact of high irradiance and short exposure times on the polymerization kinetics of sculptable and flowable bulk-fill composites

Objective: To determine the presence of exposure reciprocity in nine resin-based composites (RBCs) using fast (3 s, high irradiance) or standard light-curing modes for 10 s, 20 s, or 30 s. Methods: Five sculptable and four flowable bulk-fill RBCs were light-cured using a Bluephase PowerCure light-curing unit (LCU). The RBCs were divided into groups according to the exposure modes tested: fast (3 s mode – at 3130 mW/cm²), or for the manufacturer's recommended time on the standard (High mode at 1226 mW/cm²). The degree of conversion (DC) and reaction rates were measured at 0.2 mm and 4 mm depths for 720 s using real-time spectroscopy. Light transmission (%) was also recorded (n = 3). Parametric or non-parametric 2-way ANOVA was used depending on the data (α=0.05). Results: When compared to the values from the bottom of their 0.2 mm thick value, except for TPFlow-3 s and TPlusFlow-3 s, the bottom of the 4-mm thick specimens had a significantly different DC. Only TPF and TPFlow were statistically the same for both modes at 4 mm; ABF-3 s and OBF-3 s had the lowest DC at the bottom of 4 mm. OBF-3 s had the highest reaction rate at 0.2 mm (61.6 ± 1.2 %/s) while FOBF-10 s had the slowest rate (3.1 ± 0.1 %/s) at the bottom of 4 mm. OBF, BRFlow, TPFlow, TplusFill and TplusFlow had different light transmission patterns than the other RBCs. Depending on the brand of RBC, there was a measurable delay of 0.1 to 2.9 s at the bottom of the RBCs before the polymerization reaction started. Conclusions: Exposure reciprocity does not exist for all RBCs. Depending on the brand of RBC at the bottom of 4 mm there was a measurable delay of 0.1 to 2.9 s before polymerization started. Clinical Significance: Exposure reciprocity depends on the RBC. Short exposures (3 s) using higher irradiance should only be used when recommended by the manufacturer of the RBC.