## 4 Discussion
In line with this, one study compared digital and conventional impressions in periodontitis healthy patients to test accuracy between the two methods and then investigated soft tissue change after periodontal treatment for periodontitis patients using digital impression [15]. In that study, the difference between digital and conventional impressions in periodontally healthy patients was 70.71 \(\pm\) 25.58 \(\upmu\)m. The change of the gingival volume in periodontitis patients was 433.43 \(\pm\) 227.55 mm\({}^{3}\). Our study selected the post-extraction ridge change to compare tissue changes measured using conventional and digital impression methods. Herein, groups DI and CI presented significantly different ridge changes for the same study material, with greater values in group DI (total ridge change, 3.53 \(\pm\) 1.00 vs. 3.40 \(\pm\) 1.02 mm at the 2 mm level, 2.75 \(\pm\) 0.97 vs. 2.65 \(\pm\) 0.97 mm at the 3 mm level, and 2.08 \(\pm\) 0.87 vs. 1.64 \(\pm\) 1.12 mm at the 4 mm level). Such discrepancies should be interpreted carefully. According to some clinicians, these discrepancies might be clinically negligible based on the findings of studies that used an arbitrary 1 mm threshold value as an esthetic criterion. According to other clinicians, such differences should be treated more discretely because studies on the investigation of soft-tissue changes showed that the changes in the designated level of interest were measured to the first decimal place (mm). For example, the following findings were previously obtained: less than 1 mm of soft-tissue gain in the majority of the points of interest for root coverage outcomes in the study by [PERSON] and colleagues [18]; 0.22 \(\pm\) 0.48 and 0.83 \(\pm\) 0.61 mm of soft-tissue volume gains around the implants with specific tissue grafts in the study by [PERSON] and colleagues [19]; and 0.15-0.32 mm of soft-tissue volume changes at one year and 0.45-0.66 mm at five years after implant treatment in the study by [PERSON] and colleagues [20]. The discrepancy in the values of ridge changes between groups DI and CI was derived from the inter-group difference in the STL images at each time point (T0 and T1). At both T0 and T1, the total ridge width for the same material was greater in group DI than in group CI, and the values were not disproportionate in one aspect, either in the facial or oral aspect. This inherent discrepancy between the groups was not offset when calculating the change in the tissue surface for each modality. It should be also noted that the differences between groups DI and CI were greater at T0 than at T1. Tissue conditions might affect the recording. Gingival tissues unsupported by the bone or tooth might be susceptible to pressure from the impression material. In particular, the gingival tissue might have suboptimal tissue consistency immediately after tooth extraction. In many studies, the accuracy of intraoral scanning has been evaluated by measuring the discrepancies among different models [10; 12; 21]. Accuracy involves \"precision\" and \"trueness\". In the study context, precision is defined as the difference between digital impressions from different intraoral scanners and trueness as the difference between digital impressions from the test scanner and the reference impression. Herein, we did not evaluate the precision because it was not suitable for our study. However, given that the vinyl polysiloxane impression was regarded as a reference, comparing the groups can be considered an evaluation of trueness [15]. With such definition, the trueness of the gingiva on a full arch scale was 83.65 \(\pm\) 14.43 \(\upmu\)m in periodontally healthy patients. In the study involving only the upper jaw, the trueness of the palatal tissue was 130.54 \(\pm\) 33.95 \(\upmu\)m. Byreferring to the trueness of this previous literature, we tried to evaluate the discrepancy between the two modalities of our study.