Waiting Time Prior Adhesive Photo-Activation Affects Bond Strength To Dentin

Objective: The aim of this study was to evaluate the influence of the waiting time between application and photo-activation of an adhesive system on bond strength to dentin. 

Method: Flat dentin surfaces were produced on sound human third molars (n=24). After random allocation into four groups (n=6), the surfaces were etched with phosphoric acid for 15 s, followed by rising, blot drying and Single Bond 2 application. The solvent was evaporated by gently air-drying the surface for 5 s, and then adhesive was kept passively on the dentin surface for 0 (immediate photo-activation), 20, 40 or 60 s before the photo-activation for 10 s. Next, composite resin blocks were built up. After 24 hours, specimens with cross-sectional area of 0.81 mm² were produced and microtensile tested. Bond strength data (MPa) were submitted to one-way analysis of variance and Tukey tests (α=0.05). 

Result: The lowest bond strength values were observed when the adhesive was immediately photo-activated (24.9±7.9 MPa) compared to the other groups. There was a trend towards the reduction of bond strength as the waiting time was increased. However, a statistical significant difference was only observed between 20 (35.1±8.8 MPa) and 60 seconds (30.3±4.9 MPa) of waiting time. 

Conclusion: The immediate photo-activation of Single Bond should not be recommended. A waiting time between 20 and 40 seconds before applying the curing-light resulted in substantial improvement of bond strength.