María Inés Garchitorena¹ ORCID 0000-0001-6162-1466

10.22592/ode2019n34a5

ABSTRACT

Bioactive glasses (BG) are ceramic materials whose chemical composition allows them to induce and conduct tissue mineralization. As these glasses can be obtained with the sol-gel method and in nanometric particle sizes, their indication has been extended and enhanced.

The antibacterial properties of BG are outstanding: they are possible given the release of ions, which alkalinizes the medium, acting on the bacterial colonies.

The medical and dental applications of these materials are wide, with an emphasis on bone regeneration, remineralization of hard dental tissues and treatment of hypersensitivity. However, as they are materials with an amorphous chemical structure, their mechanical properties are not good, this being their main limitation for clinical application in restorative dentistry. In this sense, scientific research has focused on determining the possibility of including BG in various dental materials as a way to combine bioactivity with appropriate mechanical properties.

So far, it has not been possible to determine the proportion and methodology necessary to include BG in dental materials without altering their clinical behavior, which is why further research is necessary.

Keywords: bioactive glass, restorative dentistry, remineralization.

1 Restorative Dentistry, School of Dentistry, Universidad de la República, Montevideo, Uruguay.

Received on: 05 Sep 2018 – Accepted on: 07 Jul 2019

ABSTRACT

Los vidrios bioactivos (VB) son materiales cerámicos con una composición química tal que poseen la propiedad de inducir y conducir la mineralización de los tejidos. La obtención de estos

vidrios por medio del método sol-gel y la posibilidad de obtener tamaño nanométrico de partícula, han ampliado y potenciado las indicaciones de estos materiales.

Las propiedades antibacterianas de los VB son una característica sobresaliente; es debida a la liberación de iones que alcaliniza el medio, actuando sobre las colonias bacterianas.

Las aplicaciones médicas y odontológicas de estos materiales son muy amplias, destacándose la regeneración ósea, la remineralización de los tejidos duros dentarios y el tratamiento de la hipersensibilidad. Sin embargo, por tratarse de materiales con estructura química amorfa, sus propiedades mecánicas no son buenas, siendo esta característica su principal limitación para la aplicación clínica en el área de la odontología restauradora. En este sentido las investigaciones científicas se han enfocado en determinar la posibilidad de incorporar VB a diversos materiales dentales como forma de combinar su bioactividad con propiedades mecánicas apropiadas.

Hasta el momento no se ha logrado determinar la proporción y la metodología para incorporar VB en los materiales dentales sin alterar su comportamiento clínico, por lo que son necesarias más investigaciones.

PALABRAS CLAVE: VIDRIOS BIOACTIVOS, ODONTOLOGÍA RESTAURADORA, REMINERALIZACIÓN

PALAVRAS-CHAVE:                                    VIDRO         BIOACTIVO,        ODONTOLOGIA RESTORATIVA, REMINERALIZAÇÃO

¹ Restorative Dentistry, School of Dentistry, Universidad de la República, Montevideo, Uruguay. ORCID: 0000-0001-6162-1466

Authorship contribution and collaboration statement

1) Conception and design of study, 2) Acquisition of data, 3) Data analysis, 4) Discussion of results, 5) Drafting of the manuscript,6) Approval of the final version of the manuscript.

MIG has contributed in: 1, 2, 3, 4, 5, 6. INTRODUCTION

Remineralization is a process which involves restoring lost mineral ions to the dental structure, which enables the strengthening and functionality of the crystalline structure ⁽¹⁾.

Bones and teeth are incredibly complex organs, with a combination of different hard tissues (trabecular and compact bone, tooth enamel, dentin, dental cementum) and soft tissues (bone marrow, dental pulp, periodontal ligament). They have a unique hierarchical structure, with a combination of complex phenomena, such as biomolecular interactions, nutrient exchange or fluid transport ⁽²⁾.

The interaction of materials with dental tissues is determined by a series of factors such as composition, particle size, the chemistry of the released elements and the ability of tissues to respond to these agents ⁽³⁾. Today’s dentistry aims to repair damaged tissues and restore them to their natural condition, instead of replacing them with inert synthetic materials. Materials science not only studies the potential toxicity of materials but mainly focuses on the specific tissue responses they can trigger.

