This study has two-fold objectives. Firstly, to draw inferences from the collected objective findings from the present investigations and existing/known information confirmed in the scientific literature. The measurements obtained with the two imaging techniques, CBCT, and digital periapical radiography closest to their real canal length, and assessed their diagnostic validity and the possible influence of other associated factors such as degree of canal curvature. The second objective is to allow an explanation for the observed differences and alikeness, with relevant studies that used similar methodology (CBCT imaging) to obtain an evidence-based background, which might be valuable in clinical practice.
Several techniques are available for the estimation of diagnostic working length. Some protocols are more suitable for clinical use, and others are designed well as a research tool. Factors such as case difficulty and the technique sensitivity, as well as access to the various imaging techniques and imaging software, are likely to influence the choice of technique to estimate root canal length.
In this in vitrostudy, the correlation between the accuracy of root canal length estimation and its degree of canal curvature in human premolar teeth were investigated under ex vivo conditions. By using extracted teeth and by applying standard protocols, it is possible to establish a 'real' root canal length through direct clinical examination of the apical foramen. It can be used as a gold standard since the reliability, accuracy, and reproducibility of this method had previously been validated in the literature [3].
The real clinical working length was established by subtracting 0.5 mm from the actual canal length (file tip at the level of the coronal most boundary of the major foramen) and considered as a gold standard in the present study. Radiographic working length measured separately using digital periapical radiographs and CBCT images. The final radiographic working length calibrated by subtracting 1mm from the radiographic tooth length since, the ideal apical limit for all intracanal procedures is the narrowest point of the canal, the so-called apical constriction or the dentinocemental junction. This point is located 0.5 to 1.0 mm short of the radiographic apex of the root [20, 21].
The results of the present experiment found clear support for the standardized paralleling angle technique, since it is accurate and showed no significant differences in comparison with the CBCT measurement and real clinical working length value (gold standard). It is easier to do standardization for all the radiographs clinically, which allows the positioning of the film and the object in approximately the same position. However, some limitations of our implementation need to be considered, such as two-dimensional magnification, factors related to the direction of root angulation, anatomic variations, and location of the major foramen. The inspection of the third dimension of the root canal helps to increase the accuracy of working length estimation. Therefore, measurements of root canals on existing CBCT images are a relatively new method for the confirmation of the diagnostic working length, which has been studied by numerous researchers [22, 23]. However, the CBCT scan not recommended to be used routinely in endodontic practice because of dose considerations and potential radiation risks [24, 25].
In this study, neither of the digital radiography systems and CBCT showed a significant difference with the gold standard. However, the digital radiography revealed to be less accurate than CBCT imaging in correlation with the gold standard, which can be related to the dimensional distortion of the image, and the root canals frequently did not end close to the radiographical apical reference point. This anatomical variation might result in an overestimation of the canal length measured by the radiographic method. In line with these common findings, with that of several previous studies confirming the lower accuracy in root canal length estimation from periapical radiograph than that from CBCT scans [26, 27].
The present study confirmed the accuracy of the canal length measurements performed with the use of radiographic images, in various degrees of curvature. In line with previous studies, both CBCT and digital working length measurements were slightly more accurate when performed in canals having a moderate canal curvature. Although, there was no statistically significant difference in mean values of root canal length with a severe curvature angle [28]. These results explained by the fact that the exact direction and nature of root angulation and anatomic noise results with limited information on the root canal anatomy of the tooth assessed with the 2-dimensional conventional radiography. CBCT imaging overcomes this by allowing the multiplanar evaluation, which leads to a clear visualization of the root canal configuration and canal curvature.
It is crucial to highlight the fact that the results of this study are to be considered specific to the radiographic technique applied to compensate for image magnification. As a result, a long (16-inch) target–receptor distance was used. This compensation is a significant point for the broad implication of the present research in clinical practice. Although, paralleling techniques produce less projection and procedural errors than bisecting angle technique, ideal film, and x-ray tube head orientation can be difficult to achieve in some clinical situations because it can be complicated by anatomic variations [17]. On the other side, CBCT scanning evaluated, in the absence of streak and beam hardening artifacts, photon starvation caused by metal restorations, brackets, implants, and metal root fillings, motion artifacts, and anatomic noise from the opposing jaw structures. Isolating artifacts three-dimensionally has been a challenge in clinical endodontics.