## 4 Discussion
### Accuracy Evaluation of Digital Impression  
Many studies have reported better results for conventional impression [10; 15; 16], although different authors consider digital impression to be superior to the conventional one [17; 18]. In addition, many authors report the similarity of both techniques considering  
\begin{table}
\begin{tabular}{l l l l} \hline \hline
**Independent Variable** & **Gypsum Intact** & **Gypsum Fractured** & \(p\) **\textless{} 0.05** \\ \hline Specific Trueness of first & & & \(p\) **= 0.001** \\ Digital Impression & & & \\ \(<\)100 \(\upmu\)m & 7 & 0 & \\ \(>\)100 \(\upmu\)m & 1 & 6 & \\ Specific Trueness of second & & & \(p\) **= 0.001** \\ Digital Impression & & & \\ \(<\)100 \(\upmu\)m & 9 & 0 & \\ \(>\)100 \(\upmu\)m & 1 & 4 & \\ Accuracy & & & \(p\) **= 0.001** \\ \(<\)98\% & 0 & 5 & \\ \(>\)98\% & 8 & 1 & \\ \hline \hline \end{tabular}
\end{table}
Table 6: Statistical analysis of correlation between gypsum check outcomes and Specific Trueness and Accuracy.  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Patient N.** & **Specific Trueness 1** & **First Digital Impression** & **Specific Trueness 2** & **Second Digital** & **Accuracy \%** \\ \hline
1 & 3.33 & Intact & 40.00 & Intact & 99.98 \\
2 & 40.00 & Intact & 16.67 & Intact & 99.30 \\
3 & 50.00 & Intact & 96.67 & Intact & 99.27 \\
4 & 40.00 & Intact & 90.00 & Intact & 99.71 \\
5 & 283.33 & Fractured & 3.33 & Intact & 97.82 \\
6 & 446.67 & Fractured & 446.67 & Fractured & 97.62 \\
7 & 240.00 & Fractured & 186.67 & Fractured & 96.50 \\
8 & 290.00 & Fractured & 133.33 & Intact & 97.90 \\
9 & 200.00 & Fractured & 136.67 & Fractured & 97.42 \\
10 & 66.67 & Intact & 76.67 & Intact & 99.30 \\
11 & 290.00 & Fractured & 493.33 & Fractured & 98.59 \\
12 & 123.33 & Intact & 10.00 & Intact & 98.69 \\
13 & 6.67 & Intact & 3.33 & Intact & 99.95 \\
14 & 36.67 & Intact & 23.33 & Intact & 99.68 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Outcomes of gypsum check matched with Specific Trueness values.  
the accuracy [18; 19]. Accuracy is expressed as Trueness \(\pm\) Precision, so these two values should be calculated. Specifically, Trueness represents the difference between a measured distance on digital impressions and the same measurement made on the reference model scanned by a laboratory scanner. So, the Trueness will then express how far our measurement of digital impression deviates from the value most like true clinical situations. Precision will express the \"scattering\", counted as the dispersion of measurements taken with the intraoral scanner; it will show how much several scans of the same patient differ from each other. Accuracy will thus be affected by Precision and Trueness, being an indicator of the quality of the procedure and the instrument. To perform these calculations, following the ISO rules, the measurements should be conducted on at least 30 samples [8]. This indication is incompatible with clinical study, considering the difficulty of taking at least 60 impressions for each patient. In fact, most studies are conducted in vitro with superimposition software [20; 21]. So, in this study, the authors have proposed \"Specific Trueness\" because they considered the linear distance within a specific sector evaluated with three impressions.