## 4 Discussion
Nevertheless, higher percentages of penetration were registered in ST at both the apical third and the middle third of the root, so underlining as sonic energy could improve the rate of sealer penetration [3]. With attention to the apical third, our findings appear encouraging. According to [PERSON] et al. (2017), the ultrasonic devices do not perform in the apical third because, touching the walls, the phenomenon of acoustic streaming and cavitation is reduced [30]. In our case, the sonic activation, delivered for the entire duration of the filling, did not seem affected by the dentinal walls, as the shutter received sonic energy from the soniflex handpiece outside the endodontic space. Observing the sealer penetration area in some samples, a heterogeneous distribution of the sealer along the canal root perimeter emerged with a peculiar concentration along the vestibule-lingual area of each section. This phenomenon, known as the \"butterfly effect\" [4; 13; 31], is probably due to dentinal tubular sclerosis in the mesiodistal direction in some dental elements. Although the butterfly effect was not studied in this work, future studies should consider its possible influence as a confounding element [31] because it could be one of the causes of the wide penetration ranges observed in this study as in the previous ones [32; 33]. According to some recent studies about the sealer penetration [4; 11; 12; 13; 26], in our work, all analyzed parameters increased from the apical (3 mm) toward the coronal third (6 mm), probably determined either by the lesser density and diameter of the dentinal tubules in the apical third [34; 35] or by a better removal of the smear layer and more effective irrigation in the middle third rather than in the apical third [36]. The sealer penetration into dentinal tubules avoids bacterial colonization, entombs the remaining bacteria and improves the retention of canal obturation materials. Therefore, deeper tubular penetration of the sealer could represent an excellent outcome because it could allow lesser instrumentation of the apical third and the middle third of the root canal, removing those pathogens responsible for the endodontic disease, economizing on the quantity of dental materials, and reducing errors during the shaping. In addition, sonic activation of Thermafil, in addition to transferring energy waves from the carrier to the sealer, keeps gutta-percha warm and, therefore, its thermoplastic characteristics over an extended period, postponing its hardening. A recent study demonstrated that sonic and ultrasonic activations could modify the physicochemical properties of some endodontic sealers [37] when the activated carrier comes into contact with the sealer directly. This phenomenon did not happen in this study because the sonic energy was directly delivered to Thermafil without direct contact with the sealer. From our results, it has emerged that sonic activation of carrier-based obturation achieves an optimal distribution of the sealer into dental tubules and, consequently, it allows deeper levels of infiltration to be reached than those obtained with conventional Thermafil or with System B. Furthermore, the used frequency (6 Hz) and the activation time (10 s) do not negatively influence the sealer behavior. Since colonization of dentinal tubules by bacteria can reach distances of at least 300 \(\upmu\)m [38], the sonicated Thermafil presented here can be developed as a new device that combines the great adaptability of Thermafil to canal walls with the stronger sonic capability to increase penetration of endodontic materials. The main limitation of this work is due to the small sample size. Second, there could be cofounder bias due to the butterfly effect observed in some CLSM images, which may have biased some measurements. Future studies need to be conducted with larger sample sizes and to consider the role of the butterfly effect as a confounding factor.