All About Glass Ionomer in dentistry

poor hygiene can cause severe and progressive erosion in conventional GI restoration

In dentistry there is a continuous need for change in techniques and materials, depending on the change in demand from the professional perceptions of patients and with technological advancement. With time, dentists alter their perspective to establish awareness among the public that removing dental caries is not merely a mechanical method but an approach to noninvasive technique and that there are more possibilities for advancement. [1]

In 1972 Wilson and Kent added glass-ionomers (GIs) to the dental field for the first time [2]. Their potential for chemical adhesive and fluoride-releasing properties have led to their widespread use as luting materials, cavity liners and bases, as well as restorative materials. Glass ionomer cements have numerous uses in dentistry, including their use as: type I used in crowns, bridges and orthodontic braces for luting. These types are distinguished by their fast set and low film thickness. Type II a: aesthetic restorative cements, available in both conventional and resin-modified form. Type II b: Reinforced cements for restoration. They were never certainly stronger than type II a but are more resistant to wear. Type III used as lining cements and foundation featuring low viscosity and rapid setting [3]. The chemistry is generally the same for all three groups but there are differences in the powder-liquid ratio and particle size to satisfy the desired function [4]. Glass-ionomers The application and bond forming of glass-ionomers is largely in contrast to resin-based composites. In reality, the bond strength can be achieved with resin bonding systems only at 25 per cent, but at least the bond is stable and resistant to disintegration. Glass-ionomers do not require additional provisions for consistent retention or adhesion because they adhere directly to dental hard tissues, even moist ones (Fig.1).

Modification of glass ionomer cements

Successive modifications were made to regular GIC to overcome their mechanical integrity insufficiency and their ability to resist fracture loads. Thus, in addition to the high viscosity ionomer cements, and those with integration of nanoparticles, several materials have emerged with different composition such as glass ionomer cements reinforced with metal or modified with resin. All these changes were made to meet individual clinical needs and improve GICs' physicochemical properties. There's been efforts for several years to incorporate fibers as reinforcing agents into structure of these materials. But there are differences in powder-liquid ratio and particle size to satisfy the feature desired [4].

Hainomers

These are new and innovative bioactive materials developed by integration of hydroxyapatite into glass ionomer powder. They are primarily used in oral maxillofacial surgery as bone cements, and may be used in the future as retrograde filling materials. They play a part in bonding directly with the bone and influencing its growth and development [6].

Bioactive glass (BAG)

Figure6: Bioactive glass (BAG)

Larry Hench et al had invented the University of Florida's first Bioglass. It takes into consideration the fact that they form a layer rich in Ca+ and PO4 + ions around the glass on acid dissolution of glass, such a glass can form intimate bioactive bonds with bone cells and become fully integrated with the bone. For its good bioavailability, osteo-conductivity and biodegradability, BAG has been used as a restorative material for over a decade and its degradation products stimulate the production of growth factors, cell proliferation and activate osteoblast gene expression, also helps in the treatment of dentine hypersensitivity and the promotion of enamel remineralization. BAG has antibacterial effect as the pH of aqueous solutions increases (Figure 6). [7]

Reinforced-GIC
The incorporation of alumina fibers into GIC's glass powder helps to boost GIC's flexural strength. This technology is called Inorganic Matrix Material, Polymeric Rigid. It's a light-hearted GIC. It involves the incorporation into the powder of continuous alumina network scaffold and SiO2 ceramic fibers. This increases the cure depth, reduces the shrinkage in polymerization, improves wear resistance and increases the set cement's flexural strength. [9]

Zirconomer

A new class of restorative GIC is being established with increased strength and durability, it shows strength of amalgam so it is also called white amalgam.

Figure8: Zirconomer

The use of zirconia fillers in the glass portion of zirconomer strengthens the structural integrity of the restorative material & imparts higher mechanical properties for the reconstruction of the posterior teeth and for the

safety and esthetics of GIC, completely eliminating the hazards of mercury (Figure 8). 

Nanotechnology in GICs

Nanotechnology includes the development of structures, modifications, or materials with a spectrum of 1–100 nm. Uses of nanotechnology in dentistry include implant surface modifications, the development of strengthened polymeric composites through the introduction of nano-sized particles, and caries prevention. Recent studies have indicated that the inclusion ofnano-sized particles or "nanoclusters" can improve the mechanical properties of dental restorative materials, such as composites of resins (Figure 9). The GICs which have enhanced nanotechnology are as follows: [11]

1. Powder-modified nano-glass ionomers: first defined by De Caluwé et al. [12], it involves doping traditional GICs with nano-sized glass particles that can minimize setting time and increase compression strength and elastic frame. The main advantages of lower setting times of direct restore materials are increased ease of handling.

Figure9: Nano-filled resin-modified GICs

2. Nano-filled resin-modified GICs: Resin-modified GICs also have a component of polymer resin, which usually sets by a self-activated (chemically cured) or light-activated polymerization response. These were designed to improve a resin composite's mechanical properties, with the potential for GICs anticaries. Resin-modified GICs, however, have reduced mechanical properties compared with composites, including brittleness and lower strength along with aesthetics.[11] In order to overcome these disadvantages, attempts have been made to incorporate nano-sized fillers and bio-ceramic particles into RMGICs [1]. 

