Abstract
Newer scientific technological advancement in dentistry provides an array of projects such as molecular biology, cell culturing, tissue grafting, and tissue engineering. Conventional root canal treatment, apexification with biomaterials, and extractions are the procedures of choice to treat a nonvital tooth. These treatment options do not give predictable outcomes in the regeneration of the pulp tissue. This can be easily achieved by regenerative endodontics wherein the diseased or a nonvital tooth is replaced by a healthy and functional pulp-dentin complex. The rationale for regenerative endodontics follows tissue engineering techniques. This article reviews the shift in regenerative endodontic techniques.
Introduction
Tissue engineering is a newer and promising therapeutic approach in various specialties of dentistry aiming to repair and regenerate the structure and restore the function of damaged dental structures. The current regenerative concepts can reform the dental health provision. The rationale for regenerative endodontics is the placement of the diseased/traumatized/nonvital pulp tissue into a healthy and functional pulp-dentin complex. The American Association of Endodontists’ Glossary of Endodontic Terms (2012) defines regenerative endodontics as “biologically based procedures designed to physiologically replace damaged tooth structures, including dentin and root structures, as well as cells of the pulp-dentin complex.” 1 Attempts for regenerative endodontics had been started long back since 1952 where vital pulp amputation was done by the use of calcium hydroxide. 2 Revascularization of the necrosed pulp for re‑establishing a pulp‑dentin complex in permanent teeth had also been evaluated by Nygaard Östby in 1961. 3 The purpose of this article is to review novel biomimetic techniques and their scope to develop regenerative endodontic techniques (RETs).
Regenerative endodontics can be a treatment of choice in patients with a nonvital tooth with an open apex, especially in paediatric dentistry and endodontics. Complete root formation is vulnerable in a nonvital tooth that makes the tooth weak and enables the tooth to withstand normal masticatory forces, resulting in a high rate of root fractures. Moreover, there are a number of studies stated that over within 10 years after trauma, 50% of such teeth will be lost despite being endodontically treated.4-6 The contemporary treatment options for an immature tooth or open apex have not shown predictable and successful treatment outcomes, thereby resulting in root fractures.4,5 On the contrary, the use of biomimetics in regenerative endodontics aims towards a complete root development with an increase in the thickness of the root dentinal walls, thereby resulting in a healthy and functional pulp-dentin complex. 7
RETs work on the following 2 principles:
Pulp revascularization = induction of angiogenesis in an endodontically treated root canal. Pulp regeneration = pulp revascularization + restoration of functional odontoblasts and/or nerve fibres.
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The triad of tissue engineering is as follows:
Stem cells: to respond to growth factors Scaffold of the extracellular matrix Growth factors (signals for morphogenesis)
Stem Cells
These are the clonogenic and undifferentiated cells capable of both self-renewal and multilineage differentiation.
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Stem cell differentiates into 2 different types of cells: 1 daughter stem cell and 1 progenitor cell. Stem cells are of 2 types
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Embryonic stem cells: these are isolated from the inner cell mass of the blastocyst stage of development. Postnatal stem cells: they are located in and isolated from tissues such as bone marrow, neural tissue, dental pulp, and periodontal ligament.
For RET, the postnatal dental stem cells show promising results due to their remarkable odontogenic capacity as compared with other nondental stem cells and lesser chances of immune reactions.11,12 Types of postnatal dental stem cells and their sources are as follows:
Permanent teeth: dental pulp stem cells derived from the third molar.
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Deciduous teeth: stem cells from human-exfoliated deciduous teeth. The stem cells are present within the pulp tissue of deciduous teeth.
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Periodontal ligament: periodontal ligament stem cells.
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Stem cells from the apical papilla (SCAP).
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Stem cells from the supernumerary tooth mesiodens.
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Stem cells from teeth extracted for orthodontic purposes.
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Dental follicle progenitor cells.
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Stem cells from the human natal dental pulp.
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The proliferation of SCAP is much higher among all the postnatal dental stem cells. Although these cells have more promising applications, from a practical perspective, harvesting of autologous stem cells is very difficult and technically sensitive, and obtaining a specific subset of stem cells is even more difficult. 16 Current studies on the human pulps show the potential mesenchymal stem/progenitor cells with regenerative capabilities even in inflammatory conditions. 21 Epithelial cell rests of Malassez are also shown to be capable of undergoing the epithelial- mesenchymal transition. 22
Scaffolds
Scaffolds are the biomaterials that act as carriers for specific cell-type guide and support tissue regeneration. The ideal scaffold is one which facilitates cell growth, differentiation, and organization at the desired site. 23 The porosity of the scaffold should allow cell placement, resulting in an effective transport of nutrients and waste.23,24
Classification of Scaffolds
Scaffolds can be classified as follows:
Based on degradability of matrices
Biodegradable scaffolds Permanent or biostable scaffolds Based on form
Solid blocks Sheets Porous sponges Hydrogels (injectable scaffolds) Based on the presence or absence of cells
Cell-free scaffolds Scaffolds seeded with stem cells Based on origin
Biological or natural scaffolds Artificial or synthetic scaffolds
Scaffolds that have been commonly used for regenerative procedures are natural scaffolds, such as collagen, chitosan, silk, fibrin, and synthetic scaffolds, such as polyglycolide and polyglycerol sebacate. Blood clot, platelet-rich plasma (PRP)25 as well as platelet rich fibrin PRF26 have been recently tried as a natural scaffold in regenerative endodontics.
