Superimposing orthodontic records at different time points has been used widely to determine the craniofacial changes. The cornerstone of superimposition is using stable structures. Identification of stable structures in jaws without having external references in growing patients is extremely challenging. In history, metal implants have been used as reference in 2D cephalograms to explore natural stable structure [2,3]. In 3D era, implants should continually play a crucial role in CBCT [8,9,10] and digital dental model superimposition [14,15]. However, metallic implants would produce artifacts both in CBCT and IOS images, which would degrade the image quality and introduce errors. In this study, the experimental animal skulls, which are more feasible and less expensive than human skulls, were used to evaluate the reliability and validity of linear and angle measurements of 3D miniscrews on CBCT and IOS with actual values. The results of our study are applicable on human skulls as well, because the goat heads are merely platforms for miniscrew implantation. Moreover, the study is ethically impossible to be conducted on patients because of the amount of radiation exposure necessary when taking CBCT at different resolutions.
Our study showed that statistically significant overestimations of linear measurements were obtained on CBCT both at 0.12 (0.27±0.24mm) and 0.3 (0.14±0.22mm) voxels compared with actual measurements. Our results are, to some extent, consistent with several studies. Moshfeghi et al. [20] using gutta-percha, reported an enlargement by 0.10±0.99 mm in axial section and 0.27±1.07 mm coronal section at 0.3 voxels. However, the values for standard deviation were greater than our data. Tolentino et al. [21] used silica markers, but they did not observe statistical difference among voxels at 0.25, 0.3 or 0.4 mm. Variable materials used as references in different studies may attribute to the contradiction between studies. Schulze et al. [29] pointed that an extreme artifact could be produced by titanium implants. Instead of upgrading resolution, they suggested a more sophisticated reconstruction algorithm for meaningful reduction of artifacts. Moreover, when using linear measurement to evaluate the stability of miniscrews, the systematic error should be taken into consideration.
Secondly, miniscrews at two voxel settings presented reliable and accurate results on angle measurements when compared with actual values. Our result supported the use of angular measurements acquired through miniscrews in clinical applications, which is important on measuring the angle stability of miniscrews after orthodontic loading.[16]
CBCT is limited for evaluation of short-term treatment effects due to excess radiation exposure to the patients. Thus, chairside IOS is promising for this purpose. DeLong et al. [30] found that a smooth textured surface (such as the titanium miniscrews used in our study) could worsen the digitizing performance due to spectral reflection. However, our study confirmed the clinical reliability and validity of IOS for linear and angular measurements of miniscrews, which were consistent with other studies. However, these measurements were different with respect to systematic errors and their tendencies [22,23,24,25,26,27]. Our results supported that the evaluation of tooth movement on serial digital dental models from IOSs during growth or after orthodontic intervention is operable. In addition, we also found it quite interesting that the mean bias on the homolateral side was significantly larger than that on the opposite, implying unequal magnification in sagittal and transverse directions. Anh et al. [31] claimed that regions imaged later would generate more errors during configuration than regions imaged earlier. Thus, the scanning sequence could be one of the reasons for the unequal amplification effect observed in our study, and a modification is required when miniscrews are involved.
Above all, in accordance with results of literatures and this study, the following suggestions are proposed when miniscrews are used to superimpose 3D image: 1.The positional stability of miniscrews should be evaluated in order to ensure the reliability and clinical validity of the linear and angular measurements on 3D models. 2. The same CBCT machine with the same scanning settings is required when doing superimposition. 3. Systematic errors of miniscrew measurements on CBCT image and digital dental models acquired from IOS should be consider when stable structures are explored.
A limitation of this study is the exclusion of motion artifacts because this is an experiment on dry goat jaw bone. In addition, the study was conducted for a single experimental condition by testing systematic errors on a specific type of miniscrew, a single CBCT machine and one IOS. Whether the results of this study are suitable for other miniscrews, other CBCT machines at different voxel sizes ,or other IOSs is not known.