# Computerized Optical Impression Making of Fully Dentate Upper and Lower Jaws: An In Vitro Study
## 4 Discussion  
The in vitro approach of this study made it possible to provide similar, controllable scanning conditions for all scanners. In vivo scans are more difficult to perform because of limited space in the oral cavity, patient movements, moisture and soft tissue obstacles. In addition, the oral cavity is too small for the reference scanner used in this study. As reported in another study, extraoral long span scans with intraoral scanners can be more accurate than intraoral long span scans [25]. This has to be considered when interpreting the results of this study. In order not to influence the intraoral scanners' accuracy in a negative way, a non-translucent plastic material with natural tooth-colored teeth was chosen for the reference models in this study. More translucent materials such as enamel, which are present during patient care, seem to have a negative effect on the accuracy of many intraoral scanners [26]. For these reasons, the tested intraoral scanners are expected to provide lower accuracy under clinical conditions. The reference scanner has been reported to be true to 3 \(\upmu\)m [26; 27; 28], which is important because all intraoral scans have been compared to the reference scans during the trueness evaluations. The scans in this study were performed by a single, experienced dentist in order to not influence the scanning process in a negative way. The method of best-fit 3D superimposition was chosen as the most suitable because this study examined the ability of intraoral scanners to scan natural tooth shapes [29; 30]. The limit of clinical acceptance for trueness values was set to 50 \(\upmu\)m. This limit was chosen because intraoral appliances with higher differences to a patient's tooth positions can impair blood flow in the periodontal ligament [31; 32]. This can be seen as the upper limit of clinical acceptance assuming the theoretical fact that no further inaccuracies are added on top of the intraoral scan's inaccuracy along the clinical workflow. A major reason for the lower trueness of the CS compared to the TR seemed to be the lower number of datapoints, which can lead to higher calculated deviations of the reference scan file after 3D superimposition and thus to higher trueness values. However, there was no statistically significant difference between the full arch mean trueness of the CS, IT and TR (\(p<0.05\), Table 2, Figure 5). By analogy with this study, another in vitro study which evaluated the successors of the CS and TR by scanning a full dentate maxilla with unprepared teeth found comparable mean trueness values of 26.9 \(\pm\) 15.9 \(\upmu\)m with the CS 3600, 27.7 \(\pm\) 6.8 \(\upmu\)m with the TRIOS 3 and 20.8 \(\pm\) 6.2 \(\upmu\)m with the TRIOS 4 [33]. In line with this, an in vivo study evaluating the CS 3600 and the TRIOS 3 in scanning the maxillary arch found comparable trueness levels between the two successor models [34]. Another in vitro study, however, which evaluated the effect of artificial saliva on COIM of full arch  
Figure 14: Illustration of the PS virtual triangles of tooth 11 from scan number 1 and of tooth 21 from scan number 3. Black arrows indicate an alignment error. scans with unprepared teeth, found higher trueness values for the CS 3600 and TRIOS 3 than the present investigation [35]. These findings indicate that proper moisture control is more important than further developed scanner generations. The PS scanner works with the principle of optical triangulation and thus faced similar technical challenges to the CS. However, the CS can be stabilized on neighboring structures before a single scan image is taken, which has probably contributed to lower trueness and precision values of the CS compared to the PS. The PS constantly has to be moved above the scanned object at a certain focal distance because it automatically takes scan images at a higher rate compared to the CS [36]. Another examination reported the highest PS distortions in the apicocoronal axis [37].