## 2 Materials and Methods
### Teeth and Bonding Procedures  
Bovine permanent incisor teeth [22] were harvested with inclusion criteria dictating an intact labial surface, no enamel defects, no cracks, and no dental caries. The teeth were examined with a magnifier (\(\times\)10) to determine eligibility. The teeth were cleaned and polished with pumice paste (Reliance, Houston, TX, USA) at low speed for 10 s, rinsed with distilled water for 10 s, and stored in a 0.1% (wt/vol) thymol solution at 37 \({}^{\circ}\)C to avoid any bacterial growth [1; 20]. The enamel surfaces of the bovine teeth were scanned with an intraoral scanner (CS 3600, Carestream Dental, Atlanta, GA, USA) and analyzed with CS Mesh Viewert(tm) software (version; 1.0.0.13.568) (Carestream Dental, Atlanta, GA, USA). The scanned STL files were sent to the Lightforce Company (Lightforce, Cambridge, MA, USA) to generate the 3D custom-printed brackets according to the enamel surface of the teeth. The teeth were secured into polyvinyl chloride (PVC) cylinders with self-cure acrylic resin (Alumilite, Amazing clear cast, Kalamazoo, MI, USA ) (Figure 1A,B). The enamel surface was prepared with Transbond(tm) plus self-etching primer (3M, Unitek, CA, USA) for 5 s, and then, a gentle stream of oil-free air was delivered for 1-2 s on all the teeth. The ceramic and metal brackets were bonded with a light-cure composite resin (Transbond(tm) plus adhesive, 3M, Unitek, CA, USA). The 3 DBs were bonded using either composite resin (Transbond(tm) plus adhesive, 3M, Unitek, CA, USA) or a more flowable resin (Flow Tain, Itasca, IL, USA). Finally, the Assure Plus bonding enhancer (Reliance Orthodontic Products, \begin{table}
\begin{tabular}{c c c} \hline
**Bracket Type** & **Material** & **Base Size** \\ \hline
3D custom-printed (Lightforce, Cambridge, MA, USA) & Polycrystalline alumina & 12.06 mm\({}^{2}\) \\ \hline Chic Roth ceramic maxillary incisor brackets (GC Orthodontics America, Alsip, IL, USA) & Ceramic & 12.75 mm\({}^{2}\) \\ \hline Legend maxillary incisor brackets (GC Orthodontics America, Alsip, IL, USA) & Metal & 12.96 mm\({}^{2}\) \\ \hline \end{tabular}
\end{table} 
Table 1: Bracket type, material, and base size of the brackets. \begin{table}
\begin{tabular}{c c c c} \hline
**Trade Name** & **Material** & **Bonding Procedures** & **Abbreviation** \\ \hline Lightforce & Polycrystalline alumina & 3 DB + composite only (C) & 3 DBC \\ \hline Lightforce & Polycrystalline alumina & 3 DB + composite (C) + Assure Plus (A) & 3 DBCA \\ \hline Lightforce & Polycrystalline alumina & 3 DB + flowable only (F) & 3 DBF \\ \hline Lightforce & Polycrystalline alumina & 3 DB + flowable (F) + Assure Plus (A) & 3 DBFA \\ \hline Chic Roth & Ceramic & CB + composite only (C) & CBC \\ \hline Legend & Metal & MB + composite only (C) & MBC \\ \hline \end{tabular}
\end{table} 
Table 2: The experimental protocol of the 6 subgroups. It is essential to have a standardized bonding methodology when studying the bond strengths of orthodontic brackets or attachments to the enamel surface of teeth. Thus, a 1.5-inch Push-Pull Gauge (The Jonard Tools, Elmsford, NY, USA), applying a 300 gf compression, was utilized to set the brackets on all teeth with a standard force prior to the initiation of the curing process of the resin. Before curing, the resin excess was removed from the edge of the brackets with a dental exploreer [1; 20]. An LED curing light (Aceton LED, American Orthodontics, Sheboygan, WI, USA) ensured that appropriate polymerization of the resin occurred, resulting in a harder, tougher, and more stable adhesive/resin bond. This procedure was performed on each side of the bracket base for 10 s, with an intensity of 1200 mW/cm\({}^{2}\)[1; 20]. The specimens were stored in a water bath at 37 \({}^{\circ}\)C for 24 h to prevent dehydration after bonding. These processes were achieved for all samples [1]. A universal test machine (New Day Research, West Chicago, IL, USA) was operated to obtain the SBS value (Figure 2A). Each specimen was exposed to a force on its labial surface parallel to the force (Figure 2B).