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Effect of light source and specimen thickness on the surface hardness of resin composite
Richard B Price1, Tore Dérand, Robert W Loney
1Department of Dental Clinical Sciences, Faculty of Dentistry, Dalhousie University, Halifax, Nova Scotia, Canada. rbprice@is.dal.ca
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 degrees 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.