## 4 Discussion  
To avoid procedural errors in root canal preparation, information on the root canal anatomy--especially data regarding the remaining dentin thickness, narrowest and widest canal widths, and 3D direction and degree of canal curvature--is relevant. Notwithstanding the large number of root canal anatomy studies of the mandibular incisors [10; 11; 12; 13], these variables have scarcely been measured, possibly because of the lack of customized image analyzing software. In the present study, anatomical indicators were measured using MCT combined with custom-developed software, Kappa 2, which provided 3D surface models of the roots and canals and an imaginary central axis in each cross-sectional image. Based on our measurement results, clinicians can determine the initial apical file size when root canal treatment of mandibular incisor is performed. In the present study, the narrowest canal width was 0.22 \(\pm\) 0.05 mm in the SIA section, and the widest canal width was 0.40 \(\pm\) 0.11 mm in the SIA section. These results coincided with the finding reported by [PERSON] et al. that the medians of the buccolingual diameter in the apical 1 mm portion were 0.36 and 0.41 mm, respectively, in the Vertucci type I root canal and type III root canal [14]. Therefore, apical preparation should be performed at least up to the size of 0.40 mm for debridement of the necrotic pulp and removal of the infected dentin. The widest canal width of S2C (0.78 \(\pm\) 0.36 mm) was greater than that of S3C (0.59 \(\pm\) 0.38 mm). This interesting finding might result from root canal calcification in S3C and imply that these dentin collars should be removed for efficient root canal preparation. Concurrently, detailed measurement of 3D canal curvature was obtained. It was the greatest in the apical region, followed by the coronal regions. The 3D canal curvature was the straightest in the mid-root region. The overall curvature of mandibular incisors measured in this study (0.18 \(\pm\) 0.27 mm\({}^{-1}\)) was less than that of the mesibucocal canal of the maxillary first molar (0.22 \(\pm\) 0.06 mm\({}^{-1}\)) [9] and the mesibucocal (0.26 \(\pm\) 0.23 mm\({}^{-1}\)) and mesiolingual canals (0.22 \(\pm\) 0.18 mm\({}^{-1}\)) of the mandibular first molar [7]. These results imply that root canal preparation in mandibular incisors is not as difficult as in the mesibuccal canal of the maxillary first molar or the mesibuccal and mesiolingual canals of the mandibular first molar. Knowledge of the thinnest dentin, particularly in the apical part, is useful for preventing apical perforation when root canal shaping is performed. The mean (SD) value of MinDist was 0.56 (0.12) mm in the SIA section, which is susceptible to apical or strip perforation and weakening of the tooth structure. While the average mesial and distal dentin thicknesses were both 0.66 mm, the average labial and lingual dentin thicknesses were 1.13 and 1.27 mm, respectively. Therefore, instrumentation should mainly be directed toward the buccal and lingual directions, rather than the mesial and distal directions. Furthermore, the apical third (S1) has the most severe curvature, a sharp decrease in dentin thickness, and an increased frequency of the thinnest dentin being directed mesially. These features may lead to procedural errors such as transportation, ledge formation, loss of patency, and perforation. Owing to the development of the nickel-titanium rotary instruments, these iatrogenic events could be minimized by the use of flexible instruments. Previous studies reported that root canal preparation with nickel-titanium rotary files enabled the maintenance of the original canal anatomy with less canal transportation and better centering ability compared to root canal preparation with stainless-steel K files [15; 16; 17]. In the present study, detailed anatomical indicators were obtained at high precision using MCT with Kappa 2 software [18].