Effectiveness of resin composite polymerization when cured at different depths and with different curing lights

This study evaluated how photocuring devices affected the microhardness of composite resin cylinders. For this study, 120 specimens of composite were fabricated and allocated randomly into 12 groups (n = 10), according to the light source (quartz-tungsten-halogen (QTH), LED, argon laser, plasma arc) and the height of the specimen (2.0, 4.0, or 6.0 mm). Twenty-four hours after the specimens were fabricated, the Knoop microhardness test was performed on bottom and top surfaces. Statistical analysis showed significant interaction among light sources, between light sources and specimen heights, and between the surfaces. Compared to the QTH specimens, the argon laser and plasma arc specimens showed reduced polymerization on the top surface, while the plasma arc specimens showed reduced polymerization on the bottom surface. The 4.0 mm samples demonstrated higher Knoop microhardness than the 2.0 mm and 6.0 mm samples, especially when argon laser and plasma arc curing lights were used. The microhardness was always higher on the top surface than on the bottom surface. No photocuring unit was able to properly polymerize the bottom surface as completely as the top surface.