## 4 Discussion
In the literature, there are many studies evaluating the effect of different root dentin regions on the bond strength of posts to root dentin, but there is no consensus on which root region has a higher bond strength [16; 53; 54]. It was thought that the fact that the fiber post we used in our study showed a parallel structure in the coronal and middle segments and a conical structure narrowing towards the tip in the apical segment would negatively affect the maximum force applied in the apical segment of the post in the push-out test. At the same time, it was thought that this situation would negatively affect the standardization of the push-out bond strength test. In order to minimize these factors, the sections obtained from the coronal and middle thirds of the root were used in our study, as previously used in different studies [55; 56]. Some studies have suggested that the bond strength of posts is higher in the apical region than in other parts of the root canal [57; 58]. There are also studies reporting a decrease in bond values from the coronal region to the apical region [21; 22; 32; 48]. There are also studies that show the region has no effect on the bond strength of the post [16; 59]. In our study, the middle section showed significantly higher bond strength in the Panavia V5 group. These data were found to be compatible with the bond strength results of self-etch resin cement (Panavia F 2.0) material in a similar study [60]. In another study, [PERSON] et al. [32] found that the bond strength of self-etch resin cement material (Panavia F 2.0) was significantly higher in the cervical region than in the middle region for post cavities irrigated with NaOCl + EDTA and (NaOCl + EDTA) PUI. The better bond strength in the cervical segment has been explained by different theories. These theories include the effect of the micromechanical bonding mechanism with the decrease in tubule density and tubule diameter in the apical direction [61], incomplete polymerization in deeper regions of the root [62], and easier and more effective application of adhesive materials with ease of access in the coronal region [60]. In our study for the RelyX U200 group, there was no significant difference in bond strength between the sections. However, the middle group showed higher values. These data were consistent with the results of similar studies in which RelyX U200 cement was previously tested [16; 59]. In another study using RelyX U200 in post cement, the bond strength values were higher in the cervical than in the middle segment for NaOCl + EDTA-treated and PUI-treated post cavities [32]. In our study, the high bond strength values observed in the middle section may be attributed to the differences in the number, volume, and orientation of the tubules in the root dentin [63; 64]. In addition, the canal paste and gutta-percha that cannot be removed from the post cavity walls may negatively affect this value [65]. Different bonding mechanisms (micromechanical or chemical), depending on the cement material applied, may also play a role in this difference [66]. Another reason may be the differences in light transfer along the fiber post and the polymerization in the middle third of the root may have increased with sufficient light transfer [67]. It has also been reported that another reason may be that the post is better adapted in the middle root third, and the lower cement thickness in this region may reduce polymerization shrinkage [67]. One of the reasons for the measured lower bonding value at the coronal section is the increase in the C-factor. The C-factor is defined as the ratio of bonded surfaces to unbonded surfaces and is one of the most important factors that influence the success of post-endodontic restorations due to the structure of the root canal system. It can be said that restorations with a higher than three-to-one C-factor ratio may have a low chance of success [68]. The fiber post system we used in our study has the same diameter along its length in the coronal and middle regions.