## 4 Discussion
Another important finding was that the trueness and precision mostly did not differ significantly among IOSs for the groups with four teeth missing when compared with groups in which three teeth were missing, while they differed significantly from groups with two teeth missing. This may be because the absence of two teeth facilitates continuous scanning across consecutive images from the anterior to the posterior direction, hence improving recognition and creating smaller errors [34]. In groups with three or four teeth missing, the scanning process may be hampered from the canine to the first premolar, which is the turning point for different objects. Despite this, the present study still found that the scanning accuracy in the presence of the HA can be significantly improved compared with an edentulous dental arch. A recent study found that for image stitching-based IOSs, local deviations appear at the distal end of a dental arch, while the video-based IOSs are likely to be more prone to compression of the dental arch [35]. The results presented in Figure 6 show that all of the IOSs performed similarly well. That figure indicates a large deviation at the end of the arch span for the Trios 3 IOS, with a trend for expansion of the upper and lower arches. The CS3600 and 1500 IOSs mostly displayed compression of the arch according to the negative D-value, but the i500 IOS showed a greater deviation at the end of the arch span, while the CS3600 IOS was relatively harmonious. Regarding image acquisition methods, the Trios 3 IOS is a single-image capture system [13], while the CS3600 and 1500 IOSs capture images using a video acquisition system [13; 36]. These differences between the methods of image acquisition among the IOSs may have influenced the distribution of the scan distortion. [PERSON] et al. [37] compared the performance of two IOSs, one based on a single-image acquisition system (Bluecam version 4.0) and the other being a full-color video acquisition system (Omnicam version 4.2), and found that the optical performance was better for the full-color video acquisition system. Nevertheless, the graphical imaging mechanisms differ in these two scanners, as they generally do between scanners. It remains unclear which specific IOS or scanning strategy is the best, since different IOSs exhibit different accuracies based on the data capture mode, underlying principles, or software algorithms utilized [28] and it may be arbitrary to attempt to explain these observations in our study. Since both previous studies and the present study have found differences in the strategies used to capture images among IOSs [37; 38], future studies should further investigate the effects of differences between IOSs in terms of the image capture systems and graphical imaging mechanisms. Regarding clinical considerations, the first primary scan of the target area is performed when manufacturing an implant prosthesis. After trimming off the implant area, HAs are replaced with scan bodies and the second scan is performed to acquire the positions of the scan bodies for transferring to the position of the implants. However, extreme situationssuch as the presence of undercut, steep, sharp, deep, angled, or crowded surfaces make it difficult for IOSs to capture accurate point clouds [39, 40, 41]. The physical structure of scan bodies makes it difficult to scan the complete peri-implant gingiva, and so some clinicians will keep the HA in position during the first scan. After trimming off the HA area in the scan model, they remove the HA, air-dry the saliva and blood immediately, and perform the peri-implant gingiva scan, so as to guarantee the presence of the complete margin of the implant site. In addition, there is concern that the accuracy of the scan may decrease after removing the HA because, for those cases in which only residual ridges or structures with motion frenum remain, the landmarks used in the bite registration process disappear.