Glass ionomer cement

# Overview
A Glass Ionomer Cement (GIC) is one of a class of materials commonly used in dentistry as filling materials and luting cements. These materials are based on the reaction of silicate glass powder and polyalkeonic acid.  These tooth-coloured materials were introduced in 1972 for use as restorative materials for anterior teeth (particularly for eroded areas, Class III and V cavities).
As they bond chemically to dental hard tissues and release fluoride for a relatively long period modern day applications of GICs have expanded. The desirable properties of glass ionomer cements make them useful materials in the restoration of carious lesions in low-stress areas such as smooth-surface and small anterior proximal cavities in primary teeth. Results from clinical studies, do not support the use of conventional or metal-reinforced glass ionomer restorations in primary molars.

# Chemical classification
GICs are commonly classified into four principal types:
- Conventional Glass Ionomer Cements
- Hybrid Ionomer Cements (Also known as Light or Chemical Cured Resin-modified Glass Ionomer or Dual-cured Glass Ionomer Cements)
- Tri-cure Glass Ionomer Cements
- Metal-reinforced Glass Ionomer Cements

## Conventional glass ionomer cements
Conventional GlCs were first introduced in 1972 by Wilson and Kent. They are derived from aqueous polyalkenoic acid such as polyacrylic acid and a glass component that is usually a fluoroaluminosilicate. When the powder and liquid are mixed together, an acid-base reaction occurs. As the metallic polyalkenoate salt begins to precipitate, gelation begins and proceeds until the cement sets hard.These can be supplied as an ion leachable glass powder to be mixed with an aqueous mixture of polyacrylic acid or as a blend of freeze-dried polyacrylic acid and an ion leachable glass powder for mixing with distilled water. The manufacturer can also add a small amount of tartaric acid to the water that will provide a sharper, better defined setting reaction. Early release of calcium ions is responsible for the setting reaction of glass ionomer cements. The slower release of the aluminium ions is responsible for increased cross-linking, which significantly improves the strength over a period of several days. This clinical factor is important when using glass ionomer cements. Examples of traditional luting glass ionomer cements include Fuji I, and Shofu I. Recently, several faster setting, high-viscosity conventional glass ionomer cements have become available. Called viscous or condensable glass ionomer cements by some authors, these restorative materials were originally developed in the early 1990s for use with the atraumatic restorative technique in developing countries. Example here is Fuji IX GP . These materials set faster and are of higher viscosity because of finer glass particles, anhydrous polyacrylic acids of high molecular weight and a high powder-to-liquid mixing ratio. The setting reaction is the same as the acid-base reaction typical of conventional glass ionomer cements.

## Hybrid Ionomer Cements or Resin-modified Glass Ionomers or Dual-Cured GIC
These combine an acid-base reaction of the traditional glass ionomer with a self-cure amine-peroxide polymerization reaction. These light-cured systems have been developed by adding polymerizable functional methacrylate groups with a photo-initiator to the formulation. Such materials undergo both an acid-base ionomer reaction as well as curing by photo-initiation and self cure of methacrylate carbon double bonds or in other words their acid-base reactions are supplemented by a second resin polymerization initiated (usually) by a light-curing process. For this reason theyre also called Dual-Cured GIC. Developed in 1992 the resin-modified glass ionomer cements in their simplest form are glass ionomer cements that contain a small quantity of a water-soluble, polymerizable resin component. More complex materials have been developed by modifying the polyalkenoic acid with side chains that could polymerize by light-curing mechanisms in the presence of photo initiators, but they remain glass ionomer cements by their ability to set by means of the acid-base reaction.
Modern resin modified glass ionomer cements include Advance, Fuji Plus and Vitremer Luting. Most recent development in this field are the paste-paste resin modified GIC luting cement such as FujiCEM .

