## 4 Discussion
As the duration of the preparation increased, the temperature passing through the dentin also increased. The highest mean temperature was recorded in P3 mode at a 60 s time interval. This is in accordance with the readings of other investigation, where significant temperature increase was registered at extended manipulation time [13; 14; 20]. Therefore, it is worth mentioning that it is beneficial to interrupt the preparation sequence during ultrasonic activation in order to allow sufficient time for the dentin to cool down [20]. In addition to the setting parameters of the devices, the material of the fragment also affects the temperature change during removal. [PERSON] found that the removal of a NiTi filecompared to a stainless steel file induced higher temperature rise during removal [20]. In conclusion, it was recommended that lower power settings and shorter application times be applied when using ultrasonics to remove NiTi fragments compared to stainless steel ones [20]. In our experimental model, a Reciproc blue heat-treated NiTi file was broken into the root canal. Thanks to the innovative thermo-mechanical treatment, the structure modification resulted in a flexible instrument resistant to cyclic fatigue [31]. The temperature rise in our study was lower than what was presented by [PERSON] [20]. Since our results demonstrated a temperature-reducing effect of rinsing after each preparation phase, the third hypothesis was rejected. To the best of our knowledge, there are no studies investigating the combination of ultrasonic preparation and intermediate irrigation for removal of a broken instrument. Activating the ultrasonic tips without any coolant was considered to simulate the recommended clinical conditions and to present the worst-case scenario of heat generation [5; 13; 14]. To overcome the problem regarding the heat development induced by the vibration energy, a specifically designed ultrasonic unit and tip with air-spray function has been developed and is reported to effectively reduce temperature even with an application time of 120 s [15]. Another possible way to reduce the temperature without limiting the visual control is to use a rinsing solution between preparation phases. Even if the temperature exceeds the potentially harmful threshold, irrigation with a room-temperature or colder solution can facilitate the temperature decrease after a phase of ultrasonic preparation. This in vitro experimental result may be of importance for clinicians, as it suggests a similar cooling effect of agents used in the clinic for root canal disinfection and debris removal (e.g., sodium hypochlorite). This aspect may be important to protect the periodontium. As a future direction, it would be worthwhile to investigate the cooling effect of irrigation solutions at different temperatures, including refrigerated ones. Due to the favorable morphological characteristics, upper central incisors were used in this study to test the temperature increase. The dentin thickness around root canals of different teeth is dissimilar [32]. For this reason, the results of our measurements cannot be extrapolated to all teeth, since the present study used extracted upper first incisors with relatively big cross-sectional diameters. By increasing dentin thickness, increasing heat insulating effect but, at the same time, heat-storing capacity was also demonstrated [17]. In light of the fact that dentin heat-storing capacity is significant, the manipulation time is even more important. Prolonged (60 s) thermal stress (10 \({}^{\circ}\)C) was proved to induce irreversible changes in the bone [18]. Based on the results obtained in our study, the fourth hypothesis should be partially rejected, which partly assumed that the thermal conductivity of the medium surrounding the root has no influence on the temperature change. A thermal conducting medium, ProTrain gel, was used to simulate the surrounding tissues of the root, which showed a significant thermal reducing effect compared to the control group without the gel.