Effect of rapid high-intensity light-curing on polymerization shrinkage properties of conventional and bulk-fill composites

Objectives: To compare the effect of high-intensity (3 s with 3440 mW/cm(2)) and conventional (10 s with 1340 mW/cm(2)) light-curing on shrinkage properties and degree of conversion of conventional and bulk-fill resin composites, including two composites specifically designed for high-intensity curing. Methods: Real-time linear shrinkage and shrinkage force of 1.5 mm thick composite specimens were measured for 15 min after the start of light-curing using custom-made devices. From the shrinkage force data, maximum shrinkage force rate and time to achieve maximum shrinkage force rate were determined. Degree of conversion was measured using Fourier transform infrared spectrometry. Results: Flowable composites showed significantly higher linear shrinkage compared to sculptable composites (1.93-2.91 % vs. 1.15-1.54 %), as well as significantly higher shrinkage forces (18.7-24.4 N vs. 13.5-17.0 N). Degree of conversion amounted to 45.8-60.1 %. For high-intensity curing, degree of conversion was significantly lower in three out of seven composites, whereas shrinkage forces were either increased, decreased, or unchanged compared to conventional curing. For high-intensity curing, maximum shrinkage rates were 6-61 % higher, whereas times to achieve maximum shrinkage force rate were 15-53 % shorter compared to conventional curing. Composites specifically designed for high-intensity curing showed shrinkage parameters comparable to other investigated composites. Conclusion: Shrinkage behavior under conditions of high-intensity light-curing was material-dependent. Shrinkage force kinetics were more strongly affected by high-intensity curing than absolute values of linear shrinkage and shrinkage force. Clinical significance: Despite being attractive for its convenience, high-intensity curing can lead to considerably faster development of shrinkage forces in the early stage of polymerization.