Effect of reduced exposure times on the microhardness of 10 resin composites cured by high-power LED and QTH curing lights

Purpose: To compare the effect of reduced exposure times on the microhardness of resin composites cured with a "second-generation" light-emitting diode (LED) curing light and a quartz-tungsten-halogen (QTH) curing light. Methods: Ten composites were cured with a LED curing light for 50% of the manufacturers' recommended exposure time or a QTH light at the high power setting for 50% of the recommended time or on the medium power setting for 100% of the recommended time. The composites were packed into Class I preparations in extracted human molar teeth and cured at distances of 2 or 9 mm from the light guide. The moulds were separated, and the Knoop microhardness of the composites was measured down to 3.5 mm from the surface. Results: The LED light delivered the greatest irradiance at 0 and 2 mm, whereas the QTH light on the standard (high power) setting delivered the highest irradiance at 9 mm. According to distribution-free multiple comparisons of the hardness values, at 2 mm from the light guide the LED light (50% exposure time) was ranked better than or equivalent to the QTH light on the high power setting (50% exposure time) or on the medium power setting (100% exposure time). At 9 mm, the LED light was ranked better than or equivalent to the QTH light (both settings) to a depth of 1.5 mm, beyond which composites irradiated by the LED light were softer (p < 0.01). At both distances, the QTH light operated on the high power setting for 50% of the recommended exposure time produced composites that were as hard as when they were exposed on the medium power setting for 100% of the recommended exposure time. Conclusions: The ability to reduce exposure times with high-power LED or QTH lights may improve clinical time management.