## 4 Discussion  
Root canal filling materials' adhesion to dentin has been widely tested using the push-out bond strength (POBS) test, also called dislodgement resistance [26]. Well-controlled experiments are challenging when using biological samples, due to the substantial effects of the inherent biological, physical, and chemical variances imposed by natural samples. A recent study investigated the reliability of using bovine teeth as an alternative to human teeth for testing the POBS of sealers to dentin and concluded that the dentin substrate did not influence the sealers' bond strength [27]. Only one of the studies included in our review used bovine teeth [13]. The variations in push-out methodology are a concern because they prevent the comparison of results from different researchers [26]. Generally, studies have followed two philosophies: the root canals are either filled with the sealer alone or combined with gutta-percha with the filling techniques of cold lateral compaction, single-cone filling, or specific obturation systems such as Resilon/Epiphany [28; 29]. Three studies included in this review filled the samples with gutta-percha and sealer [23; 24; 25]. According to some authors, filling the canals only with sealer ensures that there are no confounding factors and that the adhesion strength tested is that of the sealer [28]. The adhesion processes are mostly influenced by the relative surface free energy, which determines the predisposition of the material's surface for establishing new interactions/bonds with the surrounding medium. In the same way, wettability is influenced by the interfacial tensions and, in turn, by the surface free energy [30]. Thus, contact angle evaluation has been widely used to measure the surface wettability of different materials [22]. The contact angle has an inverse relationship with the surface free energy (wettability), i.e., the lower the contact angle, the greater the surface free energy and, hence, likely greateradhesion [13]. [PERSON], et al. [13] observed increased surface free energy, correlated with the higher wettability of bovine dentin with AH Plus after 30 s of argon NTP, compared to the control (i.e., without NTP). They also reported a chemical change based on FTIR results. Argon plasma treatment reduced the organic compounds of dentin (amide I and II bands) and increased the inorganic component (the carbonade band), due to its ability to etch dentin surfaces; no associated topographical changes occurred. Based on these findings, the authors concluded that argon treatment favored the bonding of the sealer to dentin surfaces [13]. However, only the resinous AH Plus sealer was evaluated. There is no consensus or sufficient data about NTP's effect on radicular dentin in terms of endodontic sealers' adhesion. In the present investigation, a systematic review was conducted to answer the following PICO question: Does the NTP treatment affect sealers' adhesion to dentin compared to no treatment? A few ex-vivo studies met the selection criteria, even though some disparity in the materials and methodologies was registered, which prevented a meta-analysis from being performed. Our findings indicate that NTP on dentin root walls might positively impact sealers' adhesion, considering the increased POBS or surface energy and wettability values reported in the selected literature. The bioceramic sealer (BioRoot RCS) showed the highest POBS values, followed by the epoxy-resin-based sealer (AH Plus), after mixing helium and argon atmospheres on dentin surfaces [25]. Moreover, the NTP dentin groups showed an increase in bond strength more than two times higher than the non-plasma-treated dentin (control groups), independent of the sealer [25]. The better bond strength of the bioceramic sealer after NTP, compared to the resinous AH Plus [25], was also corroborated by other authors who stressed its good performance, particularly in the middle region of the root canal [23].