The study is planned to be conducted on a total of 30 healthy patients, aged between 18-50 years, who were diagnosed and followed up in the Department of Endodontics, Faculty of Dentistry of Yüzüncü Yıl University. The sample size was calculated in the G\*Power 3.1 program based on the data in the study of Arslan et al. In this study, the minimum required number of patients was determined as 30 by taking effect size 1.252193, alpha=0.05 and power=0.90. Mandibular first and second molars diagnosed with Chronic Apical Periodontitis will be included in the study group. Patients who do not have periapical lesions, have open apex, have a probing depth of more than 3 mm, and have bleeding during probing will not be included in the study group. RANKL and OPG levels in the gingival crevicular fluid will be determined using the ELISA test. RANKL and OPG levels in the gingival crevicular fluid of teeth with periapical lesions, which are indicated for root canal treatment, will be recorded with Periopaper (Oraflow, New York, NY).

Before taking a sample of gingival crevicular fluid, the tooth will be washed with water, dried and the plaque and deposits on it will be cleaned with cotton. Then, the tooth will be isolated with a cotton pellet and a suction, and a Periopaper (Oraflow, New York, NY) will be placed 1-2 mm into the gingival sulcus until slight resistance is felt, and a sample of gingival crevicular fluid will be taken after waiting for 30 seconds. Specimens contaminated with blood or saliva will not be included in the study. Then, these samples will be stored at -80°C. The access cavity will be opened to the teeth under local anesthesia with a 2 milliliter Ultracain D-S (Aventis, The Netherlands). The working length will be determined with a #15 K-files (Mani Inc, Tochigi, Japan) with the apex locator (DentsplyMaillefer, UK) and radiographic imaging.

The preperation of the root canals will be completed by ProTaperNext (DentsplyMaillefer, Baillagues, Switzerland) up to the X3 instrument as standard in each patient. 2 ml of 5.25% NaOCl solution will be used at each file change during preperation. During the irrigation process, a 2 milliliter dental injector and a 27 gauge dental needle tip will be used during preperation. The tip of the cannulas will be adjusted to be 1 mm shorter than the working length. Calcium hydroxide (Kalsin; Aktug Tic Bornova,Izmir, Turkey) paste is placed in the root canals after the preperation process and the tooth is temporarily closed with Cavit-G (3M ESPE, Seefeld, Germany). And the patient will be given an appointment 2 weeks later. When the patient comes to the second session appointment, the temporary restoration will be removed and the calcium hydroxide paste from root canals will be removed. Canals will be irrigated with 5 ml NaOCl 5.25% and 17% EDTA solution for the removal of calcium hydroxide. Then canals will reshaped with ProTaperNext X3 (DentsplyMaillefer, Baillagues, Switzerland). And then again 5 ml. NaOCl 5.25% and 17% EDTA solution will be used for irrigation to be ensured that calcium hydroxide is completely removed from the root canals. For final irrigation respectively 5 ml NaOCl %5,25 solution , 5 ml. % 17 EDTA solution and %5,25 NaOCl solution will be used. According to NaOCl activation (Group1: NaOCl, Er,Cr:YSSG will be activated by laser during the final irrigation; Group2 (control): Routine final irrigation procedure will be done, NaOCl will not be activated by any method) teeth will be divided into two main groups (n:15).

After final irrigation and activation procedure, root canals will be dried with sterile paper points, then all root canals will be obturated with sterile gutta-perchas and AH Plus root canal sealer (Dentsply DeTrey Gmbh, Konstanz, Germany) using the single-cone technique. Teeth with completed root canal filling will be restored with composite filling material to ensure coronal sealing. After the treatment, the patients in both groups will be called for control at periodically (1st month - 3rd month) and gingival crevicular fluid samples will be taken again with Periopaper (Oraflow, New York, NY). According to the results which is obtained with the ELISA test, the change in the RANKL and OPG levels in the gingival crevicular fluid of the patients will be evaluated.