## 4 Discussion
Defining an optimal polishing technique for PEEK is important because, regardless of the print layer thickness, mechanical finishing decreases surface roughness and can promote smooth and soft surfaces. The maximum roughness of hard surfaces within the oral cavity after polishing, as indicated by several authors [23; 24; 39; 40; 41; 42], should not exceed 2 um, which represents the threshold for bacterial plaque accumulation. Nevertheless, alternative studies suggest that a clinically acceptable limit could be set at 10 um [40; 43]. Some authors state that assessing roughness using a contact-type roughness measuring device, such as the one used in this work, is not ideal due to the risk of damaging the sample surface [21]. However, [PERSON] et al. [44] chose to evaluate roughness (Ra) using this method because the color of the PEEK samples causes inadequate light reflection, making the use of optical interferometry difficult. These authors considered this method effective due to promoting direct contact between the measuring tip and the sample surface. In the present study, the contact-type roughness measuring device was also used because the same difficulty was encountered when reading PEEK using optical interferometry. Other authors employ different approaches because there is no universal method for treating PEEK's surface. Several investigations report different values when polishing milled PEEK with SiC sandpaper, namely, [PERSON] et al. [45] (P500 to P2400) found Ra = 0.04 um, [PERSON] et al. [45] (P1200) Ra = 0.53 um, and [PERSON] et al. [45] (P600 and P800) Ra = 1.11 um. A direct comparison cannot be made since no publication was found describing the same sequence of SiC sandpapers used in this work (P80, P180, P320, P800, P1200, and P4000) for FDM PEEK. Even so, given that lower roughness values were obtained with the method used (Ra between 0.04 um and 0.11 um), it is suggested that polishing FDM PEEK with SiC sandpaper of different grain sizes could be an effective technique. [PERSON] et al. [18] conducted a study where the surface morphology of printed PEEK samples was determined. The authors evaluated both samples with three different surface treatments: (1) no polishing, (2) mechanical polishing (SiC sandpaper), and (3) sandblasting (120 um alumina). As expected, the untreated specimens had the roughest surfaces (Sa = 17.67 um), and the polished specimens had the smoothest surfaces (Sa = 0.42 um). The same trend was observed in the present study, as the samples with and without polishing differed in surface roughness. However, although the initial roughness values of Group A agree with the results of [PERSON] et al. [18] (Sa before polishing in Group A = 17.65 um), the initial roughness of the Group B samples was much lower (Sa before polishing in Group B = 6.51 um). After polishing, both groups had lower roughness than that reported by those authors (final Sa of Group A = 0.17 um and final Sa of Group B = 0.09 um). Thus, the polishing technique used in the present work seems to be more effective, since the specimens from Group A, which had an initial roughness close to that described by [PERSON] et al. [18], showed a lower roughness after polishing than that reported by those authors. The same was verified in the work of [PERSON] et al. [21], which also analyzed the surface roughness of FDM PEEK specimens. They obtained Ra values of 0.61 to 0.66 um before polishing and Ra values of 0.10 to 0.15 um after polishing the specimens. The present study obtained lower roughness results after polishing (Ra = 0.04 and 0.11 um), probably due to using a different polishing technique, although this cannot be confirmed as [PERSON] et al. [21] did not mention the polishing protocol used. Thus, it can be assumed that the polishing method employed, involving P80, P180, P320, P800, P1200, and P4000 SiC sandpapers, seems to be suitable for rendering FDM PEEK clinically usable. [PERSON] et al. [23] state that humans can detect roughness values of at least 0.5 um.