2015 Volume 58 Issue 2 Pages 101-108
Purpose: The capacity of low-frequency plasma jets to generate various active species without exerting a heat load has recently attracted interest as a potential means of medical sterilization. In a previous study, we demonstrated a marked sterilization effect in vitro with these plasma jets in an acidic environment (the reduced-pH method). In this study, we evaluated the ability of plasma to sterilize the caries-infected dentin model by the reduced-pH method.
Methods: Using extracted human teeth, a 3-mm diameter cylindrical cavity with a depth of 3 mm was prepared in the center of the occlusal surface and was autoclaved. To achieve demineralization with lactic acid for two days, Streptococcus mutans (ATCC25175) suspension (106-7 CFU/30 ml, trypticase soy broth with dextrose, 5% carboxymethyl cellulose sodium salt) was inoculated in the cavity every day for seven days. Extreme dentin softness was confirmed by dense staining obtained with caries detector solution, occurring in the outer layer of carious dentin.
Using a plot type of low-frequency plasma jet generation system, the duration of plasma irradiation was set at 30, 60, 120 and 180 s and evaluated at pH 3.5. As a control, irradiation was performed with He gas only in the cavity for 180 s. Pre- and post-irradiation samples were extracted using a round bur, recovered and placed in brain heart infusion broth, and cultured on agar plates to determine CFU. In this way, dentin was extracted from each tooth pre- and post-irradiation. The detection limit was 2 CFU/round bur in this study. Data were statistically analyzed by Wilcoxon's signed-rank test (α=0.01).
Results: Pre-irradiation viable bacteria count was 2.7±1.9×105 CFU/round bur. The detection limit was 2 CFU/round bur in this study. Pre-irradiation viable bacteria counts showed no significant pH-based differences. Under pH 3.5 conditions, bacteria count decreased from pre-irradiation baseline levels with increased duration of irradiation, becoming undetectable after 180 s of irradiation, thus indicating almost complete sterilization. In other words, a significantly strong sterilization effect was obtained dependent on irradiation duration. In contrast, no significant decrease at pH 6.5 was observed, demonstrating a lack of sterilization effect under near-neutral conditions.
Conclusion: These findings demonstrate that plasma irradiation under low pH conditions (pH 3.5) was effective for the sterilization of infected dentin with irradiation in a time-dependent manner. After 180 s plasma irradiation bacteria counts were below the detection limit.