## Tri-cure Glass Ionomer Cements
Some systems have also incorporated a chemical curing tertiary amine-peroxide reaction to polymerize the methacrylate double bonds along with the photo-initiation and acid-base ionic reaction. These materials are known as tri-cure glass ionomer cements. The chemical cure component of tri-cure cements has been shown to have a significant effect on their overall strength. Photo-initiated cements cannot be used in cases involving opaque structures such as metal substrates. The resin-modified glass ionomer cements generally have a much lower release of fluoride than the conventional glass ionomer materials.

## Metal Reinforced Glass Ionomer Cements or Cermets
Metal-reinforced glass ionomer cements were first introduced in 1977. The addition of silver-amalgam alloy powder to conventional materials increased the physical strength of the cement and provided radiopacity. Subsequently, silver particles were sintered onto the glass, and a number of products then appeared where the amalgam alloy content had been fixed at a level claimed to produce optimum mechanical properties for a glass cermet cement. Nowadays these materials are considered as old-fashioned as the conventional glass ionomer cements have comparable physical properties and far better aesthetics.
The clinical performance of cermets is considered to be inferior to other restorative materials, so much so that their use is now discouraged.

# Composition and preparation
Application involves mixtures of a powder and a liquid. 
The type of application prescribes the viscosity of the cement, which is adjusted by varying the particle size distribution and the powder-to-liquid ratio.

## GIC Powder
The powder is an acid-soluble calcium fluoroaluminosilicate glass similar to that of silicate but with a higher alumina-silicate ratio that increases its reactivity with liquid. The fluoride portion acts as a ceramic flux. Lanthanum, Strontium, Barium or Zinc Oxide additives provide radioopacity. The raw materials are fused to form a uniform glass by heating them to temperatures of 1100C to 1500C. The glass is ground into a powder having particles into a powder in the range of 15 to 50 m. Typical percentages of the raw materials are:
- Silica 41.9%
- Alumina 28.6%
- Aluminium Fluoride 1.6%
- Calcium Fluoride 15.7%
- Sodium Fluoride 9.3%
- Aluminium Phosphate 3.8%

## GIC Liquid
Originally, the liquids for GIC were aqueous solutions of polyacrylic acid in a concentration of about 40 to 50%. The liquid was quite viscous and tended to gel over time. In most of the current cements, the acid is in the form of co-polymer with itaconic, maleic or tricarboxylic acids. These acids tend to increase the reactivity of the liquid, decrease the viscosity and reduce the tendency for gelation. Tartaric acid is also present in the liquid. It improves handling characteristics and increases the working time, but it shortens the setting time. The viscosity of Tartaric Acid-containing cement does not generally change over the shelf life of the cement. However a viscosity change can occur if the cement is out of date. As a means of extending the working time of the GIC, freeze-dried polyacid powder and glass powder are placed in the same bottle as the powder. The liquid consists of water or water with Tartaric Acid. When the powders are mixed with water, the acid powder dissolves to reconstitute the liquid acid and this process is followed by the acid-base reaction. This type of cement is referred to occasionally as water settable glass ionomer or erroneously as anhydrous glass ionomer.

