MGDS1201.101_t192_24112008.ppt
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Transcript of MGDS1201.101_t192_24112008.ppt
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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.