Glass Ionomer Cement

GLASS IONOMER CEMENTS

INTRODUCTION

1972-  Wilson  &  Kent  developed the  Glass Ionomer Cement .

Often referred to as a dentin substitute or man made dentin.

It is a product of an acid base reaction in the presence of water.

Definition

WATER BASED MATERIAL THAT  HARDENS FOLLOWING AN ACID BASE REACTION BETWEEN BASIC FLUORO ALUMINOSILICATE GLASS AND AN AQUEOUS SOLUTION OF POLYACIDS

Classification of Glass Ionomer Cements.

Type 1: Luting cements

Type II- Restorative cements

      a. Esthetic – Auto cure

                         - Light cure

      b. Reinforced

Type III : Fast setting lining materials and pit & fissure Sealant.

      a. Rapid maturing base/lining cements

      b. Pit & Fissure sealant

According to Generations

First Generation

Conventional

Fast setting

Second Generation

Water hardening cement

Classification by Mclean et al, 1994

Conventional Glass ionomer cement

Metal modified glass ionomers

 -Miracle mix

 -Cermet-particle reinforced

Resin modified Glass ionomers

Poly acid modified composite resins

Highly viscous glass ionomer

  

According to clinical use

Type I- Luting

 TYPE II- Restorative

 Type III- Liner/ Base

 Type IV- Pit & Fissure Sealant

 Type V- Luting for Orthodontic Purpose

 Type VI- Core build up material

 Type VII- High fluoride releasing command set

 Type VIII- ART

 Type IX- Geriatric & Paediatric GIC

Based on types of curing

2.      

u Autocure :  Acid-base reaction

u  

u Dual-cure : Light or chemical activation followed by acid-base reaction

u  

u Tri-cure :  Light and chemical activation followed by acid-base reaction

u  

COMPOSITION

   Modern cement powders (by weight) are

                   composed of

Silicon dioxide (41.9%),

Aluminum oxide (28.6%),

Calcium fluoride (15.7%)

Aluminum phosphate (12%),

Aluminum fluoride (8%)

 sodium fluoride (9%).

Glasses are prepared by fusing the components between  1100C and 1500° C

Then pour the melt onto a metal plate or into water.

The glass is then ground to a fine powder 

The finer the particle size the more rapid the setting and the stronger the cement will be.

Si02-AI203-CaF2

Role of Alumina and Silica

The Al₂0₃/Si0₂ ratio of the glass is crucial, and is required to be 1:2 or more by mass for cement formation.

Alumina           Silica network,replace silica

Causes a negative charge on the network making it basic.

Susceptible to attack by hydrogen ions from acid.

Role of calcium fluoride

Supplemented by the addition of cryolite (Na₃AIF₆).

This flux

   -reduces the temperature at which the glass will fuse

   -increases the translucency of the set cement.

Role of Aluminium phosphate

Improves translucency.

Apparently adds body to the cement paste.

Skeletal structure of fluroaluminosilicate glass

LIQUID

The liquid is an aqueous solution of polymers and copolymers of acrylic acid.

A copolymer is a chain consisting of two

   molecules.

polyacrylic acid, is the most important acid contributing to formation of the cement matrix .

Itaconic acid

Itaconic acid promotes reactivity between the glass and the liquid.

It also prevents gelation of the liquid which can result from hydrogen bonding between two polyacrylic acid chains

Polymaleic acid

a stronger acid than polyacrylic acid

causes the cement to harden and lose its moisture sensitivity faster.

more carboxyl (COOH) groups which lead to more rapid polycarboxylate crosslinking

Tartaric acid

it extends the working time.

Strengthens and hardens the cement.

Water

It is often not considered a constituent of glass-ionomer cement.

It is the reaction medium.

Calcium and Aluminium are leached and they are transported to react with the poly acid to form polysalts in water medium.

plays a role in hydrating  the siliceous hydrogel.

