## 4 Discussion
[29] reported that with the continuous heat wave technique, there was almost no rise in temperature. The present study agrees with these authors, since we found an increase in temperature that exceeded 3.8 \({}^{\circ}\)C with the three systems used; nevertheless, no statistically significant differences were found. On the other hand, [PERSON] [30] and [PERSON] et al. [31] explained that when the technique was applied to mandibular central incisors, temperatures exceeded 10 \({}^{\circ}\)C using System B\({}^{\circledR}\). In the present study, only one sample exceeded 10 \({}^{\circ}\)C. [PERSON] and [PERSON] [23] reported an increase of more than 10 \({}^{\circ}\)C with the use of System B\({}^{\circledR}\) as a shutter system, but their time of application was 5 s and 8 s. [PERSON] et al. [24] conducted a study on mandibular molars filled with System B\({}^{\circledR}\) for 3 s and 4 s. They reported that when obturation for 3 s, the temperature of the periodontal ligament was 46,914 \({}^{\circ}\)C and when obturation for 4 s, the temperature increased by more than 10 \({}^{\circ}\)C; in conclusion, one should be careful not to extend the activation time beyond 3 s. In the present study, the plugger in all systems was set to 3 s.  
[PERSON] et al. [27] observed that bone resorption occurs when temperatures of 47 \({}^{\circ}\)C and above are reached during bone drilling. In a study on the temperature threshold for heat-induced bone tissue injury in rabbits, it was found that a temperature of 47 \({}^{\circ}\)C maintained for 1 min could cause microscopic evidence of bone remodelling and adipose tissue necrosis. [PERSON] et al. [15] showed a periapical inflammatory histological reaction in beagle dogs after an injection of thermoplasticized gutta-percha at a high temperature (160 \({}^{\circ}\)C) into the root canal. However, the tissue destruction was localized in the area around the apical foramen, while the periodontal ligament of the root surfaces remained normal. [PERSON] [32] performed a study on canine teeth filled with thermomechanical compacted gutta-percha in 17 ferrets, in which a histological evaluation of the root cement, adjacent periodontal membrane and alveolar bone was performed at time intervals of 24 h, 20 days and 40 days. At 24 h, there was no inflammatory response and no evidence of hypererania. At 20 days, 20% of the teeth showed resorption of the cement surface in the central section of the root. This resorption was not of an inflammatory nature, unlike resorptions due to trauma. All control teeth and their supporting tissues appeared normal. However, 40 days after thermomechanical compaction, 28% of the experimental teeth were affected by surface resorption; of these, 22% showed ankylosis of the alveolar bone. Therefore, these authors concluded that the heat generated by thermomechanical compaction stimulates surface resorption and long-term ankylosis. [PERSON] et al. [27], [PERSON] et al. [15] and [PERSON] et al. [32] observed alterations such as necrosis, destruction around the foramen and ankylosis in their studies. In all of these studies, temperatures above 47 \({}^{\circ}\)C or a time longer than 3 s were harmful to teeth. From these studies, it can be concluded that caution is necessary to avoid excessive heat when performing techniques with thermoplastic gutta-percha. [PERSON] et al. [33] conducted a study of a 3D-printed mandibular molar sealed by a computer system with System B(r) and Oblura II(r). They made two models: one simulating blood flow and one without blood flow. The study concluded that in the model without blood flow, the temperature in the periodontal ligament was 50 \({}^{\circ}\)C along the distal canal and 52.5 \({}^{\circ}\)C in the mesiolingual canal. However, in the model simulating the blood flow in the periodontal ligament, the peak temperature was 47 \({}^{\circ}\)C. The conclusion of the study is that the blood flow of the periodontal ligament is one of the factors to be taken into account when investigating the heat emitted during thermoplastic filling.