## 4 Discussion
According to [PERSON] et al., 9% HEDP (DR) in 3% NaOCl improved smear layer elimination at the apical level of the root canal compared with 3% NaOCl and EDTA alternating irrigation [7]. Similar results were reported by [PERSON] et al., who found that irrigation with 9% and 18% HEDP was significantly more effective for the removal of the smear layer in the apical third of the root canal than 17% EDTA [75]. In a study by [PERSON] et al., the use of a 2.5% NaOCl/9% HEDP mixture freed 95% of the dental tubules from the smear layer [76]. Although chelating agents remove the smear layer, thus enhancing the antibacterial effectiveness of irrigation and providing stronger adhesion of sealer to the root canal walls [15; 18; 19], their use may potentiate excessive removal of organic and inorganic content of the dentin, resulting in erosion of its peritubular and intertubular parts [59; 65; 77]. The sequential use of EDTA and NaOCl has been found to induce the erosion of dentinal walls [78; 79]. HEDP has also been shown to exhibit erosive potential [58; 59; 65]. With regard to the erosive potential of EDTA and HEDP, there is still no consensus in the literature. [PERSON] et al. found that irrigation with the NaOCl/HEDP (DR) mixture did not differ from the sequential irrigation with 3% sodium hypochlorite and EDTA, with no or minimal erosion in both groups [59]. Similar results were obtained by [PERSON] et al. The authors observed moderate erosion of the radicular dentin in the coronal third, whereas the middle and apical thirds showed no erosion after final irrigation with 17% EDTA or 9% HEDP, with no differences between the groups [65]. On the other hand, [PERSON] et al. identified destruction of the intertubular dentin in the middle and coronal thirds after the combined use of NaOCl and HEDP, whereas the sequential use of EDTA and NaOCl resulted in no erosive areas [58]. In contrast, according to the results of a study by [PERSON] et al., EDTA produced deeper erosion of the root canals (45.75, 41.79, and 32.25 \(\mu\)m) than HEDP (DR) [20.25, 16.40, and 15.96 \(\mu\)m] in the cervical, middle, and apical thirds, respectively [80]. In our study, complete removal of the smear layer was strongly associated with dentinal wall damage; some degree of dentin erosion was observed in all the areas free of the smear layer. In the NaOCl + EDTA group, the middle and coronal thirds of the root canals presented with completely open dentinal tubules, while in the apical thirds, there were some areas with completely closed dentinal tubules; all the thirds demonstrated moderate to severe dentin erosion. In the ID groups (all concentrations), the smear layer covered most of the root canal surfaces, and the majority of the dentinal tubules were completely or partially closed. No signs of dentin erosion were found. In all the DR groups, irrespective of the concentrations, the middle and coronal thirds had the majority of dentinal tubules open, while the apical thirds had some dentinal tubules closed. The most prominent dentin erosion comparable to that in the NaOCl + EDTA group was observed in the DR18% group. The generalizability of the results obtained in our study is subject to certain limitations. For instance, the in vitro design did not allow the estimation of the clinical relevance of the findings. The issue that was not addressed in this study was whether the detected degree of dentin erosion could significantly affect the mechanical characteristics of the root and sealer bonding. The use of bovine muscle tissue for testing the proteolytic activity of the irrigants made these findings less generalizable to human pulp tissue. Finally, the scope of this study was limited to the comparison of the tested irrigants, which is why the activating techniques were not applied. Moreover, it could be interesting to test smear layer removal in combination with other recently introduced treatments [81; 82].