Extra Bonding Layer Compensates Universal Adhesive's Thin Film Thickness

Purpose: Universal adhesives (UAs) are applied in 2-step etch-and-rinse (2-E&R) or 1-step self-etch (1-SE) mode. This study investigated whether three UAs could benefit from a highly filled extra bonding layer (EBL), turning them into 3-E&R and 2-SE UAs, respectively, thus also compensating for the commonly thin film thickness of UAs. Materials and Methods: Microtensile bond strength (μTBS) 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), applied in E&R and SE mode without/with the adhesive resin (EBL) of OptiBond FL (Opti-FL_ar, Kerr), was compared to that of the 3-E&Ra OptiBond FL (Opti-FL; Kerr), which was also employed in 2-SE mode. As a cross reference, the SE primer of Clearfil SE Bond 2 (Kuraray Noritake) was combined with Opti-FL_ar (C-SE2/Opti-FL) and again applied in 2-SE and 3-E&R mode. μTBS was measured after 1 month of water storage (37°C) and additional 25,000 and 50,000 thermocycles (TC). All μTBS were statistically analyzed using three different linear mixed-effects models with specific contrasts (p 0.05). Results: Overall, the four parameters (adhesive, bonding mode, aging, EBL) significantly influenced μTBS. G-PrB and P&Ba benefited from EBL when applied in both E&R and SE bonding modes. In E&R mode, P&Ba generally revealed the highest µTBS; C-UBq presented an intermediate and G-PrB the lowest µTBS. No significant differences were found between different bonding modes. C-SE2/Opti-FL outperformed Opti-FL in 3-E&R and 2-SE_1 month/25k. Conclusion: The overall benefit of EBL on the 1-month and TC-aged bonding efficacy differed for the different UAs tested. Keywords: bond strength, durability, hydrophobic, linear mixed model (LME), adhesive-dentin interface