Purpose: In Tsurumi University Dental Hospital, chemical cleaning (shock treatment) of dental unit waterlines (DUWL) has been carried out to maintain cleanliness. In this research, we reviewed the short-time treatment effect of low corrosion-inducing cleaning against heterotrophic bacterial contamination in DUWL, as well as the importance of daily flushing of DUWL.
Methods: As the cleaning solution, NaClO (500, 10,000 ppm) and slightly acidic water (SAW) were used. After shock treatment, water was collected from the dental unit before and after flushing on the day after the day of closure, and the free chlorine concentration and heterotrophic bacterial number were determined. Measurement was continued until 10 weeks.
Results: The free chlorine concentration of the stationary water before shock treatment was lower than the limit of tap water standard (0.1 ppm) in all DUWLs, and recovered by flushing. However, in some cases, additional flushing was needed to reach the required level of chlorine. The logarithm value of heterotrophic bacterial count in the stationary water showed a significant correlation with the free chlorine concentration. The collected water after flushing showed a significant decrease in the logarithm value of the bacterial count and a significant increase in the free chlorine concentration as compared with the stationary water. After shock treatment with NaClO at 500 ppm and 10,000 ppm, heterotrophic bacteria were not detected in the collected water. After 2 weeks of shock treatment, the number of heterotrophic bacteria in stationary water in all units exceeded 2,000 CFU/ml, which is the upper limit for water quality management in Japan. The bacterial number in flushing water samples from SAW-treated, 500 ppm NaClO-treated and 10,000 ppm NaClO-treated units exceeded the level of 2,000 CFU/ml at 3, 7 and 10 weeks after cleaning, respectively. Although cleaning using 10,000 ppm NaClO was most effective, the liquid was suspected to have a corrosive action on brass.
Conclusion: For DUWL maintenance, shock treatment with 500 ppm NaClO and reliable flushing seemed to be the most suitable method to avoid corrosive effects on the parts of the DUWL. In dental clinics where shock treatment is difficult to perform, it is important to thoroughly perform daily flushing as DUWL management.
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