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

Abstract

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.