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

To etch or not to etch, Part I: On the fatigue strength and dentin bonding performance of universal adhesives

Objectives: To characterize whether the bonding performance of resin-dentin interfaces created by universal adhesives would be affected by different extensions of H3PO4-etching using quasi-static loading tests and a modified fatigue-testing approach to more accurately predict long-term durability. Methods: Mid-coronal flat dentin surfaces with clinically relevant smear-layers were produced on sound third molars, etched with 32% H3PO4 for 0, 3 and 15 s, bonded with an ultra-mild universal adhesive (3M-ESPE) and restored with a nanofilled composite. Bonded specimens (0.8 mm2) were stored in deionized water for 24 h and sectioned into beams for microtensile testing (n=10) at 24 h and after 6 months in artificial saliva at 37 ̊C. Bar-shaped resin-dentin beams (0.9 × 0.9 × 12 mm) were tested under 4-point-flexure initially at quasi-static loads (n=22) and then under cyclic loads (n>50). The stress-life fatigue behavior was evaluated using the twin-bonded interface approach by the staircase method at 4 Hz. Fractured interfaces and the tension side of unfractured beams were evaluated under SEM, along with the micro-morphology of the etched dentin surfaces and hybrid layers. Data were analyzed by ANOVA and Tukey test and Wilcoxon Rank Sum Test (α=0.05). Results: Quasi-static loads were limited to discriminate the bonding performance of resin-dentin interfaces. Application modes significantly affected etching patterns, fatigue strength, endurance limits and hybrid layer morphology (p<0.001). Significance: Drastic reductions in fatigue strength of self-etched bonded interfaces raise concerns about the true ability of ultra-mild universal adhesives to properly bond to dentin with clinically relevant smear layers

To etch or not to etch, Part I: On the fatigue strength and dentin bonding performance of universal adhesives

Objective. To characterize whether the bonding performance and fatigue strength of resin-dentin interfaces created by a universal adhesive would be affected by different H3PO4-application times to more accurately assess long-term durability.Methods. Mid-coronal flat dentin surfaces with standardized smear-layers were produced on sound third molars, etched with 32% H3PO4 for 0, 3 and 15 s, bonded with a mild universal adhesive (3M-ESPE) and restored with a nanofilled composite. Bonded specimens (0.9 x 0.9mm) were stored in deionized water for 24 h and sectioned into beams for microtensile testing (n = 10). Resin-dentin beams were tested under tension until failure (0.5 mm/min) after 24 h or 6 month storage in artificial saliva at 37 degrees C. Bar-shaped resin-dentin beams (0.9 x 0.9 x 12 mm) were tested under 4-point-flexure initially at quasi-static loads (n = 22) and then under cyclic loads (n > 50). The stress-life fatigue behavior was evaluated using the twin-bonded interface approach by the staircase method at 4 Hz. Fractured interfaces and the tension side of unfractured beams were evaluated under SEM, along with the micromorphology of the etched dentin surfaces and hybrid layers. Data were analyzed by ANOVA and Tukey test and Wilcoxon Rank Sum Test (alpha = 0.05).Results. Quasi-static loads were limited to discriminate the bonding performance of resin-dentin interfaces. Application modes significantly affected etching patterns, fatigue strength, endurance limits and hybrid layer morphology (p < 0.001).Significance. Reductions in fatigue strength of self-etched bonded interfaces raise concerns about the true ability of universal adhesives to properly bond to dentin. (C) 2021 The Authors. Published by Elsevier Inc