PRP, an autologous first generation platelet concentrate with a rich source of growth factors, has been proposed as a potential substitute scaffold. It is a concentrated suspension of different growth factors such as platelet‑derived growth factor (PDGF), transforming growth factor (TGF‑β), insulin‑like growth factor (IGF), vascular endothelial growth factor (VEGF), epidermal growth factor, and epithelial cell growth factor. These are released via the degranulation of α-granules and stimulate bone and soft‑tissue healing. 27 PRF is a second-generation platelet concentrate named as Choukroun’s PRF after its inventor. 26 The procedure consists of drawing blood which is collected into test tubes without an anticoagulant and is centrifuged instantaneously. A tabletop centrifuge can be used for 10 min at 3000 rpm or for 12 min at 2700 rpm. PRF is considered as an ideal biomaterial for pulp-dentin complex regeneration. It has been showed that it prevents the early encroachment of undesired cells, thereby acting as a viable barrier between the desired and undesired cells. Healing is faster as it releases its growth factors steadily with the peak level reaching at 14 days corresponding to the growth pattern of periapical tissues. It also accelerates wound closure and mucosal healing due to fibrin bandage and growth factor release. This fibrin meshwork is shown to be the natural guide of angiogenesis as well as the natural support to immunity.
The most common problems encountered with respect to scaffolds are the lack of an appropriate vascularized scaffold to promote the formation of large tissue constructs. 28 Precise arrangements of cells and the formation of tissue constructs that imitate the natural tooth pulp are achieved by the 3-dimensional (3-D) cell-printing technique. 29 Once pulp tissues after tissue engineering are formed, they can be delivered to the site of action as a 3-D matrix (eg, polymer hydrogel) 29 through an injectable scaffold delivery system. The treated dentin matrix also provides a suitable environment for the regeneration of the dental tissue. 30 Enamel matrix derivatives (Emdogain) contain the amelogenins that have also been used as potential scaffolds. 31
These tissue-engineered scaffolds are used for creating a tissue construct. These are maintained in cell culture in the presence of bioactive molecules, that is, growth factors to promote the formation of a more complex tissue structure. 32
Growth Factors
Growth factors are polypeptides or proteins that bind to specific receptors on the surface of target cells (eg, bone morphogenetic protein [BMP] receptors) and induce numerous cellular activities such as migration, proliferation, differentiation, and apoptosis of all dental pulp cells, including stem/progenitor cells. 33
Various growth factors and their functions are as follows
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TGF-β. It is secreted by odontoblasts and is preserved in an active form within the dentin matrix. It also stimulates the growth and differentiation of pulp cells as well as increase in tertiary dentin matrix secretion. BMP. It regulates the differentiation of pulp cells into preodontoblasts. It also acts as a modulator of its activity on the initiation of the cytological and functional differentiation of odontoblasts. PDGF. It acts as an angiogenic growth factor. VEGF. It induces an angiogenic response in the severed pulp. It also helps in the revascularization process.
TGF-β1 and TGF-β3 are the types of growth factors that are responsible for the odontoblast differentiation and stimulation of the dentin matrix for pulp regeneration. Growth factors such as PDGF, TGF, BMPs, VEGF, IGF, and fibroblast growth factor have the potential to modulate the repair and regeneration process. 34 The TGF and BMP are the 2 most important growth factors responsible for the regeneration of the pulp-dentine complex. TGF-β1 and TGF-β3 are responsible for the differentiation of odontoblast and stimulation of dentin matrix secretion. 35 As compared with calcium hydroxide, BMPs induce the formation of quantitative and more homogeneous reparatory dentin with the well-defined odontoblastic process. Other materials such as dentin grains that release bio-active molecules and Emdogain, which is known to form the dentin, are also used. 32 These growth factors are required to be present in an optimal concentration and for optimum time at the desired site of action, which is very difficult to attain and requires further research.
Conclusion
Regenerative endodontics is still in its inception phase. Although in vitro studies have shown these strategies to be safe, effective and highly potential, still there are numerous roadblocks to be crossed before their widespread and predictable clinical applications. Considerable research is required to advance the regenerative therapeutics to the next level. With new discoveries, innovative ideas, and high-quality research, in the future, the scope of regenerative endodontics might increase to include the replacement of periapical tissues, gingiva, and even whole teeth.
Declaration of Conflicting Interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.