New materials, among them BG, have emerged, which involve the development of techniques to remineralize dental structures and, in recent years, a new paradigm has been proposed in healthcare: regenerative dentistry. It proposes repairing damaged tissues using mechanisms similar to those used by the body to renew cell populations. This approach requires using porous biomaterials, called scaffolds, which enable and favor the growth and organization of live tissue from cell cultures and appropriate biochemical factors, which induce and promote the regeneration of damaged tissue. This poses new challenges related to the development of appropriate three-dimensional scaffolds for cells to grow, proliferate and develop their specific function.BG can be used for this purpose since they meet the necessary requirements, such as osteoinduction, osteoconduction, biodegradability, biocompatibility, radiopacity, appropriate mechanical properties, ease of handling and sterilization ⁽⁴⁾. The addition of these glasses to other restorative dental materials is also being researched to provide bioactive and antimicrobial properties that could improve the prognosis for treatments.

The aim of this review is to study BG further and to determine the possibility of including them in various restorative dental materials.

METHODOLOGY

The literature review was conducted in PubMed, Timbó and Scielo. The inclusion criterion was the date of the papers, including those published since 2000 (except for articles by Larry Hench which, due to their relevance, were included despite having been published before 2000) both in English and in Spanish.

DEVELOPMENT

The conceptual and technical evolution of these materials has progressed from avoiding damage, by using inert materials, then biocompatible materials and finally regenerative materials ^(5-7). These replace dental tissues by applying mechanisms which are similar to those occurring in the body ⁽⁸⁾. Bioactive materials are those which elicit a biological response in tissues resulting in a strong chemical bond between the material and hard and soft tissues ^(2,6,9).

An ideal bioactive material should ⁽¹⁰⁾:

1. be bactericidal
2. be bacteriostatic
3. be sterile
4. stimulate dentin formation
5. preserve pulp

In 1969, Larry Hench developed bioactive glasses. Looking for a material that could bind to bone, he found a composition containing 45% by weight of silicon dioxide (SiO2), 24.5% by weight of sodium oxide (Na2O), 24.5% by weight of calcium oxide (CaO), and 6% by weight of phosphorus oxide (P2O5), commercialized under the name Bioglass® 45S5 starting in 1985 (2,11-12).

Fig. 1. The image shows the formation of hydroxyapatite on the surface of the material and closely associated with collagen fibers (Hench, L. L. Bioceramics: from concept to clinic. Journal of the American Ceramic Society 1991;74(7): 1487-1510).

The first paper on BG was published in 1971 and showed in vitro and in vivo results of the bond between the bone and the BG. This chemical bond is extremely strong because of the binding to the collagen structure; however, this bond with organic structures wasn’t discovered until 1981 ⁽¹³⁾ (Fig. 1).

Glass was originally obtained with the traditional processing method which required subjecting a mixture of reagents, at appropriate molecular concentrations, to high temperatures (above 1300^oC), melting the oxides in a platinum crucible, and then quenching it. Quenching increases the viscosity, which causes glass to solidify. This method has several disadvantages, mainly due to the high temperatures used, which leads to a highly crystalline structure, which is considered to be insoluble in the physiological medium ^(9,14-16).

Since 1991, this traditional method has been replaced by the sol-gel method, in which a chemical synthesis of silica precursors is used to form and assemble nanoparticles into a gel at room temperature ⁽¹⁷⁾. This new method has significant advantages: it requires significantly lower temperature (600 to 700^oC), the glasses obtained are more pure and homogeneous, the ability to better control particle size (enabling the production of nanoparticles), and it makes it possible to enhance bioactivity. Also, the cost is lower. Nanoporous structures (with 2- to 50-nm pores) can be generated using the sol-gel method, thus increasing surface area and, as a result, increasing bioactivity ^(5,14-18). Porosity also means the glass can be used as a scaffold for tissue regeneration ⁽¹⁵⁾ and to form capsules to carry enzymes, antibiotics and antigens ^(2,5,16).