## 4 Discussion
In the present study, we found that the bonding value of Angelus MTA was higher than other studies in the literature with a value of 23.86 \(\pm\) 8.04 MPa, although it was similar to the study of [PERSON] et al. [34] despite the extraction force during the retreatment process. According to [PERSON] et al. [37], the average bond strength value of Biodentine was calculated as 17.55 \(\pm\) 5.67 MPa, and there was no significant difference between Angelus MTA. [PERSON] et al. [35] investigated the bonding of different tricalcium silicate-based materials, they stated that there was no significant difference between MTA and Biodentine bonding ability. But, a difference was found between the failure types of the materials. While MTA Angelus mostly showed mixed failure, Biodentine demonstrated mixed and cohesive failure at similar rates. The material properties of Biodentine, enhanced by the higher tricalcium silicate ratio than MTA and the high calcium ion release of Biodentine, led to mixed and cohesive failure [38]. However, in our study, cohesive failure was observed only in the Biodentine group, which was placed into the cavity by manual condensation and prepared with a tungsten carbide bur. [PERSON] et al. [22] reported that, although the average bonding value of Biodentine was higher, there was no significant difference with MTA. Similarly, in the present study, there was no significant difference between MTA and Biodentine when placed in cavities that prepared with diamond-coated ultrasonic retro-tips. During the retreatment procedures, it is necessary to reach the apical area of the root canal. Once apical access has been achieved, all principles of endodontic treatment had to be performed. The coronal access must be completed, all previous root filling material must be removed, canal obstructions must be managed, and they must be achieved to their full working length [39]. [PERSON] et al. [15] suggested that orthograde retreatment combined with orthograde placement of an apical mineral trioxide aggregate plug is a promising long-term treatment option for teeth with postsurgical failure. In our study, 100% of apical access was achieved in MTA and Biodentine, which were placed into the cavity by manual condensation and prepared with a tungsten carbide bur. On the other hand, apical access was achieved at a rate of 100% and 70% for MTA and Biodentine placed by indirect ultrasonic vibration in the cavities prepared with tungsten carbide burs, respectively. When looking at the cavities prepared with ultrasonic retro-tips, apical access was achieved in 60% of MTA and 20% of CIS placed with manual condensation, whereas no apical access was achieved in the Biodentine group. The MTA group that placed the indirect ultrasound vibration achieved 40% apical access and 20% apical access in CIS, whereas no apical access was achieved in the Biodentine group. Biodentine showed higher bond strength than all other groups in cavities prepared with ultrasonic retro-tips, regardless of whether they were placed by manual condensation or indirect ultrasonic vibration, but no significant difference was detected. Additionally, it should be considered that other factors could influence bond strength, such as the presence of debris or substrate contamination. Different cleaning protocols were evaluated for removing sealer residues on the dentin surface [40]. Blood contamination during the bonding procedure of conventional and hydrophilic primers significantly lowers their bond strength values [41]. These protocols would be applied in future studies. [PERSON] et al. [42] evaluated the clinical outcome of orthograde endodontic retreatment after failed apicectomy. They stated that, even teeth with pre-existing retrograde fillings can be treated successfully by an orthograde retreatment, as the mean success rate for these cases did not differ significantly from the mean success rate obtained for cases without any preexisting retrograde filling.