Objectives: The main objective of this research was to determine whether the volume of residual sound coronal tooth structure has an effect on the outcome of root canal retreatments assessed using cone beam computed tomography (CBCT). This research also aims to provide an estimate of the proportion of successful outcomes of primary and secondary root canal treatments (retreatments) determined by CBCT. The effect of the reduction of radiation dose on the detection of apical radiolucencies using CBCT and the accuracy of intraoral digital scanner to determine the volume of remaining coronal tooth structure were investigated ex vivo and in vivo, respectively. Materials and methods: Reducing CBCT radiation dose was attempted by using a built-in dose reduction (DR) mode in an Accuitomo CBCT machine at 360° and 180° rotations for detecting simulated apical radiolucencies in a human dry mandible model. Volumetric measurements of the intraoral digital scanner were validated against microcomputed tomography. The differences in volume measurements and three-dimensional (3D) deviations between the two scanners were analysed using specialised surface metrology software (Geomagic Control). One-hundred and thirty-seven posterior teeth in 121 patients requiring root canal retreatment were assessed in a prospective clinical trial. All teeth were examined clinically and radiologically using periapical radiography and CBCT at baseline and 1 year after endodontic retreatment. A clinical impression was obtained for each tooth after completion of root canal retreatment, and following cast restoration placement to produce two casts. All casts were scanned using an intraoral digital scanner, and the 3D volume of remaining coronal tooth structure was calculated. In the pooled study, the one-year results for 354 teeth, including 123 primary treatments and 231 retreatments, were analysed. Results: With regard to the experimental studies, there were no significant differences in the overall accuracy between CBCT 360°DR and 180°DR when compared to the control standard dose scans, while reducing the radiation dose by more than 50%. Small volumetric differences (0.68%-1.36%) were observed between the intraoral digital scanner and μCT measurements. At one-year, overall percentage of favourable results was 88.3% using periapical radiographs, and 81.8% for CBCT (P=0.035). Teeth retaining less than 30% of their original coronal tooth structure volume had a significantly higher proportion of unfavourable treatment outcomes (P=0.040). In the pooled study, the percentage of favourable results for primary treatments (84.7%) with CBCT was not statistically different (P=0.316) from that of retreatments (77.9%). Molar teeth were significantly less successful (P=0.001) than anterior and premolar teeth pooled together. Conclusions: The radiation dose to the patients can be significantly reduced by using CBCT DR mode, while maintaining good diagnostic accuracy in detecting small periapical radiolucencies. Intraoral digital scanning accurately quantifies the residual coronal tooth structure. Root canal retreated teeth with less than 30% volume of sound remaining coronal tooth structure had a significantly higher endodontic failure rate than teeth with a remaining tooth structure volume more than 30%. The proportions of favourable results of primary root canal treatment and retreatment are comparable. CBCT detected more failures in molar teeth than anterior teeth and premolars compared with periapical radiographs.
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