## Setting Reaction
The setting reaction is an acid-base reaction between the acidic polyelectrolyte and the aluminosilicate glass. The polyacid attacks the glass particles (also called leaching) to release cations and Fluoride ions. These ions probably metal fluoride complexes react with Polyanions to form a salt gel matrix. The Al3+ ions appear to be site bound resulting matrix resistance to flow, unlike the zinc Polyacrylate matrix. During the initial setting in the first 3 hours Calcium ions react with polycarboxylate chains.
Subsequently, the trivalent Aluminum ions react for at least 48 hours. Between 20 and 30% of the glass is decomposed by the proton attack. The Fluoride and Phosphate ions are insoluble salts and complexes. The Sodium ions form a silica gel. The structure of the fully set cement is a composite of glass particles surrounded by silica gel in a matrix of Polyanions cross-linked by ionic bridges. Within the matrix are small particles of Silica gel containing fluorite crystallites.
Glass Ionomer Cements bond chemically to dentine and enamel during the setting process. The mechanism of bonding appears to involve an ionic interaction with Calcium and/or Phosphate ions from the surface of the enamel or dentine. Bonding is more affective with a cleaned surface provided cleaning does not remove an excessive amount of Calcium ions. Treating dentine with an acidic conditioner followed by a dilute solution of ferric chloride improves the bonding. The cleansing agent removes the smear layer of dentine while the Fe+3 ions are deposited and increase the ionic interaction between the cement and dentin. Also, as the initial Calcium cross-links are replaced by Aluminium cross-links, most Sodium and Fluoride ions do not participate in the cross linking of the cement, however some Sodium ions may replace the Hydrogen ions of carboxylic groups whereas the remaining ions are dispersed uniformly within the set cement along with Fluorine ions. The cross linked phase becomes hydrates over time with the same water used for mixing. This process is called maturation.
The unreacted portion of the glass particles are sheathed by a silica gel that develops during the removal of cations from the surface of the particles. Thus the set cement contains an agglomeration of unreacted powder particles surrounded by a silica gel in an amorphous matrix of hydrated Calcium and Aluminum Polysalts. Water plays a critical role in the setting of GIC. It serves as the reaction medium initially and then slowly hydrates the cross linked agents thereby yielding stable gel structure that is stronger and less susceptible to moisture contamination. If freshly mixed cements are exposed to ambient air without any protective covering the surface will craze and crack as a result of desiccation. Any contamination by water that occurs at this stage can cause dissolution of the matrix-forming cations and anions to the surrounding areas. Both desiccation and contamination are water changes in the structure during placement and for a few weeks after placement is possible.

## Manipulation
To achieve long lasting restorations and retentive fixed prostheses, the following manipulative considerations for GIC must be satisfied:
- Surface of the prepared tooth must be clean and dry
- The consistency of the mixed cement must allow complete coating of the surface irregularities and complete seating of prostheses
- Excess cement must be remove at the appropriate time
- The surface must be finished without excessive drying
- Protection of the restoration surface must be ensured to prevent cracking or dissolution.
The conditions are similar for lutting applications, except that no surface finishing is needed.

# Properties

## Setting Time
GlC sets within 6-8 minutes from the start of mixing, setting time is lesser for type I materials then Type II materials. The setting can be slowed when the cement is mixed on a cold slab but this technique has an adverse affect on strength.
GIC TYPE 1 - 5-7minutes
GIC TYPE 2 - within 10minutes

## Film Thickness
The film thickness of GICs is similar to or less than that of zinc phosphate cement and is suitable for cementation.

## Esthetics
Conventional glass ionomer cements are tooth-coloured and available in different shades. Although the addition of resin in the modified materials has further improved their translucency, they are still rather opaque and not as esthetic as composite-resins. 
In addition, surface finish is usually not as good. The colour of resin-modified materials has been reported to vary with the finishing and polishing techniques used. Potential also exists for increased body discolouration and surface staining because of their hydrophilic monomers and incomplete polymerization. Nevertheless, the demand for esthetics in the primary dentition is usually lower than in the permanent dentition.

## Water Sensitivity, Solubility and Disintegration
Like silicates the initial solubility is high (0.4%) due to leaching of intermediate products. The complete setting reaction takes place in 24 hours there the cement should be protected from saliva in mouth during this period. GIC are also more resistant to attack by organic acids. Conventional glass ionomer restorations are hence also difficult to manipulate as they are sensitive to moisture imbibition during the early setting reaction and to desiccation as the materials begin to harden. Although it was believed that the occurrence of the resin polymerization in the modified materials reduces the early sensitivity to moisture, studies have shown that the properties of the materials changed markedly with exposure to moisture. Whether it is necessary to place protective covering on resin-modified glass ionomer restorations remains controversial.

