Purpose: This study evaluated the effect of different aging methods on the repair bond strength and failure types of a microhybrid and a nanohybrid composite. Materials and Methods: Disk-shaped microhybrid (Quadrant Anterior Shine-QA) and nanohybrid (Tetric EvoCeram-TE) resin composite specimens (N = 192, n = 12/per group) were photopolymerized and randomly assigned to one of the three aging conditions: (1) immersion in deionized water (37°C, 2 months), (2) thermocycling (5000 times, 5 to 55 °C), (3) immersion in citric acid (pH: 3.0; 1 week). The control group was stored dry for 24 h at 37°C. After aging procedures, the specimens were silica coated (30 μm SiO2) (CoJet-Sand) using an intraoral air abrasion device, silanized (ESPESil) and an intermediate adhesive resin was applied (Visio-Bond, 3M ESPE). Resin composites, once of the same kind as the substrate (QA-QA, TE-TE) and once other than the substrate material (QA-TE, TE-QA) were adhered onto the conditioned substrates. Shear force was applied to the adhesive interface in a universal testing machine (cross-head speed: 1 mm/min). Results: A significant influence of the aging method was observed (p < 0.05) but the composite type did not affect the repair bond strength (p = 0.755) (2-way ANOVA, Tukey’s test). Interaction terms were significant (p < 0.05). Thermocycling showed lower results (10 ± 2.6 to 14.4 ± 4.4 MPa) than those of other aging methods (12.7 ± 5.2 to 28 ± 5.3 MPa). Using the substrate and the adherend interchangeably (QA-TE, TE-QA) did not show significant differences in the control group, but the results were significant after aging (p = 0.007) (2-way ANOVA, Tukey’s test). Interaction terms were not significant (p = 0.124). The incidence of score A (cohesive failures in the substrate) was not significant between the composite combinations in the control groups (exclusively 100%) and water-storage aged groups (92% to 100%) (p > 0.05) (chi-square). Citric acid aging yielded significantly less incidence of score A (8-75%) compared to the control group in all composite combinations (p < 0.05). Conclusion: Both microhybrid and nanohybrid composites could be used either as a substrate or as relayering composites in early repairs. Aging factors may diminish the repair quality.

Keywords: aging, microhybrid composite, nanohybrid composite, repair, silica coating, surface conditioning