## 6 Discussion and Conclusions
In this case, cracks propagated mainly along the particle-matrix boundaries, resulting in a greater susceptibility to cracking and delamination of the material. The most complex failure mechanism was observed in the case of Graphy TC-80 DP, where brittle-plastic cracking dominated, as confirmed by the SEM analysis. Although this material showed the best long-term fatigue resistance, under extreme cyclic loading, there was a gradual propagation of microcracks that merged into larger cracks, leading to the complete failure of the specimen. The structure of the fractures showed ductile fracture characteristics, suggesting that the material could operate under varying loads for an extended period of time before reaching a critical point leading to its destruction. These observations align with previous research on conventional PMMA materials, which showed that microcracks in PMMA tend to propagate more rapidly due to their inherent brittleness. In contrast, fibre-reinforced PMMA composites demonstrated better crack resistance, as reported by the team from the University of Iowa [37]. This suggests that improvements in the internal structure of dental materials, such as optimized polymer matrices and filler integration, could further enhance the fatigue resistance of 3D-printed resins. The SEM microscopic observations revealed significant differences in the internal structure of the tested materials, which directly affect their behaviour under load. Graphy TC-80 DP is characterized by a homogeneous structure with no visible fillers, which explains its higher mechanical resistance. The NextDent Denture 3D and NextDent C&B MFH Bleach materials showed the presence of filler particles, which affected crack propagation in different ways. In the case of NextDent C&B MFH Bleach, the cracks propagated mainly along the matrix and particle boundaries, indicating a weak adhesive bond between the material components. In contrast, in the case of NextDent Denture 3D, the cracks propagated through the filler particles, indicating a more complex internal structure, but at the same time possibly leading to greater material brittleness. From the conducted research, the main conclusions worth pointing out were made as follows:  
* All the tested materials exhibit low thermal expansion and high dimensional stability under cyclic heating and cooling conditions. * The Graphy TC-80 DP material shows the best mechanical and fatigue properties among those tested, making it the most suitable for clinical applications requiring high cyclic load resistance. * The NextDent Denture 3D material shows the lowest fatigue resistance, which limits its use in more demanding clinical cases. It is recommended for use in situations with lower mechanical loads. The SEM results indicate that the internal structure of the materials and the way the filler particles are bonded to the matrix have a key impact on their mechanical properties. Material homogeneity and strong bonds between the filler and polymer matrix increase the strength and durability of restorations. An uneven structure or weak bonds can lead to defects and lower material quality. Compared to conventional PMMA and acrylic materials, the results suggest that 3D-printed resins offer superior fatigue performance and resistance to crack propagation. However, further studies are needed to fully compare their long-term clinical performance under actual oral conditions. Incorporating fibre reinforcement or modifying polymer compositions could further enhance their mechanical stability, as suggested by previous studies on reinforced PMMA composites. The results of the study are clinically relevant and provide valuable guidance for clinicians in selecting the appropriate material for temporary restorations. The findings indicate that the choice of material should consider both mechanical stability and resistance to temperature changes. Graphy TC-80 DP is recommended for more demanding cases, particularly in areas subjected to high masticatory loads or for patients awaiting long-term prosthetic solutions.