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
• Glass Ionomer 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) Powder liquid 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.

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Zinc polycarboxylate cement

• a) Basic components primary zinc oxide, small
  quantities of magnesium oxide.
• b) Acidic component polyacrylic acid, which may
  be supplied
• 1) As a viscous aqueous solution of concentration
  30-40
• 2) As a dry powder, blended with the basic
  components.
• c) additionally, some products contain stannous
  fluoride

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Manipulation

• a) Polyacrylic acid solutions are more viscous
  than the liquidus of other cements, which affects
  the ease of mixing of the material.
• b) If the cement is being used to secure adhesion
  to enamel and dentin, it is important that the
  tooth surface should be clean and saliva-free

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• c) The cement should be applied to the tooth as
  soon as possible after mixing, otherwise poor
  adhesion may result. If a cement mix begins to
  cobweb on manipulation, it should be discarded.
  There is a continuous increase in cement
  viscosity during manipulation of the material
• d) Polycarboxylate cement will adhere to
  instruments, particularly those made of stainless
  steel.

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• Thus
• 1) It is useful to use alcohol as a release agent
  for the mixing spatula.
• 2) Instruments should be cleaned before the
  cement sets on them.
• 3) If cement does inadvertently adhere to a
  spatula, most of it can be chipped off quite
  easily. The remaining material can be removed in
  boiling sodium hydroxide solution.

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Setting reaction

• This involves the formation of a salt, zinc
  polyacrylate. The set material is a cored
  structure containing a considerable quantity of
  unreacted zinc oxide.
• Setting time
• a) This depends on the composition and method of
  manufacture of the powder and liquid.
• b) A faster setting time is achieve at higher
  temperatures.

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Properties

• a) These cements have very little irritant effect
  on the pulp.
• b) Low thermal diffusivity
• c) Chemical properties these cements are more
  soluble than zinc phosphate materials. Some
  products may also absorb water, which can cause
  the material to become soft and gel-like.

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• d) Zinc polycarboxylate cements are almost as
  strong as phosphate materials in compressive and
  stronger in tension
• e) The set cement is very opaque because of the
  large quantity of unreacted zinc oxide that is
  present.
• f) Biological properties is similar to those of
  zinc phosphate cement

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• Adhesion properties
• The polyacrylic acid is believed to react via the
  carboxyl groups with calcium of hydroxyapatite.

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Zinc Oxide- Eugenol cement

• Composition
• a) powder
• Zinc oxide
• Magnesium oxide may be present in small
  quantities, it reacts with eugenol in a similar
  manner to zinc oxide
• Zinc acetate in quantities up to 1 as
  accelerators for the setting reaction

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• b) liquid
• Eugenol, the major constituent of oil of cloves
• Olive oil, up to 15
• Sometimes acetic acid, to act as an accelerator
• Manipulation
• These cements are mixed by adding the powder in
  small increments to the liquid, until a thick
  consistency is obtained. A powder/liquid ratio of
  between 4/1 and 6/1 by weight will give a
  material of the required properties, with
  experience, a suitable consistency can be
  recognized without weighing the materials. As a
  rule, a thin glass slab and stainless steel
  spatula are used.

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Setting reaction

• a) Chemical reaction, to form a compound called
  zinc eugenolate
• b) Absorption of the eugenol by the zinc oxide
  may also occur.
• Other factors to be noted
• a) The setting reaction between pure zinc oxide
  and pure eugenol will not occur in the absence of
  water. Thus, a mixture of zinc oxide and eugenol,
  without added accelerators, can be kept in a
  desiccator for several days without undergoing
  much change.
• b) The set materials contains both some unreacted
  zinc oxide and eugenol.

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Setting time

• This depends on
• a) powder
• Particles size a fine powder will have a greater
  surface area exposed to the eugenol so can react
  more quickly.
• b) Accelerating additive
• c) powder/liquid ratio a thicker mix gives a
  faster setting material.
• d) Exposure to moisture on mixing or the addition
  of water will accelerate the reaction.
• e) Increase in temperature also causes faster
  setting

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Types of Zinc Oxide- Eugenol cement

• Type 1
• ZOE cement has a PH of 7 and is biocompatible
  with the pulp . The strength of temporary cement
  must be low to permit removal of the restoration
  without trauma to the teeth.

