Glass ionomer cement



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GLASS IONOMER CEMENT


TNT

(alumino silicate glass –"ASPA"alumino silicate polyalkenoate)



Composition and Setting Reaction:

- supplied as a powder and liquid or as a powder and water

-Some products are supplied as powder/liquid in a capsule
-The powder is calcium fluoroalumino silicate glass

-it contain three main components:



1-Silica (Si02) and 2-Alumina (Al2O3) mixed in a 3- flux of calcium fluoride

-The particle size of the powder depends upon the purpose for which it is used

-Barium glass may be added to some powder to give radiopacity
-The liquid is an acquous solution of polyacrylic itaconic acid copolymer

together with tartaric acid (not more than 5%).

-tartaric acid increases the working time and decreases the setting time “sharp set “
-In some products the polyacrylic acid is freeze dried and added to the powder

-In such a case the liquid may be water or a diluted solution of tartaric acid


-The setting reaction is an acid base reaction that undergoes the following stages on

mixing the powder with liquid:



1-Dissolution.

-Dissolution of glass particles by the acid i.e. H ­­+ attack to release

(Ca++ . AL+++) and fluoride ions

-Between 20% to 30% of the glass is decomposed by the acid attack.



2- Migration.

-Migration of the surface ions Ca++, Al+++ and fluoride ions complex into

the liquid.

-The divalent Ca++ ions will migrate first followed by the trivalent Al+++ ions

-The sodium ions form silica gel on the surface of the particles

3- Reaction and precipitation

-Ca++ ions will react first with the carboxylic group of the acid to form the

cross linked carboxylic salt gel leading to the initial set

-then reaction of the slowly migrated trivalent Al+++ ions occurs

-The later reaction takes longer time and results into a stronger cross linked cement

-The precipitation process of the carboxylic gel salts is a continuous process and may

take 24 hours

-Therefore, the setting material should be protected against premature exposure to

saliva as it affects the setting and the surface hardness

role of water in the setting

-water is a normal component of the liquid

-initially it serves as a reaction medium then it hydrates the matrix

-during initial set ..it is loosely bound water thus if GIC was subjected to dryness

..the reaction will not continue and the surface will crack

-when the reaction proceeds ..water become tightly bound …so if the GIC was

subjected to moisture during initial set …dissolution of the matrix will occur

-so ..both dryness and moisture contamination during initial setting will make the

GIC with lower strength , increased solubility and poor esthetics
The microstructure of the set material

-The set material is a composite cored structure consisting of unreacted glass cores

surrounded by silica gel embedded in a matrix of cross linked polycarboxylic

salt hydrogel of calcium and aluminum

-Aluminium fluorocarboxylate salts constitute the main bulk of the matrix and

provides the final strength.


Types of GIC

Type I : For luting applications

Type II : As a restorative material.

Type III: For use as a liner or base.

-Light - curable versions of GIC are also available

Properties

1-Biological properties:

1-The glass ionomer cements have a mild effect on the pulp

-In deep cavities, calcium hydroxide lining must be used under GIC

2-Anticariogenic effect : anticariogenic due to fluoride content



2- Solubility and disintegration:

-solubility of glass ionomer cements in water after 24

hours immersion is about 1.5% by weight

-protection of GIC from moisture during the 1st 24 hours decreases solubility



3- Film thickness:

-The film thickness of the glass ionomer cement is about 24 microns which is similar

to that of zinc phosphate cement
4-Mechanical properties:
a) Compressive strength properties:

-from 90-240 MPa.

-if used as a filling material it increase in strength from 160 to 280 MPa between 24

hours and one year.

-strength of GIC improves when the cementis protected from moisture during the first

24 hours after filling.



b) Tensile strength:

It is a brittle material. Its tensile strength ranges from 14-24MPa.



c) Bond strength:

-The glass ionomer cements adhere to tooth structure by the reaction of

polyacrylic acid with the calcium and phosphate content of tooth structure

-The bond strength of glass ionomer cement to tooth structure is lower than that of

the polycarboxylate cements because of the sensitivity of the glass ionomer

cements to moisture during setting

-To obtain a good bond to dentin, the surface must be treated with a conditioner to

remove any smear layer which interfere with bonding


5- Optical properties:

-They are translucent. Therefore, they can be used in anterior restoration

in low stress - bearing areas.
Modifications of glass ionomer cement:.
I) Metal modified glass ionomer:

-Trials have been made to incorporate amalgam alloy powder with the glass powder

to increase wear resistance and flexure strength, e.g: “Miracle mixture”

-such trials was not successful because it did not increase the wear resistance




II) Cermet glass ionomers:

-silver, gold, palladium were sintered with the GIC powder

-Silver is the most commonly used and by sintering it adheres intimately to the glass

particles

-The strength and wear resistance were improved markedly

Glass cermets can be used as:

i) Core build up restorations

ii) a restoration for class I and 11 in deciduous teeth.
-They have higher abrasion resistance, higher flexure strength and higher

fracture toughness than the conventional glass ionomers

-Because of the metal content, they are opaque

-They have lower fluoride release than conventional glass ionomers



III) Light cured glass ionomers:

They were first used as lining materials under composite resin, then as anterior restoratives, specially class V cavities.


These materials undergo setting reaction through two mechanisms
a) Dual cure:
i) The conventional acid - base reaction which takes place when the powder and

liquid are mixed


ii) Polymerization reaction of the resin component i.e. free radical reaction when light

is applied to the cement


-Resin modified glass lonomer is usually supplied as a powder and liquid

-the powder is rodiopaque fluroalumino- silicate glass

-the liquid is a modified polyacid with HEMA group

(Hydroxyelhyl methacrylate) which is usually added to the liquid

-The acid base reaction play a significant part of the reaction over the curing reaction
b) Triple cure:

-To ensure effective polymerization of the resin part in deep cavities, the

resin part was modified to allow self curing reaction

-In such case the newly formulated cement will set through three reactions :


i) Conventional acid - base reaction.

ii) Light cure polymerization of the resin.

iii) Chemical cure polymerization of the resin

IV) Compomer "Polyacid modified resin composite materials"

-These are supplied as one paste system

-They are considered as intermediate restoratives between glass ionomers and

composite materials

-They are more related to composite than glass ionomer materials

-They are a mechanical mixture of glass ionomer particles and composite materials

-The light curing reaction plays a significant part of the reaction over the acid - base

reaction


Light curing glass ionomers have the following advantages over the conventional types:
i) Better optical properties.

ii) Less sensitivity to moisture after setting.



iii) Superior mechanical properties.

They are used as anterior restorative materials
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