## 4 Discussion  
This study aimed to investigate the effects of different adhesive systems on the repair bond strength of a resin composite bonded to an aged composite. The results showed that adhesive systems were necessary when adding fresh composite to aged composite, and differences in the adhesive systems influenced the repair bond strength of a resin composite bonded to aged resin composite. Therefore, the first null hypothesis, i.e., that the adhesive system used does not affect the repair bond strength of a resin composite bonded to aged composite, was rejected. In order to achieve successful results in repairing composite restorations, it is necessary to create a durable bond between the old restoration and the new resin composite. The structure of the old composite, the time it spends in the mouth, and the method of repair are important factors affecting the success of the new restoration [33]. Therefore, the surface of the old restoration should be altered in order to provide a proper bonding surface [21; 22],  
Figure 4: Failure mode distribution acquired from shear bond testing of repaired composite specimens. adhesion should be provided with a suitable adhesive system, and a pairing composite resin should be used [18; 19]. This approach, as in the present study, was deliberately chosen to eliminate variables related to differences in composite composition, ensuring that the observed bond strength outcomes were attributable solely to the adhesive systems tested. The highest bond strength was obtained when the Single Bond Universal Adhesive system was used. This outcome should be interpreted in light of its chemical formulation, which includes both MDP and silane, two components known to enhance adhesion and interfacial stability. As stated by the manufacturer, Single Bond also has Vitrebond copolymer in addition to MDP and silane in its composition. Vitrebond allows the product to bond in a moist environment, which could be important in repairing aged composites, since these are fully saturated with water during the thermal cycling procedure. The results obtained from using Single Bond in the composite-to-composite repair procedure can be explained by the agent's enhanced chemical content and structure. However, the superior performance of this group must be viewed cautiously, as other adhesives demonstrated variable results that were influenced by their distinct chemical compositions. For instance, the relatively lower bond strength of PQ1 may be attributed to its lack of functional monomers and absence of silane. Similarly, the moderate performance of All-Bond Universal, despite containing MDP, could be due to the absence of silane and differences in solvent content or pH. The aging process of composites plays an important role in the bond strength of composite repairs [4; 34; 35]. Since there is no standard protocol for the artificial aging process, different aging procedures have been applied in many composite repair studies [36; 37; 38; 39; 40; 41]; thermal cycling has been the most frequently used method [39; 40; 41]. During this process, the composites and the adhesive interfaces undergo a process of degradation due to water absorption and constant temperature changes. In the present study, 10,000 thermal cycles were employed to simulate intraoral aging of the substrate composite. According to the recent literature, this protocol approximates one year of clinical service, based on the frequency of daily thermal fluctuations resulting from routine consumption of food and beverages [39; 40]. As such, the applied aging procedure reflects an early-stage degradation scenario that is representative of clinical conditions in which restoration repair may be considered. Following the thermal aging process applied only to the substrate composite surfaces, the repair protocol was subsequently performed. The repaired specimens were then stored in distilled water at 37 \({}^{\circ}\)C for 24 h prior to bond strength testing.