STAGES OF THE SETTING REACTION OF GLASS IONOMER CEMENTS  

Based on the work of Crisp and Wilson 1972-1974

Decomposition

Migration

Gelation

Post set hardening

Maturation

The setting reaction starts once Fluroaluminosilicate glass
powder and aqueous solution of polyacrylic acid are combined, producing an acid base reaction.

Surface Dissolution of glass and  Migration of these metal ions

Initially 20 to 30 % of glass is attacked

Ions locked up in the glass network are released and migrate into the aqueous phase of cement

Calcium ions predominate

During reaction calcium build up more than aluminium.

The metal ions combine with carboxylic group to form a polysalt

The pH increases, polyacrylic acid is converted to polyacrylates.

Viscosity increases.

Polyacrylic acid coil acquires charge and unwinds under the influence of electrostatic forces of repulsion.

Gelation and Vulnerability to water

Crisp et al. (1974) and Barry et al. (1979) showed that calcium polyacrylate initially predominates in the mixture

but the hardening process is derived from the slower formation of aluminium polyacrylate

This phase ultimately predominates in the mixture.

Not all the carboxyl groups of poly (acrylic acid) are converted to carboxylate groups (Coo) during the course of the reaction.

   -When most of carboxylic group are ionized the negative charge increases such that the positively charged hydrogen ions become strongly bound to the remaining unionized carboxylic groups.

   -As the density of cross links increases the metal ions are increasingly hindered in their movement towards the carboxylic groups.

Hardening and slow maturation

Hardening and the precipitation continue for about 24 hours.

There is slight expansion under conditions of high humidity and the development of translucency takes place.

The cement becomes resistant to desiccation.

Aluminium ion in the hardened matrix

Role of water

Glass ionomer cements are water-based materials

Water is the reaction medium

Plays a role in transporting calcium and aluminium to react with poly acids.

Types:

   - Loosely bound water

   -Tightly bound water

With the aging of cement, the ratio of tightly bound to loosely bound water increases

Accompanied by an increase in strength, modulus of elasticity and decrease in plasticity

Cement is only stable in an atmosphere of 80% relative humidity

In higher humidity, the cement absorbs water and the consequent hygroscopic expansion can exceed the setting shrinkage.

Cement can lose water under drying conditions, however leading to shrinking and crazing.

Susceptibility to desiccation decreases as the cement ages

This is prevented if protected for about 10 to 30 mins.

Aprox. 24% of set cement is water

Resin modified Glass ionomers

Undergoes both polymerization reaction and acid-base reaction.

The first commercial RMGICs available were liners,e.g. Vitrebond

COMPOSITION

 

POWDER

uContains radio opaque, ion-leachable fluoroaluminosilicate glass particles and encapsulated catalyst system and initiators for light curing and chemical curing

Liquid

 

uContains water and polyacrylic acid

u

uPolyacrylic acid à modified with methacrylate and hydroxyethyl methacrylate (HEMA) monomers 

u

uMethacrylate and hydroxyethyl methacrylate (HEMA) are responsible  for polymerization

Setting reaction of RMGIC
-Acid base reaction
-free radical methacrylate cure

The initial setting reaction of the material occurs by the polymerization of methacrylate group.

The slow acid-base reaction will ultimately be responsible for the unique maturing process and the final strength

Hardened mixture where HEMA & poly-carboxylate are linked by hydrogen bonding

The chemical setting reaction continues even though the reaction initiated by light is completed.

Total chemical setting is only 15%

uLess ionic activity is expected because of the reduction in carboxylic acid in the liquid of resin-modified glass ionomers.

u

uBond strength to tooth structure can be higher than that of conventional glass ionomer cements.

u

uExhibit a higher bond strength  to resin based composites.

u

uLower water and carboxylic  acid contentà reduce wettability to tooth substrates àincreases microleakage

u

uThe biocompatibility of hybrid glass ionomers is  comparable to that of conventional glass ionomer cements.

Restoration of class I, III or V .

Repair materials for damaged amalgam cores or cusps.

