CBCT scans were performed on the mandibular first molars of 1,100 patients in southern China who required CBCT imaging in the database of the department of oral radiology, Nanfang hospital, Guangzhou. With the informed consent of the patients, this study was approved by the ethics committee of Nanfang hospital. All images were collected with the CBCT imaging system between January 2018 and January 2019. All images were included in the study and further analyzed 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 absence of coronal or post and core restorations which may obscure image study;
- CBCT images of good quality, clear and without artifacts.
CBCT scanning condition
The CBCT images were taken using a Planmeca Romexis 3D CBCT scanner (Planmeca, Finland). The board-certified radiologists operated the x-ray tube at an accelerated potential of a peak voltage of 84 kv with a beam current of 14 mA and the exposure time was 12 seconds for a full arch. The voxel size was 200μm×200μm and the minimum thick-layer was 0.15mm. The detector resolution was 1024×1024 pixels and the pixel size was 127 μm×127 μm. Image data were exported in DICOM format.
Analytical method and content
All CBCT images are reconstructed and measured by the image reconstruction software of Planmeca Romexis CBCT. The running system of the software is a 32-bit windows7 system, and the display screen is a display screen of Lenovo Company. The screen resolution is 1280 × 1024. The whole CT image is observed and analyzed in the 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 2 endodontists. To confirm the reliability of the data, intraexaminer calibration was performed before the experiment. In cases of disagreement, these 2 endodontists were discussed until a consensus was reached.
MMCs 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].
TypeⅠ(3-3): Pulp chamber bottom have three root canal orifices, and always has three independent root canals, and finally there are three different apical foramens.
TypeⅡ(3-2): Pulp chamber bottom have three root canal orifices，then there are merged into two canals at a certain position of the root canal，and ends up with two apical foramens.
TypeⅡ(3-2): There are three root canal orifices at the bottom of the pulp chamber, and then they merge into two root canals, and finally come out from two different apical foramens.
TypeⅢ(2-3-1): There are two root canal orifices at the bottom of the pulp chamber, then they branch into three independent root canals, and finally they merge into one root canal, and they come out from the same apical foramen.
TypeⅣ(2-3-2): The bottom of the pulp chamber begin with two root canal orifices, then branch into three separate root canals, and finally merge into two root canals.
TypeⅤ(2-3-2-1): There are two root canals at the beginning of the pulp chamber, which are then branched into three independent root canals, and finally, the same apical foramen is formed.
TypeⅥ(1-2-3-2): There is a root canal opening at the bottom of the pulp chamber, which branches into two independent root canals at the upper part of the root canal, then divides into three independent root canals, and finally merges into two root canals.
TypeⅦ(1-3-4-1): There is only one root canal orifice at the bottom of the pulp chamber, and then it branches into three independent root canals, and then divides into four independent root canals, and finally merges into the same root canal at the apex part of root canal.
TypeⅧ(3-2-1): There are three different root canal orifices at the bottom of the pulp chamber, and then they merge into two root canals, and finally come out from the same apical foramen.
TypeⅨ(3-2-3-2): At the bottom of the pulp chamber，there are three different root canal orifices, which fuse into two root canals, then branch into three independent root canals, and finally through two different apical holes out of the root canal system.
TypeⅩ(3-4-3-2-1): There are three root canal orifices at the bottom of the pulp chamber, which are divided into four root canals, and then merged into three independent root canals, during the apical 1/3 of root canal, they are merged into two canals, and finally an apical foramen is formed.
If the types of root canal system found in our experimental sample cannot be found in the Vertucc classification or in the additional root canal classification studied by scholars from all over the world, it will be listed separately (Figure 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 . The method is: use the 3D reconstruction software of planeca romexis CBCT machine to set the root canal orifice as point a, the apical foramen as point C, draw a straight line along the root canal image from point a, and set the inflection point as point b. The acute angle of ab and bc lines was root canal curvature. The curvature of root canal can be divided into three grades: slight curvature is less than or equal to 10°; medium curvature is between 10°-30°; severely curvature is more than or equal to 30°. Measurement of root canal curved position: referring to the ratio of ab and 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 is class I, the curved region of root canal is upper 1/3; 0.5<P≤2 is class II, the curved region of root canal is middle 1/3; P > 2 is class III, the curved region of root canal is apical 1/3.
Statistical analysis was performed using SPSS 21.0, the rate is compared by chi-square test; the mean is compared with t test, with significance set at P＜0.05