## 2 Materials and Methods  
Two study models of mandibular [PERSON]'s Class I and Class III, modification 1 edentulous arches (Edentulous Study models Kavo, Kavo(r), Berlin, Germany) were used. In [PERSON]'s Class I model, rest seats were prepared in the mesial occlusal surface of tooth #44 and in the cingulum of teeth #43 and #33 (Figure 1a), whereas in the Class III modification 1 model, rest seats were prepared in the mesial occlusal surface of tooth #37, the distal occlusal surface of tooth #35, the mesial occlusal surface of tooth #46 and in the cingulum of tooth #43 (Figure 1b). The prepared models were scanned with a laboratory scanner (Medit T500, Medit(r), Seoul, Republic of Korea), and the PartialCAD software basic version (Partial Planner, Zirkonzahn(r), Gais, Italy) was used to analyze the insertion axis, eliminate the unwanted undercuts, and create the digital design of the frameworks (Figure 2). The STL files were saved and exported to the CAM equipment to begin the manufacturing process. For each study model, two frameworks were produced with CAD-CAM: (i) one in Co-Cr (Figure 3), using a Co-Cr alloy and an SLM machine (EOS GmbH, Phibo(r), Barcelona, Spain), followed by a heat treatment to improve the mechanical properties; and (ii) the other in PEEK (Figure 4), by direct milling (M5, Zirkonzahn(r), Gais, Italy) of a PEEK disc (Zirkonzahn Tecno Med 95H20; Ref. : TMAH0120--Lot:: 15922). The Co-Cr frameworks were finished with drills and rubbers, immersed in an electrolyte bath, and polished with brushes and paste. The PEEK frameworks were finished using drills to remove the supports and then submitted to rubber polishing. Figure 1: Mandibular study models with rest seat preparations: (**a**) Kennedy Class I; (**b**) Kennedy Class III, modification 1. Each framework was correctly placed on the corresponding study model, and a qualitative fit accuracy assessment method was applied. The method was based on a direct inspection of the gap between the framework components and the model surface, using image amplification and calibrated endodontic instruments that explore the entire peripheral area of the component. The framework components used as measuring sites are illustrated in Figure 5.  
Figure 4: Milled PEEK frameworks: (**a**) Kennedy Class I; (**b**) Kennedy Class III, modification 1. Figure 3: SLM-printed Co-Cr frameworks: (**a**) Kennedy Class I; (**b**) Kennedy Class III, modification 1. Figure 2: CAD design of the prosthetic frameworks: (**a**) Kennedy Class I framework design; (**b**) Kennedy Class III, modification 1, framework design. The fit assessment of the major connector was performed using a Kerr file size 50, whose tip has a 500 \(\upmu\)m diameter. An endodontic plugger size 35, with a tip diameter of 350 \(\upmu\)m, was used to assess the remaining framework components (cingular and occlusal rests and minor connectors). Misfitting was considered in the presence of a gap equal to or higher than 500 \(\upmu\)m or 350 \(\upmu\)m, respectively. To complement this analysis, whenever a misfit higher than the tip diameter of the instrument was identified, a Williams periodontal probe was used to confirm whether the gap was equal to or higher than 1000 \(\upmu\)m.  
Two types of image amplification were used for all measurements--magnifying loupes (1.8\(\times\)) (Lorben 1.8\(\times\) head magnifier) and a stereoscopic microscope (20\(\times\)) (Olympus SZ-ET, Olympus(r), Tokyo, Japan). A digital quantitative assessment of trueness through putative discrepancies between the manufactured frameworks and the original standard tessellation language (STL) files was carried out using the Geomagic software (Geomagic Control X version 2018, 3D Systems Inc., USA). For that purpose, the PEEK and Co-Cr frameworks were scanned using the S600 Arti laboratory scanner (S600 Arti, Zirkonzahn, Gais, Italy) in order to obtain the polygonal mesh of the produced frameworks in STL files (CAD framework produced, CFP).