Bactericidal effect of hydroxyl radicals generated by the sonolysis and photolysis of hydrogen peroxide for endodontic applications

The aim of endodontic root canal treatment is the elimination of bacteria and their products from an infected tooth root canal. To effectively disinfect a root canal, an ultrasonic irrigation system, in which hydroxyl radicals (HO·) generated artificially by sonolysis of H2O2, was developed previously for endodontic applications and was demonstrated to have bactericidal efficacy against Enterococcus faecalis. To improve this system, we examined the in vitro bactericidal effects of HO· generated from H2O2, activated by simultaneous irradiation with ultrasound for sonolysis and dental LED light for photolysis with a peak wavelength of 405 nm. Regarding the LED irradiation, two methods were used: (i) 'ideal' experimental conditions (irradiation close to the glass tube), and (ii) simulated endodontic conditions (more distant irradiation of a masked glass tube). In these conditions, HO· generation from H2O2 was detected by electron spin resonance (ESR) spectroscopy, and bactericidal efficacy against E. faecalis was assessed by measuring the colony forming units (CFU)/mL. The results indicated that HO· generation by ESR measurements and the bactericidal effect on E. faecalis by viable count using CFU/mL were enhanced significantly in a time-dependent manner in both conditions. In a comparison of these conditions, bactericidal activity under 'ideal' experimental conditions was similar to that under simulated endodontic conditions. Moreover, the irradiation time for effective killing of E. faecalis through the sonolysis and photolysis of H2O2 under simulated endodontic conditions was shorter than that with sonolysis alone. These results demonstrate that H2O2 activated by ultrasound and LED light may be a safe and effective disinfection technique for endodontic root canal treatment.