The effect of soft-start polymerization by second generation LEDs on the degree of conversion of resin composite

Fourier-Transform (FT)--Raman spectroscopy was used to evaluate in vitro the degree of conversion (DC) of Charisma dental composite cured by three different light curing units (LCUs) using soft-start and normal protocols. Eighty circular blocks of resin (7 mm in diameter x 2.5 mm thick) were prepared and cured using the following sources: halogen light (Degulux soft-start, n = 20, G1-G2), low power light emitting diode (LED) with transparent polymer tip (LD13, n = 20, G3-G4) and fiber optic tip (LD13, n = 20, G5-G6) and, finally, high power LED (Radii, n = 20, G7-G8). The top and bottom surfaces of the blocks were analyzed by FT-Raman spectroscopy. The respective DCs were estimated calculating the peak height ratio of the aliphatic C=C (1640 cm(-1)) and aromatic C=C (1610 cm(-1)) Raman modes. The DC at the surfaces ranged between 50% and 60% for the top and 46% and 58% for the bottom. The halogen light and high power LED LCUs with the soft-start and normal protocols produced the highest DC values of dental composite at both surfaces (p < 0.001). Curing by low power LED in the soft-start protocol did not produce adequate DC at the depth of 2.5 mm. The type of LCU light guide tip did not present a significant statistical difference in the final DC of the dental composite (p > 0.05).