The association of PDT and US represents an auxiliary resource to provide microbial reduction from the root canal system [3]. Although this benefit exists, there is a possibility that US may contribute to an even greater impregnation of the photosensitizer on the root dentin, which can reduce the BS of filling and restorative materials to root dentin. It is possible because the photosensitizer adheres strongly to the root canal walls and acts as a chemical smear layer [4]. Moreover, the association of US provides agitation of the photosensitizer, propelling this substance to the root canal walls and into the depth of the dentinal tubules, which can further increase this impregnation. Considering that the conventional PDT protocol forms a chemical smear layer that compromises the adhesion of filling and restorative material [5, 6], the present study was carried out to evaluate whether the US of the photosensitizer reduces the BS of the tested materials, even after the use of the tested final irrigation protocols.

The literature reveals that the association of 17% EDTA with US for 1 min results in effective removal of the photosensitizer from the root canal walls, as this irrigation protocol increases the bond strength of the filling/restorative materials to the root dentin [5, 6]. For these reasons, this irrigation protocol and contact time were used as a parameter in the present study. Furthermore, the present study proposes the use of 17% GA as a final irrigant, making a comparison with the 17% EDTA solution. According to Cecchin et al. [15], GA was tested as a surface pretreatment agent for dental restorative applications, presenting effective results on enamel etching and dentin surfaces. At the same time, GA revealed effectiveness for smear layer removal [8]. Thus, GA was chosen to be tested in the present study, with and without US, to evaluate its ability to promote the removal of the chemical smear layer formed by the US of the photosensitizer, and consequently increase the bond strength of the tested materials to root dentin.

The intracanal decontamination protocols performed prior to root canal filling or adhesive cementation of intraradicular posts must provide ideal conditions for BS to root dentin. This ensures effective adhesion to the root canal walls, minimizing marginal infiltration and the risk of root fracture, contributing to the longevity of endodontically treated teeth [16]. Considering that BS corresponds to the force required to displace the filling or restorative material adhered to the root dentin, the push-out test has been recommended over time to evaluate this mechanical property. It consists of applying a force to the filling/restorative material through a cross-section of the root until this material is displaced. The displacement force is uniform and simulates clinical reality, it can be performed in different thirds of the root canal, it has high reproducibility, and provides a larger tested adhesion area when compared to other tests, such as microtensile and shear tests [14, 17]. For these reasons, the push-out test was used in the present study to evaluate the bond strength of the tested materials.

The Bio-C bioceramic sealer is bioactive and releases calcium ions, providing sealing ability through stable chemical bonding, tag-like penetration into the depth of dentinal tubules, and stimulation of biomineralization [9, 18]. In turn, the Rely-X U200 self-adhesive resin cement provides recognized micromechanical and chemical retention to root dentin [19]. Considering that the US of the photosensitizer may impregnate root dentin and interfere with the adhesion of filling and restorative materials, the choice of endodontic sealer and adhesive cement for GFP plays a key role at this stage of endodontic treatment. Due to previously described properties, the Bio-C bioceramic sealer with gutta-percha and Rely-X U200 with GFP were tested in the present study after the root dentin was subjected to PDT associated with US, as well as after the tested final irrigation protocols for photosensitizer removal.

According to the results of present study, the bond strength was significantly higher in groups 2 (17% EDTA) and 3 (17% GA) when compared to control group. This is in agreement with the results of previous studies, where the use of final irrigants with ability to remove the smear layer after PDT protocol induced an increase in the bond strength of filling/restorative materials to root dentin [5, 6]. At the same time, the literature reveals that irrigation with inert solution is not enough for photosensitizer removal, leaving a chemical smear layer in the root canal walls [4]. It highlights the importance of introducing final irrigation techniques in the step-by-step of PDT protocol. Nowadays, it is not recommended. The EDTA acts by demineralizing of superficial dentin and decalcifying the root dentin [7]. The GA acts by acidic demineralization and indirect organic dissolution [8]. Therefore, both alternatives have the ability for effective photosensitizer removal from root canals, helping to improve the BS to root dentin.

The association of US with 17% EDTA and 17% GA resulted in the highest BS values for filling and restorative materials to root dentin, based on the results of present study. It confirms the first and second hypothesis of present study. The US acts through the principle of hydrodynamic turbulence, increasing the temperature and hydrostatic pressure of the irrigant inserted into the root canal. Bubbles and cavitations are then generated, and the irrigant agent is propelled more effectively against the root canal walls. It increases its cleaning potential and penetration into the depth of the dentinal tubules [20]. Similar results were observed in previous studies, also revealing that US promotes higher photosensitizer removal when compared with the isolated use of final irrigants with chelating properties, such as EDTA and QMix [5, 6]. According to van der Sluis et al. [2], the US represents a more effective supplement for cleaning the root canal system and root canal walls, when compared with traditional syringe irrigation. Therefore, the association of US with final irrigants must be considered an essential step into the PDT protocol, considering the findings of the literature and the results of present study.

Cohesive failure occurs within the filling or restorative material itself and not at the sealer-dentin or cement-dentin interface. After the push-out test in both evaluations, it was possible to observe a higher predominance of cohesive failure in all groups of present study. The Bio-C bioceramic sealer releases calcium and hydroxyl ions, which react with dentin phosphate and form hydroxyapatite at the sealer-dentin interface, creating micromechanical retention. This bioceramic sealer also exhibits volumetric expansion that fills microspaces and increases marginal adaptation [1, 11, 18, 21]. In turn, Rely-X U200 self-adhesive resin cement releases acidic monomers that demineralize and infiltrate the dentin substrate, providing micromechanical retention. At the same time, the reaction between the phosphoric acid monomers of the cement and hydroxyapatite in the dentin substrate provides chemical retention [19, 22]. All these mechanisms help to explain the high adhesion provided by the tested bioceramic endodontic sealer and self-adhesive resin cement, as well as the observed failure patterns in the present study.

Despite the fact that there is no statistically significant difference between EDTA and GA in the removal of the US-activated photosensitizer, as well as the fact that there is similarity in the potential for removing the smear layer formed by the instrumentation between the two final irrigants, GA presents some advantages over EDTA. The GA exhibits low cytotoxicity and does not induce severe damage to the mechanical properties of the dentin, even when activated by US, contrary to what is observed when EDTA is used, which can have deleterious effects in this regard [8, 23]. Considering this scenario, this study suggests the use of GA and US as part of the PDT protocol, ensuring effective photosensitizer removal from the root canal walls.

This in vitro study has some limitations. First, the experimental conditions do not fully reproduce the clinical environment, as factors such as biological fluids, functional stresses, and long-term aging were not simulated. Second, the use of standardized extracted teeth does not reflect the anatomical variability of root canal systems, which may influence both photosensitizer removal and BS outcomes. Finally, BS was evaluated only in the short term, without considering possible degradation of the dentin–sealer interface over time.

Further studies can be performed to evaluate the influence of different final irrigation protocols on photosensitizer removal and BS under simulated clinical conditions, including thermocycling, mechanical loading, and aging. The influence of these final irrigation protocols on the mechanical properties of root dentin also could be evaluated. In addition, investigations using root canals with greater anatomical complexity are needed to better reflect clinical scenarios. Finally, long-term studies assessing the durability of the dentin–sealer interface and the potential residual effects of photosensitizers over time are recommended. Based on new perspectives and tested variables, a safe protocol for removing the photosensitizer activated by US may be established.