Effect of mechanical surface pretreatment and supplementary silane on the repair bond strength of aged nanofilled resin composite: an in vitro study

Repair is a conservative alternative to replacing defective resin composite restorations; however, bonding to an aged surface remains technique-sensitive. This in vitro laboratory study evaluated the effect of mechanical surface pretreatment and a supplementary silane primer applied before a silane-containing universal adhesive on the repair bond strength of aged nanofilled resin composites. Nanofilled composite blocks (Filtek Z350XT) were artificially aged by 10,000 thermocycles (5 – 55 °C, 30-s dwell). The aged specimens were randomly assigned to eight repair groups in a 4 × 2 factorial arrangement ( n = 13 per group) by combining four mechanical pretreatments: none (NC), carbide bur (CA), diamond bur (D), and grit-blasting (air abrasion) with 50-µm aluminum oxide (SA), with or without a supplementary silane primer (-S). A silane-containing universal adhesive was applied to each repaired specimen. A non-aged positive control (PC) repaired immediately served as the descriptive benchmark. Shear bond strength (SBS) was analyzed using two-way analysis of variance (ANOVA) with Tukey’s tests, supported by one-way ANOVA across all nine groups. The degree of conversion (DC; n = 10 per time point) was compared by one-way ANOVA, and surface roughness (R a ; n = 7 per group) was compared by Kruskal–Wallis with Dunn tests. Statistical significance was set at p < 0.05. Mechanical pretreatment ( p < 0.001) and supplementary silane ( p = 0.002) significantly affected SBS, with no significant interaction ( p = 0.733). SA-S showed the highest aged-repair SBS (22.66 ± 2.84 MPa) and NC the lowest (10.97 ± 2.07 MPa); PC reached 22.76 ± 3.75 MPa. Averaged over the mechanical pretreatments, supplementary silane increased the marginal mean SBS from 15.79 to 17.61 MPa. DC did not differ across the time points ( p = 0.070). R a differed between the mechanical pretreatments ( p < 0.001); D and SA were rougher than NC and CA. Mechanical roughening was the dominant determinant of the repair bond strength in aged nanofilled composites, and supplementary silane primers resulted in a smaller improvement that did not differ significantly among pretreatments. Grit-blasting, followed by the supplementary silane, produced the strongest repair profile.