## 4 Discussion  
Endodontic treatment aims to create 3D obturation with adequate lateral and apical seals [24] to discourage bacterial growth, which can compromise the treatment outcome [3]. In order to form a monoblock, which is defined as a gap-free single-unit obturation, the sealer is necessary to bind and/or adhere to the dentin from one side and to the gutta  
Figure 4: Scanning electron microscopy photographs (at 2000\(\times\) magnification) for root canal cross-sections obturated with BC, AHP-Bio and AHP root canal sealers in the dry condition (**A–C**) and after immersion in serum (**D–F**), showing the status of the sealer layer (s), sealer/dentin interface (S/D, indicated by white arrows) and sealer/gutta-percha interface (S/GP, indicated by orange arrows). The thin arrows indicate close contact areas, and the wide arrows indicate gaps/voids. The letter “d” mentioned to dentin, letter “s” mentioned to sealer, and letter “gp” mentioned to gutta-percha. percha core from the other side [25]. It has been proposed that a mechanical interlocking force increases the sealer's adherence ability to radicular dentin [26]. The push-out test is the most common technique for assessing how well the sealer adheres to radicular dentin [7; 27; 28]. The push-out test is the most effective method for determining bond strength, which is the force needed to separate the adhesive filling material from the dentin [26]. The current study evaluated the push-out bond strength and thus the adhesion of the recently developed resin-modified bioceramic sealer AHP-Bio compared with the gold standard epoxy resin (AHP) and traditional bioceramic (BC) root canal sealers. While some studies have evaluated the push-out bond strength of a sealer when the root canal is filled with the sealer only [29; 30], most studies obturate the root canal with sealer and gutta-percha cones. In the current study, obturation with sealer and gutta-percha cones was applied, as that best simulates the clinical situation [28; 31; 32]. In the current study, the samples were selected from the middle third of obtutated root canals with a nearly identical diameter for standardization, as described in a previous study [20]. Furthermore, [PERSON] et al. [17] confirmed this selection, as the significant difference in push-out bond strength between bioceramic and AH Plus was only detected in the middle third of the root canal. So, this region is considered a critical area for evaluating the push-out bond strength of root canal sealers. The current study tried to determine the predictable reason for push-out bond strength using failure mode analysis examined by stereo microscopy and sealer/dentin adaptation using SEM. The failure mode analysis evaluates the definitive area of failure either at the sealer/dentin or sealer/gutta percha interface, or at the sealer itself. Meanwhile, SEM evaluates the predictable reason for failure either due to the sealer's failure to penetrate into dental tubules, adherence failure between the sealer and gutta-percha, or the presence of voids/gaps in the sealer layer itself. In the dry condition, the present findings show that the lowest bond strength values were recorded by AHP-Bio (1.07 \(\pm\) 0.36 MPa), while the greatest values were obtained by BC (1.49 \(\pm\) 0.45 MPa), with no significant difference between the three sealers (\(p\) > 0.05). This finding may be attributed to the percent of calcium silicate content. BC sealers are composed mainly of calcium silicate, while AHP-Bio is composed of calcium silicate and resin, as its content of calcium silicate is lower than that of BC. It was reported that the calcium silicate content of the sealer was responsible for a micromechanical interaction between the sealer and the dentin wall, and promoted a chemical interaction producing a mineral infiltration zone [20]. Echoing this finding, [PERSON] et al. [28] previously determined a greater push-out bond strength for BC than AHP.