All patients required radiographic examination of CBCT as part of their dental treatment. The images were taken as part of the routine examination, diagnosis, and treatment planning of patients that included those suffering facial trauma or maxillary sinusitis，who required oral surgery, orthodontic treatment or who needed implant treatment. With the informed consent of the patients, this study was approved by the Medical Ethics Committee of Nanfang Hospital (NFEC-2020-106). CBCT images of 1,100 patients in southern China between January 2018 and January 2019 were collected using the CBCT imaging system from the database of the department of oral radiology, Nanfang hospital, Guangzhou. All images were included in the study and further analysis according to the following inclusion criteria:
- MFMs without periapical disease;
- MFMs had not been endodontically treated;
- MFMs have no root canals with open apices or absorption;
- MFMs exhibit absence of coronal or post and core restorations, which may obscure the imaging study;
- Good quality CBCT images that are clear and lack artefacts.
CBCT scanning condition
CBCT images were obtained using a Planmeca Romexis 3D CBCT scanner (Planmeca, Finland). Board-certified radiologists operated the X-ray tube at an accelerated potential with a field-of-view size (FOV) of 8×8cm, a peak voltage of 84 kV, a beam current of 14 mA and an exposure time of 12 seconds for a full arch. The voxel size was 200 μm×200 μm, and the minimum layer thickness was 0.15 mm. The detector resolution was 1024×1024 pixels, and the pixel size was 127 μm×127 μm. The image data were exported in DICOM format.
Analytical method and content
All CBCT images are reconstructed and measured using the image reconstruction software of Planmeca Romexis CBCT. The software is run on a 32-bit Windows 7 system, and a display screen from the Lenovo Company is used. The screen resolution is 1280 × 1024. The whole CT image is observed and analysed in a dark room. Serial axial, coronal, and sagittal CBCT images were thoroughly examined from the pulp orifice to the apex. All of the images were assessed separately by two endodontists. To confirm the reliability of the data, intraexaminer calibration was performed before the experiment. In cases of disagreement, these two endodontists discussed the data until a consensus was reached. An oral radiologist provided guidance when necessary.
CBCT was used to scan the mandibular region of the patient to obtain a continuous image of the mandibular first molar and its periodontal tissue. The obtained image was input into the 3D reconstruction software Materialise Mimics 21.0 in DICOM format, and the appropriate parameters were adjusted to obtain a clear contour. Data were exported to an STL file. Then, 3D reconstruction was performed to build the part of the root canal and obtain the final 3D reconstruction diagram of the root canal.
MMC classification standard
In CBCT images, the mesial root canal system of MFMs was classified based on classic Vertucci classification and its additional root canal classification [22–25].
Type I (3-3): Pulp chamber bottom have three root canal orifices, and always have three independent root canals, and finally there are three different apical foramens.
Type II (3-2): The pulp chamber bottom has three root canal orifices. Then, these orifices merge into two canals at a certain position of the root canal to yield two apical foramens.
Type III (2-3-1): There are two root canal orifices at the bottom of the pulp chamber. They branch into three independent root canals and finally merge into one root canal that emerges from the same apical foramen.
Type IV (2-3-2): The bottom of the pulp chamber has two root canal orifices that branch into three separate root canals. These canals finally merge into two root canals.
Type V (2-3-2-1): Two root canals are located at the beginning of the pulp chamber and subsequently branch into three independent root canals. Finally, the same apical foramen is formed.
Type VI (1-2-3-2): A root canal opening is located at the bottom of the pulp chamber that subsequently branches into two independent root canals at the upper part of the root canal. Then, the canals divide into three independent root canals and ultimate merge into two root canals.
Type VII (1-3-4-1): Only one root canal orifice is located at the bottom of the pulp chamber. The orifice branches into three independent root canals, subsequently divides into four independent root canals, and finally merges into the same root canal at the apex part of the root canal.
Type VIII (3-2-1): Three different root canal orifices are located at the bottom of the pulp chamber. These orifices merge into two root canals and finally emerge from the same apical foramen.
Type IX (3-2-3-2): At the bottom of the pulp chamber, there are three different root canal orifices that fuse into two root canals, branch into three independent root canals, and finally emerge through two different apical holes out of the root canal system.
Type X (3-4-3-2-1): There are three root canal orifices at the bottom of the pulp chamber. These orifices are divided into four root canals and subsequently merged into three independent root canals. At the apical 1/3 of the root canal, they are merged into two canals, and an apical foramen is finally formed.
If the types of root canal systems found in our experimental sample cannot be identified in the Vertucci classification or additional root canal classifications studied by scholars worldwide, it will be listed separately (Figures 2-4).
Analysis of the curvature and position of the MMC
In this study, a modified Schneider method was used to measure the curvature of the MMC . This method uses the 3D reconstruction software of the Planmeca Romexis CBCT machine to set the root canal orifice as point a and the apical foramen as point C. A straight line was drawn along the root canal image from point a, and the inflection point was set as point b. The acute angles of the ab and bc lines indicate the root canal curvature. The curvature of the root canal can be classified into three grades: slight curvature was less than or equal to 10°; medium curvature was between 10° and 30°; severe curvature was greater than or equal to 30°. Measurement of root canal curved position was based on the ratio of ab to bc. Set P = ab/bc (Figure 1). According to the results of P, the curved regions of root canal can be divided into three categories: P<0.5 was class I, and the curved region of the root canal is the upper 1/3; 0.5<P≤2 is class II, and the curved region of the root canal was the middle 1/3; P >2 was class III, and the curved region of root canal was the apical 1/3.
Statistical analysis was performed using SPSS (Version 21.0; SPSS, Inc., Chicago, IL), a statistical software package for Windows. The measurement data were expressed as the mean standard deviation or percentages as appropriate for each measurement calculated at the individual and tooth levels. The chi-square test was used to examine differences among categorical variables, such as age (age<40 vs. age≥40 years), sex (male vs. female), and side (left vs. right). The mean is compared using t tests with significance set at P<0.05.