## 4 Discussion  
Heating of the NaOCl solution used as intracanal irrigant would increase cleanliness and the decontamination potential of the root canal system [6], although ideal NaOCl concentration and temperature should be further evaluated [11]. Moreover, the maintenance of a high intracanal temperature strictly depends on the heating procedures and on heat dissipation. Therefore, the aim of the present in vitro study was to compare the efficacy of different methods of NaOCl heating evaluating the temperature levels reached at different regions of root canal system as well as time interval of high temperature level persistence. According to our results, the extraoral heating method was ineffective to produce significant temperature increase at the root apex. As previously reported, preheating of irrigants resulted in a marked intracranial temperature increase during its delivery and a quick drop thereafter [12]. A slightly better temperature increase was observed when the NaOCl solution was heated at 70 \({}^{\circ}\)C. Conversely, it was reported that a 60 \({}^{\circ}\)C preheating of NaOCl resulted in significantly higher intracanal temperatures than other evaluated activation systems [13]. Accordingly, [PERSON] et al. suggested that the preheating irrigant at 60 \({}^{\circ}\)C and temperature maintenance with thermal activation may be considered an optimal irrigation protocol [14]. However, it is worth noting that temperature profiles recorded by the T5m and T10m thermocouples for the NaOCl solution heated at 70 \({}^{\circ}\)C into the glass beaker, aspirated into the syringe, and injected into the root canal were similar to those recorded by [PERSON] et al. [13] as the NaOCl water solution was directly preheated into the syringe at 60 \({}^{\circ}\)C, thus, suggesting a cooling of about 10 \({}^{\circ}\)C as the NaOCl solution at 70 \({}^{\circ}\)C was aspirated into the graduated syringe. Comparable results to extraoral heating were obtained using the intracanal heating method through the ML-12 plugger that showed slightly better results (temperature >40 and 50 \({}^{\circ}\)C) only when set at 180 \({}^{\circ}\)C. The ML-12 plugger demonstrated lower temperature peak levels when compared to F-06 and XF-30/04; on the other hand, the latter reported the highest temperature peak levels, close to the setting temperature of the heating system. The F-06 plugger demonstrated the highest temperature levels of the NaOCl solution located in the middle third region of the root (T10 mm). Similar results were obtained even with the XF-30/04 plugger that did not show significant differences between the T5 mm and T10 mm thermocouples; however, the outcomes registered at other levels of the root reported significantly lower temperature levels. In addition, negligible differences were observed in terms of temperature maintenance at several levels of the root between the F-06 and XF-30/04 pluggers, even though the time intervals were higher in the case of the XF-30/04. The obtained results support the use of intracanal heating systems that allow a better temperature persistence and may enable the use of NaOCl at a lower concentration, maintaining the ability to dissolve organic tissue [15; 16; 17]. Nevertheless, it has been reported that final irrigation with NaOCl at different temperatures resulted in similar antibacterial effectiveness, even though hot NaOCl (45 \({}^{\circ}\)C) caused higher postoperative pain levels than the same solution used cold (2 \({}^{\circ}\)C) [18]. In addition, thermal stimulation of the surrounding tissues as well as temperature dissipation should be taken into account and need to be further evaluated in future studies [6]. Noteworthy is the potential effect of temperature on NiTi endodontic files. Indeed, it has been demonstrated that the enhancement of intracanal temperature by heating NaOCl resulted in a decrease in the fatigue resistance of the rotatory endodontic instruments [19].