Bioactivity assessment of bioactive glasses for dental applications: A critical review
Graphical abstract
Introduction
Minerals ions can be removed from hydroxyapatite (HA) crystals of dental hard tissues, enamel and dentin, in case of erosive attack or carious lesions, this process is demineralization. Restoring these mineral ions to the HA crystals is called remineralization [1]. The contemporary approach to caries management has drastically changed: operative strategies are abandoned in favor of biologic approaches based on individual caries risk, preventive dentistry, and non-invasive or minimal options [2,3]. Remineralization is a non-invasive treatment to preserve dental tissues after an acidic attack. In situ remineralization could occur with a dissolution reprecipitation process in enamel or in dentin, but new tertiary dentin could also be produced by stimulation of pulp cells [4]. BAG are involved in both processes, but some studies have focused only on the first mechanism because others have focused on the second mechanism; thus, the term bioactivity for BAG remains unclear.
BAG were introduced in 1969 by professor Larry Hench (BAG 45S5), the composition was 45% silica (SiO2) — 24.5% calcium oxide (CaO) — 24.5% sodium oxide (Na2O) — 6% phosphate (P2O5). Initially, BAG were used for medical application for their ability to form a bond with bone [5]. Nowadays, the original composition (45S5) and manufacturing method of BAG are modified to create mesoporous BAG, to change particle size (nano or micro-scale) or to add additives [6]. BAG are considered bioactive materials; however, a better definition of the bioactive effect is required to improve formulations or incorporation of particles in scaffolds [7,8].
It is generally accepted that a bioactive material is a material able to induce specific biological activity and stimulate a beneficial response from the body bonding to the host tissue [5,9]. Regarding BAG, Lebecq et al. defined bioactivity by following the crystalline hydroxy-carbonate apatite (HCA) layer growth on the surface of glass particles [10]. According to other definitions, the bioactivity is the ability to form mineral deposits in biological conditions [11].
In the dental field, the definition of bioactivity depends on the clinical use from the ability to induce reprecipitation of HA on the surface of enamel and dentin to cellular effects induced by the release of biologically active substances and ions. There is no consensus on bioactivity’s definition. Vallittu et al. suggested limiting the terms ‘bioactive’ with respect to dental materials only to scientifically proven materials and material combinations that release substantial quantities of ions for specific biomineralization in the clinical environment of the material [12].
The main objective of this review was to identify and classify the different methodologies used for the BAG bioactivity quantification regarding dental hard tissues, to determine the key parameters considered for these studies and therefore, to influence dental research and clinical practice on an international basis.
Section snippets
Materials and methods
The website COVIDENCE was used to import references and view duplicates, titles, abstracts, and full texts.
Study selection
The initial electronic search using the keyword combination returned 1077 articles. After the removal of duplicates, 709 records were examined. The title and abstracts of the remaining 709 papers were screened, and 563 papers were excluded because they were irrelevant to the inclusion criteria. Finally, 146 relevant papers were scrutinized by downloading the papers and reading the full text. A consensus between the two authors was reached to determine which studies fully fulfilled the selection
Study selection
For study selection, especially search terms, the term ‘glass’ was observed in many studies of tricalcium silicates and bioceramics but not for those of BAG. Additionally, many studies were excluded after the title or abstract screening. Thirteen studies were observed in the references of other articles, and many of them were in chemistry and physics reviews; therefore, they were not mentioned in the study selection with medical electronic databases.
Particles sizes and morphology in relation to the manufacturing method
The melt-quench and sol–gel processes are the
Conclusions
According to all the in vitro studies included, BAG is a bioactive material. Bioactivity ranged from reactivity and apatite reprecipitation to pulp cell stimulation. However, the literature provides no consensus on the results of in vivo studies and no comparison between protocols to assess their bioactivity compared with other bioactive products.
A clear classification of the methods used to assess the bioactivity of BAG could improve the elaboration of protocols and rank the impact of
Funding
This research received no specific grant from funding agencies in the public, commercial, or nonprofit sectors.
Acknowledgements
The authors thank Clémence Agostini, librarian of the interuniversity health library (Université de Paris), for her assistance with the research strategy of the study. The authors also thank Charlène Chevalier (Université Claude Bernard Lyon1) for her help for the teeth illustrations of the graphical abstract.
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