## 4 Discussion  
Accuracy of digital impressions is essential in order to obtain high-quality restorations as well as to the predictability of the treatment [20]. The aim of IOSs is to increase the patient's acceptance and comfort, reduce the clinical time and also to eliminate the errors that come with the conventional impression procedure [21]. Today, with more and more IOSs being present in the dental offices, there are clinicians who are unsure whether or not there is a real need to replace the existing IOS with a newer one. Thus, the following question arises: \"Is the accuracy of the older generation of IOSs an impediment for obtaining a quality intraoral digital impression?\"  
The results of the present in vitro study were in favor of the null hypothesis that the accuracy of the latest IOSs has improved significantly when compared to older generations of scanners. The trueness and precision of intraoral scanners has been investigated in numerous scenarios by different authors. The results of this study were consistent with the results of other previous studies [22; 23; 24]. [PERSON] et al. evaluated the accuracy of three IOS (Trios 3, Trios 3 mono, Itero element) and obtained significant differences between the digital impressions [22]. [PERSON] and [PERSON] evaluated the accuracy of six IOS in the scenario of multiple single crown preparations distributed across a dentate arch. They reported the highest accuracy (trueness, precision) for Primescan (25 and 10 \(\upmu\)m), followed by Trios (40.5 and 11 \(\upmu\)m), Omnicam (41.5 and 18 \(\upmu\)m), Virtuo Vivo (52 and 37 \(\upmu\)m), iTero (70 and 12 \(\upmu\)m) and Emerald (73.5 and 60 \(\upmu\)m) [23]. [PERSON] et al. also evaluated the trueness and precision of eight IOS and concluded that there were significant differences between the digital impressions, the best values being obtained for the Primescan IOS [24]. The clinical accuracy of impressions dictates the marginal and internal fit of the final restorations. However, all the following steps involved in the fabrication of the restoration may also introduce errors [25; 26]. Even though CEREC Omnicam and Plammeca PlanScan are available on the market for almost nine years, the Omnicam IOS provides high-quality intraoral scans which are not far behind (difference of approx. 10 \(\upmu\)m) of those obtained with the latest IOSs. On the other hand, the differences in trueness between the Plammeca PlanScan IOS when compared to Primescan and Mediti i700 were significantly higher (difference of approx. 53 \(\upmu\)m). Regarding the precision of the investigated IOSs, the mean best performance was found for the Primescan IOS, followed by Mediti i700, Omnicam and Plammeca IOSs. Primescan and Mediti i700 displayed precision values that are very close within the same interval of 5 \(\upmu\)m to 15 \(\upmu\)m, with Primescan definitely keeping the median lower than 7 \(\upmu\)m.  
Figure 4: Boxplot displaying the precision values of the four intraoral scanners. Omnicam and Planmeca Planscan displayed values that differ significantly, with more outliers and resulting in a severe decrease of precision. The accuracy of digital impression is conditioned by a number of external factors including the type of preparation, fluid isolation, limited space and access for the scanner, scanning area as well as lighting conditions in the office [27, 28, 29, 30]. Being an in vitro study, there were a number of limitations. No clinical external factors were simulated (humidity, blood, patient movements). The scanning pattern was the same for all the IOSs, even though previous studies show conflicting reports regarding the influence of the scanning path on the final accuracy of digital impressions [31, 32]. As another limitation, a better material could have been used to fill the gap of the edentulous area. Scannable impression materials such as Honigum Pro Scan (DMG) would have been more suitable for the scanning procedure and could have reduced the influence of the reflection of the material.