## 4 Discussion  
The results of the present study demonstrate the influence of hardware and software development over the last decade from 2012 to 2021 on scanning accuracy. Many manufacturers offer different software updates, but at a certain point older devices are no longer supported and it is necessary to buy new hardware. Therefore, it is difficult to determine whether a decrease in transfer accuracy is solely due to the software improvement or to the hardware as well. This is a clear limitation of the current study. Moreover, newer scanner generations do not necessarily provide better results. Unlike the other IOSs examined, the current True Definition Scanner hardware and software systems showed no improvement in terms of transfer accuracy in later versions. This may be related to the computing power of the tablet computer used, particularly in the case of a portable scanner. Furthermore, it may possibly be due to the presence of the reference structures within the model, as the scanners are typically intended to capture tooth structures or scan bodies. Furthermore, it may occur by reducing the powdering, which is recommended by the manufacturer. This was particularly noticeable in preliminary tests, but an absolute statement regarding the scan algorithms is not given by the manufacturers' side, so no absolute statement regarding this possibility is feasible; this is also a clear limitation of the present study. In contrast, the combination of two acquisition principles in a single device appears to significantly increase the accuracy, as observed in the Primescan IOS [28,29]. The fact that an additional scanning principle is used in comparison to the Omnicam shows that the manufacturer is trying to further develop the scanner, which is part of the CEREC family. This may be related to inherent limitations when using only one scanning principle, which can only be overcome by combining several scanning principles. In the present study, a common problem with four implants inserted in the maxilla was simulated. With the help of the reference system, the absolute three-dimensional (3D) deviations and rotational and angulation deviations could be assessed. For comparison, a conventional impression using impression material available over the last decade was included [22,23]. The same (intraoral) scan bodies were used to obtain comparable results, as different scan bodies can lead to different transfer accuracies. Although various techniques are available for conventional implant impressions, an open-tray technique with a polyether was used for comparison. This technique was often used in previous studies and showed the highest transfer accuracy [30, 31, 32]. Measurement errors were avoided as measurements were performed only by two investigators ([PERSON] and [PERSON]). To obtain the utmost accuracy, individual scanning paths recommended by the respective manufacturers were used [10]. A lot of studies on impression and scanning accuracy rely only on surface comparisons [33, 34, 35]. However, this method may hide real differences [25,36]. Therefore, one strength of the present study was using a reference system within the IMM, which allowed the determination of the precisely x-, y-, and z-deviations for the different implant positions. In contrast to the best-fit method, this approach made it possible to obtain an accurate 3D interpretation of the results. This approach, however, requires an RC or another reference structure, which is only achievable in an in vitro setup. Though this prerequisite is a clear limitation for clinical studies, it is a strength of the present study, as we aimed to compare the different scanners in families with the utmost precision. Concerning the accuracy of the analysis, we assessed trueness and precision according to ISO 5725-1. We chose the ISO approach as a standardized method, which is very helpful for comparing our results with those of previous studies later on [16].