## 3 Discussion  
In the present case report, regenerative treatment was successful in a previously obturated, periodontitis involved tooth with ECR. A case series study [19] indicates that successful treatment of necrotized immature teeth combined with external root resorption with RET is plausible. However, other case studies indicate that RET can be applied in other challenged situations as well. [PERSON] et al. [20] illustrate a case where RET salvaged a tooth with internal resorption. [PERSON] et al. [21] report the successful management of a  
Figure 6: Radiographic follow-up: (**a**) preoperative stage; (**b**) 6-month control; (**c**) 1-year control. The arrow indicates the thickening of dentinal walls and the lengthened root. Figure 7: 24-month control CBCT: (**a**) 3D reconstruction of the healed buccal bone plate; (**b**) bucoc-lingual cross-section; (**c**) meso-distal cross-section. case, with perforation due to internal resorption, using RET. Another case demonstrates successful regenerative treatment of a previous root canal treatment [8]. Traumatic tooth injury can result in neurovascular disruption, pulp necrosis, and damage to the periodontal ligament and cementum, which might cause ECR. The exact pathomechanism of ECR is still unknown, but trauma seems to be one of the most common etiologic factors of ECR [22]. Additionally, injury of the periapical tissues can seriously negatively affect normal root development, leaving the root short and the dentin wall thin. Recent publications [23,24] have shown that removing the protective pericanal resorption-resistant sheet during root canal treatment also predisposes the tooth to a more aggressive form of ECR. Perforation of the pulp happens only in the advanced stages of ECR [25]. However, penetration of the resorption to the pulp chamber cannot be prevented after root canal treatment [23]. As a result, the continuous influx of bacteria through the perforation into the pulp chamber results in a more severe root canal infection and hinders satisfactory disinfection during regenerative procedures. It is necessary to excavate and restore the resorptive defect before disinfection of the canal, in order to provide an optimal bacteria-free environment [26]. It is challenging to avoid excessive marginal bone and sound tooth structure removal and to completely eradicate the resorptive tissue simultaneously. If the size of the portal of entry is wide and the lesion penetrates the coronal part of the root, it is advisable to use external and internal methods as therapy [24]. Therefore, it was performed using two steps. First, the defect was cleaned and sealed internally. Second, the coronal part was finalized externally and restored with composite afterwards. According to [PERSON]'s classification of root development [27], the tooth belonged to class 3 (2/3 root development with open apex). In class 3 teeth, RET is recommended because of the potential to facilitate dentinogenesis [10], contrarily to apexification. Although the success rate of periapical healing is similar, animal studies prove that teeth treated with RET can withstand more stress-induced fractures than untreated ones [7]. However, there is a lack of long-term clinical evidence. Performing RET on a tooth that has already been root-canal-treated and has persistent apical periodontitis means a significant challenge [28]. First, the materials used during the first root canal treatment are not known, and their remnants might have a deleterious effect on regeneration. Second, eliminating persistent bacteria after a failed endodontic treatment requires a more potent chemical protocol [8]. Finding the balance between optimal disinfection and maintaining the vitality of periapical stem cells is the key to successful revascularization. High concentrations of NaOCl, additional use of chlorheroidine, and antibiotics can help reduce the bacterial load in the canal [29].