## 4 Discussion  
This in vitro study evaluated the effects of changes in the exposed length of the ISB and implant placement on the image matching process and the accuracy of implant placement when using three oral scanners and one tabletop scanner. The obtained results confirmed that a shorter exposed length of the ISB caused a larger error in the virtual implant location due to inaccuracies in the image matching process performed in the dental CAD software. It was additionally found that the trueness and precision differed with the type of scanner used and the implant placement position. Therefore, the null hypothesis of this study, that the type of scanner used, the exposed length of the ISB, and implant placement position do not affect the accuracy of implant position reproduction was rejected. The accuracy of implant position reproduction tended to decrease as the exposed length of the ISB decreased in all scanner groups. In the case of trueness, the deviation started to increase at a submersion of 4.5 mm. When the implant was submerged 6.0 mm, which corresponded to the shortest exposed length of the ISB, the RMS value increased rapidly. Additionally, in the case of precision, there was no change in the deviation at the normal bone level and for a submersion of 1.5 mm, but the RMS value started to increase significantly from a submersion of 3.0 mm. The error increased rapidly in the groups with submersion of 4.5 mm and 6.0 mm, and the accuracy of implant position reproduction was affected when the ISB exposed area decreased. When the implant was placed at the normal bone level, submerged 1.5 mm, and 3.0 mm, the deviation was not large. This is because the exposed length of the ISB connected to the implant was sufficient for the area used in image matching to be accurately scanned and created. This indicates that the resulting scan data produced accurate results even in the process of image matching in the dental CAD software and did not affect the reproduction of the implant position. The accuracy of position reproduction was lower at the lateral incisor than at the first premolar and first molar in terms of both trueness and precision. This may be due to increasing errors during the stitching process in the arch of the canine area of the model during scanning, or errors accumulating with increasing scan area. [PERSON] et al. found that the first scanned area was more accurate than the last scanned area due to the accumulation of stitching errors during the scanning process [34]. [PERSON] et al. similarly compared the accuracy of implant position reproduction between a single implant model and a model with multiple implants placed, and found that the deviation increased as the scan area lengthened [35]. The results of these previous studies are consistent with our findings. Comparing the RMS values between the scanners revealed that the T500 scanner showed the highest accuracy at the normal bone level and for submersions of 1.5 mm, 3.0 mm, and 4.5 mm, whereas for a submersion of 6.0 mm, the RMS value increased rapidly to 86.0 \(\pm\) 29.0 \(\upmu\)m. In the case of tabletop scanners, although it is known that their precision is very high, the accuracy deteriorated as the exposed length of the ISB decreased. This shows that even when accurate scan data are obtained, the error increases when the area for matching the scan-body library is too small [24]. In addition, the scanning angle is smaller for tabletop scanners than for oral scanners, which also affects the accuracy [36; 37]. For the TRIOS 3 scanner, the trueness was highest at the normal bone level and for submersions of 1.5 mm, 3.0 mm, and 4.5 mm, and there were no significant differences between the i500 and CS3600 scanners. A similar trend appeared for the precision. The deviation was lowest in the T500-REF group, followed by (in increasing order) the T500, TRIOS 3, i500, and CS3600 scanners. The trends found in this study are similar to those found in previous studies conducted under similar conditions.