The effect of different exposure routines on the surface microhardness of a light-activated resin composite

Objectives: To compare the surface microhardness of a resin composite following exposure to a pulse-delayed activation routine and a standard continuous routine using a quartz-halogen lamp. Additionally, to assess the effect of increasing the distance between the activator light and the specimen using a continuous exposure routine. Method and materials: Disk specimens of a commercial resin composite restorative material were exposed to a quartz-halogen lamp in a three-step pulse-delayed routine. A continuous exposure was used as a control. Surface microhardness was recorded at the end of each step and at 1 hour after initial exposure. The data yielded four groups and the control (n = 15). Further specimens were exposed to the activator light held at a predetermined distance from the specimen. Results: The pulse-delayed routine yielded a progressive and significant increase in mean microhardness at the end of each step, and the maximum mean value was significantly lower than the control. The effect of increasing the exposure distance yielded a significant decrease in mean hardness number with increasing distance for the distances selected. Regression analysis indicated a positive linear relationship between microhardness and the natural logarithm of time and a negative correlation between microhardness and distance, respectively. Conclusion: The modified activation routines investigated resulted in significantly reduced surface microhardness numbers compared with the controls.