**2. Case selection**

The regenerative endodontic procedure is a biologically based procedure in which good case selection will have an impact on the outcomes. The recent recommendations of the American Association of Endodontists (AAE) suggest that the regenerative endodontic procedure should be carried out in teeth with necrotic pulp and immature roots, and that the root canal space should not be utilized to retain a coronal restoration in the future. In other words, the selected tooth should have enough coronal tooth structure that it will need a post to retain the coronal core [15].

It would appear that the aetiology of pulp necrosis does not play a role in case selection here; many regenerative endodontic case studies did include necrotic immature teeth with different aetiologies, whether the pulp necrosis was caused by a caries lesion [16], was secondary to trauma [17] or due to dental anomalies [9].

In view of the fact that root canal space will be occupied by a blood clot and a bacterially tight hard barrier, teeth with insufficient remaining tooth structure are not recommended to be selected, since this would require further retentive means (post). In addition, in some cases, if the tooth structure loss is massive, isolation might become problematic. Patient and/or parental cooperation is necessary, as the treatment requires multiple visits and regular monitoring and follow-up appointments. Furthermore, the consent form must be signed.

The chances of success for teeth with immature root apices to be re-vascularized are significantly higher than for teeth with fully or nearly fully formed root apices. This could be attributed to the better ingrowth of vasculatures and stem cells flow in the pulp canal.

A study by Kling et al. [18] on re-implanted teeth noticed that teeth with apical foramen diameters of 1 mm or less had no chance of re-vascularization. In different circumstances, teeth with apical foramen diameters ranging from (1.1 mm - 5.0 mm) had spontaneously vascularized 18% of the time. They noticed also that when extra-oral time was under 45 minutes re-vascularization significantly increased; 39% of the teeth re-vascularized with shorter periods, compared to 11% only with longer periods (P < 0.05).

#### *Regenerative Endodontic Procedure in Immature Permanent Teeth DOI: http://dx.doi.org/10.5772/intechopen.96986*

In addition, blood samples taken from incomplete root apices showed levels of CD 73 and CD 105 (Mesenchymal stem cell markers) almost 600 times higher than in samples obtained from the circulating blood stream, indicating the higher availability of stem cells in the apical dental papilla of immature roots [19].

Lovelace et al. did not obtain samples from mature roots for comparison. This could have produced similar results. However, the findings are supported by another study that apical papilla of the immature root apices represent a superior reliable source of stem cells that possess high surviving and cells turn over levels [20].

Immature roots with periapical pathosis of endodontic origin can undergo the regenerative endodontic procedure. Many published case reports have revealed potential root maturation in non-vital immature permanent teeth with the presence of periapical lesion or apical abscess [8, 9, 21, 22] that produced a complete resolution of the apical radiolucency in addition to a successful root maturation.

There are three possible explanations for this. The first is that immature roots with open apices allow easy communication between the root canal pulp and the periapical area. This may allow infection and inflammatory cells to reach the apex quickly while the pulp tissue is partially vital. In addition, the stem cells in the pulp and periapical papilla could survive the infection due to the high vascularity. Thus, the stem cells still can differentiate and allow root maturation in spite of the ongoing infection. The second explanation is that in young patients, the majority of the jawbone is cancellous and the bony trabeculations are larger. Therefore, resorption and periapical radiolucency may be formed in a short period and not all of the pulp tissues had turned necrotic [20]. Finally, the periapical radiolucency could be related to the apical dental papilla, where a radiolucent shadow of the dental papilla is expected and not necessarily to periapical pathology.

As a general statement, regenerative endodontic procedures should be performed if the patient is not allergic to any of the antibiotics or the medicament or irrigation agents that are usually used in sterilizing and dressing the canal in this procedure. In addition, patients should be classified as American Society of Anaesthesiologist (ASA) I or ASA II physical status. ASA I means that the patient is healthy and is a non-smoker, with no or minimal alcohol consumption. ASA II defined as with a mild disturbance without any significant functional limitations, for example if a person is fit and well but a smoker or social alcohol consumer. In addition, pregnancy, controlled Diabetes mellitus, and obesity (BMI over 30 and less than 40) all fall into ASA II classification.

This requirement is to ensure better tissue healing and a favourable response toward a stem cells differentiation, and thus continuation of root maturation without complication.
