The effects of extended curing time and radiant energy on microhardness and temperature rise of conventional and bulk-fill resin composites

Objectives To investigate radiant energy, microhardness, and temperature rise in eight resin composites cured with a blue or violet-blue curing unit, using a curing protocol which exceeded manufacturer recommendations. Materials and methods Cylindrical composite specimens (d = 8 mm, h = 2 or 4 mm, n = 5 per experimental group) were light-cured for 30 s. Light transmittance through specimens was recorded in real time to calculate radiant energy delivered to the specimen bottom. Vickers microhardness was used to evaluate the polymerization effectiveness at depth. Temperature rise at the bottom of the specimens was measured in real time using a T-type thermocouple. Results Radiant energy delivered from the blue and violet-blue curing unit amounted to 19.4 and 28.6 J/cm(2), which was 19 and 13% lower than specified by the manufacturer. Radiant energies at bottom surfaces (0.2-7.5 J/cm(2)) were significantly affected by material, thickness, and curing unit. All of the composites reached 80% of maximum microhardness at clinically relevant layer thicknesses. The benefit of using the higher-irradiance violet-blue curing unit was identified only in composites containing alternative photoinitiators. Temperature rise during curing ranged from 4.4 to 9.3 degrees C and was significantly reduced by curing with the lower-intensity blue curing unit and by increasing layer thickness. Conclusion Curing for 30 s, which can be regarded as extended considering manufacturer specifications, produced radiant energies which are in line with the recommendations from the current scientific literature, leading to adequate curing efficiency and acceptable temperature rise.