Fig. 1

Determination of appropriate field of view. （A）Transverse plane; (B)(E)(F) Sagittal plane; (C)(D) Coronal plane; (G) Cementoenamel junction curvature. The red line represents the coronal plane, the green line represents the sagittal plane, and the blue line represents the transverse plane. First, the target tooth (maxillary left central incisor) is in sight, and we adjust the red line parallel to the incisal edge and pass through the central point of the tooth (A). Then, the angle of the red line can be adjusted subtly so that the red line fits into the center of the root canal, as shown in (B). At this time, the blue and green lines are shown in (C). While keeping the red line stable in position, we gently moved the blue line down to locate the cementoenamel junction (CEJ) in the mesial-distal direction coronally (D). At this time, the blue line was seen above the CEJ in the buccal-lingual direction in the sagittal plane (E). Finally, we adjusted the blue line apically to locate the CEJ in the buccal-lingual view in the sagittal plane (F). The root length was determined at both the buccal-lingual and mesial-distal views (G). The mean value was the final root length, and the subtraction was the CEJ-C value