**1. Introduction**

It is widely recognised that full maturation of the root apex of permanent tooth is expected to occur three years after the time of eruption. During this period, an immature tooth may encounter situations such as traumatic injury, extensive caries and dental anomalies to the developing dentition, which in turn may lead to pulpal necrosis. This will hinder root maturation and will cause premature loss of the permanent dentition.

Necrotic immature teeth exhibit challenges for cleaning and shaping due to the presence of wide pulp canals, thin fragile walls and blunt root apices. In addition, it is difficult to obturate such a wide canals with large open apices to obtain hermetic apical seals.

At first, necrotic immature teeth were treated by calcium hydroxide apexification [1], which requires multiple visits to a clinic in order to change the calcium hydroxide dressing until a hard apical barrier is formed. However, long-term calcium hydroxide dressings will negatively affect the root canal dentine flexural strength and will make the tooth more susceptible to fracture [2, 4]. The apexification procedure was later modified by introducing an artificial hard barrier, Mineral Trioxide Aggregate (MTA) [3]. MTA apexification can be done in one or two sessions and provides a more reliable apical bacteria-tight seal. It also is biocompatible with periapical tissues and promotes hard tissue barrier formation [4]. However, MTA apexification does not induce root maturogensis, leaving the immature tooth

with thin root walls that are susceptible to fracture. On the contrary, regenerative endodontic procedures can help stimulate the formation of a new pulp/dentin complex in the pulp canal space, which will put the tooth in a more favourable physiological status [5].

Regenerative endodontic procedure (REP) is a biologically based treatment that aims to heal periapical periodontitis and substitutes the damaged structure including dentin, cementum, and cells of pulp/dentin complex in order to continue the tooth-growing process. Although the histological characteristics of dentin/ pulp tissues are not yet clear, radiographic evaluation has revealed a resolution of periapical lesions, an increase in root length to complete apical root formation, and a thickening of canal walls.

The idea of revitalization of lost tissues in the empty root canal was discussed first by Nyggard Ostby in 1961 [5]. He suggested that introducing a blood clot into the sterilized pulp canal would stimulate new tissue formation and heal the periapical pathosis. His hypothesis was based on the significance of blood clots in the healing of fractured bones [6].

In the histologic analysis, Ostby [5] noted connective tissue ingrowth inside the pulp canal, with scattered islands of mineralized tissue implanted into the newly formed connective tissue. In addition, the signs and symptoms of the necrotic teeth had disappeared and the apical radiolucency had healed. Although no odontoblasts were observed in the histological analysis and unwanted cells (cementoblast) were present, the presence of fibroblast and newly formed tissues was the foundation for regenerative endodontic treatment.

In 1971, Ostby and Hjortdal [7], published a case series of regenerative treatments, but they used antibiotics in the disinfection protocol. They observed an increase in root length, healing of periapical lesions, and a resolution of signs and symptoms.

The contemporary REP was first published in 2001 by Iwaya et al. [8]. In this case, the author reported on a necrotic immature premolar treated with sodium hypochlorite (NaOCl) and hydrogen peroxide (H₂O₂), then dressed with a doubleantibiotic paste (DAP), which consist of metronidazole and ciprofloxacin **Figure 1**. This treatment protocol resulted in the resolution of inflammatory signs and symptoms, full root maturation and response to cold test. The second case by Banchs and Trope in 2004 [9] used NaOCl and chlorohexidine (CHX) to disinfect the canal

#### **Figure 1.**

*On the left side, pre-operative radiograph of lower right 2nd premolar with open apex, incomplete root formation and periapical radiolucency. In the middle, radiograph taken after five months of calcium hydroxide dressing. On the right side, 35 months after treatment completion revealing complete root formation increase in root width and length [8].*

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

chemically and employed a Triple Antibiotic Paste (TAP) containing metronidazole, ciprofloxacin, and minocycline, and observed the same result of Iwaya et al. [8].

These two cases established the fundamentals for the recent regenerative case reports. The fundamentals were the removal of bacteria and disinfecting the root canal; establishment of a scaffold; the introduction the stem cells for new tissue formation; and having a coronal bacterial tight seal to prevent recontamination of the root canal system.

Several terms had been used in case reports to describe the ingrowth of new tissue inside the root canal. These are revascularization, revitalization and regeneration. Revascularization is defined as the re-establishment of vascular supply to the alreadypresent pulp of the immature tooth [10]. Revitalization is defined as the ingrowth of tissue that may differ from the initial original tissue [11]. Endodontic regeneration is defined as the replacement of lost or damaged structures including dentine, root structures and cells from pulp/dentin complex, by another structure [12].

#### **Figure 2.**

*(A) Pulp like connective tissue generated in human tooth after Regenerative Endodontic Therapy. (B) Higher magnification of the soft tissue showing the presence of collagen fibers, cells, and blood vessels in this tissue [13].*

Lately, there have been two histological case reports of teeth extracted after having endodontic regenerative treatment [13, 14]. These studies gave a clearer idea of what was happening inside the canal after regeneration.

In the first histological case report [13], the tooth became painful and symptomatic 14 months after receiving regenerative treatment. When the pulp was extirpated and examined histologically, the author found a vital loose pulp-like connective tissue **Figure 2**.

In the second histological report [14], three and a half weeks after regenerative treatment was completed, the tooth had a complicated crown root fracture that left the remaining tooth structure beyond restoration. After extracting the tooth, the pulp canal content was examined histologically; the author reported the content of the pulp canal as a loose connective tissue, similar to pulp tissue, but histologically proven to be not pulpal tissue.

The histological findings of these two case reports did not provide sufficient evidence to represent all teeth that had undergone an endodontic regenerative procedure. However, the available findings suggest that tissue regeneration is occurring inside the root canal. Accordingly, endodontic regenerative procedure is the most relevant term to describe the type of tissue grows in the pulp canal.
