## 3 Discussion  
Obtaining implant prostheses using digital technology has become a common procedure [14]. Digital implant impressions offer a viable and valid alternative to conventional impressions for single- and multi-unit implant-supported restorations. Digital impression accuracy is at the same level as conventional impression methods in the fabrication of crowns and short fixed dental prostheses [4]. However, in the case of complete arch scanning for extensive prosthetic rehabilitation, the accuracy of the digital method compared with the conventional one is controversial [15]. Current evidence on the superiority of one technique over the other is inconclusive [16]. Further in vitro and in vivo studies are needed to carefully evaluate the progress of complete arch scanning in dentistry. [PERSON] et al. [17] reported a similar marginal accuracy of conventional and digital impression methods for crown fabrication using Lava C.O.S. and iFero CAD/CAM systems and polyvinyl siloxane impressions. In their in vitro study, all measured marginal gaps were clinically acceptable. For single implants, the mean marginal gap was 61.4 \(\upmu\)m for digital impressions, which is clinically acceptable [17]. A marginal gap level of 100 \(\upmu\)m was defined as clinically acceptable [18], and 120 \(\upmu\)m was considered the maximum tolerable marginal opening [19]. Similar results have been reported in other studies [2,20]. In the process of manufacturing implant prostheses, the first step is to acquire the data using an intraoral scanner. The position and orientation of the implants are recorded by a digital impression of a scan body connected to the implant. This avoids the classic impression technique that is not preferred by patients and is also time-consuming [4]. However, low scanning quality can lead to inaccuracies in the implant's position on the virtual model [21,22]. Some studies reported that digital implant impression accuracy was influenced by implant placement depth, the characteristics of the scan body, the type of scanner, implant angulation, the distance between the implants, and operator experience [5,10]. There are cases in which the implants are placed deep in subrestal positions, and sometimes the scan bodies do not reach an optimal supragingival level so that they can be scanned accurately. This fact was also demonstrated in [PERSON]'s recent study [13] on the accuracy of implant position reproduction according to the exposed length of the body scan during optical scanning. He observed that the error increased rapidly for an implant placement depth of 4.5 mm, and was largest for a submersion of 6.0 mm. For this reason, we proposed the described technique. Our solution is a four-step technique (Scheme 1):  
* An intraoral digital impression of an analog classic transfer, which exposes a sufficiently high area above the gingival margin. Figure 8: Final designs of the crown: (**A**) frontal view; (**B**) lingual view. An extraoral digital impression of an analog transfer mounted in the transfer accessory (Wolfcraft Keyless Chuck Metal 2625000), which allows to achieve an accurate and reproducible position. This device was chosen because it has proper dimensions, is related to the analog implant dimension, has a black and opaque color, can be easily grasped by the hand during the grinding procedures, allows easy mounting of the prosthetic components, and can be sterilized due to its entirely metallic surface. We made three different marks on the chuck's head with a metal bur for better horizontal indexing of the chuck's head and the impression transfer assembly with the chuck's head and scan body assembly. The connection between the chuck and the implant analog must remain fixed so the vertical housing of the transfer and the scan body will be the same. An extraoral digital impression of the scan body mounted in the same analog in the transfer accessory.