# Different Approaches to the Regeneration of Dental Tissues in Regenerative Endodontics
## 1 Introduction
As a third component in the tissue engineering triad, the scaffold is described as a three-dimensional (3D) biocompatible material that provides mechanical support for bioactive molecules or cells, and acts as an extracellular matrix template, predisposing the adhesion and proliferation of a specific cell type [41], such as pulpal cells. Ideally, the scaffold should have high porosity to facilitate cells deposition, and to permit effective nutrient and gas exchange. Moreover, it should have the proper physical and mechanical properties, and also be entirely biodegradable. However, the scaffold degradation must be equal to a formation rate of new tissue [42,43]. There are various types of scaffolds known, based on their origin; natural scaffolds (e.g., collagen, hyaluronic acid, PRF, PRP, blood clot, chitosan) and artificial scaffolds (e.g., polymers of polyglycolic acid, polylactic acid, polyepsiloncaprolactone, glass-ceramic, and bioactive glasses) [18,42,44] differ in attributes and properties. Scaffold technology has shown promising advancements in regenerative dentistry, scientifically demonstrated in immunodeficient mice. The researchers obtained a regeneration of dentin-like tissue in disinfected and emptied root canal using a porous polymer scaffold seeded with stem cells, after transplantation into an animal model [45,46]. The development and achievements of tissue engineering have provided novel perspectives and treatment options in endodontics for the repair of the pulp- dentin complex, cement, and periodontal tissues [47]. According to The American Association of Endodontists (AAE), regenerative endodontics is defined as \"biologically based procedures designed to replace damaged structures, including dentin and root structures, as well as cells of the pulp dentin complex\" [35]. It is accepted that nonsurgical root canal therapy treatment and regenerative endodontic procedures (REP) have the same essential aim. Nevertheless, in traditional root canal treatment, the canal space is filled with foreign material, and in regenerative procedures, vital host tissue in the tooth canal is created [48], which has the ability to coordinate local immune system functions and fight bacterial infections. Regenerative endodontics brings a new perspective to treating teeth with necrotic pulp; and, furthermore, uncomplicated and inexpensive procedures can be conducted with currently used instruments and materials [49,50]. Even though, the pioneering experiments in regenerative endodontics (RET) were carried out in the 1960s by [PERSON], and despite there still being clear interest and progress in this dentistry field, there is no established standardized clinical protocol for RET [51]. In 2016, the European Society of Endodontology (ESE) [52] and the American Association of Endodontists (AAE) [53] released a clinical consideration of the regenerative procedure, however, the AAE pointed out that there are many possibilities for treatment, and that further research and reviews are needed. Thus, this review aimed to analyze selected case reports and case series conductedin the last 10 years, to provide a summary of the different approaches to regenerative endodontics. The regenerative procedure has established a new approach to root canal therapy, to preserve the vital pulp of the tooth. The presentation of numerous clinical cases allows the reader to compare the applied treatment protocols from the perspective of his own patients, and to modify his treatment methods in comparison with other dentists. The findings of this review emphasize the increasing popularity of regenerative endodontics, with the simultaneous indication of its limitations.