## 4 Discussion
In this study, sandblasting significantly increased the SBS of orthodontic brackets for both milled and 3D-printed resin composite materials, consistent with previous studies [14; 27] showing sandblasting as the most effective surface treatment for milled materials. Evidence suggests that the particle size used in sandblasting has a greater impact on bond strength than the particles' chemical composition [14]. In the present study, 50 \(\upmu\)m aluminum oxide (Al\({}_{2}\)O\({}_{3}\)) particles were used for 10 s at a pressure of 2.5 bar and a distance of 10 mm, which was sufficient to ensure strong micromechanical interlocking and chemical adhesion [14; 28; 29]. It is important to note that using higher pressures during sandblasting can damage the composite particles. Therefore, adhering to laboratory-based recommendations regarding particle size and pressure is essential in clinical practice to achieve optimal results [30]. The results of this study showed that HF surface treatment produced high SBS values comparable to sandblasting, aligning with previous studies by [PERSON] [27] and [PERSON] [23]. Hydrofluoric acid etches the glass filler particles in the resin matrix, creating microporosities that increase surface energy and improve bonding agent infiltration [31], resulting in deep primer infiltration and strong mechanical interlocking [18]. In this study, a 9.6% HF solution was applied for one minute, achieving efficient etching without compromising the composite's mechanical properties or its structural integrity [15; 16; 17]. While some studies [2; 14; 32; 33] reported lower bond strength with HF when using materials containing non-etchable zirconia fillers, the milled materials in this study had sufficient etchable glass fillers to achieve high SBS values. However, prolonged HF exposure can dissolve glass fillers, so following the manufacturer's guidelines is crucial. Nevertheless, due to the potential harm of HF to soft tissues, some orthodontists may prefer safer alternatives [20; 23]. The final surface treatment investigated in this study was surface roughening with a flame-shaped diamond bur. The high incidence of adhesive failures experienced in the diamond bur group in addition to their inferior bonding results indicated that this method was not as effective as the other two methods [34]. This finding aligns with [PERSON] et al. [34], who also found that HF and sandblasting achieved superior bond strength compared to diamond bur treatment, even after long-term water storage and thermocycling. Moreover, [PERSON] et al. [35] found that, despite enhancing bond strength by creating retentive areas, the diamond bur's abrasive effect is difficult to control and can damage the surface by exposing filler particles, making it a less favorable option overall. On the other hand, [PERSON] et al. [36] recommended diamond bur roughening as a less aggressive approach for more durable bonding at the interfaces. Adhesive failure is primarily influenced by the bond strength between the bracket and the resin cement, the bond strength between the restorative material and the resin cement, and the mechanical properties of the resin cement [19]. The ARI scores in this study indicated varying degrees of adhesive failure across different restorative materials and surface treatments, with a higher incidence of scores 0 and 1, indicating no adhesive or less than 50% adhesive remaining on the specimen's surface, respectively. This suggests that the bond strength between the bracket and resin cement was stronger than that between the restorative material and resin cement. These findings highlight the significant impact of both material composition and surface treatment on the adhesion of resin cement to indirect restorations. This study has several limitations. First, as an experimental study, it does not fully replicate intra-oral conditions, such as salivary contamination, thermal variations, and pH fluctuations, which may impact SBS results [37].