## Adhesion
By bonding a restorative material to tooth structure, the cavity is theoretically sealed, protecting the pulp, eliminating secondary caries and preventing leakage at the margins. This also allows cavity forms to be more conservative and, to some extent, reinforces the remaining tooth by integrating restorative material with the tooth structures. Bonding between the cement and dental hard tissues is achieved through an ionic exchange at the interface. Polyalkenoate chains enter the molecular surface of dental apatite, replacing phosphate ions. Calcium ions are displaced equally with the phosphate ions so as to maintain electrical equilibrium. This leads to the development of an ion-enriched layer of cement that is firmly attached to the tooth.
The shear bond strength of conventional glass ionomer cements to conditioned enamel and dentin is relatively low, varying from 3 to 7 MPa. However, this bond strength is more a measure of the tensile strength of the cement itself, since fractures are usually cohesive within the cement, leaving the enriched residue attached to the tooth. Comparisons between resin-modified glass ionomer cements and conventional materials reveal that the shear bond strength of the former is generally greater, but that they show very low bond strength to unconditioned dentin compared to conventional materials. Conditioning therefore plays a greater role in achieving effective bonding with the resin-modified glass ionomer cements. In addition, when the enamel surface is etched with phosphoric acid, the bond strength of the resin-modified materials is close to that of composite-resin bonded to etched enamel. This suggests, along with the effects of light-curing, that the bonding mechanism of resin-modified glass ionomer cements may be different from that of conventional materials.

## Margin Adaptation and Leakage
The coefficient of thermal expansion of conventional glass ionomer cements is close to that of dental hard tissues and has been cited as a significant reason for the good margin adaptation of glass ionomer restorations. Even though the shear bond strength of glass ionomer cements does not approach that of the latest dentin bonding agent, glass ionomer restorations placed in cervical cavities are very durable. Nevertheless, microleakage still occurs at margins. An in vitro study has shown that conventional glass ionomer cements were less reliable in sealing enamel margins than composite-resin. They also failed to eliminate dye penetration at the gingival margins. Although resin-modified glass ionomer cements show higher bond strength to dental hard tissues than conventional materials, they exhibit variable results in microleakage tests. Not all of them display significantly less leakage against enamel and dentin than their conventional counterparts.  This may be partly because their coefficient of thermal expansion is higher than conventional materials, though still much less than composite-resins. Controversy also exists as to whether the slight polymerization shrinkage is significant enough to disrupt the margin seal.

## Physical Strengths
The main limitation of the glass ionomer cements is their relative lack of strength and low resistance to abrasion and wear. Conventional glass ionomer cements have low flexural strength but high modulus of elasticity, and are therefore very brittle and prone to bulk fracture. Some glass cermet cements are arguably stronger than conventional materials but their fracture resistance remains low. The resin-modified materials have been shown to have significantly higher flexural and tensile strengths and lower modulus of elasticity than the conventional materials. They are therefore more fracture-resistant but their wear resistance has not been much improved. In addition, their strength properties are still much inferior to those of composite-resins, and so should not be subject to undue occlusal load unless they are well supported by surrounding tooth structure.

## Biocompatibility
The biocompatibility of glass ionomer cements is very important because they need to be in direct contact with enamel and dentin if any chemical adhesion is to occur. In an in vitro study, freshly mixed conventional glass ionomer cement was found to be cytotoxic, but the set cement had no effect on cell cultures. In another study, the pulpal response to glass ionomer cements in caries-free human premolars planned for extraction was examined. The result showed that although glass ionomer cement caused a greater inflammatory response than Zinc-Oxide Eugenol cement, the inflammation resolved spontaneously with no increase in reparative dentin formation. More recently, Snugs and others have even demonstrated dentin bridging in monkey teeth where mechanical exposures in otherwise healthy pulps were capped with a glass ionomer liner. Therefore, lining is normally not necessary under conventional glass ionomer restorations when there is no pulpal exposure. Concern has been raised regarding the biocompatibility of resin-modified materials since they contain unsaturated groups. A cell culture study revealed poor biocompatibility of a resin-modified liner. In contrast, Cox and others showed that a resin-modified glass ionomer cement did not impair pulp healing when placed on exposed pulps. As a result of this uncertainty, use of resin-modified materials in deep unlined cavities is probably not advisable.

