Tooth Repair: The Use of Bioactive Molecules and Materials This article explores the use and evaluation of the following bioactive materials and molecules as a biological basis for repair of the cells and extracellular matrices of dentine and pulp.
Introduction For several decades, dental pulp therapy of teeth degraded by carious lesions and dental wear lesions, such as abrasion, attrition and erosion, has been managed and restored using a plethora of dental materials with different properties. Biomaterials are native or synthetic polymers that act as scaffolds for tissue regeneration and when implanted in vivo, they should be non-toxic and perform the functions of the innate tissue lost (Yuan et al, 2001). Significant focus has been placed on the biocompatibility of dental materials and the International Organization for Standardization (ISO) deployed an international standard in 1997 to ascertain that this feature is considered in the development of new materials.
Overview of Biologically Based Regenerative Therapies Targeted to the Dentine-Pulp Complex Calcium Hydroxide The history of use of calcium hydroxide (Ca(OH)2) as a pulp capping agent dates back 60-70 years and it is used to elicit a regenerative response of the dentine-complex component tissues. The compound’s strong alkalinity at the site of injury is thought to result in a localised pulpal tissue necrosis and after approximately two weeks, causes pulpal migration of cells (Shroder, 1985). Consequently, odontoblast-like cell differentiation of these migrated cells results in formation of a mineralised dentine bridge.
Fairly recently, these regenerative medicinal properties of Ca(OH)2 were understood in their abilities to solubilise bioactive dentinal matrix signalling molecules such as TGF-β1. This occurs in a low pH environment, similar to that causing dentine demineralisation during carious attack, leading to the expression of ECM and cytokine genes in in vitro odontoblastic-like cells (Graham et al, 2006). The coupled expression has shown to induce dentinal bridge formation alongside more generalised tissue regeneration.
Mineral Trioxide Aggregate (MTA) Studies of rat dental pulp cultures, as carried out by Rashid et al in 2003, show increased expression of BMP-mRNA due to MTA and calcium hydroxide generating higher calcium levels in the environment. Despite the relatively non-specific manner of wound healing, studies indicate that MTA releases products which stimulate protein expression by osteoblast-like cells and fibroblasts. These include osteopontin, osteonectin and osteocalcin which are involved in mineralised tissue formation.
Growth Factors After secretion, growth factors interact with dentine extracellular matrix constituents and become withdrawn and protected in this matrix (Smith et al, 1998). Physiological and pathological processes causing matrix degradation, release these biocompatible molecules and their subsequent diffusion to the pulp cells, enables tissue regeneration as a response to injury. These growth factors are key in inducing pulpal stem cells to display odontoblast-like cell behaviour and have the ability to stimulate reactionary and reparative tertiary dentinogenesis from these matrix components. Research has provided proof that transforming growth factors (TGF-β1 and TGF-β3), Bone Morphogenic Proteins (BMP-2 and BMP-7) and Insulin-like Growth factor (IGF-1) are responsible for signalling odontoblast differentiation in vitro, however the corresponding evidence is lacking for this signalling in vivo (Bègue-Kirn et al, 1992, 1994). More recent evidence suggests that specific binding and sequestration of TGF-β1 by dentine proteoglycans may be responsible for this (Baker et al, 2009).
Biodentine™, whose chemical composition was based on tricalcium silicate, the main component of Portland cement, was developed as a dentine replacement material with an amalgamation of biocompatibility and bioactive mechanisms for repair, high mechanical and low compressive strength and short setting time as its properties. Evidence shows 100% percent success of the material as a direct pulp capping agent in adults with a healthy pulp. Its use as an indirect pulp capping agent in adult rats (BOUKPESSI, 2008) demonstrated its ability to produce high quality and quantity of protective reactionary dentine.
After setting in an alkaline environment and regulating formation of calcium salts, it exhibits the same biomimetic and sealing ability properties to MTA, however has a setting time and compressive strength far superior to those of MTA. Therefore, it is the biological properties of Biodentine™ that enable its use as pulp capping material, in radicular repair of perforations and resorptions and finally apical fillings to restore the original tissue in contact with the pulp and root.
Bioactive glasses are systems based on calcium sodium phosphosilicate (SiO2–Na2O–CaO–P2O5) components. They are defined as a material that forms a layer of hydroxycarbonate apatite (HCA), a layer structurally and chemically similar to natural tooth mineral, following surface dissolution in a physiological environment (Wallace KE et al, 1999). The release of soluble silica and calcium ions into the peripheral tissue, which are the components involved in the accelerated bonding of these materials to tissue, stimulate regenerative tissue growth and thus potential use of this material as a tissue engineering scaffold (Earl et al 2001). Originally developed to occlude dentinal tubules for hypersensitivity treatment, recent studies have shown its potential to prevent tooth surface demineralisation and/or aid in its remineralisation.
Bioglass® 45S5, under the trade name NovaMin®, is being researched by many institutions for its use in oral health care. Following its ability to bind to both soft and hard tissue, its main advantage is the potential to stimulate rapid and direct interfacial bonding to dental hard tissues (Hench LL et al, 2011). This is due to analogous biological composition of the mineralised tissue’s inorganic components and the growing hydroxyapatite on the biocompatible materials surface (Sauro S et al, 2011).
Figure 11: (Figure and caption from Mauth et al, 2007)
“Human dental pulp cells seeded onto novel bioactive glass-ceramic granules pre-treated with simulated body fluid (SBF) and analyzed by SEM (bar = 10 μm)”.
Conclusion The complexity of dental tissues, especially those of the dentino-pulpal complex, has resulted in various areas of exploitation for the use of bioactive materials and molecules to regenerate these tissues.
This area of extensive research has shown a possibility of deviation from the current chemically composed dental materials to those which have combined molecular biology in their formulation. Previous methods of pulpal therapy are being superseded by those treatments which involve implantation of bioactive molecules alongside usage of the addressed biological molecules, such as bone morphogenic proteins naturally present during repair. These are both able to recapitulate developmental events of dentine bridging and pulp healing in order to increase the prognosis of a degenerated pulp. In comparison, Biodentine™ and Bioglass® have shown succession over all previously used regenerative materials and are capable in providing a more promising prognosis for a once irreparable pulp. It is however the author’s opinion that these biological regenerative processes are combined with existing traditional approaches to increase their acceptance and reap the benefits already provided by current materials. Thus in this new era of dentistry, these findings may provide future clinicians with additional treatment options for damaged or diseased vital teeth and they will soon been an essential constituent of a clinician’s armamentarium as part of the ethos of regenerative restorative dentistry.
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