## 4 Discussion
However, the deformation and rupture of the elastomeric material during its manipulation, coupled with the difficulty of measuring its thickness due to its elasticity, are important disadvantages of this approach [15; 29; 40]. The current study used a qualitative analog method based on direct inspection of the gap between the framework components and the model surface, using image amplification and calibrated endodontic instruments (endodontic plugger #35 and file #50). A qualitative assessment using calibrated endodontic instruments was previously described by [PERSON] et al. [33]. A similar method based on using a calibrated orthodontic wire of 500 \(\upmu\)m has already been described by other authors [32]. The choice of calibrated instruments was supported by previous research by [PERSON] et al. [30], who classified a gap between 0 and 50 \(\upmu\)m as intimate contact between the structures, and a space ranging from 50 to 311 \(\upmu\)m as clinically acceptable [7; 30]. In line with this information, an endodontic plugger with a tip diameter of 350 \(\upmu\)m was used to assess the occlusal and cingular rests, and minor connectors, while an endodontic file #50 (tip diameter of 500 \(\upmu\)m) was used to assess the major connectors, for which a relief of 400 \(\upmu\)m was automatically established by the CAD software. Image amplification coupled with the calibrated endodontic instruments was a strategy to provide more accurate and reliable assessment than simple visual inspection. Interestingly, the results obtained with the two image magnification modalities--magnifying loupes and a stereoscopic microscope--were coincident in all the sites assessed, suggesting that magnifying loupes, often used in daily clinical practice, allow the identification of the same misfits as more expensive equipment. In contrast with other authors who restricted the points evaluated to the RPD rests [11; 31; 35], our qualitative assessment involved almost all the RPD components (occlusion and cingular rests, minor and major connectors) as an attempt to gain further insight into the overall fitting of the framework. Although this analog method only identifies gaps at the periphery of a specific component, not considering the intaglio surface, it enables a feasible, simple and low-cost fit assessment of a framework, which may be routinely carried out in clinical or laboratory (quality control) contexts. The ability of a technique to result in the manufacture of objects with minimal differences from the original design represents the trueness [34]. Discrepancies between the structure produced and the original digital design can result in gaps between the prosthetic framework and the supporting structures, or excess material. Both situations may compromise the fitting and clinical success of the RPD. It is important to clarify that accuracy consists of trueness (proximity of measurement results to the true value) and precision (repeatability or reproducibility) [41]. Through visual analysis of the color mapping generated by Geomagic superimpositions, it is possible to determine the direction of the displacement and understand the type of distortion observed [42]. In the current study, it was evident that the PEEK frameworks exhibited more green areas (high trueness) and increased color uniformity than the Co-Cr ones for both Kennedy classes, but especially in the Class I dedentulous condition. Digital superimposition also enabled a quantitative assessment of the trueness. This study defined trueness as the discrepancy between the frameworks produced and the original CAD design, which was expressed as RMS. This value indicates how far the discrepancies between the files are from zero, with a value of zero indicating a perfect match between the files. Thus, a high correspondence between the superimposed files depicts high trueness and occurs when low RMS values are recorded [42; 43; 44].