This study aims to evaluate the effectiveness of ultrasonic irrigation in improving root canal disinfection after chemomechanical procedures.

Although chemomechanical procedures promote a drastic bacterial reduction, many root canals remain infected, which points to the need for complementary procedures to improve root canal disinfection. Thus, this clinical study aims to evaluate the effect of passive ultrasonic irrigation as a supplementary disinfection procedure after root canal preparation. Microbiological samples of the root canals of 30 single-rooted teeth with apical periodontitis will be taken at different stages of the endodontic treatment: before preparation (S1), after chemomechanical procedures (S2) and after passive ultrasonic irrigation (S3). The samples will be submitted to the quantitative polymerase chain reaction for bacterial quantification and to the reverse transcription quantitative polymerase chain reaction for bacterial activity analysis. Data will be analyzed using the Wilcoxon test for paired samples (p < 0.05).

Inclusion Criteria:

• Teeth with necrotic pulps and asymptomatic apical periodontitis

Exclusion Criteria:

• patients who had received antibiotics during the previous 3 months or had any general disease,
• teeth that could not be properly isolated with rubber dam,
• non-restored teeth,
• periodontal pockets depths greater than 4 mm,
• previous endodontic treatment,
• open apex,
• crown/root fracture
• root resorption or calcifications.