Retrograde root filling material.

Auto Cured Resin Reinforced Glass Ionomer Cement for Anterior Restorations
 Fuji VIII

New glass ionomer cement with improved physical and aesthetic properties that makes it the material of choice for class III, V and root surface restorations.

uAdvantages

ü Reliable Bond Strength

ü Long-lasting bond due to GC Fuji VIII GP strong chemical bonding

ü Good Translucency without Light-Curing

Poly acid modified composite resins

Compomer (composite+Glass ionomer)is a one-paste material consisting of fillers and a matrix that is similar to that of composite resin.

The material contains fluoroaluminosilicate glass powder as filler to release fluoride.

contains strontium to make the material radiopaque.

contains the acidic monomer in its matrix.  

METAL REINFORCED GLASS I0NOMER

GIC lacks toughness.

 Cannot withstand high-stress concentrations that - promote crack propagation.

GICs can be reinforced

ü  By physically incorporating metal flakes or spheres ,such as silver alloy powder with glass powder, -> a silver alloy admix, ratio 1:7àmiracle mix

ü or by fusing glass powder to silver particles through sintering -> cermet.

ü  

Properties of miracle mixtures are quite inferior to amalgam.

Matrix of GIC was not strongly adhering to silver-tin particles.

Hence not well received as restorative material

CERMET

Cermet is manufactured by mixing & pelletizing under pressure a mixture of glass & metal powder such as silver , tin , gold , titanium palladium etc.

  Pellet is fused at 800°c ,then grounded to fine powder.

  The powder particles consist of regions of metal firmly bonded to glass

  5% of (by weight) titanium oxide added to restore clinically acceptable colour

Glass ionomer with alloy inclusion

Protection of cement

Various Steps in protection

   -Rubber Dam

   -Varnish

   -Petroleum jelly

   -Bonding agent

Factors affecting the rate of setting

1. Glass composition

2. Particle Size of the glass  powder

3. Addition of the Tartaric Acid

4. Relative proportions

5. Temperature of mixing

ADHESION

Mechanism of adhesion

      1. Chelation

      2. Hydrogen bonding

      3. Diffusion based adhesion

      4. Hydroxyapatite & polyacrylic acid reaction

                                                  

      5. Hydrogen bonding with dentin collagen 

                                                    

Conventional glass ionomers have an auto adhesive capacity.

Have high viscosity & wet the tooth surface well because they are hydrophilic.

They bond primarily by chelation of carboxyl groups of the polyacids with the calcium in the apatite of the enamel.

Bond strength is better in enamel because of higher inorganic content.

Polyalkenoate attacks & displaces the calcium & phosphates ions, which migrate into the cement & form an ion enriched layer.

Adhesion in RMGIC

Ionic reactivity is lower for RMGIC to coventional GIC

Acid conditioner is used, Polyacrylic acid or citric acid

Ferric chloride and aluminium chloride are occasionally used along.

Bond strength

       enamel- 2.6 to 9.6 Mpa

       dentin – 1.1 to 4.5 Mpa

  - Surface conditioners

           - remove smear layer

           - increases surface energy

           - increases wettability and decreases  

             contact angle

 

Conditioners

Polyacrylic acid- 10% for 15 sec

50% citric acid for 5 sec

25% tannic acid for 30 sec

2% ferric chloride

EDTA

ITS solution, Levine solution

FLUORIDE  RELEASE

Glass-ionomer cements are the best known fluoride-releasing material

Zone of resistance to demineralization is at least 3mm thick around a glass-ionomer restoration

The chemical composition and type of mixing are the main factors affecting the kinetics of fluoride release

Source and mechanism for fluoride release

The source of fluoride ions from glass-ionomer cements are

calcium fluoride (CaF2),

strontium fluoride (SrF2),

sodium hexaflouro-aluminate (Na3AlF6),

aluminium fluoride (AlF3)

In fully set cements, fluoride is located in the partially degraded glasses that form the glass core and in the polysalt matrix.

 fully set glass-ionomer is exposed to neutral aqueous solutions, it absorbs water and releases ions such as sodium, calcium, silica and fluoride

Rate of release is proportional to the inverse of the square root of time

All glass ionomers have been shown to have a burst effect soon after restoration placement.

