## 4 Discussion
### Fit Assessment Evaluation  
Rehabilitation with RPD involves challenges in defining an accurate pathway for insertion and removal since minor errors in the rigid framework can lead to significant discrepancies in fit [30]. Nevertheless, in many of the included studies, the fit accuracy assessment was made through a qualitative evaluation, which usually fails in reproduction due to subjectivity and difficulty of standardization or criteria definition [48]. The fit accuracy assessment based on a quantitative evaluation was applied in less than half of the included papers (twenty-five papers). Different methods used for framework production and evaluation were applied, meaning it was impossible to compare outcomes between studies directly. Most of those studies were made in the laboratory, and most clinical studies had small sample sizes. In addition, three clinical studies used experimental procedures, as different master casts for each protocol evaluated, which may reduce the probability of accurate results and can introduce bias [29; 84; 87]. Some studies used standardized values for the meaning of the distance found between framework occlusal rests and dental rest seats. Considering the cumulative errors in the production steps, some authors defined a gap from 0 to 50 \(\upmu\)m as close contact and a gap from 50 to 311 \(\upmu\)m as acceptable [37; 53]. Protocols 2 (conventional production using a physical cast digitally created) and 3 (conventional production with CAD analysis and indirect milling) emerged as those with the lowest quantitative discrepancy, and protocol 9 (casting of a digitally produced intermediate structure with additive manufacturing) was classified as \"excellent\" in qualitative analysis. However, protocols 2 and 3 were evaluated in fewer studies (one to three publications each), and protocol 9 was evaluated in three case reports with no control group and a low level of evidence. Moreover, the study that described protocol 2 used a methodology that presented impingement of the RPD frameworks on the master cast [53]. That situation can be explained by the fact that the authors did not use a reference image of the RPD framework inserted on the master cast for the superimposition protocol, thus not allowing an interpretation of the discrepancy gap values in absolute terms but in a relative way [53]. Of the remaining protocols, the one that stands out most was protocol 6 (direct additive manufacturing using conventional impression) because it was the most reported (thirty-four papers) and presented the highest number of Excellents (two) and Goods (eleven). Also, compared with protocol 4 (indirect additive manufacturing using conventional impression), it had better quantitative assessment results in four direct comparisons. Also, compared with protocol 11 (total digital flow with additive manufacturing), protocol 6 presented more Excellent or Good results (eleven against five). Considering the studies comparing protocol 6 to the conventional control group (protocol 1), eight studies showed similar or better results than protocol 6; one of those was the only clinical study with low risk of bias. Taking into account the seven reports that showed the opposite, some considerations are essential to contemplate. Most laboratory studies did not perform finishing procedures to the AM frameworks (these naturally present a rougher surface, which disfavors their results), used different master casts for each production protocol, or applied evaluation methodologies that damaged the RPD frameworks before the measurements. For the clinical studies, a qualitative fit evaluation or a quantitative assessment was made using different master casts for each production protocol, which can quickly introduce bias. Two of those seven studies declared discrepancy values for AM protocol under 200 \(\upmu\)m, a value that is like the discrepancy of the conventional protocol achieved by the studies conducted by [PERSON] (173-215 \(\upmu\)m) and by [PERSON] (193 \(\pm\) 203 \(\upmu\)m) [18,23].