The effect of LED curing units on polymerization of bulk-fill composites.

Objectives: To assess how the polymerization of bulk-fill composites depends on the light intensity and spectral distribution of LED curing units.
Methods: Composite materials Filtek Bulk Fill posterior restorative A3 (3M-ESPE), Sonic Fill Composite A2 (Kerr), Tetric Evo Ceram Bulk Fill IVA (Vivadent-Ivoclar) and flowable Filtek Bulk Fill A2 (3M-ESPE) and SDR U (Denstsply) were polymerized in a Teflon mold (4.0 mm internal diameter, 4.0 mm depth, five specimens per group) from one side using LED curing units of a lower intensity of 850 mW/cm² and a narrow emission spectrum 440-490 nm (Translux PowerBlue, Heraeus Kulzer) and a higher intensity unit of 1240 mW/cm² and a broad-dual wavelength emission spectrum 385-515 nm (BluePhase G2, Vivadent-Ivoclar). The extent of polymerization was characterized by the Knoop hardness measurement on the top and bottom surfaces of the specimens stored at 37^oC for 24 hours after polymerization. The results were analyzed using Repeated measures ANOVA and Tukey HSD post-hoc tests at α=0.05.
Results: Significantly higher hardness was found on the top-irradiated surface of Sonic Fill and bottom surfaces of Sonic Fill, Filtek Bulk Fill posterior, Tetric Evo Ceram Bulk Fill and flowable Filtek Bulk Fill cured with the BluePhase G2 unit. The bottom-to-top hardness ratio for all the composite materials exceeded the 80% level usually required for adequate polymerization.
Conclusions: Bulk-fill composites used in the study can be adequately polymerized in 4-mm increments using high-intensity LED curing units. However, increased light intensity and a broader emission spectrum of these units may improve polymerization especially of the bottom surface of the composite fillings.