“Tissue Engineering” – Future Concepts In Endodontics – A Short OverviewCorrespondence Address :
MeenakshiAmmal Dental College & Hospital
Dental caries result in the loss of the tooth structure. Therefore, the regeneration of enamel, cementum and the dentin-pulp complex is the long term goal of restorative and endodontic treatment. There is a high rate of success in the retention of teeth by endodontic therapy. The current treatment modalities consist of regenerative based approaches by the application of tissue engineering in which the diseased or necrotic pulp tissues are removed and replaced with healthy pulp tissue to revitalize the teeth. The purpose of this article is to review the components and the biological principles of tissue engineering, it is an overview of regenerative endodontics and its goals and it describes the possible techniques that will allow regenerative endodontics to become a reality.
tissue engineering, regenerative endodontics, scaffolds, stem cells
The goal of modern restorative dentistry is to functionally and cosmetically restore the tooth structure. Till recently, a variety of synthetic materials were developed to restore the damaged tooth structure. Although these materials have proved to be effective, they do not exhibit the same mechanical and physical properties as naturally formed dentine and enamel. Natural dental hard tissues, i.e. dentin, enamel and cementum exhibit little or no regenerative capability. Hence, there is a need for the replacement of the tooth tissue. Tissue engineering is a novel and highly exiting field of research. With tissue engineering techniques, it may be possible to repair damaged tissues or even create replacement organs (1). Tissue engineering can help in the regeneration of enamel and dentin to restore the lost tooth structure in future.
Dental caries remain to be one of the most prevalent young adult and childhood diseases, while the phrase â€śroot canalâ€ť is probably the most dreaded term in dentistry. There are several ways in which one can potentially engineer lost dentin and the dental pulp. The vitality of the dental pulp may be damaged by infection, exposure, trauma and chemicals. They ultimately result in premature tooth loss and therefore, diminish the quality of life.
One novel approach to restore the tooth structure is based on biology: regenerative endodontic procedures by the application of tissue engineering (2). Tissue engineering is the field of the functional restoration of the tissue structure and the physiology for impaired or damaged tissues because of cancer, disease and trauma (3). This holds the promise of the solution to a number of compelling clinical problems in dentistry that have not been adequately addressed through the use of permanent replacement devices. The key elements of tissue engineering are stem cells, morphogens and scaffolds of extra cellular matrix.
The production of dentin and dental pulp has also been achieved in animal and laboratory studies by using tissue engineering strategies. The greatest potential for these engineered tissues is in the treatment of tooth decay. Now, there is evidence suggesting that even if the odontoblasts (cells that produce dentin) are lost due to caries, it may be possible to induce the formation of new cells from pulp tissue by using certain BMPs.(4) These new odontoblasts can synthesize new dentin. The tissue engineering of the dental pulp itself may also be possible by using cultured fibroblasts and synthetic polymer matrices(5). The further development and successful application of these strategies to regenerate dentin and dental pulp could one day revolutionize the treatment of our most common oral health problem, cavities.
According to Langer and Vacanti, tissue engineering is â€śan inter disciplinary field that applies the principles of engineering and life sciences towards the development of biological substitutes that restore, maintain, or improve tissue functionâ€ť.
According to MacArthur and Oreffo, tissue engineering is defined as â€śunderstanding the principles of tissue growth and applying this to produce functional replacement tissue for clinical useâ€ť(6).
The Principles of Tissue Engineering (7):
The representation of three different tissue engineering approaches: conductive and inductive approaches and cell transplantation.
Conductive approach: This approach makes use of a barrier membrane to exclude connective tissue cells that will interfere with the regenerative process, while enabling the desired host cells to populate the regeneration site. An example of this is the dental implants and guided tissue regeneration membranes. Today, implants are considered as the standard treatment opinion in conjunction with prosthetic rehabilitation for replacing single and multiple teeth. GTR membranes are used to regenerate the periodontal tooth supporting structures and they are used a material barrier to create a protected compartment for selective wound healing(8).
Inductive approach: This approach uses a biodegradable polymer scaffold as a vehicle to deliver growth factors and genes to the host site. The growth factors or genes can be released at a controlled rate, based on the breakdown of the polymer. The inductive approach uses a biodegradable scaffold to deliver growth factor/genes at a controlled rate, based on the breakdown of the polymer. One limitation of the inductive approach is that the inductive factors for a particular tissue may not be known.
