Immediate vs delayed repair bond strength of a nanohybrid resin composite

To evaluate both the immediate and water-stored repair tensile bond strength (TBS) of a nanohybrid resin composite using different bonding protocols.

One hundred sixty half hourglass-shaped slabs were prepared. Eighty half-slabs were wet ground immediately after light curing using high-speed abrasive burs, while the other half-slabs were stored in water for one month (delayed) and then wet ground for repair. Each set of the 80 repaired slabs was split into two groups to be tested for TBS after 24 h or 1 month of water storage. For all repaired slabs, either immediate or delayed, four bonding procedures were used involving wet and dry bonding with a 3-step etch-and-rinse adhesive with or without silane pretreatment. TBS tests were performed at a crosshead speed of 0.5 mm/min. To determine the cohesive strength of the resin composite itself, which served as the reference, additional whole slabs were prepared and tested in tension after a 24-h (n = 10) and a 1-month storage period (n = 10). Failure modes were evaluated using a stereomicroscope at 40X magnification. Three-way ANOVA was run to test the effect of water storage, testing time, bonding protocols, and their interactions on the repair TBS, which was given as a percentage of the reference values.

For the immediate repair groups, the repair TBS ranged from 40% to 61.9% after 24-h storage and from 26% to 53.1% after 1-month water storage compared to the TBS of the whole slabs. For the delayed repair group, the repaired TBS ranged from 47.2% to 63.6% for the 24-h repairs and from 32.2% to 44.2% for the test groups stored in water for 1 month. Three-way ANOVA revealed that water storage had no significant effect on the repair TBS (p = 0.619). Both testing time and bonding protocols had a significant effect on the repair TBS (p = 0.001). The interactions between the independent variables (water storage, testing time, and bonding protocols) had no significant effect (p = 0.067).

The repair bond strength was consistently and highly significantly less than the cohesive strength of the composite. A delay of 1 month before carrying out the repair had no effect on the bond strength, irrespective of the bonding procedure used. Silane treatment did not improve the repair bond strength. In all instances, except for the immediate wet bonding plus silane procedure and delayed dry bonding, the bond strength of the repairs significantly dropped after 1 month of storage in water.