The results of the present study showed that, in the range of ± 0.5 mm from the apical foramen, the EAL ProPex Pixi was the best accurate modality among the experimental ones. Among the CBCT measurements, the accuracy in the range of ± 0.5 mm was lowest in the Romexis Viewer (the largest voxel size) measurements, using the 3D Endo software.
The important role of the working length determination had never diminished, especially in the existing of more and more modern effective instruments for preparation17,18, the complexity of the root canal system19. There was fixed bias between the ProPex Pixi and the AL measurements with the means of the EAL measurements were constantly lower than the latter, across the entire range of measurements.
The results of the present study showed that there were the 3D-PL measurements at voxel size of 0.15 mm and the 3D-CL measurements at voxel size of 0.10 mm agreed with the AL. All other 3D Endo measurements, there were fixed biases, like that of the EAL measurements.
The results of the present study also showed that the accuracies of the 3D-CL measurements were higher than that of 3D-PL measurements at voxel sizes of 0.10 and 0.075 mm. The 3D-CL measurements were the WL after correcting the proposed lengths by the operator. The virtual rubber stop on the file was corrected using the anatomic landmarks on the occlusal surface by the operator in the effort of reaching to the most accurate length. The results showed that, at smaller voxel sizes, the adjustments of the rubber stop positions were easier, leading to more accurate correct lengths. This agreed with the results of the previous studies6,9,11.
The conventional CBCT Romexis Viewer measurements in the present study obtained the more accurate at smaller voxel sizes, like that of the previous studies6,11. The present study used the Bland–Altman plots for analysis of the agreement of the measurements with the AL, differed from the previous studies6,11. The 3D Endo measurements also confirmed the similar results in the present study, that the smaller voxel sizes yielded more accurate measurements.
Studies on determination of WL with CBCT commonly used human extracted teeth in dry mandible or in jaw model6,20,21. The setting with the dry human mandible is better than other contexts in controlling of some clinical variations such as artifacts caused from position or motion of patient, beam hardening from other surroundings, or noise from other anatomic structures21. The arrangement of teeth in the impression tray of the present study does not eliminate completely artifacts from the neighboring teeth in the tray. However, CBCT images are clear and anatomic landmarks are defined easily and exactly owing to of high resolution11. The Romexis Viewer measurements agreed with the AL with good mean differences, just higher than that of the best method in the present study. At the present voxel sizes of the CBCT image, the Romexis Viewer measurement was reliable modality for WL determination. This result differed from the other previous studies1,6,11,21.
Although the human extracted teeth seem appropriately for evaluation the accuracy of CBCT WL determination, the artificial endodontic training tooth still completely satisfies requirements of this purpose22. Authors just select the actual root canal length of the artificial tooth as the gold standard in evaluation of the accuracy of the CBCT WL22. The 3D Endo software can improve accurate 3D root canal length determination, however, the operator should check, control, and maintain continuously the working length during the preparation phase to detect changes, especially in severe curved canal22.
One important shortcoming with CBCT in endodontic WL determination on the heavily metallic restored tooth is the significant artifact23. More artifact means a greater approximate range of length, and in these cases, CBCT provides only an estimate of the length23.
Proper knowledge of root canal anatomy and morphology is indispensable for every clinician in endodontics for locating the root canal orifices. CBCT imaging has supplied an exact, noninvasive approach for clinical chairside assessment of root canal anatomy4. The 3D Endo software is an effective, quick, and easy modality for identification and visualization of canal trajectories in three dimensions. This software reveals promise in supporting operator for quantifying anatomical complexities preoperatively10,24.
Endodontics can be performed at a high level without CBCT imaging, but it cannot be practiced at the highest level23. Image-guided endodontics with minimally invasive access and instrumentation is recently recommended by authors for better in preservation as much tooth structure as possible23.
The radiation exposure for dental CBCT has been a dramatic reduction as compared to the conventional medical CT. However, this is not the major key to use for the routine examination in endodontics. The benefits from the examination far outweigh the risks related to ionizing radiation exposure should be carefully considered23. Although the radiation exposure for each dental examination is less than dose of radiation from other sources, the exposure time is so short, leading to the augmentation of damage. Diagnostic examinations should be performed at the lowest dose of radiation, following the ALARA principle: as low as reasonably achievable12. The American Association of Endodontists statement suggests that the risk–benefit ratio is too high for CBCT to be a routine screening tool, even though the radiation levels are low with focused-field device25. Therefore, the application of CBCT only for root canal length measurement is not advocated22.
The present study used three different voxel sizes for evaluation of the accuracy and agreement of the CBCT measurements compared to the AL. This differed from the previous studies using 3D Endo software9,22. One important feature of the present study was the root canals of the human extracted molars with appropriately calculated sample size used for the evaluation, that differed from the other studies9,11. However, the present study did not include the dry jaws to simulate the clinical situation and use the intact human molars for a real situation with the pre-existing CBCT data of the patients. Further studies should perform to confirm the effectiveness of the 3D Endo software with better conditions to simulate the clinical situations such as dry jaws, intact human premolars, molars.
The CBCT measurements using 3D Endo with the proposed length and corrected by the software and Romexis Viewer with different voxel sizes did not reach to 100% accuracy in the range of ± 0.5 mm from the actual root canal length.