Anatomical assessments

First, a lateral cephalograph was constructed from each CBCT. An accurate technique was adopted to draw the maxillary and mandibular planes because the assessment of dental inclination requires the measurement of the angle formed between the maxillary and mandibular planes and the longitudinal axis of the respective tooth. On lateral cephalographs, the palatal plane of the maxilla is defined as the line connecting the anterior nasal spine (which appears as a point on the superior or inferior contour of the nasal spine where it has 3 mm thickness) to the posterior nasal spine (at the tip of the posterior spine of the palate where soft and hard palates merge). The mandibular plane is defined as the line connecting the gonion (the midpoint of the contour connecting the mandibular body and the ramus) to the gnathion (the midpoint of the inferior border of the mandibular symphysis). However, a noteworthy issue is that in CBCT assessment, a line cannot be generalized to the entire 3D space. On the other hand, the landmarks gonion, gnathion, anterior nasal spine, and posterior nasal spine are not visible on the sagittal views of the respective incisor tooth. To solve this problem, high-resolution CBCT volumes were used. The palatal plane and the anatomical mandibular plane were adjusted parallel to the horizon such that in all images, the horizontal line indicated the maxillary or mandibular plane (Fig. 1).

Paralleling the mandibular plane with the horizontal line

Afterward, to find the best section for the evaluation of incisors, the most prominent points in the buccal and lingual sides of each incisor were identified by a trained dental student on the cervical part of each tooth in the axial view, and connected with a line. This line indicated the sagittal plane, i.e., the most appropriate section for the evaluation of BLI of the tooth and marginal bone height (Fig. 2).

Axial view: Selecting the most appropriate section for the measurements in the sagittal plane

Next, a separate file was allocated to each incisor and coded to ensure a blind examination. For this purpose, all 248 CBCT scans of incisors were mixed, and randomly coded 001 to 248 through a simple randomization method. Then, the patients’ names were eliminated from the images to prevent possible bias by the radiologist and orthodontist and ensure a blind assessment.

Subsequently, an orthodontist drew the longitudinal axis of each tooth by connecting the incisal edge to the apex. The buccolingual inclination (BLI) of each tooth was estimated by measuring the angle formed between the longitudinal axis and palatal plane in the maxilla, and the longitudinal axis and mandibular plane in the mandible (Fig. 3).

Sagittal plane: Measuring dental inclination

To measure the buccal heigh deficiency (BHd) and the lingual heigh deficiency (LHd), a radiologist blinded to the BLI of the teeth drew a line from the CEJ in the buccal and lingual sides, perpendicular to the longitudinal axis of the tooth. Finally, a line was drawn from the bone crest in the buccal and lingual sides perpendicular to the longitudinal axis. The distance between the drawn lines and reference lines was recorded as the BHd and LHd in millimeters (Fig. 4).

Sagittal view: Measuring the alveolar crest height on the buccal and lingual plates