The fluoride levels vary for different types of ionomers at 0.16 µg/mm² to 0.42µg/mm²

Studies showàAfter 60 days, the concentration of fluoride released had slowed from 15.3 -155.2µg/ml at day 1 to 0.9-3.99µg/ml.

Increase in fluoride release is observed at low pH values

Fluoride recharging

Glass ionomers may have synergistic effects when used with extrinsic fluorides.

In the presence of an inverse fluoride concentration gradient, glass ionomers may absorb fluoride from the environment and release it again under specific conditions

Topical APF (acidulated phosphate fluoride), with fluoride rinses and fluoridated dentifricesà recharging takes place

Amalgam alloy admixed cements were found to release higher or equal amounts of fluoride to those of conventional glass-ionomers due to increased microporosity.

In cermet cements, where silver particles are sintered to glass particles, the effective contact area between the glass particles and polyacrylic acid is reduced, leading to a reduction in fluoride release

Fluoride release from GIC’s

PHYSICAL PROPERTIES

Manipulation of GIC

Dispensing and Mixing

The powder-to-liquid ratio is important and varies from manufacturer.

Dispensed cement powder should be mixed in two equal portions.

Principal objectives

Mix on a dry slab/ mixing pad

Do not spread the mix or spatulate heavily

Gently incorporate by rolling the powder rapidly into the liquid within 10 secs.

Include second part and mix within 15 secs.

The finished mix should be glossy wet.

Encapsulated productsà typically mixed for 10 secs, in a mechanical mixerà dispensed directly onto the tooth & restoration

Working time

Cement must be used immediately

Working time after mixingà about 2 minutes at room temperature.

Do not use the cement once a “skin” forms on the surface OR when viscosity increases noticeably.

Loss of gloss/slump test

Setting time

GIC sets within 6-8 minutes from the start of mixing

24 hr compressive strengthà 90-230 Mpa.

Tensile str. Lowà because of brittle nature of glass ionomers

Compr. Str. Of GICà increases b/w 24 hours to 1 year (160-280 Mpa)

Advantages of glass ionomer cements

-        Rapid set in common with other dental cements.

-        Low temperature rise during setting

-        High compressive strength

- Adhesiveness to Enamel and Dentine

       

-        Resistance to Acid Erosion

-        Fluoride release

-        Slight initial plasticity

- Blandness to the pulp

Short comings of glass ionomer cements

Lack of toughness and abrasion resistance

Early water sensitivity – surface protection

Porosity leading to poor surface polish

Post-operative sensitivity

In restorative applications, not as esthetic as dental composites.

       

Indications

Restoration of erosion and abrasion lesions without cavity preparation.

Sealing and filling of occlusal pits and fissures

Restoration of class III and V carious lesions

Lining of all types of cavities in which biological seal and cariostatic action are required.

      Eg: In patients with high caries index

Dentin substitutes or the attachment of composite resins using acid-etch technique.

Restoration of approximal, buccal, lingual and occlusal cavities in which minimal cavity preparation can be done.

Restoration of deciduous teeth: (Strengthened cement materials).

Cementation of crowns and inlays particularly in patients with a high caries incidence. 

Core build up for badly broken teeth.

Provisional restorations where future veneer crowns are contemplated

Sealing of root surfaces for over dentures.

Repair of defective margins- cavities and crown margins.  Deep proximal boxes as a cervical lining under amalgam restorative materials.

Root end filling materials

For the fabrication of intracoronal periodontal splints.

Contra Indications

High stress bearing areas like marginal ridges of posterior teeth.

Restoration of occlusal surfaces having extensive faciolingual and mesio-distal dimensions.

Restoring class IV cavities involving that incisal angle.

Thank you..!