Lec. 3 Mat. Cements

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Dental Materials

Types Of the Cements

Introduction


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Silicate Cements

Silicate cements were introduced in 1903 asanterior esthetic filling materials. They aretranslucent .

Silicates are attacked by oral fluids and in timedegrade. The average life of a silicaterestoration is four years.

1- 25 years. Silicates are rarely used nowadays. This is due

to development of better materials likecomposite resin and glass ionomer cements.


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Applications

Esthetic restoration of anteriorteeth

Intermediate restoration incaries active mouths.


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Anticariogenic Properties

The anticariogenic property is due to presenceof 15% flouride. Flouride release is slow andoccurs throughout the life of the restoration.

Mech. Of anticariogenic effect are :

It makes the adjacent tooth structure moreresistant to acid decalcification.

It acts as an antibacterial agent. It acts as anenzyme inhibitor and thus prevents themetabolism of carbohydrates.


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Zinc Phosphate Cement

Applications :

Luting of restorations (cementation)

High strength bases Temporary restorations

Luting of orthodontic bands and

brackets.


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Classification: Type - I: Fine grained for luting, Film

thickness should be 25 um or less. Type-II : Medium grain for luting and

filling, Film thickness should not be morethan 40 urn.

Available As Powder and liquid system . Capsules of preproportioned powder and

liquid.


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Composition

Powder Zinc oxide 90.2%

Principal constituent

Magnesium oxide 8.2%Aids in sintering Other oxides (like bismuth trioxide, calcium oxide, barium oxide,

etc.) 0.2%

Improves smoothness of mix Silica 1.4%

Filler, aids in sintering


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Liquid

Phosphoric acid 38.2%Reacts with zinc oxide

Water 36.0%

Controls rate of reaction

Aluminium phosphate or sometimes zincphosphate 16.2%

Buffers, to reduce rate of reaction Aluminium 2.5%

Zinc 7.1%.


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Setting Time5 to 9 minutes

Control of Setting Time by:


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A/ Manufacturing process:

1. Sintering temperature: The higher thetemperature, the more slowly the cement sets.

2. Particle size: Finer particles react more

quickly as a greater surface area is exposed tothe liquid.

3. Water content of liquid: Presence of excesswater accelerates, whereas insufficient water

retards the reaction.4. Buffering agents: When added slow down

the reaction.


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B/ Factors under control of operator:

Temperature: Higher temperaturesaccelerate the reaction

P/L ratio: More the liquid, slower the

reaction Rate of addition of powder to liquid: the

reaction is slower if the powder is

incorporated slowly. Mixing time: The longer the mixing time

(within practical limits), the slower is therate of reaction


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Properties

Compressive Strength : Zinc phosphate cementis stronger (103.5 MPa) than zinc oxide-eugenolcement but not as strong as silicophosphate. Theset cement gains 75% of its maximum strength in

the first hour. Maximum strength is attained in thefirst day.

The strength of zinc phosphate cement issufficient when used as a base or luting agent.

However, when it is exposed to the oralenvironment, e.g. temporary restorations, itsbrittleness and low strength causes it to fractureand disintegrate.


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Modulus of Elasticity :It is comparatively high . This

makes it stiff and resistant to elasticdeformation. This is necessary when itis employed as a luting agent for

restorations that are subjected to highmasticatory stresses.


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Solubility and Disintegration : This property is

important for cements used for permanentcementation., it shows low solubility (0.06% Wt).

However, in the mouth they show greaterdisintegration over a period of time. This shows

that other factors are involved .

Film Thickness :

For Type I - Film thickness not more than 25 um ,

for Type II - Film thickness not more than 40 um.


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Thermal Properties: Zinc phosphatecements are good thermal insulators

and may be effective in reducinggalvanic effects.

Adhesion Property : The retention of acemented restoration is by mechanical

interlocking of the set cement withsurface roughness of the cavity andrestoration.


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Biological Properties : Pulp response-moderate. The acidity is high at the time ofinsertion . Three minutes after mixing, thepH is 3.5. It approaches neutrality in 24 to48 hours.

A thickness of dentine as great as 1.5mm can be penetrated by the acid of thecement. If dentine is not protected againstinfiltration of this acid, pulpal injury mayoccur, especially during the first few hours.Pulp protection In deep cavities is needed

Optical Properties :The set cement isopaque.


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Manipulation :

Spatula used Stainless steel. Mixing time 1min. 15 seconds.

Powder-Liquid Ratio: 1.4 gm/O.5 ml :A cool glass slab is used in order to delaythe setting and allow more powder to be

incorporated before the matrix formationoccurs. The liquid should be dispensed justbefore mixing.


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Procedure


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Maximum amount of powder should beincorporated in the liquid to ensure minimumsolubility and maximum strength. Note: An

appropriate consistency is attained by addition ofmore powder to the liquid and not by allowing athin mix to thicken.


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Insertion

The crown should be seatedimmediately and held underpressure till set. Field of operation

should be dry. Varnish is appliedat the margins, where the cementis exposed.


