## 4 Discussion
The immediate shear bond tests of the samples after 24 h showed cohesive failure patterns in the aged composite, indicating that the bond strength was higher than the inherent strength of the aged composite. The required reference value for the ideal bond strength needed after composite repair must be in the range of 15 to 25 MPa [42]. In this study, all the groups, except for the control and PQ1, yielded optimal repair bond strength. It should be noted that in the present study, the repaired specimens were stored in distilled water for 24 h prior to bond strength testing. While this short-term storage period allows for the assessment of immediate bond performance, it may not sufficiently reflect the long-term hydrolytic and mechanical degradation observed clinically. Previous studies have demonstrated that extended water storage and post-repair thermocycling can adversely affect bond durability by promoting water sorption, plasticization of the resin matrix, and interfacial degradation over time [35; 37]. Therefore, incorporating additional aging protocols following the repair procedure would decrease the predictive value of in vitro testing and may hinder a clearer evaluation of the clinical performance of adhesives. The adhesive systems used in this study were selected based on their distinct chemical compositions to enable a comparative evaluation of bonding strategies in composite repair. Most of the adhesives contained 10-MDP as a functional monomer, while others included both MDP and silane, or monofunctional methacrylate [10; 11; 19]. PQ1, a conventional etch and rinse adhesive lacking both MDP and silane, was deliberately included to represent earlier-generation adhesive systems. Its inclusion allowed for the assessment of how modern formulations particularly those incorporating silane, either as a separate pretreatment or within universal adhesives perform in comparison to traditional products. Notably, PQ1 was also selected because it permits combination with separate silane application, allowing the study to isolate and examine the specific contribution of silane to the repair bond strength. Accordingly, an additional group utilizing silane pretreatment followed by PQ1 application was tested. This group exhibited significantly higher bond strength compared to PQ1 alone, underscoring the essential role of silane in promoting chemical adhesion to aged composites [2]. Silane molecules contain two functional groups: silanol groups that bond with the inorganic filler particles, and organofunctional methacrylate groups that copolymerize with resin monomers [43]. This dual reactivity enhances both chemical bonding and surface wettability, which may explain the improved performance observed in the silane/PQ1 group [2; 44]. The results obtained from Single Bond Universal and Clearfil Universal Adhesive were comparable to those of the silane/PQ1 group, indicating that both strategies effectively enhanced the repair bond strength of aged resin composites. This suggests that the application of silane either as a separate step or incorporated into universal adhesive formulations can significantly improve bonding performance [45]. Given the similar outcomes observed between these two approaches, silane-containing universal adhesives may be preferred in clinical practice due to their simplified application, reduced technique sensitivity, and lower risk of procedural errors. Therefore, the second hypothesis stating that \"the use of separate silane pretreatment would not significantly differ in effectiveness from universal adhesives containing silane\" was accepted. Additionally, the GC Composite Primer group, which contains monofunctional methacrylate according to the manufacturer, showed bond strength values comparable to the silane-containing groups. In contrast, adhesives without silane (e.g., PQ1) exhibited significantly lower bond strength, further emphasizing the importance of silane in achieving effective chemical bonding to aged composites.