## 1 Introduction  
Regenerative endodontic procedures (REPs) have emerged as a treatment option for immature necrotic teeth with apical periodontitis to allow the reestablishment of a newly formed vital tissue and enable continued root development. There is no standard protocol of REPs; hence, the American Association of Endodontists (AAE) as well as the European Society of Endodontology (ESE) suggest Quality Clinical Guidelines for a regenerative procedure which was adapted in both of the case reports hereby described [1; 2]. The clinical protocol of REPs differs mostly in terms of concentrations of sodium hypochlorite irrigant, intracanal medication, the method to induce the bleeding, and the use of biological matrices (Table 1). One of the goals of REP is root thickness and length augmentation to reinforce root canal walls [3]. The appropriate management of infected canals can create a favorable environment for tissue regeneration since open-apex immature teeth favor the reestablishment of a rich blood supply and stem cells [4]. Considerable advances in disinfection techniques that combine the balance between antimicrobial effect and biocompatibility with mesenchymal stem cells (MSCs) have been developed. Similarly, it has been demonstrated that intracanal bleeding evoked from the apical tissues brings substantial numbers of MSCs into the canal system during REPs in immature teeth [5]. However, the characteristics and phenotype of this tissue in the presence of inflammation are still largely unknown [6]. \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline
**European Society of Endodontology Position Statement:** & **American Association of Endodontists Clinical Considerations** \\
**Revalidation Procedures** & **for a Regenerative Procedure** \\ \hline
**First Appointment** & **First Appointment** \\ \(\bullet\) & Perform clinical diagnostics according to a clinical and radiographical assessment checklist. \\ \(\bullet\) & Tooth cleaning, local anesthesia (optional), field isolation and disinfection (with, e.g., Iso-Betadine). \\ \(\bullet\) & Prepare access cavity. \\ \(\bullet\) & Remove loose or necrotic pulp tissue using suitable endodontic instruments. \\ \(\bullet\) & Avoid mechanical instrumentation of the root canal walls. \\ \(\bullet\) & Irrigate with 1.5-3\% sodium hypochlorite (20 mL, 5 min), use of side-vented needle, place 2 mm above vital tissue (which can be controlled using an operating microscope or when the patient reports pain). \\ \(\bullet\) & Bleeding or exudate (control with paper points) may require extended irrigation; irrigate with sterile physiological saline (5 mL) to minimize the cytotoxic effects of sodium hypochlorite on vital tissues. \\ \(\bullet\) & Dry with paper points. \\ \(\bullet\) & Irrigate with 20 mL of 17\% EDTA. \\ \(\bullet\) & Insert a non discolouring calcium hydroxide product homogenously into the root canal. The antibiotics, mainly triple antibiotic paste (TAP) consisting of ciprofloxacin, metronidazoleze and muscolcemic were used with good results. Drawbacks such as discoloration, cytotoxicity, sensitization, development of resistance and difficulty of removal from the root canal need to be considered. \\ \(\bullet\) & Place coronal seal directly onto intracanal dressing with a minimum thickness according to the material selected. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison between intraoperative clinical aspects of European Society of Endodontology (ESE) _Position Statement: Revitalization procedures_ adapted from ESE [1] and _Clinical Considerations for a Regenerative Procedure_ adapted from American Association of Endodontists (AAE) [2]. The apical papilla is the source of a particular type of MSC, namely, stem cells of the apical papilla (SCAPs), which plays an important role in physiologic root development and may also contribute to further root development during REPs [4,7,8].