On the degradation mechanics and fatigue strength of resin-dentin interfaces: Identifying weak links of adhesive strategies after prolonged ageing.

OBJECTIVES

To evaluate the mechanical behavior of resin-dentin bonded interfaces submitted to cyclic loading after a prolonged hydrolytic challenge to better understand long-term failure mechanics under fatigue.

METHODS

Resin-dentin twin-bonded interfaces (TBI) were produced on sound midcoronal dentin beams by a mild universal adhesive used in self-etch or etch-and-rinse (H3PO4 etching for 15 s) modes. Composite buildups were made with a nanofilled composite. Bar-shaped TBI specimens (0.9 ×0.9×12 mm) were stored for 24 h or seven years in artificial saliva at 37 °C and tested under 4-point flexure at quasi-static (n = 16) and cyclic loads (n = 30) until failure. The stress-life fatigue behavior was evaluated using the staircase method at 4 Hz. Crack initiation sites at bonded interfaces and fracture patterns were evaluated by SEM. Cyclic-loaded data was analyzed by Kruskal-Wallis on Ranks (α = 0.05).

RESULTS

No significant differences in quasi-static 4-point flexures were identified between adhesive strategies for both storage periods (p > 0.05). The etch-and-rinse strategy produced significantly higher fatigue life distributions at both storage periods (p < 0.05). Long-term storage significantly reduced fatigue life distributions for both adhesive strategies (p < 0.05). After ageing, the endurance limit for the etch-and-rinse strategy was 2.1-fold higher compared to the self-etch strategy. Crack-initiation sites mostly shifted from hybrid-layer surrounding areas for unaged samples to the restorative composite bordering bonded interfaces after ageing.

SIGNIFICANCE: The adhesive strategy plays an important role on the long-term degradation mechanics of universal adhesives. Hydrolytic degradation of methacrylate-based restorative composites emerges as a direct cyclic-weakening contributor to resin-dentin interfaces.