## 4 Discussion  
When removing broken instruments from the root canal, an important goal is to minimize the associated complications, such as root perforation, excessive tooth loss, and damage to the periodontium caused by excessive temperature rise on the external root surface [25]. Our null hypotheses assumed that the temperature on the external root surface would not change during the removal of a fractured Reciproc blue file using Gates-Glidden drills and an ultrasonic device applied at different setting parameters. According to the results, the first and the second hypotheses were rejected, since both the preparation with Gates-Glidden drills and the ultrasonic preparation set at different parameters increased the temperature on the root surface. A modified ultrasonic technique [26] ensures a so-called staging platform providing sufficient space around the fragment to allow the use of an ultrasonic tip [27]. A concern with this technique is the heat generated by the Gates-Glidden burs in the root canal, which is transferred to the external root surface. This may damage the periodontal ligaments and the alveolar bone, resulting in minor injuries, such as hyperemia and fat cell deposition, or more significant damage, including ankylosis, bone resorption, and bone necrosis [18; 19; 28]. [PERSON] and [PERSON] found a temperature increase of 2.7 \({}^{\circ}\)C using GG at 2000 rpm [21], which is in line with our results when using the first GG. However, during the platform preparation with the modified GG, the average temperature increase was only 2.1 \({}^{\circ}\)C. These values cannot be considered harmful to the periodontium [18; 19]. Ultrasonic tips have been recommended as the most common instruments to remove broken files [3; 21] after the platform preparation. The ultrasonic tip rephines the dentin around the fragment, and, furthermore, the vibration against the broken segment finally can loosen it. Although this technique was reported to have a success rate of 70.5%, some complications can occur, including heat generation [25; 29]. According to our results, the temperature rise during ultrasonic fragment removal was dependent on the setting parameters of the unit, the time interval of the manipulation, and the irrigation applied between the preparation phases. The measured temperature values in the present study are comparable with the findings of [PERSON] et al. [14], who described a 0.6-11.2 \({}^{\circ}\)C temperature rise, which was also dependent on the power setting, application time interval, and the temperature recording site of the root. In general, strong influence of the setting parameters was detected, as previously published [14; 15]. In our study, endodontic function at a power level of 5 (E5) and periodontal function at a power level of 3 (P3) were selected to represent different frequencies and amplitudes of vibration. The frequency of the ultrasonic device used in this study was 28 kHz \(\pm\) 3 kHz and the amplitude was <200 \(\upmu\)m. According to the manufacturer's, these parameters were determined based on dental experiences. P3 mode represented a higher frequency with a higher vibration amplitude compared to E5 mode. Although using the combination of P3 mode and U-file caused a higher temperature rise, it did not reach the potentially harmful threshold of 10 \({}^{\circ}\)C. In addition, the vibration was so powerful, leading to damage or even fracture of the U-file during the experimental preparation. The endochuck adaptor used to capture the U-file increased the length of the tip, thus increasing the risk for the working frequency to change. This may shift the resonance frequency outside of the working range of the piezoelectric crystal [30]. E5 mode represented lower frequency and amplitude of the vibration, leading to a much lower temperature rise. At this lower energy level, the operating time interval had no strong effect on the temperature rise, however, the impact of the manipulation time on temperature rise was significant using P3 mode.