## 4 Discussion  
In this in vitro pilot study, the PEEK frameworks obtained by milling showed a better fit accuracy to the corresponding models than the SLM-printed Co-Cr frameworks in both Kennedy classes, with the PEEK framework of a mandibular Kennedy Class I presenting the best scores. RPD manufacturing through CAD/CAM technologies has emerged as a promising strategy. Since it may involve either subtractive or additive methods that can be combined with a variety of materials, a thorough evaluation of the frameworks produced by CAD/CAM is still limited and sometimes inconsistent, demanding further research [11; 12; 16]. To the best of our knowledge, no available studies compare milled PEEK with SLM-printed Co-Cr frameworks. Nevertheless, several studies have addressed these materials and manufacturing techniques separately. Consistent with our data, [PERSON] et al. observed a better fit accuracy of milled PEEK frameworks compared with Co-Cr frameworks produced using a conventional method, with PEEK showing clinically acceptable gaps for occlusal rests (86.2 \(\pm\) 22.6 \(\upmu\)m) and major connectors (52.8 \(\pm\) 44.6 \(\upmu\)m) [37]. According to [PERSON] et al., PEEK frameworks manufactured by milling also showed clinically acceptable values (<311 \(\upmu\)m) at the rests (70 \(\pm\) 90 \(\upmu\)m) and major connectors [21]. In turn, [PERSON] et al., assessing Co-Cr frameworks fabricated using different techniques, observed that the SLM-printed Co-Cr frameworks presented the poorest fitting when compared with Co-Cr frameworks produced with (i) the lost-wax technique from a stone model, (ii) 3D printing from a stone model and (iii) the lost-wax technique from a resin model [7]. When the fit accuracy of PEEK frameworks to oral structures was subjectively rated, adequate fitting scores were reported [23]. Also, patients showed higher satisfaction compared to the use of Co-Cr RPDs [38]. Although this evidence provides essential clues, the diversity of the assessment methods and manufacturing techniques preclude a direct comparison with our results. From a clinical perspective, the gap between the inner surface of the framework and the supporting structures represents the fit accuracy of an RPD [29]. Various methods for assessing this gap have been described in the literature [7; 39], but a gold-standard method is still unreported. The fitting of an RPD framework can be analyzed either qualitatively or quantitatively [31]. As they are simple and fast, qualitative assessment methods, such as visual inspection or pressing tests, are widely used, but the information \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Co-Cr Framework** & **PEEK Framework** \\ \hline Maximum & 454.4 & 302.1 \\ Minimum & \(-\)455.4 & \(-\)300.9 \\ Mean & \(-\)33.3 & \(-\)26.9 \\ Standard Deviation & 101.9 & 53.2 \\ Root Mean Square (RMS) & 107.2 & 59.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Quantitative assessment of trueness (expressed in \(\upmu\)m) for [PERSON]’s Class III modification 1 frameworks. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Co-Cr Framework** & **PEEK Framework** \\ \hline Maximum & 594.5 & 430.1 \\ Minimum & \(-\)594.8 & \(-\)430.1 \\ Mean & \(-\)73.3 & 25.4 \\ Standard Deviation & 128.9 & 64.4 \\ Root Mean Square (RMS) & 148.3 & 69.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Quantitative assessment of trueness (expressed in \(\upmu\)m) for [PERSON]’s Class I frameworks. collected needs to be more precise and objective [31]. In quantitative assessment methods, a direct measure of the gap using analog instruments has proven difficult, as it involves sectioning the RPD, which may not reflect the overall fitting [7]. Therefore, an indirect quantitative assessment of the gap has often been applied through a silicone mold of this discrepancy, followed by the measurement of its weight or thickness [3; 39; 40].