## 2 Materials and Methods  
The radiographs were selected from the archives of the postgraduate clinic of the Department of Orthodontics, School of Dentistry, National and Kapodistrian University of Athens, Greece. These radiographs were required for diagnosis and treatment planning. Only gender and age were recorded for each subject.  
The sample consisted of 474 radiographs who met the following inclusion criteria: good image quality for visibility of the 2 nd, 3 rd and 4 th cervical vertebrae (C2-C4), belonging to children, adolescent or young adults with unremarkable medical history, having no inherited or acquired craniofacial deformities, and having no previous craniofacial trauma and no previous orthodontic treatment. Radiographs from subjects on long-term medication or with syndromes, congenital anomalies (such as absence of pedicle [29], cleft lip and/or palate), metabolic or developmental diseases or nutritional problems that may have an impact on craniofacial and vertebrae development, were excluded.  
All cephalometric radiographs were acquired with the same x-ray setup [Planmenca Promax (Helsinki, Finland), max KV 84, 26 mm Al filter, 1700 VA] using identical source-subject-film distances. Image distortion was not considered since linear measurements were not performed.  
The skeletal maturation was assessed according to [PERSON]'s method [1]. This 6-stage method (CS1-CS6) is based on the assessment of anatomical changes of the 2 nd, 3 rd and 4 th cervical vertebrae (C2, C3 and C4). The method evaluates two parameters: (a) the presence or absence of concavity of the lower border of the vertebrae and (b) their shapes. Four distinct shapes are recognized, i.e., square, trapezoid, rectangular vertical and rectangular horizontal (Figure 1).  
All radiographs were cropped, in order to depict only the cervical vertebrae area, and randomized. A Microsoft Excel database was formed and distributed to the raters, who were blinded to name, sex and age.  
The images were evaluated independently by one medical radiologist (MR), one dentomaxillofacial radiologist (DR), two orthodontists (OR1 and OR2), one dentist (D) and one 3 rd year orthodontic postgraduate student (OPS). The calibration procedure consisted of two sessions: (a) a theoretical session, which included a presentation of the CVM method, and (b) a clinical session. During the latter, the raters evaluated 40 images (not included in the sample). The results were analyzed, and the disagreements discussed [1; 30]. In case of uncertainty, the earlier maturation stage between two consecutive stages was chosen. The ratings sessions were performed no later than 5 days after the calibration session and under the same conditions in a diagnostic monitor. A time limit was not imposed on the rating session's duration; however, a maximum number of 30 images was allowed in each session in order to minimize errors due to raters' fatigue. Each examiner re-evaluated all images after one month. The images' order was randomly changed for this second rating.  
Statistical analysis was performed using the STATISTICA 10 software package (TIBCO Software Inc., Palo Alto, California, CA, USA) and MedCalc software (v. 14.10.2) for MS Windows (Microsoft, Washington, DC, USA). \(p\)-values \(\leq\) 0.05 were considered statistically significant. Inter-rater agreement was measured using the intraclass correlation coefficient and intra-rater agreement was assessed by [PERSON]'s weighted kappa. Method's reliability was evaluated based on intra- and inter-rater agreement. A factorial (2 \(\times\) 6) analysis of variance was used to investigate the correlation between the CVM stages and the chronological age, using mean Cervical Maturation Stage (CS; between the 6 ratings) and gender as grouping factors and chronological age as the dependent variable. Furthermore, the potential of this CVM method in chronological age assessment was evaluated by general linear model regression analysis using age as the dependent variable and gender and mean CS as the predictor variables.