## 2 Detailed Case Description
It was determined on CBCT and verified using an electronic apex locator (Woodpex III, Guilin Woodpecker Medical Instrument Co., Ltd., Guilin, China). The canal was mechanically prepared with gentle brushing motions with a #60 K-file (VDW, Munich, Germany). During chemical irrigation, 15 mL of 1.25% NaOCl was used and activated with ultrasound. Flushing with sterile saline was followed by irrigation with 20 mL of 17% EDTA. A side-vented needle, 1 mm short of the working length, was used to avoid extrusion of the solutions into the periapical area. The canal was dried and  
Figure 1: The initial situation: (**a**) presenting discoloration of the right central incisor; (**b**) preoperative periapical X-ray highlighting inadequate obturation sealing with a short root canal filling. dressed with Ca(OH)\({}_{2}\)[15]. The access cavity was closed with GIC and an acrylic temporary restoration. Four weeks later, there were no clinical signs or symptoms of inflammation. Local anesthesia without vasoconstristors (Scandonest 3% plain; Septodont Ltd., Saint Maur des Fausses, France) was given to avoid insufficient bleeding before absolute isolation. After accessing, the canal was irrigated with 10 mL of 0.12% hyperpure ClO\({}_{2}\) to remove the Ca(OH)\({}_{2}\) and disinfect the canal again. Ultrasonically activated EDTA (17%) was used to liberate fossilized growth factors in the dentin [16; 17]. The apical tissues were lacerated with a #35 Hedstrom file to initiate bleeding into the canal space, which provides a blood clot as a scaffold. At the same time, the influx of mesenchymal stem cells and platelet-derived growth factors make pulp tissue regeneration possible. The level of the blood clot was established at 3 mm below the cementoenamel junction (Figure 4a). A resorbable collagen sponge (Collaplug; Zimmer Bionmet Dental, Palm Beach Garden, FL, USA) was placed on the top of the blood clot to function as a matrix to facilitate the placement of the Biodentin. A 3 mm thick coronal plug of Biodentin (Figure 4b) was created to provide a bacteria-tight seal. Figure 3: Sections of the CBCT: (**a**) bucco-lingual cross-section; (**b**) 3D reconstruction of the buccal bone plate; (**c**) horizontal cross-section indicating the localization of the cervical resorption. Figure 2: Perforation management at the first session: (**a**) clinical photo illustrating the perforation across the buccal wall; (**b**) closure of the perforation with Biodentin. After confirming the barrier, the tooth was restored with an extended composite direct veneer using the layering technique (Micerium Hri; MICERIUM S.p.A., Avegno GE Italy) (Figure 5). The patient was recalled at six months, one year, and two years. Clinical follow-up was performed according to ESE quality guidelines [18]. The patient remained symptom-free. After six months, an X-ray showed no periapical radiolucency. After one year, an increase in root thickness in the apical third and a decrease in size of the apical foramen were visible (Figure 6). At the 24-month control, the tooth was clinically and radiologically asymptomatic; no sensitivity to percussion or palpation was detected. However, there was no reaction to cold, probably due to the technical difficulty of stimulation, as the revascularized pulp tissue was 3 mm below the bone level covered with Biodentin. A small field-of-view CBCT image (Figure 7) indicates that the resorption did not progress further. A calcified bridge formation was observed at the apex. Figure 4: Blood clot formation and Biodentin placement: (**a**) blood clot formed at the level of cementoenamel junction; (**b**) blood clot sealed with Biodentin. Figure 5: Composite restoration.