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Glass ionomer Cements

INTRODUCTION

 

GIC was developed by Wilson and Kent England, in 1972.

This material was introduced in US as ASPA (Alumino Silicate Poly Acrylate) in 1977.

The GIC have been evolved as a hybrid from silicate cement as a powder and polyacrylate as a liquid.

ISO terminology for cement was extensively used as a dentin replacement material. Dentin has also been referred as man made dentin or dentin substitute.

The greatest advantage of this cement is –

Chemically adherent to tooth structure and anticariogenic property.

Now a days GIC is used for different application such as – Luting, restoration, baseliners, core builders and cementation of orthodontic band and brackets and also form atraumatic restorative treatment.

 

 

 

 

 

 

HISTORICAL DEVELOPMENT

 

Summary of the historical evolution of glass-ionomer cements. The

original cement (GIG) is hydrophilic because of the water content required to dissolve the polyacrylic acid chains and maintain the ion-cross-linked hydrogel. Hydrogels (bottom middle) are neither as strong nor as esthetic as dental composites (upper right). Early experiments focused on replacing some of the fluoroaluminosilicate filler with metal or cement particles. These metal-modified glass-ionomers (MM-GIC) were not esthetic, but have been used as cores. Replacing part of the hydrogel with water-soluble, light-curing monomers and polymer phases generated resin-modified glass-ionomer (RM-GIC or RMGI). Complete replacement of the matrix with typical composite chemistry but inclusion of the fluoride-releasing matrix phases or glass produced compomers (composites capable of releasing F ions). Modification of compomers by blending in precured glass-ionomer phases as particles produced giomers. Original glass-ionomer "as is" or modified with small additions of polymer resin or more F-enriched glasses generated resin-reinforced glass ionomers (RR-GIG or RR-GI), which have been extremely popular as ART and temporary materials. Fuji IX (GC Corporation) is a noteworthy representative of this category.

 

 

REQUIREMENT OF IDEAL RESTORATIVE MATERIALS

1. Restoration of aesthetics
2. Maintenance of the physical strength of the crown.
3. Preserving the anatomy of the occlusal surface and thus preserving the inter relationship with the opposing and adjacent teeth.
4. Prevention of further ingress of bacteria or their by products into the micro space between the restoration and the tooth.
5. Long term adhesion between the restoration and the tooth to ensure complete isolation.

 

SYNONYMS OF GIC

v    Poly (alkenoate) cement

v    GIC (Glass ionomer cement)

v    ASPA (Alumino silicate polyacrylic acid)

 

CLASSIFICATION

The following is the accepted classification –

 

Type I – luting

v    Cementation of crowns, bridges and orthodontic devices.

v    Powder : Liquid ratio approximately 1:5:1

v    Radio – opaque

 

Type II – restorative

v    Type II.1 – Restorative aesthetic

v    All types of aesthetic restorations

v    Auto – cure or resin modified

v    Radio  – opaque generally

v    High physical properties

 

Type II.2 – Restorative

v    Restorations under high occlusal load

v    Auto cure or resin modified

v    Powder : Liquid ration 3:1 or greater

v    Radio opaque

v    Used as a dentin substitute or interim restoration

 

Type III – Lining or base

v    Simple lining under a metallic restoration

v    Powder: liquid ration 1:5:1 only

v    Auto cure

v    Radio – opaque

 

COMMERCIAL NAMES

1. Aquacem, fugi I – Type I
2. Chem. Fil – Type II
3. Ketac bond – Type III
4. Vitra bond – Light cure GIC

 

 

 

Available as:

1. Powder / liquid in bottles
2. Pre- proportioned powder / liquid in capsules
3. Light cure system
4. Powder / distilled water (water settable type)

 

COMPOSITION

 

1. Powder

 

The powder is an acid soluble calcium fluoro – alumino silicate glass.

Silica (SiO)2 – 41.9

Alumina (Al2O3) – 28.6

Aluminium fluoride (AlF3) – 1.6

Calcium fluoride (CaF2) – 15.7

Sodium fluoride  (NaF) – 9.3

Aluminium phosphate  (Al Po4) – 3.8

The fluoride component acts as a ‘ceramic flux'

Lanthanum, strontium, barium or zinc oxide addition provide radio opacity.

Liquid:

 

Earlier the liquid was a 50% aqueous solution polyacrylic acid. It was very viscous and has a tendency to gel.

