Do Universal Adhesives Benefit from an Extra Bonding Layer?

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. 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). 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. The overall effect of EBL on immediate and aged bonding efficacy depended on the specific universal adhesive and its bonding mode.