## 2 Materials and Methods
### Accuracy Analysis  
In the beginning, after reconstructing the digital model on the CAD-CAM station of each three impressions, the authors manually measured the distance between each transfer axis in the conventional impression and between each center of the scanbody in the digital impression. The distance between the scanbodies' centers in the first optical impression was indicated as Digital Impression 1 (DI1), and the same distance for the second optical impression was indicated as Digital Impression 2 (DI2). The distance between transfers of conventional impression was indicated as RV (Reference Value) because it was considered as the reference value. First of all, to evaluate the reliability of the optical impression and its comparison with the analogic impression, Trueness, Precision, and Accuracy were calculated. Trueness is defined as the closeness of measurements to the reference data. In this study, it is the difference between the distance of the two transfers of the analogic impression and the distance of the two scanbodies of the optical impression. According to ISO rules [8], Trueness must be calculated on a minimum of 25 measurements, which is usually possible with superimposition software using the best-fit algorithm [13], but it is incompatible with a clinical investigation, as it could be very invasive to take 25 different impressions on the same patient. So, in this study, ISO Trueness was intended as the differ ence between the mean digital distance of 2 optical measurements and the conventional impression. Authors called the Trueness of two measurements \"Specific Trueness\", which is calculated as follows:  
Specific Trueness \[(\mu\mathrm{m})=\left|\mathrm{digital\ distance}-\mathrm{real\ distance}\right|\]  
Precision is defined as the closeness of repeated measurements, calculated as the distance between multiple measurements of the same object performed by the same instrument under the same conditions, calculated as follows:  
\[\mathrm{Precision}\ (\mu\mathrm{m})=\sqrt{\frac{\left(\mathrm{distance}_{1}- \mathrm{mean\ distance}\right)^{2}+\left(\mathrm{distance}_{2}-\mathrm{ mean\ distance}\right)^{2}}{\mathrm{measurement\ number}}}\]  
After Trueness and Precision calculation, the Accuracy value can be calculated for each patient. From the ISO definition, Accuracy could be defined as a linear distance: the range between the Trueness--Precision value and Trueness + Precision value. By definition, Accuracy is represented by the formula \"Trueness \(\pm\) Precision\" and is expressed in \(\mu\mathrm{m}\).  
In order to simplify the readability, allowing an immediate understanding of such value, the authors proposed to convert a distance value to a percentage value. To do so, Dimensional Error was considered, and can be calculated as follows:  
\[\mathrm{Dimensional\ Error}\ (\%)=\frac{\left|\mathrm{mean\ digital\ distance}-\mathrm{real\ distance}\right|}{\mathrm{real\ distance}}\ x\ 100\]  
Thus, the Accuracy percentage is calculated as = 100 \(-\) Dimensional Error. The last step of this research was to correlate the Accuracy of the digital models with the clinical situation. For each patient, three gypsum gigs were fabricated, respectively, for the two digital and the conventional impressions, assessing the impact of impression Accuracy on the passive fit of future rehabilitation. The gypsum check was inserted with a controlled torque of 10 N. If clinicians could insert it without complications, the impression was considered reliable. If partial or total fracture occurred, the impression would not allow the passive fit of future rehabilitation, so it was considered inadequate and had to be retaken. Patient information about single measurement, Trueness, Precision, Accuracy and percentage Accuracy, and the final clinical check of gypsum gigs were collected into a database.