## 4 Discussion  
The reviewed literature included only nine of the eleven possible protocols to produce RPD metal frameworks that combine digital and conventional techniques. The results show that all protocols are accurate, and protocols 4, 6, and 11 (total digital workflow) are the most frequently used in the reported articles. Digitally produced frameworks were clinically acceptable, but the fit accuracy outcomes were heterogeneous among studies since different assessment methods were used.  
RPD frameworks are a highly personalized and complex structure, with a wide range of structural designs, created specifically for each patient. For this variation, there are additional difficulties in carrying out an investigation that quantitatively analyzes a standardized error that appears in the RPD production [19].  
RPD framework accuracy assessment can be done by testing the fit in the plaster cast or in the patient [19; 38]. From the sixty-two included papers, the most used assessment methods were qualitative, among which the visual inspection predominated. Quantitative assessment methods based on the distance between the RPD framework and support structures, measured directly or indirectly by using silicone, or based on the discrepancy of the produced metal framework to the STL design, are also described. The difficulties  
Figure 4: Risk of bias summary of the randomized clinical studies [30; 84; 87].  
Figure 3: Risk of bias summary of the non-randomized clinical studies [4; 29; 47; 51; 64; 66; 70; 71; 77; 80].  
of a quantitative general analysis of an RPD metal framework led analog fit accuracy evaluation to be mostly limited to occlusal rests, components that reflect the global fit of the RPD [29; 55].  
With the development of digital technologies, it is possible to use reverse CAE techniques which allow quantitative analysis not only by means of specific digital measurements but also by digital superimposition of the entire RPD framework, enabling better quality of production control [53]. However, the application of these techniques on a clinical daily basis is limited mainly by inaccurate soft tissue measurement with current intra-oral scanner technology and the patient exposure to X-ray radiation when a computerized tomography scan is used [31; 55; 90; 91; 92].  
Compared to the early-1970s development of digital fixed prosthesis production, the first case of digital production of an RPD metal framework was only published in 2004. [36; 93]. More than half of the studies included in the present work were published in the last four years, and among these, we found greater scientific evidence, which indicates the current late interest in digital RPD frameworks. Nevertheless, scientific evidence is still weak, as most are case reports (thirty papers) or in vitro studies (nineteen papers). From the thirteen clinical studies, eleven performed a fit accuracy evaluation, and the majority of these (eight papers) presented a reduced sample size (\(\leq\)15 patients). Also, it is important to consider that in the thirteen studies with higher scientific evidence level, only one presented low risk of bias and five presented high, serious, or even critical risk. Therefore, a first appreciation of this systematic review is the need for future research on this topic with a higher level of scientific evidence and lower risk of bias.