# Factors Affecting Trueness of Intraoral Scans: An Update
## 4 Discussion  
There is controversy among the studies analyzed regarding trueness comparison among IOS devices. Both studies that assessed crown preparations described that TRIOS scanners (3 Shape A/S) presented the best trueness (lowest difference values) [19; 22]. This finding is in accordance with a previous study comparing IOS devices for preparations [14]. On the other hand, another study on inlay preparations described that Primescan (Dentsply-Sirona) had the best trueness [20]. Regarding IOS of dental arches, it was found that the IOS devices included by the studies varied considerably, which prevented this review from drawing any conclusions about which devices actually have the best trueness. On the other hand, one study comparing two different devices (Primescan and Omnicam) of the same brand (Dentsply-Sirona) found that the device with the newest technology (Primescan) had the best IOS trueness for defendulous arches [23]. This can be interpreted as evidence that IOS technologies are significantly improving to scan an defendulous patient, which has been considered the biggest challenge for IOS as compared to conventional impressions [28]. Despite the abovementioned promising findings for IOS of defendulous patients, there is agreement among the articles assessed herein regarding the fact that span length is inversely proportional to IOS trueness [24; 26]. This finding is also in agreement with previous studies [14; 28] and with another article assessed herein that described the scan size as a factor significantly inversely associated with IOS trueness [22]. Furthermore, the operator's experience with IOS has also been found to be directly proportional to IOS trueness [22]. The operator's educational level, however, is suggested not to be a factor affecting IOS trueness, as similar results were reported for dentists and dental students [21]. This can be considered evidence of the usefulness of IOS and digital dentistry in dental education [29]. Besides the abovementioned factors, rescanning and post-processing scans as well as the conditions of the preparations and the presence of adjacent teeth are also considered in this review as significant factors affecting IOS trueness. Furthermore, research published prior to the dates considered in the present inclusion criteria had also identified IOS scanning strategy as a factor affecting IOS trueness [30]. This contrasts with the present update review, which did not find any trueness comparisons among different scanning strategies. Instead, most of the included studies used the main scanning strategies recommended by the manufacturers, which are usually scientifically validated for clinical use. Among the main limitations of the present systematic review is that only a short time span was covered in an attempt to include only recent studies using the most novel IOS technologies. Furthermore, not all IOS devices available in the market were assessed by the studies included. It is also important to note that this systematic review did not include studies assessing the indirect impact of the trueness of IOS systems on the accuracy of implant placements and other clinical outcomes. For this purpose, further clinical long-term prospective studies would still be recommended to address each type of clinical impact occurring due to changes in IOS trueness. Finally, only three studies were in vivo, which suggests that future prospective in vivo studies are recommended to confirm the clinical impact of the factors described herein on IOS trueness. In conclusion, within the limitations of this systematic review, the present findings on current IOS technologies suggest that the device, scanning distance, operator experience, rescanning and post-processing scans, conditions of the preparations and presence of adjacent teeth are factors significantly affecting IOS trueness. Conceptualization, A.R.G.C. ; methodology, J.N.-C. and A.-M.A. ; formal analysis, A.R.G.C., J.N.-C. and A.-M.A. ; investigation, A.R.G.C., J.N.-C. and A.-M.A. ; resources, A.-M.A.