## 4 Discussion
### Mechanical Properties
It has also been reported that the hardness values of 3D-printed resins are lower than those of thermopolymerized PMMA resin, which was also the case when ZrO\({}_{2}\) NPs were incorporated [30]. When zwitterionic materials are added, the hardness also decreases as high doses deteriorate the mechanical properties [19]. PMMA resins with graphene also exhibited lower hardness than the other groups, as some studies have already shown that PMMA-based materials have lower hardness values than other materials investigated in this study [26]. When Ag-TNP were incorporated into urethane acrylate-based resins, the hardness increased with the increase in nanoparticle concentration, except when the concentration was above 1%, as there were problems with printing in this case. When zirconia was incorporated, the hardness increased [31], as did nanodiamonds, which showed a low wear rate compared to the control group [36; 37]. The flexural strength of a material is related to the flexibility of the material before it reaches its limit. The bending forces to which the materials are subjected are related to the forces that occur in clinical situations, i.e., the ability of the materials to withstand bending and torsion. Therefore, when dental materials are subjected to permanent deformation, it is important that they have high flexural strength [41]. When analyzing the articles included in this systematic review, it was found that there were differences in the values of this parameter due to the different compounds contained in the resins. In most of the articles, the flexural strength decreased when the compounds were incorporated into the resins. When graphene nanoplatelets were added, the flexural strength was found to decrease at concentrations above 0.1%. This has also been shown in other articles [33], as when the graphene concentration is above 0.1%, zones of force concentration are created, which affect flexural strength and ultimate strength [14]. Thus, it was described that at lower graphene concentrations, such as 0.01% graphene, the flexural strength of the resin becomes more similar to that of the resin without incorporation [33]. The same occurred when mesoporous silica nanoparticles reinforced with silver were incorporated into the acrylic resin: As the concentration increased, the flexural strength decreased because clusters of these compounds formed in the polymer network. Although the flexural strength of the resin with nanoparticles was impaired, the flexural strength value corresponded to that for dental restorations (ISO 20795-1) [15]. In terms of fracture strength, the resins with this compound showed higher values than the control group, as the nanoparticles impaired the propagation of cracks and fissures in the resin [15]. In the case of zwitterionic materials in resins, some studies have already reported that their high dose in dental materials may negatively affect the mechanical properties, which is related to the gelation process of zwitterionic materials in high concentrations, which affects the polymerization process of the resin. However, it is important to note that the results presented in this article are in accordance with ISO 20795-1 [19]. For the zirconia nanoparticles (ZrO\({}_{2}\)-NP), the increase in flexural strength in the ASIGA resinsdepends on the concentration of ZrO\({}_{2}\)-NP, as this parameter also increases with increasing concentration [42]. In the NextDent resins, however, the flexural strength is higher in the group with 1% ZrO\({}_{2}\)-NP, while it decreases in the other groups, so that the difference in the flexural strength values could be due to the different compositions of the resins tested. This article also described that the 3D-printed resins fulfill the ISO recommendations (65 MPa) as the minimum value for the flexural strength of this type of material [30].