Microtensile bond strength and dentin–adhesive interface of universal adhesives before and after artificial aging: an in vitro study

Universal adhesives (UAs) are widely used for resin composite restorations; however, the durability of the dentin–adhesive interface after aging remains a key clinical concern, as interfacial degradation may compromise bond stability over time. Therefore, this study evaluated the microtensile bond strength (µTBS) and dentin–adhesive interface of four universal adhesives before and after artificial aging. Occlusal thirds of human third molars were sectioned and randomly assigned to four groups: A, Palfique Universal Bond; B, Single Bond Universal; C, All-Bond Universal; D, One Coat 7 Universal. All UAs were used in self-etch mode. Palfique Universal Bond was not light-cured (chemical activation); All-Bond Universal was applied in two coats; the others in one. The total sample comprised 144 sticks. Of these, 120 were tested for µTBS (30/group) at 1 mm/min and subdivided with/without artificial aging (n = 15/subgroup; 10,000 thermocycles between 5 ± 2 °C and 55 ± 2 °C). Failure mode was examined under a stereomicroscope. Adhesive-interface thickness was measured on 24 sticks (n = 6/group) by scanning electron microscopy (SEM). Data were analysed by two-way ANOVA for µTBS, followed by simple comparisons when the interaction was significant. For adhesive-interface thickness, comparisons among adhesives within each aging condition and between aging conditions within each adhesive were evaluated using one-way ANOVA with Scheffé post hoc tests (p 0.05). Aging reduced µTBS for A and D (p 0.05), while D had a thicker interface than A and B (p < 0.05; η2 = 82.5%). After aging, A presented a thinner interface than B–D (p < 0.05; η2 = 93.5%) and was the only UA showing a reduction versus its non-aged counterpart (p = 0.003). Failure modes were predominantly adhesive for A and B, and mainly mixed for C and D (p < 0.001). After artificial aging, Palfique Universal Bond showed lower microtensile bond strength and a thinner adhesive interface, whereas Single Bond Universal and All-Bond Universal maintained comparable bond strength with no reduction in adhesive-interface thickness; One Coat 7 Universal showed a significant decrease in bond strength without a reduction in adhesive-interface thickness. Within the limitations of this in vitro study, the 10-MDP–containing adhesive systems evaluated showed greater interfacial stability after aging than the 3D-SR–containing system tested; however, this finding should be interpreted cautiously because the materials also differed in overall formulation and application protocol.