Photoinitiator dependent composite depth of cure and Knoop hardness with halogen and LED light curing units

Light curing units (LCUs) are used for the polymerization of dental composites. Recent trends in light curing technology include replacing the halogen LCUs with LCUs using light emitting diodes (LEDs) reducing curing times and varying the LCUs light output within a curing cycle. This study investigated the time dependence of the Knoop hardness and depth of cure of dental composites polymerized with a halogen LCU (Trilight) and two LED LCUs (the commercial Freelight and custom-made LED LCU prototype). The halogen LCU was used in the soft-start (exponential increase of output power) and standard mode. Four dental composites (Z100, Spectrum, Definite, Solitaire2) were selected, two of them (Definite, Solitaire2) contain co-initiators in addition to the standard photoinitiator camphorquinone. The depth of cure obtained with the Trilight in the standard mode was statistically significantly greater (p < 0.05) than that obtained with the LED LCUs for all materials and curing times. The custom made LED LCU prototype (LED63) achieved a statistically significantly greater depth of cure than the commercial LED LCU Freelight for all materials and curing times. There was no statistical difference in Knoop hardness at the 95% confidence level at the surface of the 2 mm thick sample between the LED63 or Trilight (standard mode) for the composite Z100 for all times, and for Spectrum for 20s and 40s curing time. The composites containing co-initiators showed statistically significantly smaller hardness values at the top and bottom of the samples if LED LCUs were used instead of halogen LCUs. The experiment revealed that the depth of cure test does not and the Knoop hardness test does discriminate between LCUs, used for the polymerization of composites containing photoinitiators in addition to camphorquinone.