## Anticariogenic effect by way of fluoride release
Fluoride is released from the glass powder at the time of mixing and lies free within the matrix. It can therefore be released without affecting the physical properties of the cement. Since it can also be taken up into the cement during topical fluoride treatment and released again, the cement may act as a fluoride reservoir over a relatively long period. As a result, it has been suggested that glass ionomer cements will be clinically anticariogenic. This assumption is supported by some in vitro studies using an artificial caries model in which less decalcification has been found in cavities restored with glass ionomer cements. The amount of constant fluoride release did not differ much between brands of conventional glass ionomer cements. The fluoride release of some resin-modified materials is at least the same as conventional materials but varies amongst different commercial products. Nevertheless, the critical amount of fluoride released from a restoration that is required to be effective in inhibiting caries has not yet been established. Despite the constant fluoride release of glass ionomer restorations, results from clinical studies are not so promising. Kaurich and others researches compared glass ionomer and composite-resin restorations over one year and concluded that there was little clinical advantage in using glass ionomer cement. Tyas examined cervical composite-resin and glass ionomer restorations five years after placement and found no significant difference in recurrent caries rates. More clinical studies would therefore be needed to confirm the anticariogenic effect of glass ionomer cements.

# Clinical Success in Primary Molars
Clinical trials investigating the longevity of glass ionomer restorations in primary molars are mostly short-term studies of less than three years. The longest survival rates for glass ionomer restorations are in low stress areas such as Class III and Class V restorations. In an early study, Vlietstra and others reported that 75% of conventional glass ionomer restorations in primary molars were intact after one year, and that margin adaptation, contour and surface finish were all satisfactory. The longest clinical study has been conducted by Walls and others who compared conventional glass ionomer restorations with amalgam restorations in primary molars. Although they reported no significant difference in overall failure rates after two years, follow-up of the restorations up to five years showed that glass ionomer restorations had significantly inferior survival time to amalgam. The importance of long-term clinical studies should therefore not be overlooked.
Other short-term trials also show poor success rates of conventional glass ionomer restorations in primary molars. Ostlund and others compared Class II restorations of amalgam, composite-resin and glass ionomer cement in primary molars and reported a high failure rate for glass ionomer cement of 60% after one year. In contrast, the failure rates for amalgam and composite-resin restorations were eight and 16% respectively. Fuks and others compared the clinical performance of a glass ionomer cement with amalgam in Class II restorations in primary molars. Only nine of 101 glass ionomer restorations met all quality criteria after one year, whereas 90% of the amalgam restorations met all the evaluation criteria after three years. Papathanasiou and others investigated the mean survival time of different types of restorations in primary molars and found that the mean survival time for glass ionomer restorations was only 12 months compared to more than five years for stainless steel crowns and amalgam restorations.
In a recent study, the median survival time for Class II glass ionomer restorations in primary molars was also reported to be significantly shorter than for amalgam restorations. The results of these studies indicate that conventional glass ionomer cement is not an appropriate alternative to amalgam in the restoration of primary molars unless the teeth are expected to exfoliate in one or two years. Short-term clinical studies have shown that the performance of Class II glass cermet restorations in primary molars is significantly worse than conventional materials. Although Hickel and Voss2 found no significant difference in the cumulative failure rates between glass cermet and amalgam restorations in primary molars, they did find that the loss of anatomical form was more severe with glass cermet cement, concluding that amalgam should be preferred in restorations with occlusal stress.
Only limited data are available for resin-modified glass ionomer restorations in primary molars and they are mostly in the form of clinical experience or abstracts. The initial results show that these restorations perform better than conventional materials in short-term comparisons. Long-term trials would be required to confirm their efficacy. Until then, the choice of resin-modified glass ionomer restorations in primary molars remains a relatively empirical one and should therefore be restricted to cavities well supported by surrounding tooth structures, such as small Class I and Class II restorations. In cases where high occlusal load is expected, other alternatives such as amalgam or stainless steel crowns should be considered.
Advantages
- Inherent adhesion to tooth structure
- High retention rate
- Little shrinkage and good marginal seal
- Fluoride release and hence caries inhibition
- Biocompatible
- Minimal cavity preparation required hence easy to use on children in and suitable for use even in absence of skilled dental manpower and facilities (such as in ART)
Disadvantages
- Brittle
- Soluble
- Abrasive
- Water sensitive during setting phase.
- Some products release less fluoride then conventional GIC
- Not inherently radiopaque though addition of radiodense additives such as barium can alter radiodensity
- Less aesthetic then composite

