## 2 Detailed Case Description  
A 12-year-old girl with pain related to biting was referred to the Department of Restorative Dentistry and Endodontics by a pediatric general practitioner, for treatment of the upper right central incisor. This clinical case report was conducted in accordance with the ethical principles outlined in the Declaration of Helsinki. Prior to any study procedures, the patient received comprehensive written information regarding the study objectives, procedures, potential risks, and benefits of the treatment being reported. Informed consent was obtained from the patient, and it was made clear that participation was entirely voluntary, and the patient had the right to withdraw consent at any time without repercussions. No medicaments or systemic disease influencing dental treatments were found in her medical history. Her dental history revealed a bike accident when she was eight years old, which resulted in the fracture of the crown of tooth 11 (FDI notation). A year later, due to the experience of pain related to biting, the dentist performed a root canal treatment. At age 12, the pain recurred, and she was referred to our department. On clinical examination, the crown of tooth 11 had a multisurface composite filling with dark discoloration (Figure 1a). The tooth was sensitive to palpation and percussion. No swelling was observed in the vestibule. It did not react to the sensibility test compared with the reference tooth. On the preoperative periapical X-ray, an insufficient root canal filling was visible in the short, immature root with a wide-open apex, and there was a large periapical radiolucency around it (Figure 1b), at class five according to the periapical index (PAI). Based on these findings, the case was diagnosed as a tooth with failed root canal treatment, chronic apical periodontitis, and acute exacerbation. REI was chosen for therapy because of its potential for strengthening the root by finishing the dentinogenesis. At the first session, a silicone impression was taken of the front teeth, to make a chairside immediate provisional direct restoration at the end of the session, after the removal of the previous large composite filling from the tooth. After administering local anesthesia (Lidocaine-adrenaline 20 mg/0.01 mg/mL, Egis, Budapest, Hungary), tooth 11 was isolated with a rubber dam, and the canal was accessed. While removing the poorly-fitting gutta-percha with a size #30 Hedstrom file (Dentsply Maillefer, Ballaigues, Switzerland), a perforation filled with granulomatous tissue was found bucally at the cervical area involving the coronal third of the root (Figure 2a). The granulomatoustissue was removed with a small excavator, and the perforation was sealed internally with Biodentin (Figure 2b). After the Biodentin (Septodont Ltd., Saint Maur des Fausses, France) had set (\(\sim\)12 min), the canal was irrigated with 10 mL of 1.25% NaOCl (University Pharmacy, Semmelweis University, Budapest, Hungary) and 2 mL of 0.12% hyperpure ClO\({}_{2}\) (Solumium Pental, Solumium Kft, Budapest, Hungary) to complete the irrigation. Next, the canal was dried and dressed with Ca(OH)\({}_{2}\) (Calcipast; Cerkamed, Stalowa Wola, Poland) and sealed with polytetrafluoroethylene tape and glass-ionomer cement (GIC) (Ketac Molar; 3 M ESPE, Seefeld, Germany). Finally, an acrylate provisional restoration was fabricated and cemented. A small field-of-view cone beam computed tomography (CBCT) image was taken to evaluate the extent and nature of the perforation and to create an appropriate treatment plan (Figures 2b,c and 3a). The sections of the CBCT confirmed a type 2 Ap (based on Patel classification) [14] perforating external cervical resorption. In the second session, the resorptive cavity was explored externally to finalize the restoration of the cervical defect under visual control. In absolute isolation, restoration of the coronal part of the defect was undertaken and filled with composite (Filtek Z250; 3M ESPE, St. Paul, MN, USA). The working length was established at 1 mm short of the apex.