The main objective of this study is to compare the healing rate of periapical lesions after non-surgical endodontic retreatment in teeth obturated with gutta-percha versus those obturated with mineral trioxide aggregate (MTA), with a 12-month follow-up. Secondary objectives include evaluating differences between groups in healing time, postoperative pain, and tooth discoloration.

Visit 1: Eligible adult patients requiring non-surgical endodontic retreatment and presenting with apical periodontitis will be recruited. After providing written informed consent, participants will undergo clinical and radiographic screening to confirm eligibility criteria. Baseline assessment will include clinical examination, periapical radiography using a standardized parallel technique, and cone beam computed tomography (CBCT) imaging to evaluate the cause of failure of the previous root canal treatment and to measure the size of the periapical lesion. In addition, tooth color will be measured before treatment using a spectrophotometer in order to establish baseline values for the evaluation of possible discoloration during follow-up. Participants will then be randomly assigned to one of two treatment groups according to the obturation material used during retreatment: gutta-percha with resin-based sealer or mineral trioxide aggregate (MTA). Randomization will be performed using block randomization to ensure balanced group allocation. Visit 2: Non-surgical endodontic retreatment will be performed under rubber dam isolation and dental operating microscope by experienced endodontists. The previous root canal filling material will be removed using ultrasonic tips, manual instruments, and nickel-titanium rotary instruments. Working length will be determined using an electronic apex locator and confirmed radiographically. The canals will be irrigated with sodium hypochlorite and prepared using standard chemomechanical techniques. Calcium hydroxide will be placed as intracanal medication for a minimum of three weeks, and a temporary restoration will be placed Visit 3: The temporary restoration and intracanal medication will be removed and final irrigation will be performed. Root canals will then be obturated either with gutta-percha using the continuous wave technique or with mineral trioxide aggregate (MTA), according to the allocated study group. The treated tooth will then receive a definitive restoration. Patients will complete a questionnaire to record postoperative pain following the treatment. Visit 4, 5, 6 and 7: Follow-up visits will be scheduled every three months for up to 12 months or until further intervention is required. At each follow-up visit, clinical examination and standardized periapical radiographs will be performed to evaluate periapical healing. Tooth color will also be reassessed during follow-up. When complete healing is suspected radiographically, CBCT imaging will be performed to confirm three-dimensional healing.

Inclusion Criteria:

Patients ≥18 years old American Society of Anesthesiologists (ASA) physical status I and II Teeth requiring non-surgical endodontic retreatment Periapical lesion ≤10 mm in maximum diameter on periapical radiograph Apical canal diameter ≤ size 50 Teeth without periodontal defects or mobility

Exclusion Criteria:

Smokers Teeth with insufficient remaining tooth structure Teeth requiring surgical retreatment Primary endodontic treatment complications (fractured instrument, perforation, ledge or apical transportation)