2011 Volume 54 Issue 6 Pages 424-431
In dentistry, ultrasonic waves have been used to remove calculus and plaque attached to the teeth and for root planing, and are an indispensable tool for removing calculus in periodontal tissue. It has been reported that plaque is removed by the cavitation effect caused by exposure to ultrasonic waves. However, little is known about the detailed effects of ultrasonic waves on periodontal tissue. In this study, we examined the influence of ultrasonic waves, which can remove the biofilm, on gingival epithelial cells. First, Streptococcus mutans biofilms were exposed to ultrasonic waves at the two frequencies of 1 MHz or 3 MHz, duty ratio of 0-90% and exposure time of 1 minute or 3 minutes, and the optimal conditions for biofilm removal were identified. We examined the effect of ultrasonic waves on the human gingival epithelial cell line Ca9-22 on cell proliferation, cell toxicity, production of IL-1β and number of cell deaths. Exposure to ultrasonic waves at 1 MHz and duty cycle of 60% for 3 minutes removed biofilms most efficiently. On the other hand, the biofilm removal rate using 3 MHz was highest at 40% duty cycle for 3 minutes. To explore the cytotoxicity of ultrasonic waves, we measured LDH in culture supernatant. Exposure to 3 MHz ultrasonic waves at 40-90% duty cycle increased LDH at all times. At 1 MHz frequency, LDH in gingival cells exposed to ultrasonic waves at 50-90% duty cycle for 1 minute or 3 minutes was significantly high. We performed the WST assay to clarify the effect of ultrasonic waves on cell proliferation. Exposure to 1 MHz and 3 MHz ultrasonic waves at 80 and 90% duty cycles for 3 minutes suppressed cell proliferation significantly. Furthermore, we examined the conditions of ultrasonic wave exposure at 1 MHz frequency for 1 minute (duty cycle 40-60%) and 3 minutes (duty cycle 50-90%), and the biofilm removal rate was more than 40%. We measured IL-1β production by ELISA and the number of dead cells stained by trypan blue under these conditions. Exposure to ultrasonic waves for 3 minutes increased IL-1β production and the number of dead cells. These data indicated that exposure to ultrasonic waves at 1 MHz frequency and 40-60% duty cycle for 1 minute had a beneficial effect on biofilm removal and was safer for gingival epithelial cells. In this study, we identified safer conditions for ultrasonic wave exposure yet with a high biofilm removal rate. It is necessary to examine not only cytotoxicity, but also the functions of cells and other cells.