 

 

In most current cements, the liquid contains –

v    Polyacrylic acid – in the form of co – polymer with iticonic acid, maleic acid and tricarballylic acid

v    Tartaric acid

v    Water

 

Copolymerizingwith iticonic, maleic acid, etc tends to increase reactivity of the liquid, decrease viscosity and reduce tendency for gelation. Tartaric acid improves the handling characteristics, increase working tissue and shorter setting time. Water is the most important constituent of the cement liquid, it is the medium of reaction and it hydrates the reaction products. The amount of water results in a weak cement. Too little water impairs the reaction and subsequent hydration.  

 

SETTING REACTION

 

 

 

Leaching:

 

When the powder and liquid are mixed together, the acid attacks the glass particles. Thus calcium, aluminium, sodium and fluoride ions reach out into the aqueous medium.

 

Calcium cross – links:

 

The initial set occurs when the calcium ions cross – links (binds) the polyacrylic acid chains. This forms a solid mass.

 

Aluminium cross – links:

 

In the next phase( next 24 hours) the aluminium also begins to cross – link with polyacrylic acid chains.

Sodium and fluorine ions:

 

These ions do not take part in the cross – linking, some of the sodium ions may replace the hydrogen ions in the carboxylic groups. The rest combine with fluorine to form sodium fluoride which is uniformly distributed within the cement.

 

Hydration:

 

Water plays a very important role in the cement. Initially it serves as the medium, later, it slowly hydrates the matrix, adding to the strength of the cement.

 

Silica gel sheath:

 

The unreacted glass (powder) particle is sheathed (covered) by a silica gel. It is formed by the leaching of the ions (Ca2+, Al3+, Na+, F-) from the outer position of the glass particle.

 

STRUCTURE OF SET CEMENT

 

The set cement consists of agglomerates of unreacted powder particles surrounded by silica gel and embedded in an amorphous matrix of hydrated calcium and aluminium polysalts.

 

 

 

 

MANIPULATION

 

v    Conditioning of tooth surface

v    Proper manipulation

v    Protection of cement during setting

v    Finishing

 

PREPARATION OF TOOTH SURFACE 

 

The tooth should be clean for effective adhesion of cement. The smear layer present after cavity preparation tends to block off – the tooth surface, and so should be removed to achieve adhesive bonding.

 

This is achieved by

v    Pumice wash

v    Polyacrylic acid (The objective is to remove the smear layer but still leave the collageous tubule plug in place. This plug act as a barrier to the penetration of acid from the cement).

Apply 10% polyacrylic acid for 10 to 15 seconds. Next, rinse with water for 30 seconds, very deep areas of the penetration should be protected by a dab of calcium hydroxide.

 

Eroded areas

 

The dentin and cementum are first cleaned with a pumice slurry followed by swabbing with polyacrylic acid for 5 seconds or more. After conditioning and rinsing, the surface is dried but not desiccated. It should be kept free of contamination with saliva or blood as these will interfere with bonding. It contaminated, the whole procedure is repeated.

 

PROPORTIONING AND MIXING

 

Powder / liquid ratio:

 

Generally 3:1 by weight (manufacturers recommendation should be followed) low powder / liquid ratio reduces mechanical properties and increase the chances of cement degradation. Moisture contamination alters the acid – water balance. Most manufacturers provide a plastic scoop, which is useful for measures.

 

Spatula Used

v    Agate Or Plastic

                                  

 

 

MANUAL MIXING

 

The powder bottle is tumbled gently. The powder and liquid is dispensed just prior to mixing. A cool and dry glass slab is preferred as it allows all the powder to be incorporated into the mix and yet maintain its plasticity.

 

The powder is divided into two equal increments. The first increments is incorporated into the liquid rapidly with the stiff bladed spatula to produce a homogenous milky consistency. The remainder of the powder is then added. The mixing is done in a folding method in order to preserve the gel structure.

 

Mixing Time

v    45 seconds

 

Insertion

 

The mix is immediately packed into the cavity with a plastic instrument.

 

MECHANICAL MIXING

 

GIC supplied in capsule form containing prepropertioned powder and liquid is mixed in an amalgamator which is operated at a very high speed. The capsule has a nozzle, and so the mix can be injected into the cavity.

 

 

 

Advantages:

1. Better properties due to controlled powder / liquid ratio
2. Less mixing tense required
3. Convenient delivery system

 

Disadvantages:

v    Cement quantity limited by the manufactures

v    Shade selection is limited, colours can not be blended.

 

HAND MIXING

 

Divide the dispensed powder into two equal parts. Gently spread the liquid drop a little over the glass slab. Roll the first half of the powder into the liquid and incorporate the two together rapidly. This is com­pleted in 10 seconds and the rest of the powder is brought into the mix. No attempt should be made to try and dissolve the powder into the liquid. It is only necessary to wet the surface of each particle so that ion release can occur leading to the initiation of the acid/base setting reaction. The final mix should be completed within 25-30 seconds.

