# Accuracy of Extraoral Digital Impressions with Multi-Unit Implants
## 4 Discussion  
This study did not identify statistically significant differences between the extraoral scanners used, between the impression materials or between the Coltene silicone and the corresponding stone casts. Thus, the first two null hypothesis are accepted, and the third null hypothesis is partially rejected, since statistically significant differences were reported for the Zhermack silicone when compared with the corresponding stone casts. Figure 6: Boxplot diagram of the study groups. Accuracy is defined by trueness and precision as precision corresponds to reproducibility between repeated measurements and trueness describes the closeness of real dimensions [20; 21]. To evaluate the accuracy between the master model and the virtual models, a digital software (Geomagic X) with a best-fit superimposition algorithm was used. This method has been recommended for accuracy analysis, although some authors report that it does not adequately show real divergences as positive and negative deviations between reference and virtual models could drive results to cancel each other out [15; 22; 23; 24]. On the other hand, the global best-fit superposition also does not allow one to determine the direction of the discrepancies [6]. Another technique used is the local best-fit superposition in which certain areas of overlap are defined. Nevertheless, the global best-fit algorithm presents a higher degree of repeatability, whereas the local best-fit algorithm involves error-prone manual selection. [24]. The use of rigid custom trays is not a critical factor in the accuracy of implant impressions because elastomeric materials have high degrees of dimensional stability and satisfactory degrees of inherent rigidity [15]. However, in our study, we used rigid custom trays with holes to accommodate the impression copings to facilitate and standardize the impressions made. Splinted open-tray impressions were used in our study and were found to be more accurate than non-splitting closed-tray impressions for completely edentulous patients [20]. However, the splinting technique used in this study to minimize polymerization shrinkage cannot be used in full-arch immediate-loading implant protocols. The sectioning of the splint 24 h after polymerization and the subsequently addition of more resin would require a second appointment [4]. Regarding the impression material, both polyethers (PEs) and polyvinylisoloxanes (PVSs) are excellent impression materials for implant rehabilitations [4]. In the present study, all the impressions were made using polyvinylisoloxanes. A study by [PERSON] et al. showed RMS values of 85.9 \(\pm\) 30.2 \(\upmu\)m in the best-fit superimposition of stone casts created using an intraoral scanner [9]. In our study, in the silicone groups, the RMS values ranged between 69.90 and 87.43 \(\upmu\)m. This study also showed RMS values of 111.8 \(\pm\) 26.1 \(\upmu\)m in casts printed with a stereolithographic 3D printer [9]. In our study, in the stone model groups, the RMS values ranged between 81.74 and 110.62 \(\upmu\)m.  
The optical impressions proved to be reliable for short-spans cases. However, when considering long-span cases with more than five implants, the accuracy of the impressions made via intraoral scanning is not comparable to the accuracy of the conventional impressions [22]. In these cases, the use of laboratory scanners for the digitalization of impressions can be a valid alternative, as demonstrated in our study in which it showed better accuracy than the corresponding stone casts for one of the studied elastomers, Zhermack silicone. In the present study, the differences between the Coltene and Zhermack groups might be related to differences in optical or mechanical properties between the silicones. However, this information is not supported by the literature. The digital acquisition of implant impressions allows for the elimination of some laboratory phases, leading to error reduction.