This study was approved by the research ethics committee of Shanghai JiaoTong University Schooh of medicine affiliated Shanghai Ninth People Hospital, Shanghai, China (SH9H-2019-T74-1). Ninety human mandibular first molar that were extracted for periodontal reasons were selected. All procedures were performed with the informed consent of each patient. All collected teeth were washed with saline, remaining tissue and dental crown were removed and stored in distilled water until use.
Fourteen human mandibular first molar (mesial buccal and mesial lingual root canal, twenty-eight mesial root canal in total) were chose by the following inclusion criteria: (i) tooth defects do not involve the root canal orifice to the apex, (ii) no pulp calcification occurred, (iii) fully formed apices (iv) X-ray showed the shape of the root canals (v) moderate curvature of root canals: 20-35 degree of canal curvature by using picpick software (NGWIN) combined with Schneider’s method[8].
Fourteen specimens are randomly divided into 2 groups, which are MTwo-R and M3-RT group. Each group has 7 teeth, which included 14 mesial root canals in total.
1.1 Initial root canal treatment
14 teeth were decoronated to standardize the root lengths to 15mm. The working length was established using 10 K-file and it was set 1.0mm shorter of the measurement. The instrumentation procedure used M3 PRO 10/04 file for root canal glide path. The specimens were then prepared by ENDO-MATE DT endodontic motor (Shanghai Yujing Trading Co.Ltd,China) and M3 PRO shaping files 15/06, 25/04 and 25/06 files (Shanghai Yirui Dental Equipment Co., Ltd.,China), at a speed of 350 r/min, torque of 1.5 N•cm through the working length. Root canals were irrigated with 2ml 2.5% NaOCl between each use of instruments, followed by a 2 minutes irrigation of 17% EDTA (Pulpdent,USA) and a final rinse of 1ml saline solution. After root canals were dried with paper points, 25/06 gutta-percha master cone (Dentsply,USA) was used along with AH plus root canal sealer (Dentsply,USA) and obturated with cold lateral condensation technique. Radiograph showed dense obturation materials from the orifice to the apex. All specimens were sealed with Caviton temporary filling materials (GC,Japan) and teeth were stored at 37。C in 100% humidity for 30 days.
1.2 Retreatment technique
MTwo-R: After removing the temporary filling material, the reinstrumentation of MTwo-R was carried out by using ENDO-MATE DT endodontic motor, adjusted at 280r/min, 2.0 N•cm torque according to the manufacturer’s instruction. MTwo-R 25/05 files (VDW-Dental,Germany) were used initially to remove the cervical third of the obturation material, 15/05 was then used with gently pecking motion toward the working length. The teeth were shaped again with 25/06 in the final step until no filling materials could be seen on their surface.
M3-RT: The reinstrumentation was carried out by the same endodontic engine, adjusted to 350 r/min, 2.0 N•cm torque according to the manufacturer’s instruction. The specimens were instrumented as follows: 1) RT3 30/09 (Shanghai Yirui Dental Equipment Co., Ltd., China): removed the filling materials at the cervical third, 2) RT2 25/08: removed the filling materials at the middle third, 3) RT1 20/07: removed the filling materials at the apical third up to the working length, 4) 25/06: final access of instrumentation until no gutta-percha could be seen on their surface.
No gutta-percha solvents were used during retreatment procedures, only lateral brushing movement against the root canal wall was applied to remove the obturation materials, followed by 2.5% of NaOCl irrigation between each use of instruments. Time required for retreatment was recorded. All treatments were operated by one clinician and each file was used in one root canal only.
1.3 Micro-CT analysis
Micro-CT images were reconstructed with the software NRecon v. 1.6.9 (Bruker microCT, Kontich, Belgium), through axial and transverse cuts of the internal structure, artefact reduction, smoothing and beam hardening correction. The quantitative evaluation by three-dimensional reconstruction of the volume of root canal filling material (Figure 1) and horizontal transportation at different level (1, 3 and 5mm from the apex) (Figure 2) was obtained using the software plug-in three-dimensional analysis tool CTAn v.1.14 (Bruker microCT, Kontich, Belgium) and three-dimensional reconstruction with CTVol v.2.2.1(Bruker microCT, Kontich, Belgium). This procedure was performed for all canals, before and after retreatment.
1.4 Evaluation method
1.4.1 Efficiency of the root canal filling removal: time required for retreatment is measured by a stopwatch. Retreatment time taken did not require the duration of files change and irrigation.
1.4.2 Clearance rate of obturation materials: (the volume of root canal fillings before retreatment – the volume of root canal fillings after retreatment) / the volume of root canal fillings before retreatment X 100%.
1.4.3 Apical transportation at the position 1, 3, 5mm from the apex= | (X1- X2) - (Y1- Y2) |. X1 is the minimum thickness of distal root canal wall before retreatment; X2 is the minimum thickness of distal root canal wall after retreatment; Y1 is the minimum thickness of mesial root canal wall before retreatment; Y2 is the minimum thickness of mesial root canal wall after retreatment. Apical transportations were calculated by comparing the position of the apex before and after the retreatment procedures. (Figure 3)
1.4.4 The deformation of retreatment files: observe every retreatment files with naked eye, head-mounted magnifier dental loupe (X3.5) (Carl Zeiss, Germany) and dental microscope (X10.0) (Carl Zeiss, Germany) to identify deformation of files (file fracture or unwinding).
1.5 Statistical analysis
All statistical analyses were performed by using IBM SPSS version 17.0 . The quantitative data between groups were compared using independent sample t-test and expressed by using x±s, while the qualitative data between groups were compared using Chi-squared test. The level of significant was set at P <0.05.