## 4 Discussion
### Mechanical Properties
When zirconia was incorporated into the resin, the printing direction affected the flexural strength values, as in the 0\({}^{\circ}\) printing direction, the control group showed a higher flexural strength, while in the 90\({}^{\circ}\) printing direction, the group with 3% zirconia showed the highest value. This study showed that the flexural strength value of the resin with 3% zirconia printed at 90\({}^{\circ}\) was in line with the ISO10477 recommendation [31]. The introduction of bioactive glasses reduces flexural strength and fracture resistance [35]. When aminated nanodiamonds were incorporated into the resin, their incorporation was found to increase flexural strength, as it has already been described in several studies that their addition increases mechanical strength due to their strong covalent bonds and the fact that they are evenly distributed in the polymer [37]. When investigating the incorporation of graphene into PMMA resins, it was found that nano-ceramic resins (LU) and lithium disilicate ceramics (EX) had higher flexural strength values than the other materials, namely polyurethane resins. PMMA, which contained graphene. In the LU group, the higher flexural strength was due to the formation of zirconia/silica agglomerates; in the EX group, the high flexural strength was related to the homogeneity of the crystals in the ceramic. It was also described that further clinical studies on flexural strength after the incorporation of graphene into PMMA resins are needed [26]. Finally, the flexural strength of titanium dioxide and PEEK nanoparticles and silver-reinforced titanium dioxide nanoparticles in acrylic resins was also investigated. For the silver-reinforced titanium dioxide nanoparticles, the flexural strength was only impaired when the concentration of this compound was greater than 1% because agglomerates formed, resulting in poor dispersion of this compound in the resin matrix [38]. In the case of titanium dioxide and PEEK nanoparticles, their incorporation into the PMMA resin leads to an increase in flexural strength compared to the control group. However, in group 2 (TiO\({}_{2}\)-1%-PEEK-0%), problems occurred during the printing process as agglomerates of TiO\({}_{2}\) nanoparticles formed, especially in the polymerization phase of the resin, which affected the flexural strength value and impaired the polymerization phase of the resin [13]. It was also reported that the incorporation of PEEK reduced the agglomeration of the nanoparticles and thus improved mechanical properties [16]. When cellulose nanocrystals reinforced with silver were incorporated, it was found that their concentration was a critical factor because, although the nanocrystals were well incorporated into the resin matrix, their agglomeration on the surface could affect the flexural strength [17]. The accidental fall of dentures is one of the most common causes of fractures. It is therefore important that dental materials have good impact resistance [30]. The impact strength of resins with zirconia nanoparticles has increased because they are evenly distributed in the acrylic resin matrix. However, as with flexural strength, there were also differences in impact strength between the two resin types. The NextDent resin showed an increase in impact strength, while the ASIGA resin did not, which could be related to the printing process [30]. When titanium dioxide and PEEK nanoparticles were incorporated into the resins, it was found that the groups with higher concentrations of these compounds achieved higher impact strength values than the control group. These results could be related to the fact that the compounds incorporated into the resin matrix are able to absorb large amounts of fracture force and prevent the absorption of energy through the destructive cracks present in the material. However, to observe this behavior, it is important that the nanoparticles do not aggregate in the polymer matrix [16].