 

 

PLACEMENT OF GIC AS A RESTORATIVE MATERIAL AND REMOVAL OF EXCESS

 

The restorative cement mixture is applied by a plastic instrument or injected on the prepared tooth surface. Tooth cavities should be slightly overfilled with cement. After placement, the surface should be covered with a plastic matrix to protect the setting cement from losing or gaining water during the initial set. The matrix is left in place for at least 5 minute, although this time varies according to the product, based on the setting rate. Upon removal of the matrix, the surface must immediately be protected while the excess material is trimmed from the margins. Further finishing procedures, if needed, should be delayed for at least 24 hours. However, because this is clinically unrealistic, finishing, of the restoration should be completed in the same appointment. Thus faster setting cement are desirable. Even so, the longer the dentist waits to properly protect the surface, the more mature the cement becomes, the lower the risk for surface cracks, and the lower the tendency, for the restoration to become slightly more opaque.

 

In the case of resting applications, no matrix protection is needed. The excess cement can be removed immediately upon seating or after a length of time as prescribed in the manufacturer's instructions.

 

PROTECTION OF CEMENT DURING SETTING

 

GIC is extremely sensitive to air and water during setting. Thus, immediately after placement into the cavity, a preshaped matrix is applied to:

1. Protect the cement from the environment during initial set.
2. Providing maximum contour so that minimal finishing is required.

The matrix is removed after five minutes. Immediately after removal, the cement surface is again protected with :

1. A special varnish supplied manufacturer, or
2. An unfilled light cured resin bonding agent, or
3. Cocoa butter.

 

This protects the cement from drying while the dentist proceeds with the finishing. Failure to protect the cement surface from contact with air results in a chalky or crazed surface.

 

The cause for chalky or crazed surface:

v    Inadequate protection of freshly set cement (from air)

v    Low powder / liquid ratio

v    Improper manipulation

 

 

 

 

 

 

 

Finishing

v    Excess material is trimmed from the margins, hand instruments are preferred to rotary, tools to avoid ditching. Further finishing if required is done after 24 hours.

 

Protection of cement after setting

 

Before dismissing the patient, the restoration is again coated with protective agent, to protect the trimmed areas. Failure to protect the cement from saliva for the first 24 hours can weaken the cement.

 

Precautions

1. A good mix should a glossy surface. This indicates the presence of residual polyacid (which has not been used in the setting reaction) and ensures proper bonding to the tooth. A mix with dull surface is discarded as it indicates prolonged mixing and reduces the adhesion.
2. If the liquid contains polyacids, it should not be placed in a refrigerator as it becomes very viscous.
3. The restorations must be protected from drying at all times, even when other dental procedures are to be carried our later.
4. The glass slab should not be below dew point as moisture may condense on the slab and change the acid water balance.

 

POSTOPERATIVE PROCEDURES

 

Before the patient leaves, the type II GIC restoration should be coated again with a protective agent, because the exposed cement around trimmed areas and margins is still vulnerable to the environment until it reaches full maturity. If these recommended procedures for providing protection to the setting cement are not followed, a chalky or a crazed surface will inevitably result.

 

In summary, protection of glass ionomer restoration depends on meticulous attention to recommended procedures to: (1) conditioning to the tooth surface (2) proper manipulation, and (3) protection of the cement during setting and during potential situations when desiccation might occur. When these parameters are controlled, high quality restorations should be produced.

 

Setting time

v    Type – I – 4-5 minutes

v    Type – II – 7 minutes

 

 

 

PROPERTIES

 

1. Compressive strength: (150 MPa), it is a less than silicate tensile strength : (6.6 MPa), higher than silicate

 

2. Hardness: (49 KHN), less harder than silicates. The wear resistance is also less when compared to composites.

 

3. Fracture toughness: A measure of energy required to produce fracture. Type II GIC's are inferior to composites in this respect.

 

 

 

4. Solubility and disintegration:  Like silicates, the initial solubility is high (0.4%) due to leading of intermediate products. The complete setting reaction takes place in 24 hours. Therefore the cement should be protected from saliva in the mouth during this period. Glass ionomer cements are more resistant to attack by organic acids.

 

5. Adhesion: It adheres well to enamel and dentine. Mechanism of adhesion: Glass ionmer bonds chemically to tooth structure. The exact mechanisms has not been fully understood. The bonding is due to the reaction between the carboxy groups of the polyacids and the calcium in the enamel and dentine.

 

The bond to enamel is always higher than to denture, probably due to the greater inorganic content of enamel and its greater homogeneity.