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Zinc Oxide-Eugenol Cement

These cements have been used extensivelyin dentistry since the 1890's. Depending ontheir use they vary widely in theirproperties.

To improve the strength many modifiedzinc oxide-eugenol cements have beenintroduced, e.g. EBA-alumina modified andpolymer-reinforced zinc oxide-eugenolcements.

Recently non-eugenol zinc oxide cementshave become available. They are suitablefor patients sensitive to eugenol


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Classification: 4 Types

Type I ZOE - For temporary

cementation . Type Il ZOE - Permanent cementation

.

Type III ZOE - Temporary filling andthermal insulation .

Type IV ZOE - Cavity liners .


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Composition

Powder Zinc oxide 69.0% Principal

ingredient

White rosin 29.3% To reducebrittleness of set cement Zinc stearate 1.0% Accelerator,

plasticizer Zinc acetate 0.7% Accelerator,

improves strength Magnesium oxide Is added in some

powders, acts with eugenol in a similarmanner as zinc oxide.


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Liquid

Eugenol 85.0 Reacts with zinc

oxide

Olive oil 15.0 Plasticizer.


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Setting Reaction

ZnO + H20 Zn(OH)2

Zn(OH)2 + 2HE ZnE2 + 2H20

Base Acid Salt(Zinc hydroxide) (Eugenol) (Zinc eugenolate)


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Setting Time 4-10 minutes. Thecomplete reaction between zinc oxide

and eugenol takes about 12 hours.This is too slow for clinicalconvenience.


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Factors Affecting Setting Time

1. Manufacture: The most active zinc oxide powdersare those formed from zinc salts like zinc hydroxideand zinc carbonate by heating at 3000C.

2.Particle size: Smaller zinc oxide particles set faster.

3.Accelerators: Alcohol and water.4.Heat: Cooling the glass slab, slows the reaction.

5.Retarders: The set can be retarded with glycol andglycerine

6.Powder to liquid ratio: Higher the ratio, faster theset.


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Properties:Mechanical Properties

Compressive strength:. They arerelatively weak cements. The strength

depends on what it is used for. The compressive strength therefore ranges

from a low of 3 to 4 MPa upto 50-55 MPa.Particle size affects the strength.

In general, the smaller the particle size,the stronger the cement. The strength canalso be increased by reinforcing withalumina-EBA or polymers


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Tensile strength: Ranges from 0.32 to5.3 MPa.

Modulus of elasticity This is animportant property for those cementsintended for use as bases.


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Thermal Properties are approximately thesame as for human dentin

Solubility and Disintegration :Thesolubility of the set cement is highest

among the cements (0.4% wt). Theydisintegrate in oral fluids. This break downis due to hydrolysis of the zinc eugenolatematrix to form zinc hydroxide and eugenol.

Solubility is reduced by increasing the P/Lratio ..


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Film Thickness : The film thicknessof zinc oxide-eugenol cements (25um) is higher than other cements.

Adhesion :They do not adhere well toenamel or dentin. This is one reasonwhy they are not often used for finalcementation of crowns and bridges.

The other reasons are low strengthand high solubility.


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Biological Properties : pH and

effect on pulp: (pH is 6.6 to 8.0). Theyare the least irritating of all cements.

the Bacteriostatic and obtundant

properties: They inhibit the growth ofbacteria and have soothing effect(obtundant) on the pulp in deepcavities, reducing pain.

Optical Properties :The set cementis opaque.


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Manipulation

The bottles are shaken gently.Measured quantity of powder and

liquid is dispensed onto a cool glassslab. The bulk of the powder isincorporated into the liquid andspatulated thoroughly in a circular

motion with a stiff bladed stainlesssteel spatula. Smaller increments arethen added until the mix is complete.


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Setting Time 4-10 minutes.

ZOE cements set quickly in the mouth

due to moisture and heat.


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Modified Zinc Oxide-EugenolCements

EBA-Alumina modified cements

Polymer reinforced.


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EBA-Alumina Modified Cements

Composition

Powder : Zinc oxide-70% & Alumina-

30%.Liquid :EBA 62.5% (orthoethoxy

benzoic acid) Eugenol-37.5%.

Properties :Its properties are betterthan that of unmodified ZOE.

Setting Time 9.5 minutes


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Polymer Reinforced Zinc Oxide-EugenolCement

Uses

1. Luting agent

As base

As temporary filling material and

As cavity liner.


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Composition Powder

Zinc oxide 70%

Finely divided natural or synthetic resins. Liquid

Eugenol

Acetic acid acceleratorThymol antimicrobial


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Properties These cements have improved mechanical

properties and the Pulp response Similar tounmodified ZOE-moderate

Manipulation The proper powder/liquid is dispensed on a

dry glass slab. The powder is mixed into theliquid in small portions with vigorousspatulation.

Working timeThese cements have a longworking time.

Lec. 3 Mat. Cements

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