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Dental Cements for Bonding

Application

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Types of cements

Zinc phosphate cement

Zinc silicophosphate cement

Zinc polycarboxylate cement

Zinc Oxide- Eugenol cement

Resin-based cement

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Zinc phosphate cement

General description:

Zinc phosphate is the oldest of the

cementation agents and thus is the one that

has the longest track record. It consists of

powder and liquid in two separate bottles.

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Composition

1) Powder :

Zinc oxide (90%)

Magnesium oxide (10%).The ingredients of the powder are sintered attemperatures between 1000C and 1400 into a cakethat is subsequently ground into fine powders. The

powder particle size influences setting rate.Generally, the smaller the particles size, the fasterthe set of the cement.

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b)Liquids: Phosphoricacid,water,aluminumphosphate,andinsomeinstances,zincphosphate.Thewatercontentof

mostliquidsis33%5% Settingreaction: Whenthepowderismixedwiththeliquid,thephosphoricacidattacksthesurfaceoftheparticlesandreleaseszincionsintotheliquid.Thealuminum,

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Factors Influencing Working and

Setting Time1) pwder:liquid ratio:

Working and setting times can be increased by

reducing the powder: liquid (P:L) ratio. Thisprocedure, however, is not acceptable means ofextending setting time because it impairs the

physical properties and results in a lower initialPH of the cement. The reduction in compressivestrength, along with the decrease in the P:L ratio.The initial PH of the mixture also decreases withincreasing P:L ratio.

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2) Rate of powder incorporation:

Introduction of small quantity of the powderinto the liquid for the first few increments

increases working and setting times by

reducing the amount of heat generated and

permits more powder to be incorporated

into the mix. Therefore, it is the

recommended procedure for zinc phosphate

cement.

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3) Spatulation time:

Operators who prolong the spatulation timeare effectively destroying the matrix that

was forming. Fragmentation of the matrix

means extra time is needed to rebuild the

bulk of the matrix.

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4) Temperature of mixing slab:

The most effecting method of controlling theworking and setting times is to regulate the

temperature of the mixing slab. Cooling the

slab markedly retards the chemical reaction

between the powder and the liquid so that

matrix formation is retarded. This permits

incorporation of the optimum amount of

powder into the liquid without the mixdeveloping an unduly high viscosity.

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Physical and Biological properties

Two physical properties of the cement that arerelevant to the retention of fixed prostheses

are the mechanical properties and thesolubilities. The prosthesis can becomedislodged if the underlying cement isstressed beyond its strength. High solubility

can induce loss of the cement needed forretention and may create plaque retentionsites.

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Zinc phosphate cements, when properly

manipulated, exhibit a compressive strengthof MPa and a diametral tensile strength of

5.5MPa . Zinc phosphate cement has a

modulus of elasticity approximately 13GPa.

Thus, it is quite stiff and should be resistant

to elastic deformation even when it is

employed for cementation of restorations

that are subjected to high masticatory stress.

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The recommended P:L ratio for this zincphosphate cement is about 1.4g to 0.5 ml.the increase in strength attained by additionof powder in excess of the recommendedamount is modest as compared with the

reduction incurred by decreasing theamount of powder in the mix. A reductionin P:L ratio of the mix produces a markedlyweaker cement. A loss or gain in the water

content of the liquid reduces thecompressive and tensile strengths of thecement.

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Zinc phosphate cements show relatively low

solubility in water when they are tested inaccordance with ADA specification.

Retention :

Setting of the zinc phosphate cement does notinvolve any reaction with surrounding hard

tissue or other restorative materials.

Therefore, primary bonding occurs by

mechanical interlocking at interface and not

by chemical interaction.

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Biologic properties

As might be expected from the presence of the

phosphoric acid, the acidity of the cement is quite

high at the time when a prosthesis is placed on aprepared tooth. Two minutes after the start of the

mixing, the PH of zinc phosphate cement is

approximately 2. The PH then increases rapidly

but still is only about 5.5 at 24 hours. The PH islower and remains lower for a longer period when

thin mixes are employed.

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Zinc phosphate cement probably occurs

during the first few hours after insertion.

However, studies of zinc phosphate cements

prepared with liquids containing radioactive

phosphoric acid indicate that in some teeth

the acid from the cement can penetrate adentin thickness as great as 1.5 mm. Thus, if

the underlying dentin is not protected

against the infiltration of acid via thedentinal tubules, pulpal injury may occur.

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Manipulation

1) It is probably not necessary to use

measuring device for proportioning the

powder and liquid, because the desiredconsistency may vary to some degree with

the clinical situation. However , the

maximum amount of powder possible forthe operation and should be used to insure

minimum solubility and maximum strength.

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2) A cool mixing slab should be employed.

The cool slab prolongs the working and the

setting times and permits the operator to

incorporate the maximum amount of the

powder before the matrix formation

proceeds to the point at which the mixturestiffens. The liquid should not be dispensed

onto the slab until mixing is to be initiated,

because water will be lost to the air byevaporation.

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3) Mixing is initiated by addition of a small amount ofpowder. Small quantities are incorporated initially withbrisk spatulation. A considerable area of the mixing slabshould be used. A good rule to follow is to spatulate eachincrement for 15 seconds before adding another increment.The mixing time is not unduly critical. Completion of themix usually requires approximately 1 minute and 30seconds. As stated previously, the appropriate consistencyvaries according to the purpose for which the cement is to

be used. However, the desired consistency is alwaysattained by adding more powder and never by allowing athin mix to stiffen. For a fixed partial denture, additional

time required to apply the cement. Therefore, a slightlydecreased viscosity should be used.

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4) The casting should be seated immediately with avibratory action if possible, before matrix

formation occurs. After the casting has beenseated, it should be held under pressure until thecement sets to minimize the air spaces. The fieldof operation should be kept dry during the entire

procedure.

5) Excessive cement can be removed after it has set.It is recommended that a layer of varnish or othernonpermeable coating should be applied to themargin.

The purpose of the varnish coating is to allow thecement more time to mature and develop anincreased resistance to dissolution in oral fluid.

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Zinc silicophosphate cement

Zinc silicophosphate cement (ZSP) cements consist of amixture of silicate glass, a small percentage of zinc oxide

powder, and phosphoric acid. The clinical indications for

this cement are similar to those of zinc phosphate cement.Its strength is somewhat superior, the other majordifference is that set ZSP cement appears somewhattranslucent and releases fluoride by virtue of the silicateglass. Aesthetically, it is superior to the more opaque zinc

phosphate cement for cementation of ceramic restorations.The use of ZSP cement is declining, as practitioners havechoices of other more esthetically pleasing materials, suchas resin and glass ionomer cements.