To etch or not to etch, Part II: On the hydrophobic-rich content and fatigue strength of universal adhesives

Objective: To determine whether smear layer management, via conservative etching pro-tocols, and the hydrophobic-rich content of hybrid layers would affect the fatigue strength of resin-dentin interfaces.Methods: Bar-shaped dentin beams obtained from sound third molars were wet-polished for 30 s. Dentin was etched with 32 % ortho-phosphoric acid for 3 or 15 s, 10 % meta -phosphoric acid for 15 s or by a prime-and-rinse application using a mild universal ad-hesive (Scotchbond Universal, 3M ESPE). Self-etch application served as control. Coating was performed with a solvent-free bisGMA-based resin. Composite buildups were made with a nanofilled composite. Resin-dentin beams with twin-bonded interfaces were sec-tioned and stored in deionized water for 24 h at 37 celcius before 4-point flexural quasi-static monotonic testing (n = 16). Stress-life fatigue behavior was evaluated under cyclic loading (n = 35) by the staircase method at 4 Hz. The tension side of cyclic-loaded unfractured beams were evaluated under SEM, along with the micro-morphology of etched dentin surfaces. Monotonic data was analyzed by two-way ANOVA followed by the Tukey Test and cyclic-loaded data by Kruskal-Wallis on Ranks (alpha = 0.05).Results: Etching protocols and higher hydrophobic-rich content produced significantly higher fatigue life distributions (p MPA 15 s > OPA 3 s > P + R > SE. Less aggressive etching and coating reduced crack formation at hybrid layers.Significance: Current oversimplification trends in resin-dentin bonding constitute a trade-off between hybridization quality and easier adhesive handling. Controlled dentin etching and increasing the hydrophobic-rich content of hybrid layers may be necessary to extend the longevity of mild universal adhesives. (c) 2022 Published by Elsevier Inc. on behalf of The Academy of Dental Materials. CC_BY_NC_ND_4.0</p