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• Type 2
• On is based on the addition of alumina to the
  powder and ortho-ethoxybenzoic acid to the
  eugenol liquid, and the second based on the use
  of a polymer .
• The compressive strength improved ZOE cements but
  overall the mechanical properties are inferior
  to those of other cements.

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Resin based cement

• General description
• A varietly of resin-based cements have now become
  available because of the development of the
  direct-filling resins with improve properties,
  the acid-etch technique for attaching resins to
  enamel, and molecules with a potential to bond to
  dentin conditioned with organic or in organic
  acid. Some are designed for general used and
  other for specific uses such as attachment for
  orthodontic brackets or resin bonded bridge.

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Glass-ionomer (Glass polyalkenoate)

• 1) Presentation
• Traditional form powder and liquid.
• Preproportioned capsules.
• Water settable cement the polyacrylic acid is
  freeze-dried and added to the powder. In such a
  case the liquid may be distilled water or a
  diluted solution of tartaric acid.

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Composition

• The powder is of the same composition as that of
  silicate cement
• N.B Barium is added to give radiopacity.
• The liquid is the same composition as that of the
  polycarboxylate cement.

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Setting reaction

• The setting reaction is an acid base reaction
  that undergoes the following stages on mixing the
  powder with liquid
• 1) Dissolution .
• 2) Migration
• 3) Reaction and precipitation

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• Dissolution
• Dissolution of the surface glass particles by the
  acid i.e H attack to release cations (ca ,
  AL) and fluoride ions . Between 20 to 30 of
  the glass is decomposed by the acid attack.
• 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.

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• Reaction and precipitation
• The migrated 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. This is followed by the reaction of
  the slowly migrate trivalent Al ions. 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.

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Role of water in the setting process

• Water is an important constituent of the cement
  liquid. It serves initially as a reaction medium
  then it slowly hydrates the cross-linked matrix,
  thereby increasing the material strength. During
  the initial set, it is known as loosely bound
  water. As the reaction proceeds, it becomes
  tightly bound. Thus, if glass ionomer cement is
  subjected to dryness during the initial set, the
  reaction reaction will not go to completion and
  the surface will crack.

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• On the other hand, if glass ionomer cement is
  subjected to moisture contamination during the
  initial set, dissolution of the matrix will
  occur. Consequently, either the condition yield
  glass ionomer cement with reduced strength,
  increased solubility and poor aesthetics.
  Therefore, glass ionomer cement must be protected
  against water changes in the structure during
  setting process.

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The microscopic 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
  poly salt hydrogel of calcium and aluminum.
  Aluminum fluoracarboxylate salts constitute the
  main bulk of the matrix and provides the final
  strength.

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Types of glass ionomer cement

• There are three types of GIC based on their
  formulations and their potential uses. These are
  designated as follows
• Type I for luting applications.
• Type II as a restorative material.
• Type III for use as a liner or based.
  Light-curable versions of GIC are also available.

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Properties

• Biological properties
• a) The glass ionomer cements have a mild effect
  on the pulp. In case of deep cavities, calcium
  hydroxide lining must be used under glass ionomer
  cement.
• b) Anticariogenic effect these cements have the
  potential for inhibiting secondary caries due to
  the presence of fluoride.

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Solubility and disintegration

• Glass ionomer cements are susceptible to attack
  by water during its setting. Therefore, it is
  necessary to coat the restoration immediately by
  varnish to protect the cement from premature
  exposure to the saliva. Value of solubility and
  disintegration of the glass ionomer cements in
  water after 24 hours immersion is about 1.5 by
  weight.

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Film thickness

• The film thickness of the glass ionomer cement is
  about 25 microns which is similar to that of zinc
  phosphate cement.

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Mechanical properties

• Compressive strength properties
• The 24 hours compressive strength of glass
  ionomer cements ranges from 90-240 MPa. A glass
  ionomer cement as a filling material showed an
  increase in strength from 160 to 280 MPa between
  24 hours and one year. The strength of the glass
  ionomer cements improves more rapidly when the
  cement is protected from moisture during the
  first 24 hours after filling.

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• Tensile strength
• It is a brittle material. Its tensile strength
  ranges from 14-24 MPa.
• Bond strength
• The glass ionomer cements bond chemically to
  tooth structure by the reaction of carboxylic
  group of polyacrlyic acid with the calcium and
  phosphate content of tooth structure. The bond
  strength of glass ionomer cement to tooth
  structure is lower than 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.

