The effect of different light-curing units on fatigue behavior and degree of conversion of a resin composite

Objectives The aim of this study was to investigate the effect of different light-curing units and irradiation modes on the mechanical fatigue strength and degree of conversion of a restorative resin composite. Methods Conventional halogen, plasma arc and blue LED light-curing units were used for polymerization of a resin composite (Tetric® Ceram, Ivoclar, Vivadent, Liechtenstein). Initial fracture strength (FS) and flexural fatigue limit (FFL) as well as degree of conversion (DC) were measured. The FFL was determined under cyclic loading for 105 cycles in terms of a staircase approach. The specimens were stored for 14 days in 37 °C distilled water prior to testing. The curing efficiency was observed with Fourier transform infrared micromultiple internal reflectance spectroscopy. The measurements were carried out at 0.5 and 2.5 mm distance from the directly irradiated surface after 14 days storage in dark and dry conditions at 37 °C. Results The highest FS, FFL and DC were observed from high energy curing devices and from extended curing intervals. The conventional halogen light exhibited the most homogenous in-depth curing efficiency along with a low loss of mechanical resistance under cyclic fatigue. Significance Evaluation of flexural fatigue limit and curing efficiency correlate in terms of decreased mechanical strength due to insufficient light-curing intervals or light intensities. Initial promising fracture strengths do not correlate with a clinically more relevant fatigue loading and with the in-depth degree of conversion, both accounting for a significantly reduced strength performance.