## 4 Discussion  
This study revealed the effects of impression modalities (digital vs. conventional impressions) on evaluating tissue changes at the soft-tissue level and potential factors affecting these tissue changes. We found that (1) the total tissue changes were significantly greater with digital impression than with conventional impression, and (2) the discrepancy between the two modalities was significantly greater immediately after tooth extraction than at two months after extraction. Information regarding tissue changes is currently a major research interest, especially in implant dentistry. Until recently, hard-tissue changes were primarily investigated via cone-beam computed tomography and the resultant acquisition of DICOM files [16]. However, there was methodological difficulty in measuring soft-tissue changes. Previously, dental impressions were taken for soft-tissue analysis. [1; 2; 3] Much of the data we referred to were produced using this method. However, digital impressions using the intraoral \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & **T0** & **T1** & \\ \cline{3-5}  & & \
\begin{tabular}{c} **Mean \(\pm\) SD (mm)** \\ **Median (IQR) (mm)** \\ \end{tabular} &
\begin{tabular}{c} **Mean \(\pm\) SD (mm)** \\ **Median (IQR) (mm)** \\ \end{tabular} & \(p\)**-Value** \\ \hline \multirow{4}{*}{Total} & H2 & 0.48 \(\pm\) 0.23 & 0.35 \(\pm\) 0.19 & Total \\  & 0.42 (0.33, 0.53) & 0.33 (0.22, 0.55) & Total \\ \cline{2-5}  & H3 & 0.41 \(\pm\) 0.24 & 0.31 \(\pm\) 0.21 & \textless{}0.001 * \\  & 0.35 (0.23, 0.55) & 0.22 (0.15, 0.46) & \textless{}0.001 * \\ \cline{2-5}  & H4 & 0.42 \(\pm\) 0.26 & 0.31 \(\pm\) 0.23 & 0.033 * \\  & 0.45 (0.24, 0.61) & 0.28 (0.15, 0.44) & 0.019 * \\ \hline \multirow{4}{*}{Facial} & H2 & 0.23 \(\pm\) 0.12 & 0.16 \(\pm\) 0.10 & Facial \\  & 0.23 (0.15, 0.32) & 0.15 (0.11, 0.23) & Facial \\ \cline{2-5}  & H3 & 0.20 \(\pm\) 0.10 & 0.13 \(\pm\) 0.11 & 0.001 * \\  & 0.22 (0.13, 0.27) & 0.11 (0.06, 0.18) & 0.001 * \\ \cline{2-5}  & H4 & 0.18 \(\pm\) 0.12 & 0.12 \(\pm\) 0.10 & 0.005 * \\  & 0.24 (0.07, 0.28) & 0.11 (0.01, 0.22) & 0.003 * \\ \hline \multirow{4}{*}{Oral} & H2 & 0.24 \(\pm\) 0.18 & 0.19 \(\pm\) 0.12 & Oral \\  & 0.18 (0.14, 0.32) & 0.18 (0.11, 0.23) & Oral \\ \cline{2-5}  & H3 & 0.21 \(\pm\) 0.18 & 0.18 \(\pm\) 0.14 & 0.277 \\  & 0.17 (0.12, 0.33) & 0.18 (0.11, 0.23) & 0.060 \\ \cline{2-5}  & H4 & 0.24 \(\pm\) 0.18 & 0.19 \(\pm\) 0.14 & 0.347 \\  & 0.22 (0.12, 0.37) & 0.20 (0.11, 0.27) & 0.139 \\ \hline \hline \end{tabular} H2: 2 mm below the ridge crest, H3: 3 mm below the ridge crest, H4: 4 mm below the ridge crest, T0: immediately after tooth extraction, T1: at 2 months after extraction, SD: standard deviation, IQR: interquartile range. * Statistically significant difference (\(p<0.05\)).  
\end{table} 
Table 2: Discrepancies between the two impression techniques at each time point. scanning technology have been propagated, excluding some steps in previous methods and seemingly enhancing the accuracy of the soft-tissue analysis. Some studies compared digital and conventional impressions for palatal soft tissue registration [13; 17]. In one study, a digital impression was compared with a vinyl polysiloxane impression, resulting in 130.54 \(\pm\) 33.95 \(\upmu\)m difference between them [13]. Of note, this difference was greater than the value from the area of full dentitions (80.01 \(\pm\) 17.78 \(\upmu\)m) even though the difference between digital and conventional impressions appeared to be minor. In the other study, the digital impression was compared with an irreversible hydro-colloid impression, exhibiting that the difference between them was 0.02 \(\pm\) 0.07 mm [17]. Those studies are limited to the evaluation performed at a one-time point. More recently, the interest regarding soft tissue seems to be focused on the change following a specific treatment or over time.