Purpose: This study examined the effect of delayed activation of light-cured resin composites on the microtensile bond strengths of two all-in-one adhesives to sound dentin.
Materials and methods: Flat dentin surfaces prepared from caries-free third molars were bonded with either Prompt L-Pop (ESPE) or One-Up Bond F (Tokuyama). Each adhesive was divided into 6 subgroups containing three teeth each. A hybrid composite was left to contact the cured adhesive surfaces for 0, 2.5, 5, 10 or 20 min before light activation. In the 6th subgroup, the cured adhesive was covered with a layer of nonacidic bonding resin before contacting the composite for 20 min. Composite-dentin beams, with average cross-sectional areas of 0.85 mm2 were prepared for microtensile bond testing. Representative fractured beams from the 0, 10- and 20-min delay subgroups were prepared for SEM examination. Additional specimens were prepared in the 0- and 20-min delay subgroups by replacing the hybrid composite with either a flowable composite or a composite liner to facilitate TEM preparation and examination.
Results: Kruskal-Wallis ANOVA and Dunn's multiple comparison tests showed that significant differences existed among the subgroups of each adhesive. Regression analyses showed that for each adhesive, there was an exponential decline in mean bond strength with increasing delay in light activation, and the correlation was highly significant (r = -0.99, p < 0.005 for Prompt L-Pop; r = -0.96, p < 0.01 for One-Up Bond F). SEM examination of fractured interfaces in the immediately light-cured subgroups revealed the absence of voids within the fractured composite. However, voids were consistently observed in the 10 min- and 20 min-delay subgroups. They corresponded to soapsuds-like blisters that were observed with TEM in the 20 min-delay subgroups.
Conclusion: Adverse interactions between acidic adhesive resin monomers and tertiary amines in light-cured composites normally do not occur because of the fast rate of free-radical generation in photochemical redox reactions. However, such interactions can occur in all-in-one adhesives on prolonged contact of light-cured composites with the cured adhesive layer.