 

6. Esthetics :Esthetically they are inferior to silicates and composites. They lack translucency and have a rough surface texture. They may accumulate stain with time.

 

7. Biocompatibility:Pulpal response mild – Type II glass ionomer are relatively biocompatible. The pulpal reaction is greater than that of zinc oxide – eugenol cements but less than that produced by zinc phosphate cement. Polyacids are relatively weak acids.

 

The water settable cements show higher acidity. Type I GIC is more acidic than type II, because of the slower set and lower powder / liquid ratio.

 

Pulp protection :In deep cavities, the smear layer should not be removed as it acts as a barrier to acid penetration. Deep areas or protected by a dab of calcium hydroxide cement.

 

8. Anticariogenic properties:Type II glass ionomer releases fluoride in amounts comparable to silicate cements initially and continue to do so over an extended period of time.

 

In addition, due to its adhesive effect they can have the potential for reducing infiltration of oral fluids at the cement – tooth interface, thereby preventing secondary caries.

 

MODIFICATION OF GLASS IONOMER CEMENTS

 

1. Fast setting materials

They are also called as highly viscous GIC or condensable GIC. The powder is chemically modified during manufacturing to decrease the calcium content & thus limit the production of calcium polyalkenoate chains which are highly water soluble. This allows faster maturation of the material but decreases the translucency.

 

2. Water settable glass ionomer cements

The polyacrylic acid copolymers are freeze dried & coated onto the powder particles. The liquid contain tartaric acid and water. These cements are called as ‘water settable GIC'. They set faster than the conventional GIC.

 

 

3. Resin modified GIC 

Also called ‘hybrid ionomers, polyacid modified resin (PAMR)'. 5% of resin matrix is added by modifying the liquid component of the GIC.

 

 

Various organic compounds have been used like

v             Polymerisable monomers

v             Polymerisable polyalkenoid vied

v             Preppolymer substitution or addition to polyalkenoi acids

v             Acid monomers

 

The setting reaction is essential an acid base reaction. The resin component is activated by light, chemicals or both. As the ion exchange system is available for adhesion, a resin bonding system need not be used. The acid base reaction continues even in the absence f light activation.

 

4. Metal modified GIC

GIC have been modified by inclusion of filler particles in an attempts to improve their strength, fracture toughness and wear resistance. Two methods have been employed. The first approach involves mixing of spherical silver amalgam alloy particles with glass inomer powder. This cement is called siturs alloy admix or miracle mix.

 

The second method involves the fusion the silver particles to the glass ionomer particles by sintering at high temperature. This is called as ‘CERMET'.

 

 

5. Compomers (Polyacid modified resins)

 

The word compomer is derived from composites & glass ionomers. They are resin composites in which the filler is a glass similar to the aluminofluorosilicate glass used for GICs. A variable quantity of dehydrated polyalkenois acid is incorporated along with the fitter but this is not available for reaction with the glass until there is some water uptake into the restoration. The initial setting is by a light activated system similar to the composites. As water is absorbed from the saliva into the there is a limited degree of acid bare, which releases small uantities by fluoride. However, the adhesion is based on the acid technique using premirs because chemical adhesion does not occurs.

 

 

Advantages

v             Inherent adhesion to tooth structure

v             High retention rate

v             Little shrinkage and good marginal seal

v             Fluoride release and hence caries inhibition

v             Biocompatible

v             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

v             Brittle

v             Soluble

v             Abrasive

v             Water sensitive during setting phase.

v             Some products release less fluoride then conventional GIC

v             Not inherently radiopaque though addition of radiodense additives such as barium can alter radiodensity

v             Less aesthetic then composite

 

USES:

 

1. 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:

v    Mixing Time: 15 seconds

v    Setting Time: 2 minutes

v    Working Time: 2 minutes

v    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.

 

2. 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.

 

3. 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 100µ 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.

 

5. 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:

v             Intrinsically adhesive

v             Fluoride release - but this does not guarantee freedom from 2° decay (Figure VIII)

v             Similar coefficient of thermal expansion to tooth

 

Disadvantages:

v             Considerably weaker than amalgam and composite

v             Tendency to crack worsened by early instrumentation

v             Silver containing materials offer little improvement in physical properties

v             Some materials radiolucent

 

Recommendations:

v             Excellent filler but relies on having sufficient dentine to support crown

v             Where used as a build-up, best to leave tooth preparation until next appointment

v             Good material on which to bond restorations with resin cement

 

 

 

 

6. 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 they're also used frequently as in non-undercut cavities, with reliance being placed upon their adhesive characteristics to ensure their retention.

 

7. 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.

 

8. For 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.

 

9. As Restorations for Decidous 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.

 

 

 

 

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