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• Optical properties
• They are translucent. Therefore, they can be used
  in anterior restoration in low stress-bearing
  area.

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Modifications of glass ionomer cement

• Modifications have been made in order to improve
  the mechanical properties, abrasion resistance,
  and optical properties of glass ionomers.

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Metal modified glass ionomer

• Trials have been made to incorporate amalgam
  alloy powder with the glass powder in order to
  increase wear resistance and flexure strength.
  E.g (miracle mixture) such attempt was not
  successful because it did not increase the wear
  resistance.

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Cermet glass ionomers

• Fine precious metals such as silver, gold ,
  palladium were sintered with the glass ionomer
  powder. Silver is the most commonly used by
  sintering it adheres intimately to the glass
  particles. The strength and wear resistance were
  improved markedly.
• Glass cermets can be used as
• 1) Core build up restorations or as
• 2) A restoration for class I and II 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 .

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Light cured glass ionomers ( resin modified glass
inomer or hybrid ionomer )

• They were first used as lining materials under
  composite resin, then they gained a wide
  acceptance as anterior restorative, 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

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• ii) Polymerization reaction of the resin
  component i.e. free radical reaction when light
  is applied to the cement.
• Resin modified glass ionomer is usually supplied
  as a powder and liquid, the powder is radiopaque
  ion leachable fluroalumino-silicate glass while
  the liquid is a modified polyacid with
  methacrylate end group, the HEMA ( Hydroxyethyl
  methacrylate) which is usually added to the
  liquid. The acid base reaction play a significant
  part of the reaction over the curing reaction.

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• b) Triple cure
• To ensure effective polymerization of the resin
  part in deep cavities, the 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

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Compomer ( polyacid modified resin compsite
materials)

• These are supplied as one paste system and not as
  power and liquid. They are considered as
  intermediate restoratives between glass ionomers
  and composite 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. The later being minimal.

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• 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.
• N.B. polyacide modified resin composite materials
  are more related to composite resin rather than
  glass ionomer materials.

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Cavity varnish and liners

• Varnishes and liners are used for coating the
  freshly out tooth structure of the prepared
  cavity.
• The cavity varnish is natural gum such as copal,
  rosin or a synthetic resin dissolve in an organic
  solvent such as acetone, chloroform or an ether.

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• The cavity varnish is applied to the cavity
  preparation with a brush or cotton pedget, the
  solvent is allowed to evapoate leaving a thin
  coating resin film on the surface. This process
  may be repeated two to three times to give a
  uniform resin layer.
• The cavity liner is a liquid in which calcium
  hydroxide and some zinc oxide are suspended in a
  solution of natural or synthetic resin.

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Application

• 1) To seal the dentinal tubules and prevent
  penetration of chemicals into the pulp.
• 2) To act as a temporary protection against the
  loss of constituents from the surface of a
  filling material. Cavity varnishes are used as a
  surface coat over glass ionomer restoration.
• 3) To seal the dentinal tubules under amalgum
  restorations and prevent penetration of metallic
  ions into enamel and dentin thus reducing
  discoloration of the teeth, around amalgum
  restorations. A film of varnish under a metallic
  restoration is not an effective thermal insulator.

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Calcium hydroxide cements

• This material is supplied as two pastes in two
  collapsable tubes. One paste consists of a
  mixture of calcium hydroxide, zinc oxide and
  sulphonamide, the other paste consists of glycol
  salicylate, titanium dioxide and calcium
  sulphate. Light activated calcium hydroxide
  cements have become available.

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Properties

• 1) The freshly mixed cement is alkaline with a PH
  of 11-12. it has the ability to stimulate the
  pulp to lay down secondary dentin. Thos
  characteristic is utilized in very deep carious
  lesions where calcium hydroxide cement is used as
  a pulp capping agent. i.e. it can be placed
  adjacent to the pulp and it is capable of
  destroying micro-organisms found in carious
  lesions.

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• 2) Solubility and disintegration the calcium
  hydroxide is highly soluable since it is
  dissolved if left at the cavity wall and margin,
  this will lead to increase marginal leakage.
• 3) The compressive strength of calcium hydroxide
  liner is very low about 5 PMa. Therefore in deep
  cavities a thin sublining of a calcium hydroxide
  cement and then a base of zinc phosphate cement
  should be placed before condensation of amalgam.