## 4 Discussion
All-Bond contains 10-MDP, which is the main component of many resin-based materials and is the most frequently used acidic monomer. The dihydrogen phosphate group of MDP is used for etching tooth substrates, and the methacrylate group is used for cross-linking with other resin monomers [46]. 10-MDP is known to interact most intensively with hydroxyapatite and to form a hydrolytically stable bond with calcium [46]. Indeed, many promising results have been obtained from various investigations on the enamel and dentine bonds of 10-MDP [10]. In the present study, the shear bond strength of All-Bond in the repair process was significantly lower than that of Clearfil Universal and Single Bond Universal. This result can be attributed to the lack of hydroxyapatite in the resin composite samples for MDP to bond with. In this study, only the effect of adhesive systems on repairing composite resins was tested, so the repair composite used was chosen to be the same as the aged composite. Usually, in repair studies, the substrate and the adherent composites are chosen to be different in order to reflect clinical conditions wherein the type of composite to be repaired is unknown [47]. It has likewise been stated in a study [48] that the composite type influences the repair bond strength of the restorations. For instance, [PERSON] and [PERSON] [48] demonstrated that the type of composite plays a critical role in the repair bond strength, with significant differences observed between nano-hybrid and microhybrid composites. Although using the same composite material for both aged and repair specimens' controls for compositional variability and isolates the effect of adhesive systems, clinical situations often involve repair on unknown or different original composites. A recent systematic review and meta-analysis found no significant difference in repair bond strength between similar and dissimilar composite combinations, suggesting that well-established repair protocols may still be effective even when the substrate composite is unknown [49]. Nonetheless, it remains important to test mismatched composite pairings in future studies to further validate clinical applicability across a wider range of materials. Additionally, with the increasing adoption of three-dimensional (3D) printed dental composites in restorative dentistry, there is a growing need to investigate whether conventional adhesive strategies, including silane application, perform similarly on these novel materials. Recent studies have reported distinct surface characteristics and layer-by-layer structural differences in 3D printed composites compared to traditionally fabricated resin composites, which may affect the repair bond strength and adhesive behavior [50]. Therefore, future studies should investigate various composite types, including 3D printed dental composites, to validate the applicability of the present findings across different clinical scenarios. However, in this study, we preferred to repair the substrates with the same resin composite to eliminate bonding failures that can be caused by differences in the compositions of resin composites. Nevertheless, it should be acknowledged that the use of different composite materials in clinical situations may result in varying repair outcomes due to differences in filler morphology, resin matrix composition, and degree of conversion. These material dependent variables can influence the effectiveness of the adhesive interface and the chemical compatibility between the aged and fresh composites [49]. Therefore, future research evaluating repair bond efficacy among various composite combinations would yield a more thorough comprehension relevant to practical clinical situations. Ideally, the repair bond strength should be similar to the cohesive strength of the repaired resin composite. In this study, all failure types were cohesive in the aged composite for all groups, except for one in the PQ1 group (93.3%) and one in the All-Bond Universal group (93.3%).