Heat Generation and Temperature Increase in the Root during Electromagnetic Apical Treatment

    

    

    Figure 6: Temperature change. (a) The intracanal temperature increased rapidly during the first energization and was low in the subsequent energization; (b) The
root surface temperature did not exceed 45°C during the first energization and did not increase to a large extent during the subsequent energization for a time
interval of ›3.0 s.
    

(a) The intracanal temperature increased rapidly during the first energization and was low in the subsequent energization.

(b) The root surface temperature did not exceed 45°C during the first energization and did not increase to a large extent during the subsequent energization for a time interval of ›3.0 s.

Discussion

We demonstrated egg white coagulation at the tip portion of the active electrode, although studies [23,24] have suggested heat generation at the apical portion. The current effective value increased by increasing the K-file insertion depth (p ‹ 0.001), and no differences were observed in the K-file diameter (Figure 4b). However, we observed coagulation at K-file insertion depth of ‹3.0 mm, which became more remarkable in a smaller-diameter K-file (Figure 4a). The current effective value increased as the file insertion depth increased. However, because the coagulation of egg white decreased, the increase in the current density and contact resistance was assumed to be caused by the small contact area between the egg white and file, primarily affecting the Joule heat generation. Hence, to more effectively apply the Joule heat, the K-file diameter is should be as small as possible and the energizing zone should be within 3.0 mm; therefore, the current density can be increased.

When energizing using the Endo Training-Bloc, the heat generation feature differed from that with energizing inside a cylindershaped container, and coagulation was observed at the tip portion of the K-file only when it approached the vicinity of the AF portion. This phenomenon became more prominent when the AF diameter became smaller (Figure 4c). The Joule heat amount, according to the Joule’s first law, increases as the current increases. Moreover, according to the Ohm’s law, the current decreases as resistance increases. Additionally, the resistance is proportional to the length of the electrode and inversely proportional to the cross-sectional area. In the case of an Endo Training-Bloc, the current was energized only through AF. Therefore, smaller the diameter of AF and longer the distance up to AF, higher is the electrical resistance; consequently, no heat generation was considered when a location far from the AF was energized. Thus, to effectively generate the Joule heat, several factors, such as file diameter, insertion depth, solution amount in the root canal, and AF diameter, should be considered.

The agar depression diameter could be expanded in a timedependent manner up to 1.0 s, but was not significantly (p ‹ 0.01) different from a case of 1.5-s energization (Figure 5a,b). When heated for ›1.0 s, the electric current was not conducted and no heat was generated from the results of the evaporation of the agar. Hence, when high-frequency current is applied in a root canal, it might not be necessary to energize for ›1.0 s. In this experiment, 1.0% agar plate was employed as a sensible heat substance because the egg white adheres to the K-file.

Special caution should be taken regarding the Joule heat at the root surface when high-frequency current is clinically applied. Excess temperature increase can irreversibly damage the periodontal ligament [25,26] and unnecessary heat might scald the periodontal tissue [27,28]. Bone tissue exhibits irreversible changes on heating at 50°C for 1 min or 47°C for 5 min; thus, as bone resorption progresses, the bone is eventually replaced with adipose tissue [29]. To avoid severe bone tissue damage and decrease in regeneration function, heating condition has been determined at 44–47°C within 1 min [30]. Upper limit temperature for the periodontal tissue might be ~45°C [26]. To predict the influences on periodontal tissues, we measured intracanal and root surface temperature changes. We used the mandibular anterior tooth [31] because it has a relatively thin root dentin, which easily transfers the intracanal temperature to the root surface. When energization was intracanally conducted ten times using a K-file with the conductive area limited to the tip portion of 3.0 mm, the temperature increased only at the uncoated tip portion, e.g., to evenly cauterize a root canal wall, energization should be performed once or several times by moving the K-file rather than energizing multiple times at the same position. However, the temperature at the root surface did not exceed 45°C during the first energization, and high temperature increase was not observed during the subsequent energization for a time interval of ›3.0 s (Figure 6b). Hence, if the interval is ›3.0 s, the temperature at the root surface will not continue to overheat to ›47°C for ›1 min. The root surface temperature will possibly be lower than that in vitro because of heat diffusion by convention flow via the blood vessels within the periodontal ligament and/or heat dissipation by the heat conductivity of the periodontal ligament. The possibility of periodontal tissue damage should be considered as low; moreover, heat generation might cauterize and coagulate bacteria and infected debris because of high-frequency energization.

The following optimum conditions were identified to design and operate the EMAT system.

EMAT device

Based on egg white coagulation evaluation, K-file #10/.02T is selected as the active electrode based on the high heat generation. Considering the Joule heat generation, K-file was coated with a nonconductive material, such as parylene, except for its tip portion (3.0 mm).

EMAT procedure

After a chemomechanical root canal preparation (Figure 7a,b), a high electroconductive liquid, such as physiological saline solution or sodium hypochlorite solution, is placed into the root canal, and then energization is conducted for 1.0 s intermittently at 3.0- s intervals to prevent overheating, indicated by the root surface temperature (Figure 7c). To eliminate the scatter in the conductivity of the K-file, it should be pulled out from the root canal for each energization process, and if protein-coagulated layer adheres to the K-file, it should be cleaned using a wet gauze. In this study, the agar depression diameter was approximately 0.81 mm when the current flow was energized with 500-kHz frequency for 1.0 s into the agar plate. Hence, in cases without a periapical lesion, the point that is 1.0~2.0 mm apart from AF in the coronal direction is defined as an energization start point to avoid the direct heating of the periodontal ligament. To avoid excess heating, a 1.0-s energization is conducted 1-3 times with 1.0-mm intervals while pulling the K-file toward the crown. When a periapical lesion should directly be heat-denatured, additional energization can be performed on AF and inside the apical lesion (Figure 7d). Regarding the bactericidal effect, the Joule heat generated by high-frequency energization can denature around the K-file; thus, by applying the abovementioned EMAT system, highfrequency energization should yield the bactericidal effect in the root canal and the so-called uninstrumented area, which is considered difficult to reach during a chemomechanical root canal preparation. Moreover, this system provides a useful nonsurgical treatment method for refractory cases. Note that heat generation using the EMAT system is confirmed to cause few adverse events. Studies are required on the clinical applications and safety considerations of this system.

    

    

    Figure 7: The electromagnetic apical treatment procedure.
(a) Apical periodontitis with periapical lesion (b) Chemomechanical root canal preparation (c) 1.0-s energization in root canal 1–3 times with 1.0-mm intervals (d)
Additional energization on AF and inside the apical lesion when necessary.