Examining exposure reciprocity in a resin based composite using high irradiance levels and real-time degree of conversion values

Abstract

Objective Exposure reciprocity suggests that, as long as the same radiant exposure is delivered, different combinations of irradiance and exposure time will achieve the same degree of resin polymerization. This study examined the validity of exposure reciprocity using real time degree of conversion results from one commercial flowable dental resin. Additionally a new fitting function to describe the polymerization kinetics is proposed. Methods A Plasma Arc Light Curing Unit (LCU) was used to deliver 0.75, 1.2, 1.5, 3.7 or 7.5 W/cm2 to 2 mm thick samples of Tetric EvoFlow (Ivoclar Vivadent). The irradiances and radiant exposures received by the resin were determined using an integrating sphere connected to a fiber-optic spectrometer. The degree of conversion (DC) was recorded at a rate of 8.5 measurements a second at the bottom of the resin using attenuated total reflectance Fourier Transform mid-infrared spectroscopy (FT-MIR). Five specimens were exposed at each irradiance level. The DC reached after 170 s and after 5, 10 and 15 J/cm2 had been delivered was compared using analysis of variance and Fisher's PLSD post hoc multiple comparison tests (alpha = 0.05). Results The same DC values were not reached after the same radiant exposures of 5, 10 and 15 J/cm2 had been delivered at an irradiance of 3.7 and 7.5 W/cm2. Thus exposure reciprocity was not supported for Tetric EvoFlow (p < 0.05). Significance For Tetric EvoFlow, there was no significant difference in the DC when 5, 10 and 15 J/cm2 were delivered at irradiance levels of 0.75, 1.2 and 1.5 W/cm2. The optimum combination of irradiance and exposure time for this commercial dental resin may be close to 1.5 W/cm2 for 12 s. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.