LED Curing Units: More Power - Better Polymerization?

Methods: Class II cavities were prepared in extracted molars and filled/cured in three 2 mm increments using a metal matrix. Two composites (Tetric Evo Ceram (IvoclarVivadent) and ELS(Saremco)) were cured with three LED curing units (Bluephase C8 [1600mW/cm2], Bluephase [1800mW/cm2] and Bluephase 16i [2400mW/cm2] (IvoclarVivadent)) using curing cycles of 5, 10, 20 and 40 seconds. Twelve teeth were filled for every composite-curing time-curing unit combination. The teeth were thermocycled (1000 cycles at 5º and 55ºC). After a seven day incubation period Knoop microhardness was determined in a longitudinal direction 100, 200, 500, 1000, 1500, 2500, 3500, 4500µm from the occlusal surface at a distance of 150 and 1000µm from the metal matrix. The total degree of polymerization of a composite specimen was determined by calculating the area under the hardness curve.

Results: Multiple linear regression showed that material, curing time and curing unit had a significant influence on the degree of polymerization (p<0.0001). Hardness values at 150µm from the matrix were lower than those at 1000µm. There was an increase of the microhardness at the top of each increment and decrease towards the bottom of each increment. Longer curing times were generally combined with harder composite samples. Higher light intensities may have some advantage in curing capabilities of composite.

Conclusion: Higher light intensities allowed to shorten the curing time. But this effect was not without limits. For some composites it is advantageous to proceed a light curing with lower energy but with longer curing time. The total energy concept could not be confirmed with this study.