## 4 Discussion
Based on the findings of the J-T trend test, the second hypothesis was partially rejected. For the Trios 3 IOS, a significant trend was found in all groups with a positive value of the Standard J-T Statistic, which indicated an upward trend from an anterior to a posterior tooth position. All Trios 3 groups tended to present extension of the maxilla and mandible when compared with the reference model. For the i500 IOS, a significant trend was found in all groups with a negative value of the Standard J-T Statistic, which indicated a downward trend from anterior to posterior positions. Since the values of the differences between the measurement and reference scans were mostly negative, with a numerically downward tendency, this indicates increased distortion in the posterior area, whereas the measurement scan models tended to show compression when compared with the reference scan models. To sum up, both the Trios 3 and i500 IOSs displayed variation trends in the distortion from anterior to posterior positions, with the Trios 3 IOS tending to show an extension form while the i500 IOS appeared to be more prone to a compression form of the upper and lower arches. In contrast, applying the J-T trend test to the CS3600 IOS showed no significant trend in either group. [PERSON] et al. [30] evaluated the effect of the position of a coded healing abutment (CHA) on trueness of intraoral scanning. Two CHAs were placed at mandibular left first and second molar positions and then scanned by an intraoral scanner (TRIOS, 3 Shape). The trueness of the first molar site was found significantly greater than of the second molar site. In our study, similar results were found in terms of the posterior sites. In Figure 6, the trends of distortion in Trios 3 and i500 were found steady in the anterior tooth position yet suddenly rose in the posterior tooth position especially at position 6 and position 7. Therefore, the most easily distorted area may be distributed in the posterior tooth site. As evident in Figure 6, for the Trios 3 IOS, groups HA-3 and NHA-3 seemed to exhibit lower distortion along the arch span when compared with the other groups. This might have been due to the existence of the second molar in these two groups. For metal restorations, the scanner light that impinges on a surface without a powder coating tendsto refract and scatter, which hinders the ability of the scanner to determine the depth of the object [31]. Therefore, the accuracy tends to be lower for metal restorations than for natural teeth. Some studies have found that applying a powder coating to the surface will produce more accurate and consistent results when scanning materials with different layers [32; 33]. However, the present study focused on the effects of the HA itself, and so other interference factors were minimized even though the outcomes for the accuracy of IOSs could have been better. [PERSON] et al. [20] reported that the precision of the scan data was significantly lower when there were greater than or equal to five missing teeth on the dental arch. Our study found the trueness and precision among the IOSs mostly decreased in the following order: HA-2 > HA-3 > HA-4, NHA-2 > NHA-3 > NHA-4. In addition, [PERSON] et al. [19] found the trueness for short-arch scans was <103 \(\upmu\)m for edentulous scans and <56 \(\upmu\)m for dentate scans. Comparing to our study, it seems that, despite the presence of the HA, the presence of a larger number of natural teeth in the model resulted in higher accuracy in the scanning process. Therefore, the anatomical morphology of natural teeth might be easier for IOSs to detect, resulting in superior results compared with HAs. The molar teeth constitute more complex anatomical structures, especially in the occlusal plane, which may also facilitate the scanning process.