# Uses
The general use-based classification of GICs is as follows:
- Type I	- For luting cements
- Type II	- For restorations
- Type III - Liners and bases
- Type IV	- Fissure sealants
- Type V	- Orthodontic Cements
- Type VI	- Core build up
Additionally GICs may be also used for:
- Intermediate Restorations
- Adhesive Cavity Liners (Sandwich Technique)
- ART (Atraumatic Restorative Technique)
- Restorations for deciduous teeth
The type of application prescribes the viscosity of the cement, which is adjusted by varying the particle size distribution and the powder-to-liquid ratio. The maximum particle size is 15 m for lutting agents and 50 m for restorative cements.

## As Luting Agents
Glass Ionomer Luting Cement is excellent for permanent cementation of crowns, bridges, veneers and other facings. It can be used as a liner under composites. It chemically bonds to dentine/enamel, precious metals and porcelain restorations. It has good translucency and universal yellow shade, with early high compressive strength. It releases fluoride ions and reduces sensitizing by giving a firm foundation for composites, pulp protection and insulation. It mechanically bonds to composite restorative materials. It reduces the incidence of micro-leakage when used to cement composite inlays or onlays. It is easy to mix with good flow properties. It is fast setting with low fill thickness and low viscosity. It reaches the neutral pH fast, following placement on the tooth. It is used for cementation of orthodontic bands.
Typical Physical Properties:
- Mixing Time: 15 seconds
- Setting Time: 2 minutes
- Working Time: 2 minutes
- Total Time: 4.5 minutes at 23 C
Mixing Directions:
2 Scoops of powder and 3 drops of liquid. The powder should be placed separately on the mixing pad. The first scoop of powder should be incorporated into the liquid and as soon as it is fully wet, add the second scoop and mix smoothly to a smooth, creamy state ready for cementation. The cement in this glossy state should be applied immediately to clean dry restoration which is sealed on the dry prepared tooth. The excess cement is trimmed away at its rubbery stage, just prior to the final set.

## As Orthodontic Brackets Adhesives
Currently the most commonly used adhesive for orthodontic bracket bonding are based on composite resin. However Glass Ionomer systems have certain advantages. They bond directly to tooth tissue by the interaction of Polyacrylate ions and hydroxyapatite crystals, thereby avoiding acid etching. In addition they have anticariogenic affect due to their fluoride leaching ability. Nevertheless their use in orthodontic bracket bonding has been limited due to inferior mechanical properties, in particular bond strength.

## As Pit and Fissure Sealants
Another suggested use of glass ionomer cements is as fissure sealants. The material is mixed to a more fluid consistency to allow flow into the depths of the pits and fissures of the posterior teeth. Early cements were found to be unsuitable for use as sealants if the fissures were less than 100 meter wide. The large glass particles of cement prevented adequate penetration of fissures with a bur.

## 4. As Liners and Bases
GICs have a number of advantages as cavity lining as they bond to dentine and enamel and release fluoride which not only helps in prevent decay and therefore minimizing the chance of appearance of secondary carries, but also promote the formation of secondary dentine. They can be used beneath both composite resin and amalgam.

