This cross-sectional study evaluated the CBCT scans of the maxilla and mandible of adults between 18 to 35 years (both males and females) retrieved from the archives of a radiology clinic. The study was approved in the ethics committee of Kermanshah University of Medical Sciences (IR.KUMS.REC.1397.525). A written informed consent was obtained from all patients.
Sample size was calculated to be 66 records (n = 22 in each group) according to a study by Shewinvanakitkul et al, (27) assuming the standard deviation of canine inclination in class I and class II patients to be 3.6 and 4.5, respectively, accuracy (d) of 0.5, alpha = 0.05 and power of 90%.
All CBCT scans had been obtained with NewTom VGi CBCT system (Verona, Italy) for orthodontic or surgical treatment planning. The inclusion criteria were CBCT scans taken with 15 × 15 cm field of view in natural head position and maximum intercuspation. The CBCT scans were selected using convenience sampling.
CBCT scans of patients with a history of orthodontic treatment, orthognathic surgery, craniofacial syndromes such as the cleft lip or palate, facial asymmetry, hemi-hypertrophy of the mandible, pathologies involving the upper airways, upper airway infection, chronic mouth breathing, permanent snoring, history of trauma, missing of more than 4 teeth in each jaw, tonsillar hypertrophy, adenoids, history of tonsillectomy and respiratory problems were excluded. CBCT scans on which the critical cephalometric landmarks could not be identified were also excluded.
The CBCT images were evaluated in axial, sagittal and frontal views. The axial view was used to assess the cross-section of teeth. The frontal view was used to assess the transverse pattern of the jaws, inclination of teeth and the curve of Wilson and the sagittal view was used to assess the anteroposterior relationship and the vertical relationship of the jaws.
All CBCT images were obtained with 300 µm spatial resolution, 110 kV and 78.59 mAs. The CBCT data were exported in DICOM format using NNT Viewer software. The Mimics Medical Software (version 19; Materialise, Leuven, Belgium) was used to reconstruct lateral and posteroanterior cephalograms. In order to standardize the images and minimize errors in measurements, all images were reoriented using NNT Viewer Reorientation software (version 19; Materialise, Leuven, Belgium) such that the Frankfurt horizontal plane and the interorbital line (a line connecting the inferior points of the orbital rims) were parallelized to the horizontal line. By doing so, the head position was standardized in all records and all angles were measured based on this line (Fig. 1). For cephalometric analysis, the following hard tissue reference points were identified:
Orbitale (Or): The most inferior point of the orbital rim
CI: The incisal edge of canine
CA: The apex of canine
MO: The central point of the buccolingual width of the occlusal surface of molar tooth
MC: The central point of the buccolingual width of the cervical part of the anatomical crown
MBM1/MBM2: The mesiobuccal cusp tip of the maxillary first and second molars
MLM1/MLM2: The mesiolingual cusp tip of the maxillary first and second molars
The cephalometric indices used for assessment of the sagittal pattern included the ANB angle and the Wits appraisal; according to which, the samples were divided into Class I (ANB: 0–4°; Wits 0 to -1), class II (ANB > 4°, Wits > 0) and class III (ANB < 0°, Wits < -1) groups.
The measurements of inclination angles and the curve of Wilson were made using NNT Viewer and Mimics software. In assessment of the buccolingual inclination of molars, the MO point was used as the reference point in order to eliminate the effect of morphology of the cusp of molars. The MC point was used as the reference point to eliminate the effect of root morphology. On the frontal view, the MO-MC line was drawn to determine the buccolingual inclination of posterior teeth (Figs. 2 and 3). The CI-CA line was drawn to determine the inclination of canine teeth (Figs. 4 and 5). Its angle in the maxilla and mandible was determined by drawing a line parallel to a line connecting the two orbitale points.
In order to measure the curve of Wilson, the mesiobuccal and mesiolingual cusp tips of the maxillary first and second molars were connected along the buccal groove and the formed angle was measured (Fig. 6).
Measurements made by an examiner and an experienced radiologist on 20 CBCT scans were repeated again after 2 weeks to assess the intra-examiner reliability. The lowest intra-class correlation coefficient was 0.969, which was considered excellent according to the Cicchetti’s classification (31). The Dahlberg’s formula was used to assess the method error. The maximum value was found to be 1.01.
Normal distribution of data was evaluated using the Kolmogorov-Smirnov test. The chi-square test was used to compare the study groups in terms of gender. Since data were normally distributed, ANOVA was applied for statistical analysis. For variables with non-homogeneity of variances, the Welch ANOVA was used. Tukey’s post hoc test was applied for pairwise comparisons. All statistical analyses were carried out using SPSS version 18 (SPSS Inc., IL, USA) at 0.05 level of significance.