Nano bio-ceramic impregnated GIC

It is well-documented that the integration of nano-sized particles in powder-modified nano-glass ionomers enhances its mechanical properties. De Caluwé et al has shown that doping conventional GICs with nano-sized glass particles can reduce setting time and increase compression strength and elastic-modulus. Improving the mechanical properties adds to GIC's quality and reliability and shelf-life, as they can more easily tolerate the masticatory and occlusal forces. Adding apatite to GIC powder increases the GIC set's crystallinity and thus, enhances chemical stability and water insolubility [10].

Nano-ionomer

Nowadays, nanotechnology has been implemented in the dental field, also known as molecular nanotechnology or molecular engineering, offering a cosmetically acceptable reconstruction with excellent mechanical and physical properties. A new generation of glass ionomer cements modified in resin (RMGICs) was introduced in 2007. The manufactures describe Ketac nano (3 M ESPE) as (nano-ionomer). Nanotechnology incorporation in these materials enhances its physical properties [13]. Nanoiomer blends nanofillers and clusters of particles from the fluoroaluminosilicate glass to enhance color characteristics and polishability. This new resin-modified nano-filled glass ionomer restorative material was introduced into permanent teeth to restore permanent teeth and small cavities. It is based on the same manufacturer's (Vitremer) resin-modified glass ionomer with a simplified dispensing and mixing system that requires the use of a priming step, but no separate conditioning step. Its primary healing mechanism is by activating light, and no redox or self-healing occurs during setting. This system allows for the integration of a highly packed filler material, of which about two thirds are nanofillers. The fluoride release profile of the modified glass ionomer cement with nano-filled resin is similar to that of the RMGIC [4].

Addition of titanium dioxide nanoparticles

The addition of apatite and titanium nanotubes to resin-based cements has been found to increase fracture toughness, flexural strength and compressive strength, as well as resin-based cements' hardness and elasticity modulus, without altering their radiopacity or biocompatibility [14]. In 2011, Elsaka et al evaluated the effect of adding titanium dioxide nanoparticles to conventional glass ionomer cement in 3, 5 and 7wt percent ratios. In contrast to unmodified glass ionomer cement, they found that glass ionomer cement containing 3 and 5wt percent titanium dioxide nanoparticles showed improved fracture toughness, flexural strength and compression strength. However, for glass ionomer cement containing 7wt per cent titanium dioxide nanoparticles, a decrease in mechanical properties was found. Glass ionomer containing nanoparticles containing 5 and 7wt per cent titanium dioxide had reduced surface micro hardness. Time arrangement of glass ionomer nanoparticles containing titanium dioxide was approved. The application of titanium dioxide nanoparticles to modern GIC did not weaken its frequency of bonding with dentin or its release of fluorides. Compared with the unmodified glass ionomer, glass ionomers containing TiO2 nanoparticles processed the most potent antibacterial activity against streptococcus mutants [4,15] .  

Not only is glass ionomer bioactive, but it also has characteristics of an intelligent substance. Glass-ionomer can be called active as it releases fluoride, it can be called smart because it releases fluoride in proportion to the acidity. It has a buffer capacity of pH to some extender. Their brittleness and poor wear resistance are common problems of modern glass-ionomers. Even the latter is pH-dependent, meaning proper oral hygiene is vital. Unlike resin bonding, glass-ionomer's adhesion to tooth structure is not sensitive to technique, and its performance increases with time. Their constant urge for innovations in dentistry stems from changing professional perceptions and changing patient demands with increasing awareness that dental caries treatment is not just a technique, but also requires a less invasive bio-medical approach. In the course of its inception, this new family of GIC restorative materials has held many nifty facets and still hold the baton in the never-ending quest for excellence in clinical dental research. GIC will make everything happen. With the introduction of nanotechnology into GIC to improve its mechanical properties, it can be concluded that commercially available nano-RMGICs have no substantial advantage or disadvantage over conventional restorative materials in terms of surface mechanical properties.

References 

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8. Piekar C, Rajnikanth S. An In-Vitro assessment of role of tooth mousse in preventing wine erosion. Aust Dent J 2008:53:22-5

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11. NajeebáS, KhurshidáZ, ZafaráMS, KhanáAS, ZohaibáS, MartÝáJM, etáal. Modifications in glass ionomer cements: Nano-sized fillers and bioactive nanoceramics. Int J Mol Sci 2016;17:1134.

12..Deá CaluwÚá T, Vercruysseá CW, Fraeymaná S, Verbeecká RM. The influence of particle size and fluorine content of aluminosilicate glass on the glass ionomer cement properties. Dent Mater 2014;30:1029-38.

13.Coutinho E., et al. “Bonding effectiveness and interfacial characterization of a nano-filled resin-modified glass-ionomer”. Dental Materials 25.11 (2009): 1347-1357.

14.Elsaka SE., et al. “Titanium dioxide nanoparticles addition to a conventional glass-ionomer restorative: Influence on physical andantibacterial properties”. Journal of Dentistry 39.9 (2011): 589-598.

15. Mitra SB., et al. “An application of nanotechnology in advanced dental materials”. Journal of the American Dental Association 134.10(2003): 1382-1390.