Cell transplantation: This strategy uses a similar vehicle for delivery in order to transplant cells and partial tissues to the host site. The cell transplantation strategy truly reflects the multidisciplinary nature of tissue engineering that requires a clinician, a bioengineer and a cell biologist.
Clinician: - A biopsy of the tissue sample, containing the cells of interest.
Cell biologist: - Multiplies the cells and maintains their function.
Bioengineer: - The manufacturer of the tissue, the bioreactor and the material onto which the cells will be placed for transplantation. Lastly, the clinician transplants the engineered tissue- polymer scaffold degrades and is remodeled by host and transplanted cells resulting in complete natural tissue.
Tissue Engineering Triad(9):
Tissue engineering is the employment of biological therapeutic strategies which are aimed at the replacement, repair, maintenance and/or the enhancement of tissue function.
(Table/Fig 1): Tissue engineering triad Stem cells:
Tissue engineering is generally considered to consist of three key elements (Table/Fig 1)
1. stem cells/progenitor cells
2. scaffolds or extra cellular matrix
3. Signaling molecules.
Stem cells are commonly defined as cells that have the ability to continuously divide and produce progency cells that differentiate into various other types of cells that differentiate into various types of cells or tissues.
Dental stem cells which were used in the initial tooth tissue engineering studies were obtained from immature, unerupted tooth buds which were isolated from animals like pig, rat, etc.
The basic role of scaffolds in tissue engineering is to act as carriers for cells, to maintain the space and to create an environment in which the cells can proliferate and produce the desired tissue matrix.
Types of scaffolds
1. Natural scaffolds.
2. Mineral scaffolds
3. Synthetic scaffolds
1. Natural scaffolds: The examples for natural scaffolds are collagen, hyaluronic acid, chitosan and chitin. These natural scaffolds have been used in several craniofacial and dental applications. These lack the desired structural rigidity for use in the load bearing region.
2. Mineral scaffolds: These are composed of calcium phosphates in the form of hydroxyapatite or β tricalcium phosphate. These scaffolds are brittle and hence, are prone to fracture.
3.Synthetic scaffolds: The most widely used synthetic materials are polymers of polyglycolicacid, polylacticacid and polydioxanone. These scaffolds lack critical cell signaling capabilities and can interfere with new tissue growth.
Signaling molecules: These are the molecules that transmit signals between cells, functioning as stimulators/inhibitors of growth, as well as the modulators of differentiation. These consist of growth factors (PDGF, TGF-β), differentiation factors (BMPs) and stimulating factors.
The Major Approaches To Tissue Engineering(11):
Ex-vivo approach: In this technique, the target tissue is created in a laboratory by culturing cells in biodegradable scaffolds in the presence of specific trophic factors before their transplantation into the body.
In â€“ vivo approach: This technique involves the induction of intrinsic healing activity at the site of the tissue defect by using these three elements (cells, scaffolds and signalling molecules).
Tissue engineering is the employment of biological therapeutic strategies which are aimed at the replacement, repair, maintenance and/or enhancement of tissue function. Today, the field of tissue engineering has established the essential foundations for the design and fabrication of neo tissues in two or three dimensions for transplantations. Tissue engineering holds the promise of the solution to a number of compelling clinical problems in dentistry that have not been adequately addressed through the use of permanent replacement devices. The regeneration or replacement of oral tissues which are affected by inherited disorders, trauma and neoplastic or infectious diseases is expected to solve many dental problems.
(Table/Fig 2): Factors contributing for success of regenerative endodontic procedures
Regenerative dentistry including periodontics, endodontics and maxillofacial surgery is a new field that seeks to apply the concepts of tissue engineering to the management of lost oral tissues by using various types of stem cells, growth factors and scaffolds. Within the next 25 years, unparalled advances in dentistry and endodontics are set to take place, with the availability of artificial teeth, bone, organs and oral tissues(1).
Regenerative endodontic procedures can be defined as biologically based procedures which are designed to replace damaged structures including dentin and root structures, as well as the cells of the pulp-dentin complex7. The factors contributing to the success of regenerative endodontics comprises of the research on adult stem cells, growth factors, organ tissue cultures and tissue engineering materials (Table/Fig 2). The objectives of the regenerative endodontic procedures are to regenerate pulp-like tissues: ideally, the dentin pulp complex; regenerated damaged coronal dentine.
