Effect of light source and specimen thickness on the surface hardness of resin composite

Purpose: To compare the Knoop hardness of different thicknesses of resin composite irradiated using either a plasma arc curing (PAC) light, or a quartz tungsten halogen (QTH) light. Materials and Methods: A 2.5-mm diameter hole was drilled through the center of 2, 3, 4, and 5-mm thick discs of enamel and dentin. This hole was filled with either Filtek P60 or Prodigy Condensable composite and irradiated in bulk using: (a) a PAC light with an 8-mm, 470 nm light guide for 3 seconds, (b) a QTH light with an 8-mm standard light guide for 40 seconds (QTH+S), or (c) a QTH light with a 13/8-mm Turbo light guide for 40 seconds (QTH+T). Immediately after irradiating the composite, the surface hardness was measured at the bottom and then the top. Hardness values were remeasured after 24-hour and 7-day storage in water at 37°C. The hardness values were compared using the Analysis of Variance for Repeated Measures and Least Squares Means test for multiple comparisons (LSM test) at P< 0.05. Results: The power density delivered by each light was: PAC over 1999 mW/cm², QTH+S 588 mW/cm², and QTH+T 844 mW/cm². Both the thickness of the composite and the light source had a significant effect on the hardness. Overall, using the QTH+T produced the highest hardness values and the PAC light the lowest values. For all 2-mm thick specimens, irrespective of the light source, the hardness values at the bottom were not significantly different from their top hardness values measured at the same time. For the 3-mm thick specimens using the QTH+T, the hardness values at the bottom of both composites were not significantly different from their top hardness values at all time intervals. For the 3-mm thick specimens using the QTH+S, the bottom hardness values were not significantly different from those at the top for P60 only, and only at 24 hours and 7 days, but not when measured immediately. For all the 4 and 5-mm thick specimens, the hardness values at the bottom were all significantly different from those at the top measured at the same time interval.