## 4 Discussion  
The bond between brackets and tooth surfaces is crucial for successful orthodontic treatment. The bond strength must be strong enough to prevent accidental debonding during treatment yet low enough to allow for easy removal without excessive force [20]. The current experimental study evaluated the effect of various surface treatment protocols on the shear bond strength (SBS) of metal orthodontic brackets bonded to different indirect composite resin restorative materials. The findings revealed that both the type of restorative material and the surface treatment method significantly influenced SBS, leading to the rejection of the null hypothesis. Shear bond strength (SBS) testing is the most commonly used laboratory method for assessing the bond strength of orthodontic brackets to tooth surfaces [21]. In this study, SBS testing was performed following a standardized protocol described previously [21]. An adequate SBS value between metal orthodontic brackets and enamel surfaces is considered to be 6-8 MPa [22]. Since the mean SBS values for all surface treatment protocols in this study exceeded this range, they can be deemed effective for clinical applications. Additionally, all specimens underwent thermocycling in distilled water between 5 \({}^{\circ}\)C and 55 \({}^{\circ}\)C for 5000 cycles, simulating a clinical service period of six months in the oral environment, to evaluate the performance of the bonded interfaces under continuous hydrothermal stress [23]. Regarding the material, the milled composite resin materials demonstrated significantly higher SBS values compared to the 3D-printed resin materials, likely due to differences in material composition, particularly the amount and type of inorganic fillers [18]. It is believed that the organic matrix of any composite material primarily contributes to chemical bonding and copolymerization with the uncured organic matrix of the adhesive resin [24]. However, the microstructure and high degree of conversion in CAD/CAM composite materials reduce their copolymerization ability with other organic matrices. As  
Figure 4: Distribution of adhesive remnant index (ARI) scores (%) observed for tested groups after debonding. Abbreviations: Group C (control, no surface treatment), Group HF (treated with 9.6% hydrofluoric acid), Group DB (mechanical roughening by a diamond bur), and Group SB (sandblasting using aluminum oxide). Scores: 0 = no adhesive remained on the bonded surface; 1= less than 50% of the adhesive remained on the bonded surface; 2 = more than 50% of the adhesive remained on the bonded surface; 3 = 100% of adhesive remained on the bonded surface. a result, bonding to these materials relies heavily on achieving strong micromechanical interlocking by interacting with their inorganic filler content. This is supported by the highly significant differences observed in the surface treatment factor [2]. The higher amounts of inorganic fillers in both LU and GR materials (80 wt.% and 86 wt.%, respectively) likely explain their superior SBS values compared to the lower filler content in CT material (30-50 wt.%). Additionally, the type of inorganic filler plays a crucial role in the micromechanical interlocking process. Since certain fillers, such as zirconia particles, cannot be etched by hydrofluoric acid, the presence of sufficient amounts of etchable filler particles, like silica and glass, is essential for achieving strong bonding strength [18]. Overall, all surface treatment methods examined in this study significantly increased SBS compared to the control group. This improvement in SBS can be attributed to the increased surface roughness, which enhances the impregnation of the bonding agent and promotes micromechanical retention [18; 25; 26]. These findings align with those of other studies [14; 18; 27], which also demonstrated the role of both mechanical and chemical surface treatments in enhancing SBS values for milled composite materials.