# Factors Associated with the Proximity of the Incisive Canal to the Maximally Central Incisor
## 2 Materials and Methods
### Data Acquisition and Measurements  
CBCT images were acquired using a cone-beam X-ray CT system (KaVo 3D eXam, KaVo, Biberach, Germany). The scan protocols were set at 120 kVp, 5 A, and 0.3-mm voxel size (0.3 \(\times\) 0.3 \(\times\) 0.3 mm), and the rotation (scan) time was 17.8 s. The field of view (FOV)was set to 230 \(\times\) 170 mm. The data were stored in the Digital Imaging and Communications format, subsequently imported into the imaging software, Invivo 6 (Anatomage, San Jose, CA, USA), and reconstructed into three-dimensional (3D) images for further processing and analysis. The following procedure was used to measure the linear distance between the left and right MCI root apexes and the IC using 3D images. All the dimensional measurements of MCI root apexes and the IC were measured in axial slices parallel to the Frankfort horizontal (FH) plane based on a previous study [20]. The FH plane was defined according to previous studies as the plane passing through the bilateral polion (Po) and left orbita (Or) (Figure 1). The height of the measurement was taken at the level of the MCI root apex (Figure 2). The linear distance between the maxillary central incisor (MCI) root apex and the incisive canal (IC) on each side, defined specifically as the shortest linear distance between the lateral boundary of the MCI root and the lateral boundary of the IC cortical bone, was measured by tracing the center of the MCI root and IC cross-sections, based on the methodologies of previous studies [30] (Figure 3). When the lateral cortical bone boundary of the IC was in contact with the root, the distance between the MCI and the IC was recorded as zero. Left and right averages were used to compare the explanatory variables. Figure 1: Maximillary central incisor root apexes and incisive canal are measured in axial slices parallel to the Frankfort horizontal plane. Figure 2: The linear distance between the maxillary central incisor root apexes and the incisive canal is measured at the height of the maxillary central incisor root apex. Lateral cephalometric radiographs were used for the anatomical morphology of the maxillofacial region. A Collimator Type R20J (SHIMADZU, Kyoto, Japan) was used for lateral cephalometric imaging and Cephalometrics A to Z ver. 22.0 (YASUNAGA Computer Systems Company, Fukui, Japan) was used for lateral cephalometric analysis. Lateral cephalograms of the patients were acquired when their teeth were in centric occlusion. The lateral cephalometric reference points and lines are listed in Table 1 and illustrated in Figure 4.  
This study used two distance and five angle measurements of the cephalometric variables to evaluate the relationship between the MCI and distance to the IC. The seven cephalometric variables are listed in Table 2. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Landmark** & \multicolumn{2}{c}{**Definition**} \\ \hline N & Nasion & Most anterior point of the nasofrontal suture \\ S & Sella & Center of the sella turcia \\ Or & Orbitale & Most inferior point of the lower margin of the orbit \\ Po & Porion & Most superior point of the external \\  & & auditory meatus \\ Me & Menton & Most inferior point on the symphysis \\ Pog & pogonion & Most anterior point on the symphysis \\ A & point A & Point of the deepest concavity anteriorly on the \\  & & \multicolumn{1}{c}{} \\ B & point B & Point of the deepest concavity anteriorly on the \\ ANS & anterior nasal spine & Most anterior point of the anterior nasal spine \\ PNS & posterior nasal spine & Most posterior point of anterior nasal spine \\ Ptm & pterygomaxillary fissure & Lowest point of the pterygopalatine fossa \\ U1 & upper incisor & Maximillary central incision edge \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Lateral cephalometric reference points. Figure 3: The measurement of the linear distance between the maxillary central incisor root apex and incisive canal of the left and right is measured on each side by tracing the center of the maxillary central incisor root and incisive canal cross-sections.