Dental Cements for Bonding Application

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Dental Cements for Bonding Application

• Dr. Waseem Bahjat Mushtaha
• Specialized in prosthodontics

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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 at
  temperatures between 1000C and 1400 into a cake
  that is subsequently ground into fine powders.
  The powder particle size influences setting rate.
  Generally, the smaller the particles size, the
  faster the set of the cement.

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• b) Liquids
• Phosphoric acid, water, aluminum phosphate, and
  in some instances, zinc phosphate. The water
  content of most liquids is 33 5
• Setting reaction
• When the powder is mixed with the liquid, the
  phosphoric acid attacks the surface of the
  particles and releases zinc ions into the liquid.
  The aluminum, which already forms a complex with
  the phosphoric acid, reacts with the zinc and
  yields a zinc aluminophosphate cement is a core
  structure consisting primarily of unreacted zinc
  oxide particles embedded in a cohesive amorphous
  matrix of zinc aluminophosphate.

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Factors Influencing Working and Setting Time

• 1) pwderliquid ratio
• Working and setting times can be increased by
  reducing the powder liquid (PL) ratio. This
  procedure, however, is not acceptable means of
  extending setting time because it impairs the
  physical properties and results in a lower
  initial PH of the cement. The reduction in
  compressive strength, along with the decrease in
  the PL ratio. The initial PH of the mixture also
  decreases with increasing PL ratio.

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• 2) Rate of powder incorporation
• Introduction of small quantity of the powder into
  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 time are
  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 the
  working 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 mix developing an unduly high
  viscosity.

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Physical and Biological properties

• Two physical properties of the cement that are
  relevant to the retention of fixed prostheses are
  the mechanical properties and the solubilities.
  The prosthesis can become dislodged if the
  underlying cement is stressed beyond its
  strength. High solubility can induce loss of the
  cement needed for retention and may create plaque
  retention sites.

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• Zinc phosphate cements, when properly
  manipulated, exhibit a compressive strength of
  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 PL ratio for this zinc phosphate
  cement is about 1.4g to 0.5 ml. the increase in
  strength attained by addition of powder in excess
  of the recommended amount is modest as compared
  with the reduction incurred by decreasing the
  amount of powder in the mix. A reduction in PL
  ratio of the mix produces a markedly weaker
  cement. A loss or gain in the water content of
  the liquid reduces the compressive and tensile
  strengths of the cement.

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• Zinc phosphate cements show relatively low
  solubility in water when they are tested in
  accordance with ADA specification.
• Retention
• Setting of the zinc phosphate cement does not
  involve 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 a prepared 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 is lower 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 a dentin thickness as great as 1.5 mm.
  Thus, if the underlying dentin is not protected
  against the infiltration of acid via the dentinal
  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 desired consistency may vary to some
  degree with the clinical situation. However , the
  maximum amount of powder possible for the
  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
  mixture stiffens. The liquid should not be
  dispensed onto the slab until mixing is to be
  initiated, because water will be lost to the air
  by evaporation.

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• 3) Mixing is initiated by addition of a small
  amount of powder. Small quantities are
  incorporated initially with brisk spatulation. A
  considerable area of the mixing slab should be
  used. A good rule to follow is to spatulate each
  increment for 15 seconds before adding another
  increment. The mixing time is not unduly
  critical. Completion of the mix usually requires
  approximately 1 minute and 30 seconds. As stated
  previously, the appropriate consistency varies
  according to the purpose for which the cement is
  to be used. However, the desired consistency is
  always attained by adding more powder and never
  by allowing a thin mix to stiffen. For a fixed
  partial denture, additional time required to
  apply the cement. Therefore, a slightly decreased
  viscosity should be used.

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• 4) The casting should be seated immediately with
  a vibratory action if possible, before matrix
  formation occurs. After the casting has been
  seated, it should be held under pressure until
  the cement sets to minimize the air spaces. The
  field of 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
  other nonpermeable coating should be applied to
  the margin.
• The purpose of the varnish coating is to allow
  the cement more time to mature and develop an
  increased resistance to dissolution in oral fluid.

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Zinc silicophosphate cement

• Zinc silicophosphate cement (ZSP) cements consist
  of a mixture 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
  major difference is that set ZSP cement appears
  somewhat translucent and releases fluoride by
  virtue of the silicate glass. 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 have choices of other more
  esthetically pleasing materials, such as resin
  and glass ionomer cements.