Our study had some limitations that should be addressed. First, in our study, antero-posterior location of the mandibular canal was defined by the distance from the mental foramen. Tooth positions could not be used as a reference, because premolars and molars were totally or partially missing in considerable number of the cases. Although areas 1–3 were considered mostly to correspond to the area from the second premolar to second molar, it might not be true for some cases due to anatomical variations for the position of the mental foramen. Second, we did not evaluate the difference of the visibility of the mandibular canal by age or gender. According to the study by Miles et al. [4], the visibility was significantly lower in females than in males. It was also affected by age depending on the location. Although we applied power analysis to determine the sample size, the sample size was not sufficient for such analysis. Third, we could not confirm the actual positions of the mandibular canal walls because we used CBCT images of clinical cases. Thus, it might be possible that a few cases with misinterpretation were included in our data.