Until now, conservative treatment for irreversible pulpitis or necrosis in adult teeth 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 Ny 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]. Performing REPs, it may be possible to restore these properties.

However, some conditions must be considered regarding the age of the patient. 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 cementum. 2. Changes in the functionality and potential of stem cells. Over the years, the ability of multipotent stem cells to replace damaged tissues decreases [20]. This could be related to the fact that the differentiation capacity of MSCs decreases over time, but not to the amount of inflow of MSCs into the root canal system in adults, because this amount does not change with age [28]. 3. Smaller diameter of the apical constriction in mature teeth. Currently, there is no consensus regarding the optimal size of the apical foramen for REP [14]. It should be as small as possible without affecting cell migration and reinnervation [29]. In 2007, Murray suggested that, in teeth with closed apexes, revascularization by inducing bleeding may require an apical instrumentation of 1 or 2 mm to allow the flow of bleeding [18]. Later, some authors proposed it as feasible to succeed apical diameters smaller than 1 mm [30] or equal to 0.5 mm [18].

Three key elements are essential for 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].

REPs’ scientific evidence in adult teeth is limited. Currently, there is no specific clinical protocol for the management of these teeth. Regarding the promising future of these techniques, it is necessary to analyze the considerations that must be considered for the treatment of these teeth and the results obtained up to nowadays.

2. Development and Findings