Composite monomer conversion and microhardness using short curing times

This study evaluated how short curing times affect monomer conversion and microhardness of a bulk-fill composite. Two light-curing units (LCUs) were used: a laser LCU (Monet, AMD Lasers) and a multi-peak LED LCU (PowerCure, Ivoclar Vivadent). A bulk-fill composite (Tetric PowerFill, Ivoclar) was cured for 1, 2, or 3 seconds with the laser, and for 3 or 20 seconds with the LED. Monomer conversion was measured via real-time FTIR at the bottom of 4-mm composite cylinders. Microhardness was assessed at 100µm, 1mm, 2mm, 3mm, and 390µm depths. Spectral irradiance, radiant emittance, and radiant exposure of each LCU were recorded using a calibrated system. Data were analyzed using ANOVA and Tukey's test (α = 0.05). Radiant exposures for the laser LCU were 2.5, 5.0, and 7.5 J/cm² for 1, 2, and 3 seconds, respectively. The LED LCU delivered 9.3 J/cm² (3 s) and 22.3 J/cm² (20 s). Monomer conversion at 4 mm increased with more prolonged exposure for both LCUs. The LED LCU at 20 seconds showed significantly higher conversion than at 3 seconds. Microhardness decreased with depth, regardless of curing method or duration. Monomer conversion at 4 mm was higher with increased curing time, correlating with radiant exposure. Only the 20-second LED exposure provided a bottom-to-top microhardness ratio of 0.84, indicating sufficient curing at depth.