Purpose: To evaluate the influence of different curing distances with three types of lights in terms of the surface microhardness of a resin composite as a function of power density.
Methods: 90 cylindrical light-cured resin composite specimens were polymerized with three light curing units: (a) Mini LED AutoFocus with a fast curing mode (1955 mW/cm2); (b) LEDemetron I (1541 mW/cm2) and (c) Optilux 401 QTH (1294 mW/cm2) for 40 seconds. Polymerization was performed with the curing tip at a distance of 0 mm, 3.0 mm, 6.0 mm, 9.0 mm, 12.0 mm and 15.0 mm from the top surface of the specimen. 15-25 minutes after photocuring, the top and bottom specimen's surface KHNs were determined. Microhardness values were measured again after 24-hour storage in distilled water in a light proof container at 37 degrees C. Data were submitted to ANOVA and Multiple Comparisons with a Tukey test. The KHNs before and after storage in distilled water were analyzed using a paired t-test. Linear regression analysis was also performed and all statistical analyses were performed at a significance level of 0.05.
Results: There was a linear relationship between the logarithm of the power density and the curing distance. The light curing unit and curing tip distance had a significant effect on the composite microhardness. Specimens cured with the Mini LED AutoFocus light provided higher KHNs than those photoactivated with the LEDemetron I light and Optilux 401 light. There was no statistically significant difference in KHNs between the LEDemetron I group and the Optilux 401 group. The mean Knoop microhardness values decreased significantly as the curing tip distance increased. Fifteen minutes after light cure, effective hardness percent values (> 80%) were achieved in a few experimental groups. A linear correlation was found between the logarithm of the power density and the hardness percent values. Twenty-four hours after light irradiation, most of the experimental groups obtained valid hardness percent values. After storage in distilled water for 24 hours, the KHN of both the top and bottom surfaces in most of the groups increased significantly. There were no significant increases in the mean relative hardness ratios related to surface or light curing device.