## 4 Discussion
### Production Protocols Used in the Studies  
Digitization of the information by scanning is always needed for the application of digital techniques. The studies that described the information acquisition by extra-oral scanning, without conventional impression, were in vitro studies that used simulation models of maxillary arches. As is well known, in a clinical approach, extra-oral scanning is always preceded by a conventional impression. Thus, the protocols presented in those papers were defined as if they started with a conventional impression. The intra-oral scanning technique involves highly developed digital tools for clinical practice. However, the equipment technologies still have limitations on the trueness of entire arches and extensive edentulous edges related to the intra-oral environment and the camera position [92; 94; 95]. Moreover, a digital dynamic impression is still impossible digitally [44; 51]. There are other methods for acquiring digital information, such as Cone Beam Computer Tomography (CB/CT) or Computer Tomography Scan (CT Scan), but these were not considered by the researchers due to inadequate patient exposure to irradiation [40; 96; 97]. The protocol with the most research (thirty-four papers) was protocol 6 (direct additive manufacturing using conventional impression). This protocol differs from protocol 11 (direct additive manufacturing using intra-oral scanning) by using conventional impressions to acquire information, and diverges from protocol 4 (indirect additive manufacturing using conventional impression) since it enables direct metal fabrication, making the fabrication of the intermediate structure unnecessary. The production of an intermediate structure, a crucial step in the conventional protocol, is an additional source of potential errors; hence, there is nowadays a preference for digital protocols that skip this step. In the present study, the protocol applied by [PERSON] and colleagues [54] was defined as protocol 6, even though there was no conventional impression, as the scanning of the information of the simulation model was performed with an extra-oral scanner. In the study by [PERSON] and colleagues [53], the intraoral scanner was used to perform an extra-oral scanning of the simulation model. Hence, the protocols in question were considered as 2 (conventional impression and production using CAD analysis) and 6, not as 7 (conventional production using iOS and CAD analysis) and 11 (direct additive manufacturing using iOS), respectively. After digitizing the information, the initial digital RPD framework designs implied the combination of several types of software. Currently, the analysis and design software already have all the tools combined, allowing integration of the digitized information, analysis of the path of insertion, and design of the RPD framework using libraries of components that can be customized. Nevertheless, the digital design software for RPD frameworks is still under development and improvement [4]. The protocols that involve milling to obtain an RPD metal framework are less investigated, since protocols 3 (indirect milling using conventional impression) and 5 (direct milling using conventional impression) were only described, in four and two studies, respectively. Protocols 8 (indirect milling) and 10 (direct milling), both using intra-oral scanning, are not mentioned in the included papers. Despite the possibility of a better finish than the additive technology, milling techniques are described as inappropriate for producing RPD metal frameworks because of the disadvantages of material waste and limitations in complex designs [28; 36; 48; 57; 96; 98]. In the scientific literature, the production of RPD frameworks by milling techniques is related mostly to non-metallic materials such as polyether-ether-ketone (PEEK) [99; 100; 101]. The most reported digital technology for directly producing an RPD metal framework was selective laser melting (SLM), and the most described equipment was the EOSINT M270.