Effect of irradiance and light source on contraction stress, degree of conversion and push-out bond strength of composite restoratives

Purpose: To evaluate the influence of five curing methods on contraction stress, stress rate, and degree of conversion (DC) of a composite and on bond strength of composite restoratives. Methods: For the stress test, composite was applied between two 5-mm diameter glass rods, mounted in a servohydraulic machine. Stress rates were calculated as the change in stress vs. time. DC was measured by FTIR. Bond strength testing was performed using a push-out test in bovine incisors. The C-factor was 3.0 for all tests. Five methods were evaluated: High Intensity LED (LED HI), Continuous Halogen Light (QTH CL), Medium Intensity LED (LED MI), Low Intensity LED (LED LI), and Pulse Delay Halogen Light (QTH PD). Results were analyzed by ANOVA and Tukey's test (a = 0.05). Results: Stress values ranged from 9.25 MPa (QTH PD) to 10.46 MPa (LED MI). No statistical difference was observed among the methods. Bond strength values ranged from 24.6 MPa (LED HI) to 35.4 MPa (QTH PD), with the QTH PD presenting a statistically higher value compared to the other methods. Stress rate and bond strength presented an inverse linear correlation (r2 = 0.79). LED HI presented the highest maximum stress rate, followed by LED MI, QTH CL, LED LI, and QTH PD. The reduction in stress rate observed for the low intensity groups was associated with a eeneral increase in bond strength, with no adverse effect on the degree of conversion of the restorative composite.