Polymerization of composite resin using different light-curing units by direct and indirect techniques

Purpose: The aim of this study was to evaluate the curing efficiency, through tooth slices (0, 1.2, 1.5 and 2.0 mm), of three different light sources. Materials and Methods: Three caries-free mandibular 3rd molars were used. After the roots were cut from the coronal portions, slices of different thickness (1.2 mm, 1.5 mm, 2.0 mm) were made by cutting the buccal face of the dental crowns. Then, samples were made in a metallic mold (4 mm diameter and 2 mm thickness) with a microhybrid composite resin (Charisma; Heraeus-Kulzer). The samples (n = 140) were divided into 3 groups: group I - halogen LCU for 20 and 40 s; group II - argon laser LCU for 20 and 40 s; group III - LEDs LCU for 20, 40, and 60 s. The samples were cured either directly (no tooth slice intervening; control), or indirectly (through tooth slices of different thickness). The samples were stored dry at 37°C for 24 h, and then Vickers microhardness testing was performed. Results: The data were submitted to ANOVA (p < 0.05). Standard deviation values from the microhardness measurements were less than 1% of the mean and showed a statistically significant difference among the light curing units and the cure through the tooth slices of different thicknesses. Conclusion: In the indirect photoactivation technique, it was necessary to increase the exposure time independent of the LCUs used mainly for the bottom surface. Under these experimental conditions, the LED curing unit promoted the lower microhardness values when compared to the halogen and argon laser LCUs