# Evaluation of the Impact of Various Functional Fillers on Key Properties of Dental Composites
## 4 Discussion  
The null hypothesis of this study was rejected, as the inclusion of hydroxyapatite, calcium fluoride, and nanosilver as fillers in dental composites was found to have a significant impact on their mechanical properties across all evaluated cases. The differences observed were statistically significant. Hardness is a crucial property for composite materials, as it reflects the material's resistance to deformation. It is frequently used as an auxiliary parameter to assess mechanical performance. Based on the hardness measurements obtained in this study, it can be concluded that the incorporation of hydroxyapatite, calcium fluoride, and nanosilver fillers increased the hardness of the flowable composites (89 ShD). This increase in hardness is consistent with findings from another study [43], which examined a composite based on urethane dimethacrylate (UDMA), achieving a hardness value of 85 ShD. Other studies by [PERSON] [34] and [PERSON] [44] also confirm the enhancement of mechanical properties, likely due to the strengthening effect of fluoride ions on hydroxyapatite. A key characteristic of hydroxyapatite is its ability to exchange ions, including its interaction with fluoride [44,45]. When hydroxide ions are replaced by fluoride, fluorapatite is formed. According to [PERSON], [PERSON] et al. [46], fluorapatite exhibits superior crystallinity, hardness, and resistance to chemical and physical damage, and are less soluble. As confirmation of this statement, it was observed that composite materials containing fluoride fillers exhibited lower hardness values compared to composites that also included hydroxyapatite. Furthermore, composite samples containing nanosilver without hydroxyapatite or fluoride showed a significant decrease in hardness (54.8 ShD). This reduction in hardness may be attributed to the polymerization process. [PERSON] and colleagues [47] suggest that silver nanoparticles may influence the polymerization process in dental materials, leading  
Figure 7: Box plots of tribological wear (10\({}^{-4}\) mm\({}^{3}\)/Nm) for modified flowable composite resins (\(n\) = 5) (\(\circ\), \(\arcdeg\) symbols represent outliers). to an increase in the amount of elutable substances. This process can be attributed to the interaction between silver nanoparticles and various physical and chemical mechanisms. One possible explanation is the reflection and scattering of light from the curing lamp on the surface of the nanoparticles. Chemically, electrons from the metallic silver nanoparticles may react with unpaired electrons in the photoinitiator system. Silver nanoparticles can transfer an electron to the photoinitiator system, accept it, and then store the electrons [47; 48]. This effect may hinder the penetration of light into the deeper layers of the composite material. To further investigate this and gain a deeper understanding, future studies should focus on examining the impact of individual fillers on the polymerization process. In order to assess the materials' resistance to cracking under dynamic loading, an impact test was performed. The results revealed that incorporating fillers such as hydroxyapatite, nanosilver, and calcium fluoride led to a decrease in the impact strength of the material (0.095 J/cm\({}^{2}\)), which was proportional to the increasing hydroxyapatite content. A similar reduction in impact strength relative to HAp content was observed in previous research [47], where a different composite based on urethane dimethacrylate resin was used. However, when a hybrid composite containing 2% hydroxyapatite with fluoride and nanosilver was tested (0.128 J/cm\({}^{2}\)), a higher impact strength was recorded compared to the control sample (0.072 J/cm\({}^{2}\)). Given that specific occlusal forces are present within the oral cavity, amounting to approximately 100-150 N in the incisor region [16; 49], it is crucial to analyze the forces acting on teeth and dental fillings.