Objective:To evaluate the bactericidal effect of ultrasonic treatment combined with metronidazole

chlorhexidine and physiological saline in root canal preparation and the clinlical evaluation after treatment.Methods:A totle of 400 chronic periapicalperiodontitis teeth in clinic were selected and then divided them into four groups

metronidazole combined with ultrasonic technique group

chlorhexidine combined with ultrasonic technique group

physiological saline combined with ultrasonic technique group and control group.Each group had 100 teeth.The incidence of endodontic inter-appointment emergencies(EIE) after treatment was compared.Twenty teeth of each group were selected and anaerobic bacteria colony counting before and after root canal preparation were examined.Results:Anaerobic bacteria of each group were significant reduced(P<0.01)

and the effects of metronidazole and chlorhexidine groups were better(P<0.05)

but there was no statistical difference between the two groups(P>0.05).The metronidazole and chlorhexidine groups had lower incidence of EIE than the other two groups

but there was no statistical difference between them(P>0.05).Conclusions:Metronidazole and chlorhexidine were ideal root canal ultrasonic irrigations and they could kill bacteria of root canal and prevent the incidence of postoperative pain

so it can increase success of root canal therapy.