## 4 Discussion
Analyzing the RMS values obtained, it was possible to conclude that the discrepancies between the produced frameworks and the original CAD design were lower in the PEEK than in the Co-Cr frameworks, in both Kennedy Classes, indicating a higher trueness of the milled PEEK frameworks. Although a complete correlation cannot be established due to diverse experimental conditions and specimens, these results are in agreement with those obtained by [PERSON] et al., who concluded that PEEK frameworks produced by milling (43 \(\pm\) 23 \(\upmu\)m horizontal and 38 \(\pm\) 21 \(\upmu\)m vertical) showed lower distortion and increased precision in comparison with frameworks fabricated with SLM (365 \(\pm\) 205 \(\upmu\)m horizontal and 363 \(\pm\) 133 \(\upmu\)m vertical) [11]. [PERSON] et al. also found that PEEK frameworks produced directly by milling exhibited low discrepancies (17.36 \(\pm\) 4.32 \(\upmu\)m) and uniform color maps [18]. In addition, evaluating metal frameworks produced indirectly by milling and additive techniques, [PERSON] et al. found that subtractive techniques displayed fewer deviations and therefore greater trueness (150.1 \(\pm\) 20.5 \(\upmu\)m) than additive techniques (179.0 \(\pm\) 13.7 \(\upmu\)m) [39]. Other work conducted by [PERSON] et al. reported that RPD frameworks fabricated with SLM had a better trueness (RMS = 350 \(\upmu\)m) than frameworks produced using the conventional method [2]. In that study, the discrepancy levels were higher than those obtained in the current investigation for SLM-printed Co-Cr frameworks (\(-\)73.3 \(\pm\) 28.9 \(\upmu\)m for Class I and \(-\)33.3 \(\pm\) 101.9 \(\upmu\)m for Class III, modification 1). Although relevant insights have been provided, our study has some limitations that should be mentioned. They are related to the accuracy of the laboratory scanners used for image acquisition and the difficulties found during the scanning of the external and internal surfaces of the produced frameworks to capture all points of interest. The precision level achieved by the manufacturing equipment and the settings used in the CAM machine may have also influenced our results. The finishing and polishing procedures may have had an impact on the final quality of the frameworks that should not be neglected. Furthermore, since our work involved two materials and two production techniques, isolated comparisons regarding either the material or the manufacturing method _per se_ cannot be established. The decision of using 3D printing for Co-Cr manufacturing instead of a milling technique was supported by the high costs and rapid wear of cutting tools observed in the latter, which impair its routine use in clinical practice. As this was a pilot study, the reduced number of models and frameworks assessed constitutes a bias of the current work, precluding any conclusions regarding the reproducibility of the manufacturing systems used. Finally, the absence of a control group, which may include RPD frameworks produced by the corresponding conventional methods, is a drawback of this study. In future research, besides increasing the sample size, a digital fit accuracy assessment method should be considered in order to enable a more objective and straight forward analysis. Apart from improving the in vitro data collection, clinical research is mandatory to evaluate the oral performance of these RPD frameworks, focused on the effects on both supporting tissues and patients' comfort/satisfaction levels.