## For Core Build Up
Some dentists favour glass ionomers cements for cores, in view of the apparent ease of placement, adhesion, fluoride release, and matched coefficient of thermal expansion. Silver containing GICs (eg the cermet, Ketac Silver, Espe GMbH, Germany) or the 'miracle mix' of GIC and unreacted amalgam alloy have been especially popular. Some believe the silver within the material enhances its physical and mechanical properties, however, in-vitro studies are equivocal and a study of a cermet used to fill deciduous teeth showed that it performed less well than a conventional GIC. In the days when many GICs were radiolucent, the addition of silver conferred radiopacity without which it would be difficult or impossible to diagnose secondary caries. Nowadays, many conventional GICs are radiopaque and are easier to handle than the silver containing materials. Nevertheless, many workers regard GICs as inadequately strong to support major core build-ups.  Hence the recommendation that a tooth should have at least two structurally intact walls if a GIC core is to be considered. In our view it is best to regard GIC as excellent filler but a relatively weak build-up material. In order to protect a GIC core the crown margin should, wherever possible, completely embrace 1-2 mm of sound tooth structure cervically. Extension of the crown margin in this way is termed the 'ferrule effect' and should ideally be used for all cores.
Advantages:
- Intrinsically adhesive
- Fluoride release - but this does not guarantee freedom from 2 decay (Figure VIII)
- Similar coefficient of thermal expansion to tooth
Disadvantages:
- Considerably weaker than amalgam and composite
- Tendency to crack worsened by early instrumentation
- Silver containing materials offer little improvement in physical properties
- Some materials radiolucent
Recommendations:
- Excellent filler but relies on having sufficient dentine to support crown
- Where used as a build-up, best to leave tooth preparation until next appointment
- Good material on which to bond restorations with resin cement

## For Intermediate Restorations
Because of their inherent adhesive nature and brittleness and about satisfactory aesthetics GICs are also widely used to restore loss of tooth structure from the roots of teeth either as consequence of decay or the so called cervical abrasion cavity. Abrasion cavities were once though to be the product of over zealous tooth brushing, possibly in association with the use of an abrasive dentifrice. It is now recognized that both dietary factors and functional loading of teeth (causing the teeth to bend) can be co-factors in their aetiology. In addition theyre also used frequently as in non-undercut cavities, with reliance being placed upon their adhesive characteristics to ensure their retention.

## As Adhesive Cavity Liners (Sandwich Technique)
The so called sandwich technique involves using GIC as dentine replacement and a composite to replace enamel. These purpose designed lining materials set quickly and can be made receptive for the bonding of composite resins simply by washing the material surface if the material is freshly placed (excess water results in some of the GIC matrix being washed out from around the filler particles giving a microscopically rough surface to which the composite wall will attach in an analogous manner to etched enamel). This surface should be coated either with an unfilled resin or a DBA to optimize attachment. It is only necessary to etch a GIC with acid if the restoration has been in place for some time and has fully matured. The sandwich technique has a number of attractions but it should be undertaken as planned procedure rather than as method to improve the appearance of unsatisfactory GIC restoration.

## ART (Atraumatic Restorative Treatment)
ART or the Atraumatic Restorative Treatment is a method of caries management developed primarily for use in the Third World countries where skilled dental man power and facilities are limited and the population need is high. The technique uses simple hand instruments (such as chisels and excavators) to break through the enamel and remove as much carries as possible. The cavity is loaded using cotton rolls. When excavation of carries is complete (or as complete as can be achieved) the residual cavity is restored using a modified GIC. These GICs are reinforced to give increased strength under functional loads and are radio opaque. There aesthetic properties are poorer with the material being optimally opaque.

## As Restorations for Deciduous Teeth
Because of their high fluoride release and minimal cavity preparation requirement GIC is now widely the materials of choice for the restoration of carious primary teeth. Restoring carious teeth is one of the major treatment needs of young children. A restoration in the primary dentition is different from a restoration in the permanent dentition due to the limited lifespan of the teeth and the lower biting forces of children. As early as 1977, it was suggested that glass ionomer cements could offer particular advantages as restorative materials in the primary dentition because of their ability to release fluoride and to adhere to dental hard tissues.  And because they require a short time to fill the cavity, glass ionomer cements present an additional advantage when treating young children. However, the clinical performance of conventional and metal-reinforced glass ionomer restorations in primary molars is disappointing. And although the handling and physical properties of the resin-modified materials are better than their predecessors, more clinical studies are required to confirm their efficacy in the restoration of primary molars.