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= Glass Ionomer Cement =

Introduction
Over the years, as the use of silver amalgam restorations have drastically decreased, the need for durable and biocompatible long-lasting materials is becoming necessary. It is important for a dental material to have several criteria such as to bond reliably to tooth structure, potentially reduce biofilm formation and also inhibit dental diseases to protect the tooth. Glass-ionomer cements (GICs) which belong to the class of materials known as acid-base cements, are able to fulfill many of these qualities. They are the product of reaction of weak polymeric acids with powdered basic glasses with setting reaction occurring in concentrated solutions in water. The final structure of the product contains a substantial amount of unreacted glass which acts as filler, reinforcing the set cement. Polymers used in glass-ionomer cements are either homopolymer polyacrylic acid or the copolymer of acrylic acid and maleic acid. Polymer plays an important role in influencing the properties of the glass-ionomer cement. With the presence of high molecular weights polymer in GICs, strength of the set cement increases. However, solutions of high molecular weight polymers have high viscosities which causes difficulty in mixing. One of the essential components of glass-ionomer cements is glasses, which should be basic, i.e., capable of reacting with an acid to form a salt. Alumino-silicate glasses with fluoride and phosphate additions are generally used in practice which fulfill the requirement. Both alumina and silica contribute to the material’s basic character, making the glass susceptible to attack by acids. The Table below shows an example of the glass composition.

Another most common variation of GICs is Resin-Modified Glass-Ionomers which contain the same essential components as conventional glass-ionomers (basic glass powder, water and polyacid), but also include monomer component, 2-hydrocyethyl methacrylate and camphorquinone as the initiator. Resin-modified glass-ionomers set by the both acid-base reaction and addition polymerization. However, RM-GICs have markedly compromised biocompatibility compared with conventional glass-ionomers due to the release of HEMA monomer in the first 24 hours. HEMA is able to diffuse through human dentine and is cytotoxic to the cells of the pulp.

Types of Glass Ionomer Cements
There are various uses of glass-ionomers within dentistry. Glass-ionomers cements are used as full restorative materials, especially in the primary dentition, and also as liners and bases, fissure sealants and as bonding agents for orthodontic brackets. They can be classified into a type 1, type 2 and type 3. Type 1 is luting and bonding cements, which are used for the cementation of indirect restorations including crowns, bridges and orthodontic brackets. They are able to achieve a good thin film thickness in the order of 20µm with relatively low powder/liquid ratio (from 1.7:1 to 3.8:1 when the acid has been dehydrated to a powder form). Type 2 is restorative materials which use high powder/liquid ratio and are radiopaque. Last but not least, type 3 which is lining or base cements often used as a thin layer beneath restorations and serve as a thermal insulator or dentine replacement. There are a few delivery methods for both conventional GICs and RM-GICs.


 * Powder/ liquid
 * Capsule
 * Paste/ paste system

Fluoride Release and Remineralisation
The pattern of fluoride release from glass ionomer cement is characterised by an initial rapid release of appreciable amounts of fluoride, followed by a taper in the release rate over time. An initial fluoride “burst” effect is desirable to reduce the viability of remaining bacteria in the inner carious dentin, hence, inducing enamel or dentin remineralization. The constant fluoride release during the following days are attributed to the fluoride ability to diffuse through cement pores and fractures. Thus, continuous small amounts of fluoride surrounding the teeth reduces demineralization of the tooth tissues. A study by Chau et al. shows a negative correlation between acidogenicity of the biofilm and the fluoride release by GIC, suggestive that enough fluoride release may decrease the virulence of cariogenic biofilms. In addition, Ngo et al. (2006) studied the interaction between demineralised dentine and Fuji IX GP which includes a strontium – containing glass as opposed to the more conventional calcium-based glass in other GICs. A substantial amount of both strontium and fluoride ions was found to cross the interface into the partially demineralised dentine affected by caries. This promoted mineral depositions in these areas where calcium ion levels were low. Hence, this study supports the idea of glass ionomers contributing directly to remineralisation of carious dentine, provided that good seal is achieved with intimate contact between the GIC and partly demineralised dentine. This, then raises a question, “Is glass ionomer cement a suitable material for permanent restorations?” due to the desirable effects of fluoride release by glass ionomer cement.

Glass Ionomer Cement in Primary Teeth
Numerous studies and reviews have been published with respect to GIC used in primary teeth restorations. Findings of a systematic review and meta-analysis suggested that conventional glass ionomers were not recommended for Class II restorations in primary molars. This material showed poor anatomical form and marginal integrity, and composite restorations were shown to be more successful than GIC when good moisture control could be achieved. Resin modified glass ionomer cements (RMGIC) were developed to overcome the limitations of the conventional glass ionomer as a restorative material. A systematic review supports the use of RMGIC in small to moderate sized class II cavities, as they are able to withstand the occlusal forces on primary molars for at least one year. With their desirable fluoride releasing effect, RMGIC may be considered for Class I and Class II restorations of primary molars in high caries risk population.

Glass Ionomer Cement in Permanent Teeth
With regard to permanent teeth, there is insufficient evidence to support the use of RMGIC as long term restorations in permanent teeth. Despite the low number of randomised control trials, a meta- analysis review by Bezerra et al [2009] reported significantly fewer carious lesions on the margins of glass ionomer restorations in permanent teeth after six years as compared to amalgam restorations. In addition, adhesive ability and longevity of GIC from a clinical standpoint can be best studied with restoration of non- carious cervical lesions. A systematic review shows GIC has higher retention rates than resin composite in follow up periods of up to 5 years. Unfortunately, reviews for Class II restorations in permanent teeth with glass ionomer cement are scarce with high bias or short study periods. However, a study [2003] of the compressive strength and the fluoride release was done on 15 commercial fluoride- releasing restorative materials. A negative linear correlation was found between the compressive strength and fluoride release (r2=0.7741), i.e., restorative materials with high fluoride release have lower mechanical properties.

Conclusion
With the increasing acceptance and use of GIC, more studies and research need to be conducted to improve the current limitations due the inferior physical properties of GIC compared to other materials. Future improvements will be necessary to increase their longevity to be used as permanent materials. However, for now, due to the inconclusive, low-grade evidence claiming the superiority of GIC in restorations, this page does not support the use of GIC in permanent restorations.