Clinical Evaluation of Underwater Discharge Plasma as a Root Canal Irrigant: A Randomized Pilot Study on Efficacy and Safety

Abstract

Background/Objectives: Root canal therapy (RCT) aims to eliminate intracanal infection and promote periapical healing through mechanical instrumentation and chemical disinfection. Conventional irrigants, such as sodium hypochlorite (NaOCl), are effective but may exhibit limited penetration into anatomically complex root canal systems and carry the risks of cytotoxicity if extruded beyond the apical foramen or into surrounding periodontal tissues. In this pilot study, we evaluated the clinical effectiveness and safety of underwater discharge plasma (UDP) as a biocompatible alternative to NaOCl for root canal irrigation. Methods: A prospective, randomized clinical trial was conducted involving 30 patients who required root canal treatment. Patients were randomly allocated to the UDP (n = 15) or NaOCl (n = 15) group. All treatments were performed by a single operator following standardized protocols. Pain was assessed using the visual analog scale (VAS), and periapical healing was evaluated using the Periapical Index (PAI) at baseline, 2 months, and 4 months. Statistical analyses included the Friedman test, Mann-Whitney U test, and Fisher's exact test. Interobserver agreement for radiographic readings was evaluated using quadratic-weighted Cohen's kappa coefficient. Results: A total of 28 patients completed the study. VAS scores significantly decreased over time in both groups (p < 0.05), with no significant difference between the groups at any time point (p > 0.05). At 4 months, radiographic healing was observed in 71.4% and 92.9% of patients in the UDP and NaOCl groups, respectively (p > 0.05). PAI score changes and clinical success rates were comparable between groups. No adverse effects or thermal damage was reported when using UDP. Conclusions: UDP demonstrated short-term clinical efficacy and safety comparable to that of NaOCl. Thus, UDP may serve as a biocompatible alternative for root canal disinfection. Further large-scale and long-term studies are warranted to confirm its clinical utility.

Keywords: endodontics; hydroxyl radical; non-thermal plasma; root canal; underwater discharge plasma.

Figure 1

Representative radiographic images with corresponding Periapical Index (PAI) scores. The images depict various degrees of periapical radiolucency, assessed according to the PAI scoring system, introduced by Ørstavik et al. [24], ranging from a healthy periapical status (score 1) to severe periodontitis with exacerbating features (score 5). These reference images were used to assist the blinded radiologists in evaluating periapical healing in this study.

Figure 2

Serial periapical radiographs illustrating the progression of periapical healing in representative cases from the UDP (test) and NaOCl (control) groups. (a–d) Images from a UDP-treated tooth showing: (a) preoperative periapical radiolucency, (b) immediate post-obturation status, (c) 2-month follow-up status, and (d) 4-month follow-up status with evidence of healing. (e–h) Images from a NaOCl-treated tooth at similar time points. Radiographs were acquired using a standardized digital system (RIOS Sensor; Healdens, Yongin, Republic of Korea) and were used for evaluating PAI scores by blinded evaluators.

Figure 3

Schematic of the impedance monitoring mechanism in the UDP system. The circuit diagram illustrates how an alternating voltage ($V_{AC}$) is applied and the resulting current ($I_{AC}$) is measured to calculate the total impedance (Z) of the root canal contents. The components include the resistance of organic material (R), inductive reactance ($X_L$), and capacitive reactance ($X_C$). The total impedance is expressed as $\mathrm{Z}=\sqrt{{{(X}_C-X_L)}^2+{\mathrm{R}}^2}\mathrm{\Omega }$. The system uses real-time impedance feedback to adjust energy output dynamically and ensure safety. When the impedance exceeds 5000 Ω, the device automatically ceases energy delivery to prevent thermal injury to surrounding tissues.

Figure 4

User interface of the UDP device (PLAZEN RCT^®; Dentory, Seoul, Republic of Korea). The screen displays a preset maximum plasma discharge duration of 1.5 s, a built-in safety feature that regulates energy delivery. This function ensures precise temperature control during plasma activation within the root canal, minimizing the risk of thermal injury.

Figure 5

Fluorescence emission analysis of terephthalic acid (TA) solution following underwater discharge plasma (UDP) treatment. (a) Emission spectra of the TA solution following pulsed plasma discharge using the PLAZEN RCT^® device (Dentory, Seoul, Republic of Korea) (25 W, 1.2 kV, burst wave mode). The black line represents the control sample without plasma exposure, while the red line indicates the plasma-treated sample, which exhibits a distinct emission peak at 425 nm. (b) Photographic image of TA solutions under 310 nm LED illumination before (left) and after (right) plasma treatment, captured using the Horiba FluoroMax+ spectrofluorometer (Horiba, Kyoto, Japan) (excitation: 310 nm; emission range: 360–600 nm; increment: 1 nm; slit width: Ex/Em = 3/3 nm).

Figure 6

Schematic representation of hydroxyterephthalic acid (HTA) formation through the reaction between terephthalic acid (TA) and hydroxyl radicals (^•OH). TA reacts with ^•OH generated by underwater discharge plasma, producing HTA, which emits fluorescence upon UV excitation. This reaction serves as an indirect indicator of the presence of ^•OH in the solution.