## 4 Discussion
Therefore, a prosthesis fabricated in a position with a (+) Signed difference may show high occlusion and the reverse may show infra occlusion [1]. [PERSON] et al. reported that there was no significant difference between the conventional methods and various IOSs when compared rotation of VIR in a complete dentate quadrant arch scan [9]. In our study, a tilting effect was observed in the quadrant arch scan with the absence of posterior support or with more than two missing teeth. A possible explanation for this finding is the lack of landmarks in the edentulous area and the limitation of the scanner tip size, both of which could have contributed to the compromised estimation [17]. In addition, [PERSON] et al. reported that the tilting effect could occur in the area away from the section recorded by VIR in the full-arch scan [11]. In other studies, a similar effect was observed mainly in the posterior area [12; 18; 19]. [PERSON] et al. explained that the cumulative errors in superimposition and stitching processes contributed to the angular deviation in the posterior region [18]. They also stated that even a small angular deviation could lead to a large vertical deviation, which is crucial in the fabrication of FPDs. In our study, it is noteworthy that the tilting effect was observed in the estimation of quadrant scan with the absence of posterior support or the presence of more than two missing teeth. The results of our study suggested that the accuracy of arch scan data obtained by the IOS could affect the accuracy of VIR [7; 12]. In this study, we calculated the trueness and precision of the maxillary quadrant arch scan data to assess the effect of the distortion of the scan data on the VIR. Based on our study, trueness and precision were affected by edentulous conditions and scanners. Depending on the edentulous conditions, the deviation showed statistically difference. In addition, according to the scanner used, significant difference was shown. In Trios, signed and unsigned differences were lower than those of the other IOSs due to lower distortion relative to other intraoral scanners despite the significant deviation by the edentulous conditions. In the case of i500, the deviation of trueness and precision was higher compared to the other IOSs, also the rotation of the mandibular quadrant arch occurred with deviation. In other words, obtaining accurate scan data of both arches might be one of the key factors for increasing the accuracy of the VIR. The median trueness and precision of the four experimental groups were 24.3-85.5 and 17.4-73.9, respectively. [PERSON] et al. reported the mean trueness and mean precision of the quadrant scan as 23.0-49.8 and 17.0-43.2, respectively [3]. Considering the possibility of the scan body to function as an artificial landmark in the edentulous area [20], the presence of the scan body in previous study could explain the better results of [PERSON] et al. as compared to our study. [PERSON] et al. assessed the precision of full-arch scan under varying edentulous conditions [16]. There was a discrepancy in the precision between the partial edentulous conditions. The resulting discrepancy in precision was 44.4-115.7. Considering that the full-arch scan was performed in this study, our study result seems comparatively more precise compared to the previous study. This indicated that a quadrant arch scan data might be more useful than a full arch scan data in terms of a more accurate VIR. Previous studies about the accuracy of the VIR had a limitation in that there was no evaluation for the distortion of the arch scan data itself, even though the full arch model was used to assess the VIR [9; 12; 13; 15]. [PERSON] et al. evaluated the accuracy of VIR using the total occlusal surface area [12]. However, the author considered the precision of the scan data by comparing only the distance of the key points and not the scan data itself. [PERSON] et al. [15] also used the number of occlusal contacts to investigate the VIR.