# Endodontic Regenerative Procedures in Necrotic Adult Teeth
## 1 Introduction  
Regenerative endodontic procedures (from now on REPs) have been defined as 'biologically based procedures that intend to physiologically replace damaged tooth structures, including dentine and root, as well as cells of the pulp-dentin complex' [1]. REPs were first exclusively developed for the treatment of immature teeth, with the purpose of achieving a complete root development, increasing the root length, thickening the root wall, and accomplishing apical closure [2]. The American Association of Endodontists (AAE) [3], the European Society of Endodontology (ESE) [4], and several systematic reviews [5; 6; 7] agree that these techniques are an alternative with a huge potential in the treatment of immature permanent necrotic teeth, although they underline the poor quality of the available evidence and the need for further investigation [2; 5; 8; 9; 10; 11; 12]. These procedures can not only achieve a tooth with open apex completing its development, decreasing its susceptibility to fracture, and increasing its longevity, but also can restore tooth vitality and pulp functions thanks to the pulp tissue regeneration. Thus, sensibility, immunity, healing, and restorative properties of dental pulp would also be recovered [13], in turn reducing the chances of reinfection. Besides, it is believed that regenerated tissues may structurally be more resistant to fracture than endodontically treated teeth [14]. For this reason, the feasibility of this treatment in mature teeth should be further studied, as until now, conservative treatment for irreversible necrosis or pulpitis has been limited to conventional endodontic therapy. The high success of this technique is undeniable [15; 16], with success percentages raging, according to the systematic review made by [PERSON] et al. in 2010, from 83 to 93%, with a follow-up of 2 to 10 years [17]. Nevertheless, although the treatment would be successful, the tooth does not remain vital and, therefore, it is more susceptible to fracture and reinfection [14]. Three key elements are essential for performing REPs: stem cells, scaffolds, and activators [18]. These techniques begin with the disinfection of the root canal system, followed by the induction of bleeding in the periapical region with the purpose of obtaining a blood clot. This mass would behave as a natural scaffold for the migration of undifferentiated stem cells that come from outside the apex, mostly from the alveolar bone and periodontal ligament [3], while providing growth factors that stimulate cell differentiation and proliferation, inducing the formation of new tissue. However, the composition and concentration of the clot is unpredictable, and the uncertainty increases with age [18; 19]. When bleeding is insufficient and it does not provide an ideal blood clot, an alternative material should be used. Evidence supports the use of autologous platelet concentrates [20; 21; 22; 23], such as platelet-rich plasma (PRP) and fibrin-rich plasma (PRF) [21], which are the most popular ones. These concentrates can be used as a scaffold, by their own or in combination with a blood clot [11]. Although a closed apex is found in most of the adult teeth, it is possible to find immature teeth that physiologically should have already completed their root development, mainly due to a dental trauma, caries, or developmental anomaly, which led to the necrosis of the dental organ [24]. In Figure 1 is an adult patient's immature central incisor, necrotic due to a dental trauma. Because of the age of the patient, in these cases, some conditions must be considered. 1. Reduced revascularization potential. On a systemic level, angiogenesis is reduced by degenerative changes that occur in vessels and nerves over time [25]. In the pulp-dentin complex, age can imply a compromise of its vascular supply [26], as the size of the pulp chamber decreases due to the apposition of dentin, while calcifications tend to occur more frequently [27], as well as the apical narrowing due to the gradual apposition of dentin and cement. 2.