Regenerative Approaches In Endodontics(7)
There are several techniques for the application of regenerative endodontics.
These techniques are:
1. Root canal revascularization via blood clotting.
2. Post natal stem cell therapy.
3. Pulp implantation.
4. Scaffold implantation.
5. Injectable scaffold delivery.
6. Three dimensional cell printing.
7. Gene therapy.
These regenerative endodontic techniques are based on the basic principles of tissue engineering.
Root canal revascularization via blood clotting: The development of regenerative endodontic procedures may require the re-examination of many of the closely held percepts of traditional endodontic procedures. The revascularization method assumes that the root canal space has been disinfected effectively by the use of intracanal irrigants, with the placement of antibiotics for several weeks. Several case reports have documented the revascularization of the necrotic root canal systems by disinfection, followed by establishing bleeding into the canal system via over instrumentation(12).
Post natal stem cell therapy: The simplest method to administer the cells of appropriate regenerative potential is to inject the post natal stem cells into the disinfected root canal systems after the apex is opened. The post natal stem cells can be derived from multiple tissues including skin, buccal mucosa, fat and bone. One recent approach could be to use the dental pulp stem cells that have been taken from the umbilical cord, which are mostly disease and pathogen free.
Pulp implantation: In pulp implantation, the cultured pulp tissue is transplanted into cleaned and shaped root canal systems. The pulp tissue is grown in sheets in vitro on biodegradable polymer nanofibers or on sheets of extracellular matrix proteins such as collagen I or fibronectin(13). The limitation of this technique is that specialized procedures may be required to ensure that the cells properly adhere to the root canal walls.
Scaffold implantation: Pulp stem cells must be organized into a three-dimensional structure that can support cell organization and vascularization. This can be accomplished by using a porous polymer scaffold which is seeded with pulp stem cells(14). In pulp-exposed teeth, dentin chips have been found to stimulate reparative dentin bridge formation. Dentin chips may provide a matrix for pulp stem cell attachment and they may also be a reservoir of growth factors15. The natural reparative activity of the pulp stem cells in response to the dentin chips provides some support for the use of scaffolds to regenerate the pulp dentin complex.
Injectable scaffold delivery: Tissue engineered pulp tissue is seeded into the soft three-dimensional scaffold matrix, such as a polymer hydrogel. Hydrogels are injectable scaffolds that can be delivered by syringe16, they have the potential to be noninvasive and are easy to deliver into the root canal systems. In theory, the hydrogel may promote pulp regeneration by providing a substrate for cell proliferation and differentiation into an organized tissue structure. Despite these advances, hydrogels at are at an early stage of research and this type of delivery system, although promising, has yet to be proven to be functional in vivo.
Three dimensional cell printing: The three-dimensional cell printing technique can be used to precisely position cells and this method has the potential to create tissue constructs that mimic the natural tooth pulp tissue structure17. The ideal positioning of cells in a tissue engineering construct would include placing odontoblastoid cells around the periphery to maintain and repair dentin, with fibroblasts in the pulp core supporting a network of vascular and nerve cells.
Gene therapy: Gene therapy has been recently used as a means of delivering genes for growth factors, morphogens, transcription factors and extracellular matrix molecules locally to the somatic cells of individuals, with resulting therapeutic effect3. The gene can stimulate or induce a natural biological process by expressing the molecules which are involved in the regenerative response for the tissue of interest. Both an in-vivo and ex-vivo approach can be used for gene therapy. One use of gene delivery in endodontics would be to deliver mineralizing genes into the pulp tissues to promote tissue mineralization. Gene therapy is a relatively a new field and evidence is lacking to demonstrate that this therapy has the potential to rescue the necrotic pulp.
Developmental Approaches For Regenerative Endodontic Techniques. (Table/Fig 3)(7):
One of the most challenging aspects of developing a regenerative endodontic therapy is to understand how the various component procedures can be optimized and integrated to produce the outcome of a regenerated pulp-dentin complex. For regenerative endodontic procedures to be widely available and predictable, endodontists will have to depend on tissue engineering therapies to regenerate pulp dentin tissues.
Each one of the regenerative techniques has advantages and disadvantages and some of the techniques are even hypothetical, or at an early stage of development. The future development of regenerative endodontic procedures will require a comprehensive research program which is directed at each of these components and their application to our patients. The authors believe that regenerative endodontics is an inevitable therapy and the endodontic profession to pool the resources to hasten its development.
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