## 4 Discussion  
This study evaluated the enhancement effect of an HA when making digital impressions with three commonly used IOSs and investigated the distribution of distortion across different tooth sites. The effect of the HA was assessed by determining how the accuracy varied according to whether or not an HA was applied in dedentulous areas. The distribution of the distortion according to tooth sites was evaluated by comparing the distances between the upper and lower hemispheres at each tooth site. The accuracy was significantly higher for groups with an HA. Therefore, the first null hypothesis was rejected, since the HA did affect the accuracy of virtual records obtained using IOSs. However, for the Trios 3 IOS, pairwise comparisons showed no significant difference between HA-3 and NHA-3. This may be attributable to both the HA-3 and NHA-3 scans obtained using the Trios 3 IOS showing higher trueness than for the other scanners, since high accuracy would mean that the presence or absence of the HA would not greatly influence any discrepancies in the accuracy. The results for all the scanners  
Figure 6: The difference values (D-Values) between the measurement scan distance (MSD) (acquired by IOSs) and the reference scan distance (RSD) (acquired by T500 scanner) for each tooth position. The three lines in each graph are for two, three, and four missing teeth. The numbers on the x-axes represent the position of the hemisphere. For the Trios 3 IOS, the D-value in all groups increased significantly as the tooth site moved posteriorly (\(p<0.05\)). For the i500 IOS, the D-Value in all groups decreased significantly as the tooth site moved posteriorly (\(p<0.05\)). Data are mean and standard deviation values. validated that the HA can be used as a landmark to facilitate the accuracy of scanning in dentulous areas. The accuracy of an IOS was previously found to be lower in a long dentulous area, which would be improved if an artificial landmark was applied [18]. This result may arise from the lack of stable features that could guarantee accurate stitching in an edentulous area [25]. The presence of auxiliary geometric structures may enhance the outcomes of digital impressions of edentulous areas [21]. When compared with the partially edentulous groups, the presence of an HA provided more complex geometry features for scanners to detect and allowed images to be stitched together smoothly, resulting in a significant enhancement in the scanning accuracy. [PERSON] et al. [18] reported the accuracy of IOSs could be improved when applying an artificial landmark on the edentulous area. [PERSON] et al. [26] evaluated the effect of an auxiliary geometric device (AGD) on the accuracy of intraoral scanning and they found the trueness and precision were significantly improved when the AGDs were applied on the edentulous jaws. However, considering clinical applicability, it can be impossible to employ additional artificial landmarks to improve the accuracy of scanning when making a digital implant impression. Therefore, according to the results of our study, simply keeping the HA present during the first scan may be desirable for improving the scan accuracy. In addition, complex geometric structures on the dental arch were found to contribute positively to the outcomes of image alignment [20], which might have ameliorated the results in the HA groups in the present study. After applying an HA, the accuracy of the aligned arch increased significantly. Several previous studies have found that the span and location of the edentulous area interferes with the accuracy of IOSs [12; 27; 28; 29], and that the accuracy of the scanned data decreases with increasing distance between the scan bodies [14]. The J-T trend test was applied in the present study to analyze how the distortion was affected by increasing the span length from the initial scanning position. The second hypothesis stated that the distortion in the accuracy of the scan does not vary from an anterior tooth position to a posterior tooth position.