## 4 Discussion
### Effect of Printing Orientation on Mechanical Properties  
Among different parameters, the printing orientation affects the mechanical performance of PEEK [29; 41; 46]. [PERSON] et al. [3] investigated the influence of printing orientation on the mechanical properties of PEEK/HA composite scaffolds fabricated via MEX 3D printing technology. For the mechanical testing, scaffolds were compressed parallel and perpendicular to the printing orientation. They concluded that samples represented higher Young's modulus and lower compressive strength along the \(Z\) printing orientation [3]. The results of this study were not comparable to ours, as the samples were built not only with PEEK; HA was also included. Fabricated samples were also cubes with different pore sizes and underwent compression testing. In contrast, our samples were printed with a theoretic infill percentage of 100% in dog bone shape and underwent tensile testing.  
[PERSON] et al. [29] investigated PEEK samples printed in different orientations via MEX technology. They concluded that vertically printed samples had a poorer mechanical performance [29]. [PERSON] et al. [47] also mentioned that they had the lowest print void and best mechanical performance in horizontally printed samples out of PEEK material, which were dried for 48 h at 140 \({}^{\circ}\)C before printing. Even though the drying parameters of PEEK filaments were not similar to our study, results of mechanical test were in accordance with the results of our study, as we had no fractures in groups 1 and 2, with maximum tensile strengths of 91.613 MPa and 101.82 MPa, respectively; fractures were seen in group 3 with a tensile strength of 89.748 MPa.  
The effect of the printing path on the mechanical properties of 3D-printed PEEK was also investigated by [PERSON] et al. [48], in a study which presented similar results to ours. They mentioned that printed samples along horizontal 0\({}^{\circ}\) exhibited the best mechanical properties and showed plastic deformation during tensile testing, which was loaded parallel to the printing orientation. Samples printed in this orientation did not completely rebond themselves, which may have affected the mechanical strength. Other samples printed in two different orientation paths, including horizontal 90\({}^{\circ}\) and vertical 90\({}^{\circ}\), were loaded perpendicular to the printing orientation. They showed flat fracture morphology, which may have shown tearing at the interlayer interface [48]. Some other studies also reported that the orientation of printing had an essential impact on the tensile performance of 3D-printed PEEK samples. Thus, by printing in the horizontal orientation, we could achieve near bulk mechanical properties [9; 29; 46; 49; 50; 51].  
In the current study, pairwise comparisons of the tensile strength values of subgroups 3.1 to 3.8 showed a significant difference between subgroups 3.6 and 3.8; the means of the tensile modulus in groups 1, 2, and 3 were not significantly different (\(p\)-value = 0.063). This implied that the elastic behavior was the same, but differing durability could be seen due to voids generated during the milling process of creating dog bones out of printed towers or voids between layers during the printing process.