## 2 Materials and Methods  
### Participants, Eligibility Criteria, and CBCT  
This study focused on adult participants 20 years or older who had received an orthodontic screening examination and had their initial CBCT images taken during their orthodontic screening appointments at the Arizona School of Dentistry & Oral Health Postgraduate Orthodontic Program since 2017. Images were taken using the head support, head strap, and chin cup to align the patients' heads in an upright sitting position to obtain the Frankfort horizontal (FH) plane parallel to the floor and with the patients instructed to bite in maximum intercuspation [11]. Participants aged 20 years or older of either gender were included. In addition, only those participants who had CBCT taken with a large field of view (FOV) at their initial orthodontic screening visit at the clinic were considered for this research. The following exclusion criteria were applied: (1) a history of TMDs or active TMD symptoms as evaluated through a questionnaire based on the Axis I clinical diagnostic criteria [12], (2) prior TMJ surgery or trauma, (3) previous orthodontic treatment, (4) craniofacial anomalies such as cleft lip and palate, (5) extracted permanent teeth other than third molars, (6) severe skeletal asymmetry, (7) head and neck neoplasms, (8) systemic disease that could cause structural or physiological changes to TMJ or surrounding structures [13; 14; 15]. After applying these criteria, a total of 135 participants were selected for the study. Table 1 shows the sample distribution consisting of 135 participants, 64 males and 71 females. Race was grouped as Caucasian and non-Caucasian. For purposes of analysis, the participants were divided into three age groups: early (20 \(\leq\) y \(<\) 30), middle (30 \(\leq\) y \(<\) 40), and late (40 \(\leq\) y). They were divided into three vertical skeletal groups based on the angle between sella-nasion and mandibular plane (SN-MP): hydrodivergent (SN-MP \(<\) 29\({}^{\circ}\)), normovergent (29\({}^{\circ}\)\(\leq\) SN-MP \(\leq\) 36\({}^{\circ}\)), and hyperdivergent (SN-MP \(>\) 36\({}^{\circ}\)) [16]. Additionally, they were divided into three horizontal skeletal groups based on the ANB angle: Class I (1\({}^{\circ}\)\(\leq\) ANB \(\leq\) 4\({}^{\circ}\)), Class II (ANB \(<\) 1\({}^{\circ}\)), and Class III (ANB \(>\) 4\({}^{\circ}\)) [10]. All CBCT imaging was conducted using the i-Cat FLX (Dental Imaging Technologies Corp, Hatfield, PA, USA). The scanning parameters were standardized to ensure consistency across all scans. The FOV was set at 23D \(\times\) 17H cm with a voxel size of 0.3 mm. The CBCT  
\begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline  & & \multicolumn{3}{c}{**Gender**} & \multicolumn{3}{c}{**Age**} & \multicolumn{3}{c}{**Race**} \\ \cline{3-8}  & **Variables** & **Total** & **M** & **F** & **Early** & **Middle** & **Late** & **Caucasian** & **Non-Caucasian** \\ \cline{3-8}  & & & & **20 \(\leq\) y \(<\) 30** & **30 \(\leq\) y \(<\) 40** & **40 \(\leq\) y** & & \\ \hline Gender & Male & & 64 & & 29 & 15 & 20 & 27 & 37 \\  & Female & 135 & & 71 & 22 & 25 & 24 & 26 & 45 \\ Age & Early & 51 & 29 & 22 & & & & 18 & 33 \\  & Middle & 40 &machine was operated at 120 kVp and 5 mA. Scans were performed using standardized \"ortho\" settings, allowing for uniform image acquisition [3; 10]. The constructed CBCT images were orientated with the Frankfort horizontal (FH) plane. The FH plane was constructed from the right portion and two orbitales on both sides (Figure 1A). The TMJ slices were constructed using Dolphin imaging version 12.0 (Dolphin Imaging and Management Solutions, Chatsworth, CA, USA) by orienting the center point in the middle of the condylar head and by aligning the axis perpendicular to the condylar head. The slices had a thickness of 1.0 mm, a width ranging from 35 to 45 mm based on condylar head size, and a total of 18 cuts. The slices were oriented in both sagittal and coronal planes (Figure 1B) [4; 5]. Institutional review board approval to conduct the study was granted by A.T.