###### Abstract  
The consequences of traumatic injuries are pulp necrosis, periapical inflammation, and possible external cervical resorption (ECR). The concomitant cessation of root maturation and dentinogenesis in premature teeth result in a fragile tooth. Revascularization of the pulp might be an alternative treatment option. A 12-year-old patient was referred for retreatment of chronic apical periodontitis with acute exacerbation after root canal treatment of an upper central incisor. During gutta-percha removal, an ECR was detected. First, it was closed internally with Biodentine, followed by an external composite closure after a cone beam computed tomography evaluation. In the first session, chemomechanical cleaning was aided with sodium hypochlorite and hyperpure chlorine dioxide, and the canal was dressed with Ca(OH)\({}_{2}\) until the next session. In the next session, only the low-toxicity, but adequate, bactericidal hyperpure chlorine dioxide was applied for irrigation, before the provocation of periapical bleeding into the canal. The blood clots were covered with Biodentine, and the tooth was restored with composite filling. During the 24-month follow-up, clinical signs/symptoms had disappeared, and the periapical lesion was resolved. Increased root thickness in the apical third and decreased size of the apical foramen were detected. In conclusion, regenerative endodontic procedures could potentially be used to retreat immature teeth with persistent apical periodontitis and external cervical resorption.  
Keywords:dental trauma; external cervical resorption; regenerative endodontic procedure; retreatment; hyperpure chlorine dioxide +
Footnote †: journal: Medicalstem cells, growth factors, and scaffold simultaneously, is to induce bleeding into the root canal via laceration of the periapical area [6].  
This novel treatment modality offers several benefits, including the resolution of periapical lesions, ongoing root development, improved crown-root ratio, thickened root canal walls, and, consequently, increased fracture resistance [7; 8]. Furthermore, successful regeneration can restore the immune defense function and sensibility of the tooth once vital tissue reoccupies the root canal.  
However, regeneration can only occur after disinfection and creation of a microenvironment adequate for stem cell survival and differentiation [9]. Complete root canal disinfection is always complex during primary treatment [10] but even more difficult during retreatment. Therefore, revascularizing the tooth after previous endodontic therapy is challenging, and thus, only a few cases of using revascularization for re-treatment have been reported [8]. Hyperpure ClO\({}_{2}\) does not inhibit the viability of periodontal ligamental stem cells [11] at the toxic concentration for microbes due to its size-selective properties [12]. Therefore, it could be an excellent candidate for disinfecting the canal before revascularization.  
External cervical resorption (ECR) could further complicate a traumatic case. This progressive resorptive condition can adversely affect the success of regenerative treatment if it establishes a connection between the oral cavity and the root canal. To the best of our knowledge, no reported case studies documenting regeneration in the presence of ECR exist. In such a case, cleaning the resorptive defect and hermetically sealing the perforation are essential to establish a bacteria-tight barrier before the regenerative procedure [13].  
This report aimed to showcase the positive outcomes of the regenerative treatment approach performed on an immature upper central incisor with external cervical resorption. The tooth had a poorly obturated root canal and exhibited radiographic and clinical signs of apical periodontitis with acute exacerbation.