The goal of this clinical trial is to learn if adjuvant antimicrobial photodynamic therapy (aPDT) can help in the treatment in patients with pieces with apical periodontitis and the presence of fistula, in terms of reducing symptoms, resolution of fistula and the clinical signs of inflammation at the oral mucose, as well as, evaluation radiographic parameters.

Participants will undergo conventional endodontic treatment at Group I (n=15) and will undergo conventional endodontic treatment combined with antimicrobial photodynamic therapy at Group II(n=15).

The calculation of the sample size was conducted with the statistical software G-Power 3.1 applying the chi-square homogeneity test. Identical sample sizes were considered for the aPDT group and the sham group. The percentage of type error was set at 5% and the statistical power at 80%, obtaining n = 30 considering 10% losses. The distribution of data within each group and the homogeneity of the variants will be verified for the choice of an analysis of variance or not. With this information the most proper statistical test will be conducted. The sample size calculation is based on the literature and a 5% significance level will be adopted

Inclusion Criteria:

Patients must have a single-rooted tooth with apical periodontitis and the presence of fistula.

Patients must be over 18 years of age.

Exclusion Criteria:

Patients with comorbidities such as cancer, diabetes, coagulation diseases, and anemia; Patients undergoing orthodontic treatment; Patients with periodontal pockets deeper than 4 mm; Patients taking antibiotics; Pregnant or lactating patients; Patients with parts that present the impossibility of performing absolute isolation.

All participants will receive conventional endodontic treatment. The study group will receive antimicrobial photodynamic therapy with methylene blue application followed by active diode laser illumination. The control group will receive methylene blue solution and a simulation of laser application, which will be achieved by using an aluminum foil barrier placed at the tip of the equipment to block light in the control group. During this process, the participant will receive googles which aid eye protection and helps masking. The operator will wear protective glasses, which will aid in protection. Pre-irradiation and irradiation times will be the same at both groups.