Halting the progression of defects by early treatment plays a fundamental role in the prognosis of teeth with invasive cervical (
19) or inflammatory (
20) root resorption defects. Therefore, early and accurate detection of root resorption can significantly increase the treatment success. On the other hand, conventional radiographs do not provide sufficient diagnostic value and therefore, digital diagnostic systems were introduced due to their inherent advantages (
21). Assessment of root resorption using CBCT is limited to patients in whom defects have been previously detected by radiographic examination and they have taken 3D scans for therapeutic purposes (
12). Considering the importance of early detection of root resorption defects in increasing the treatment success rate, this study aimed to assess the effect of voxel size on the diagnostic accuracy of root resorption by CBCT.
Previous studies have shown that conventional radiography does not provide sufficient accuracy for detection of small external root resorption defects in buccal or lingual surfaces (
6,
8-
10). Thus, researchers have been in search for more efficient techniques for detection of external root resorption defects. In 2007, Da Silveira et al. evaluated the advantages of multislice CT to detect external root resorption defects and reported that this modality had a high sensitivity and specificity in detecting external root resorption defects on the buccal root surfaces (
11). However, defects in the apical third of the roots were significantly more difficult to detect. On the other hand, in 2009, Liedke et al. showed that CBCT had a very high sensitivity and specificity for detection of external root resorption defects and they reported no significant difference in this regard based on the size of the defect, its position or the plane of section (
16).
The current study showed that CBCT with different voxel sizes had relatively equal diagnostic efficacy in terms of sensitivity and specificity for detection of variable-size defects in different areas and surfaces of the roots and this technique had sufficient diagnostic value for detection of external root resorption. Although small differences in sensitivity and specificity were found in 150, 200, 250, and 300 μm voxel sizes, they were not significant.
The results of the current study are in line with those of Liedke et al. in 2009. They also reported equal specificity and sensitivity values for different CBCT voxel sizes. However, they only evaluated defects on buccal root surfaces and used iCAT CBCT system (
16). In 2010, Kamburoglu et al. evaluated two CBCT systems with different voxel resolutions for detection of external and internal root resorption defects and showed that high resolutions of both systems had similar efficacy for detection of internal root resorption defects and their efficacy was superior to that of low resolutions in one of the systems (
18). In addition, the results showed that the diagnostic accuracy of different voxel sizes of CBCT increased as the size of defects increased, but no significant difference was found between small and medium-size defects. In 1998, Goldberg et al. evaluated the detection accuracy of simulated external root resorption defects in the maxillary incisors and showed that detection of small defects was more difficult than that of medium and large size defects. This result was in accord with our findings (
9). In 2007, Da Silveira et al. evaluated the diagnostic accuracy of CBCT for detection of external root resorption defects and showed accurate detection of apical small resorption defects in 28.6%, medium-size defects in 86.66%, and large defects in 100% of cases. The diagnostic accuracy of CT was higher for larger defects (
11). Moreover, in 2009, Hahn et al. used flat panel volumetric CT (fpVCT) and showed that non-cavitary defects were accurately diagnosed in 53% of cases. Small resorption defects were accurately diagnosed in 69% of cases. These values were 96% for medium size and 89% for severe, large defects (
22). In 2012, Neves et al. evaluated external root resorption using different voxel sizes of CBCT and showed that by an increase in size of defects, the accuracy, sensitivity, positive predictive value and negative predictive value of diagnoses increased as well (
23,
24).
In order to three dimensionally reconstruct images using data retrieved from the axial scans, each initial voxel must be dimensionally converted to several cubic voxels. This process, called interpolation, creates same-size cubic voxels that occupy the same volume. The CT number of these cubes is equal to the mean initial voxel CT number. Creation of these new cubic voxels allows image reconstruction at each surface with no reduction in resolution.
Use of CBCT, depending on the device model and the protocol applied, significantly decreases the patient radiation dose (
25). Similarly, radiation dose has a direct correlation with the number of slices. Thus, it has been suggested that minimum number of slices should be used for diagnostic purposes (
14). Based on the results of the current study, all three voxel sizes had almost equal diagnostic efficacy and accuracy for detection of external root resorption defects. Thus, 300 μm voxel size, with the least patient radiation dose and the shortest scanning time compared to other voxel sizes may provide sufficient diagnostic accuracy for external root resorption defects in the buccal and lingual root surfaces.
Advent of CBCT revolutionized dental imaging because with only a slight increase in the patient radiation dose, high level of diagnostic information with adequate quality and quantity is obtained compared to conventional radiography and in most cases, this slight increase in dose is justifiable by taking into account the value of diagnostic information obtained. However, if this imaging modality cannot significantly increase the detection accuracy, this increase in patient radiation dose is not accepted. The patient radiation dose in CBCT scans is 3-7 times higher than that of conventional radiography. Moreover, CBCT is time consuming. But, the patient radiation dose in dental CBCT is much lower than that of medical CT.
On the other hand, the diagnostic advantages of each imaging technique must be considered by taking into account the risk of exposure to excess radiation. This is particularly important in younger patients since excess radiation adversely affects the development of organs (
25). According to Farman in 2005, the “as low as reasonably achievable” (ALARA) principle is a fundamental rule for diagnostic radiographies that also applies to CBCT and some new principles must also be added for CBCT (
26). At the same time, clinicians should try to minimize the patient radiation dose without negatively affecting the quality of images. Future studies are required to compare the diagnostic accuracy of different CBCT systems for detection of external root resorption as well as other defects.
The main limitation of this study was its in vitro design; thus, generalization of results to the clinical setting must be done with caution.