## 4 Discussion
Conversely, some studies have found that the bond strength of AHP is greater than that of the BC sealer [20; 30], with no significant difference between them [17]. Volumetric changes during sealer setting could affect the sealer's bond to dentin and/or gutta-percha. It has been suggested that BC sealers exhibit slight expansion during setting [17], which promotes good adaptation to dentin, while AHP undergoes a chemical bond to dentin collagen fibers [33; 34], with low polymerization shrinkage [35; 36] at the AHP/GP interface, creating a gap and adhesive failure at this interface. It is possible that the adhesion of AHP to dentin is stronger than its adhesion to gutta-percha, which could explain our results of 80% adhesive failure at the AHP/GP interface. The current study's finding of a lower bond strength for AHP-Bio, with 70% cohesive failure, may be attributed to its porous structure, as described by [PERSON] et al. [37], which may result in a fragile material. Different categorization systems for the failure modes of root canal filling materials have been used in previous studies. The most commonly used system includes three categories: cohesive failure of the sealer, adhesive failure at the S/D interface and mixed cohesive/adhesive failure [5; 35; 38]. In another study, a five-category system was used, adding adhesive failure at the S/GP interface and mixed adhesive failure (at both the S/D and S/GP interfaces) [27; 39]. However, the current study used a system of four categories, excluding mixed adhesive failure, as that type was limited to only one case of BC sealer and could have affected the statistical analysis. In the dry condition, the predominant failure modes of both bioceramic (BC and AHP-Bio) sealers were cohesive failure and mixed failure, with greater cohesive failure values in AHP-Bio (70%). In support of these findings, the iRoot Bioceramic sealer has previously displayed greater cohesive failure within the sealer layer [35]. This result may be attributed to the nature of the material's particle size and/or the presence of voids within the sealer layer. BC has a small particle size, allowing the particles to coalesce with each other and contain fewer voids [9; 40]. Interestingly, AHP-Bio was found to have a matrix of elongated and rounded larger particles containing more pores between them [37]. Furthermore, it has been suggested that the large particle ratio within the sealer layer may yield a weak bond [29]. Adding to these previous findings, the current study's SEM images revealed voids within both the BC and AHP-Bio sealers. The large porous structure of the AHP-Bio sealer could yield a fragile material and be responsible for the lower bond strength and 70% cohesive failure. No adhesive failure at the AHP-Bio/D interface was detected. This may be attributed to the presence of dimethyl sulfoxide, which promotes good adhesion to dentin [41]. SEM images of the dry condition revealed close contact at the BC/D interface in most cases. However, there were varying degrees of gaps in certain areas of the S/D interfaces of all groups, particularly in certain areas of the BC samples (14.2-28.4 \(\upmu\)m). Both AHP and AHP-Bio exhibited thin gaps at the S/D interface, ranging from 1.64 to 3.28 and from 1.59 to 5.31 \(\upmu\)m, respectively. Voids were also detected within the AHP and AHP-Bio sealer layers. The great voids detected in BC may be attributed to the setting of the BC sealer in the dry condition, as a poor crystalline porous structure has been previously found for this type of sealer [9]. In the BC group, due to the presence of voids both within the sealer layer and in some areas of the BC/D interface, both cohesive failure (40%) and adhesive failure (30%) were detected at the S/D interface. The voids detected in all sealer groups may be attributed to the matched cone technique used in this study. Similarly, [PERSON] et al. [42] found voids within the obstruction when using the single-cone technique with both BC and AHP.