Do universal adhesives benefit from an extra bonding layer?

Purpose: Universal adhesives use a combined primer/bonding resin applied either in 2-step etch-and-rinse (2-E & R) or 1-step self-etch (1-SE) mode. This study investigated whether three universal adhesives would benefit from an extra bonding layer (EBL), essentially making them 3-step E & R (3-E & R) and 2-SE adhesives, respectively. Materials and Methods: Microtensile bond strengths (μTBSs) to bur-cut dentin of Clearfil Universal Bond Quick (C-UBq, Kuraray Noritake), G-Premio Bond (G-PrB, GC) and Prime & Bond Active (P & Ba, Dentsply Sirona) were compared to those of the 2-SEa Clearfil SE Bond 2 (C-SE2, Kuraray Noritake) which was also employed in 3-E & R mode. A split-tooth design was applied with each adhesive used in E & R and SE mode on each tooth, half without/with EBL (C-SE2 Bond). All adhesives were light cured after application of the final layer. The μTBS of one-third of the specimens was measured after 1-month water storage without thermocycling (TC), with the remaining two-thirds aged with 25,000 and 50,000 TCs. All μTBSs were statistically analyzed using a linear mixed-effects model with specific contrasts (p < 0.05). Results: Overall, the four variables (adhesive, bonding mode, EBL, aging) significantly influenced μTBS. Solely G-PrB benefited from EBL when applied in SE mode. In E & R mode, P & Ba generally resulted in the highest μTBSs, C-UBq presented with intermediate and G-PrB with the lowest μTBSs, where the latter was significantly lower than the μTBSs recorded for C-SE2. In SE mode, P & Ba generally resulted in the highest μTBSs in 1- and 2-SE mode. Equally high μTBSs were recorded for G-PrB in 2-SE mode with EBL. Lower μTBSs were recorded for G-PrB in 1-SE mode and for C-UBq in 1- and 2-SE mode. Conclusion: The overall effect of EBL on immediate and aged bonding efficacy depended on the specific universal adhesive and its bonding mode.