## 4 Discussion  
This study aimed to evaluate the fatigue performance of post-core and endocrown restoration types applied to different ferrule configurations by means of finite element analysis. As a result of the FEM, all of the configurations studied were found to affect the fatigue performance. As such, the initial hypothesis was rejected. This study demonstrated the effect of residual coronal structure on fatigue performance in the rehabilitation of maxillary central incisors following endodontic treatment. These data are supported by other studies [8; 24; 25; 26] and confirm the favorable effect of the ferrule effect on mechanical strength and prognosis. The existence of the ferrule effect is expected to increase resistance to lever loads and positively affect the structure by distributing stresses more uniformly [27]. As can be seen in Figure 1, the ECIR and PC0 groups, which had no ferrule effect, had the greatest loads on the restoration materials, while the most successful group was PC4, where all four walls were intact. However, the presence of even one coronal wall (PC1) is critical for fatigue performance (Figure 2), as shown in the study by [PERSON] et al. [28]. These situations are also supported by estimated fatigue performance cycles (Table 2). In addition, the results found for resin cement were similar. This may also indicate that in configurations where the ferrule effect cannot be created, decementation will occur faster. However, the Pmax value detected in resin cement for ECIR was relatively low. This may be because the stresses exerted by the internal vertical walls providing internal retention are transferred to the restoration rather than to the resin cement. Although PC4, PC3, and PC2 showed similar results when looking at post loads, PC1 and PC0 showed a rapid increase in Pmax values. There must therefore be at least two intact walls to ensure an ideal distribution of the load on the post. As the number of intact walls decreased, the incoming loads increased rapidly. This result is consistent with the work of [PERSON] et al. [26]. The Pmax values in dent in tissue were lower in post-core restorations. These findings also support those of other studies [12; 29; 30]. Endocrown restorations, a reconstruction option for endodontically treated teeth with extensive coronal damage, are particularly indicated for molars [13; 31]. In endocrowns, the elimination of the hybrid post/composite core component reduces the number of interfaces that need to be connected, thereby reducing the negative effects of hybrid layer disruption [13]. Endocrowns are also thicker  
Figure 9: The estimated fatigue performance cycles for the dentin; ECIR: no ferrule, endocrown; ECER: 2 mm circumferential ferrule, endocrown; PC4: 2 mm circumferential ferrule, post-core; PC3: 2 mm mesial, palatal, and distal ferrule, post-core; PC2: 2 mm mesial and distal ferrule, post–core; PC1: 2 mm distal ferrule, post–core; PC0: no ferrule, post–core; Pmax: maximum principal stress values; LS: lithium disilicate; PICN: polymer-infiltrated ceramic network. in the occlusal section and can also be expected to have higher fracture resistance than conventional crowns [32]. However, these findings apply to molar endocrowns. To simulate the clinical conditions as closely as possible, the lateral and axial movements of the tooth in the alveolar socket should be simulated [33]. This was attempted by adding the visco-hyperelastic structure of the PDL to the study [34]. When comparing restorative materials, LS showed a higher stress distribution in the restoration material, while the PICN showed higher values in all other groups tested. These results are in accordance with previous studies [35; 36]. This means that the LS material absorbs the incoming loads more in its structure while transmitting less to other parts. Reducing load transfer, particularly to the dentin and post, may be important to avoid catastrophic failure.