## 4 Discussion  
In our study, the fillings in retrograde cavities prepared with ultrasonic retro tips were found to have significantly higher bond strength after retreatment compared to prepared with tungsten carbide bur, thus confirming the first hypothesis; however, no difference could be found between cements after retreatment and the second hypothesis was not confirmed. The cross-sectional plane should be considered during root resection. Ideally, the inclination as perpendicular as possible to the long axis of the tooth (0\({}^{\circ}\)) preserves the root length and exposes fewer dental tubules, thus reducing the number of tubules exposed to the periphery, resulting in less microleakage. In our study, a (0)\({}^{\circ}\) level was given for the same reasons [23]. The marginal adaptation has been identified as the proximity degree and the clamping of a filling material to the cavity wall [24]. It is possible that the failure of marginal adaptation of the material and the presence of cracks or gaps at the interface between the material and the dentin wall may lead to apical leakage, resulting in treatment failure [25]. [PERSON] et al. [26] demonstrated that there is no significant difference between preparation with ultrasonic tips or micro-handpiece preparations in the sealing of retrograde fillings. The ideal root-end filling should prevent the exit of any bacteria, bacterial products, or toxic materials into the surrounding periradicular tissues while sealing the contents of the root canal system within the canal [2]. [PERSON] et al. [27] investigated persistent cases of periapical lesions after endodontic surgery. Due to defective sealing at the interface between the root-end filling and the cavity, the failure of apical surgery occurred. Such a gap would support a continuous leakage of bacteria from the infected root canal system into the periapical tissue, thus sustaining inflammation. This results in the reoccurrence of periapical disease after endodontic surgery. Nevertheless, there are limited studies for retreatment after endodontic surgery [15; 16]. Retrograde filling is not available in these studies. The fact that the bond strength values of the ultrasonic cavity group were significantly (\(p<0.000\)) higher than the tungsten carbide cavity group regardless of retrograde filling material is important. This study was inspired by the ability to prepare smaller, centralized and desired depth cavities as shown in the study of [PERSON] et al. [28]. Figure 6: Fracture types in cavities prepared with ultrasonic retro tips. CIS: Glass ionomer cement, MC: Manual compaction, BIO: Biodentin, IJV: Indirect ultrasonic vibration. The popularity of bioceramic-based retrograde filling materials has recently increased [4,29]. It was demonstrated that MTA has superior properties in terms of sealing ability, biocompatibility, and periradicular tissue regeneration when compared to conventional retrograde filling materials [30]. Among some studies in which the average bonding value of MTA in retrograde cavities prepared by traditional methods were examined, [PERSON] et al. [31] reported 5.08 \(\pm\) 2.41 MPa, [PERSON] et al. [32] calculated 5.2 \(\pm\) 0.4 MPa after a curing time of 24 h, 9.0 \(\pm\) 0.9 MPa after curing for 7 days, and [PERSON] et al. [33] calculated 4.5 \(\pm\) 1.5 MPa. Despite the removal force during the retreatment process, the bonding value of MTA placed in the traditional retrograde cavity prepared with a tungsten carbide bur was calculated as 4.35 \(\pm\) 3.68 MPa in our study, similar to other studies in the literature. Several studies have investigated filling bonding in the retrograde cavity using diamond-coated ultrasonic retro tips. [PERSON] et al. [34] calculated the bond strength of MTA as 19.18 \(\pm\) 4.7 MPa, [PERSON] et al. [35] calculated the bond strength of Angelus MTA as 1.82 \(\pm\) 0.47 MPa, and [PERSON] et al. [36] calculated the bond strength of MTA as 7.77 \(\pm\) 1.34 MPa.