## 2 Material and Methods
### Sample Selection  
Ten of each of seven types of vestibular brackets of size 0.022 \(\times\) 0.028-inch for tooth 2.1 were investigated, making a total of 70 brackets. Specifically, the slot heights of five types of commercially available brackets were measured, each bracket from a different batch, while two types were 3D-printed in-house (Table 1). The sample was divided into four different groups according to the bracket material and the method of holding the archive, as follows:  
1. Conventional metal brackets: Primo SWM (Sweden and Martina, Italy) and Legend Mini (GCOrthodontics, Breckerfeld, Germany).
2. Self-ligating metal brackets: SIX 3D careers (Target Ortodonzia srl, Garbagnate Milanese, Italy) and Damon (Ormco, Glendora, CA, USA).
3. In-house 3D-printed resin brackets (IH3 DB): IH3 DB1, manufactured using Sprint-Ray Pro 95 (SprintRay Inc., Los Angeles, CA, USA), and IH3 DB2, manufactured using EnvironTec D4K (EnvisionTec, Gladbeck, Germany).  
The in-house 3D-printed resin brackets were digitally designed (CAD) using the professional 3D drawing software Rhinoceros 7 (Robert McNeel and Associates, Washington, WD, USA), and 3D printed (CAM) via two different desktop 3D printers using DLP (Digital Light Projection) technology. In brief, once the bracket had been designed, the.stl file was imported into dedicated 3D printing software, namely RayWare 2.8.1 (SprintRay Inc., Los Angeles, CA, USA), for the SprintRay Pro 95 printer (SprintRay Inc., Los Angeles, CA, USA) and the Envision One RP software (EnvisionTec, Gladbeck, Germany) for the EnvironTec D4K (EnvisionTec, Gladbeck, Germany).  
For both printers, the various samples and supports were digitally designed and positioned on the printing plate as per Figure 1. The resins used to print the brackets were the following:  
* Verseo Smile Temp (BEGO GmbH and Co. KG, Bremen, Germany) biocompatible resin, certified IIa with flexural strength \(\pm\)80 Mpa and colour A2 on the Vita scale, calibrated for the SprintRay Pro 95 printer at 50 \(\upmu\)m on the Z axis.
* C&B MFH (NextDent B.V. Soesterberg, Netherlands) biocompatible resin, certified IIa, with flexural strength \(\pm\)107 Mpa, colour A2 on the Vita scale, and calibrated for the EnvironTec D4K printer at 100 \(\upmu\)m on the Z axis.  
\begin{table}
\begin{tabular}{l l c c c} \hline \hline  & \multicolumn{4}{c}{**Brackets Selected for the Study**} \\ \hline
**Type of Bracket** & **Manufacturer** & **Tooth** & **Slot Height** & **Torque** \\ \hline Primo SWM & Sweden \& Martina & 2.1 & 0.022-inch/0.558 mm & 17\({}^{\circ}\) \\ \hline Legend Mini & GC Orthodontics & 2.1 & 0.022-inch/0.558 mm & 17\({}^{\circ}\) \\ \hline Damon Q2 & Ormco & 2.1 & 0.022-inch/0.558 mm & 12\({}^{\circ}\) \\ \hline Carriere SLX 3D & Carriere & 2.1 & 0.022-inch/0.558 mm & 17\({}^{\circ}\) \\ \hline Inspire ICE & Ormco & 2.1 & 0.022-inch/0.558 mm & 17\({}^{\circ}\) \\ \hline IH3 DB 1 & In-office (Sprintray Pro 95) & 2.1 & 0.022-inch/0.558 mm & 17\({}^{\circ}\) \\ \hline IH3 DB 2 & In-office (EnvisionTec D4K) & 2.1 & 0.022-inch/0.558 mm & 17\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Description of the sample for each type of bracket investigated.  
Azurea pin gauges (Azurea Technologies S.A., Switzerland) with increments of 0.002 mm were used to measure the height of each slot (Figure 2). The gauges were inserted inside the slot of each bracket, beginning with the gauge with the greatest diameter (0.584 mm), and an Optika B500 optical microscope (Optika S.R.L, Ponternica, Italy) was used to check that the gauge was in full contact with the bottom of the slot (Figure 2). The slot height was recorded as the diameter of the gauge that filled the slot completely from top to bottom (Figure 2). Measurements were conducted by a single operator (CC) and repeated two weeks apart.