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Dental Material Science > Polyalkenoate Cements > Flashcards

Polyalkenoate Cements Flashcards

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1
Q

Polyalkenoate cements subsets

A

-Glass ionomer cements and resin-modified glass ionomer cements

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2
Q

GICs definition and general properties

A

-water-based, plastic direct dental restorative cement formed from an acid-base reaction between a poly-alkenoic acid and ion-leachable calcium fluoro-aluminasilicate glass particles with strontium for radioopaqueness

  • Versatile materials
  • Good biocompatibility
  • Chemical cure, with no shrinkage stress
  • Light curing versions are available
  • Self adhesion to enamel and dentine
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3
Q

Applications of GIC

A
  • Restorations (but not in load bearing areas unless using a sandwich technique)
  • Luting (dental cement connecting underlying tooth structure to a fixed prosthesis)
  • Bonding/conditioning
  • Protection
  • Suited for minimally invasive procedures
  • Deep carious lesions
  • Fissure sealing
  • Protection of root surfaces against caries
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4
Q

Composition of GICs

A
  • Poly-alkenoic acid (such as polyacrylic acid, polyitaconic acid or polymaleic acid or copolymers)
  • This is the liquid phase (acid)
  • Glass powder: Ion leachable calcium fluoro-aluminosilicate glass particles
  • This is the base
  • Tartaric Acid
  • Water
  • Heavy metals such as strontium added for radioopacity
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5
Q

Ions released from the base and roles

A
  • Calcium fluoroalumino silicate glass particles
  • Acid soluble, ions are leached out
  • Silica affects transparency
  • Alumina affects opacity and setting time and can increase the compressive strength of the cement
  • Calcium fluoride- fluoride ions reduce fusion temperature, increase the strength of the cement and have a therapeutic effect

-Structured ionically as a tetrahedral complex with centrally located aluminium ion and closely localized alkaline earth cations (sodium, potassium, calcium and strontium) to maintain electronegativity

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6
Q

Examples of the acid parts

A
  • Poly-alkenoate acids
  • Polyacrylic acid
  • Polymaleic acid
  • Polyitaconic acid

OR

-Poly acrylic co maleic acid
Poly acrylic co itaconic acid

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7
Q

Role of tartaric acid

A
  • Improves handling
  • Extends the working time
  • Sharpens the set
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8
Q

Chemical Reaction of GICs

A

-Acid-base neutralisation reaction

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9
Q

Why is water required for the reaction

A

-Acids dont behave like acids in the absence of water

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10
Q

Stage 1 of the setting reaction

A

Dissolution

  • Outer layers of the glass particles are attacked by the acid
  • Release of aluminium, fluoride, calcium, sodium and strontium ions

-Leads to diffusion based adhesion between the glass and matrix

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11
Q

Stage 2 of the setting reaction

A

Gelation and Hardening

  • Initial setting (4-10 mins)
  • Divalent Calcium ions bind to the caboxylate groups, forming a clinically hard surface
  • Calcium ions also able to chelate with carboxyl groups
  • Silicaeous hydrogel is formed, maturing over 24hs with further cross-linking with aluminium ions
  • Process can cause a volumetric shrinkage of up to 3%
  • Surface protection at this stage is advisable to ensure regulation of the movement of water in and out of the hydrogel, therefore permitting maturation to occur
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12
Q

Stage 3 of setting reaction

A

Hydration

  • Hardening can take over a week
  • Aluminium ions become involved and aluminium polyacrylates are formed
  • 4-6 months
  • Improvement of properties
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13
Q

How does GIC appear on dental clinic

A
  • Generally supplied as powder and liquid constituents
  • Ratio of powder to liquid produced on hand-mixing is susceptible to operator induced variability

-Powder and liquid phase
OR
-Powder and acid (freeze dried) and water
OR
-In capsules: pre-dispensed amount of powder and liquid phase

  • Encapsulated systems marketed to overcome this variation
  • Allows accurate dispensation of powder and liquid
  • Also allows uniform proportioning and mixing resulting in a homogenous cement
  • Initial viscosity of the paste in the encapsulated systems can affect physical properties
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14
Q

Benefits of the aqueous system in GIC

A
  • Provides ion transport
  • Acid base setting reaction
  • Fluoride release
  • Bound into the set cement
  • Affects ultimate stability
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15
Q

Water balance in GICs

A
  • GICs susceptible to hydration and dehydration during the early stages of setting
  • Early water contamination: Loss of polyacrylate chains, Al3+ ions may diffuse out, absorption of water, loss of translucency and loss of physical properties
  • Dehydration: Cracking and fissuring of the cement, softening of the surface and loss of matrix forming ion
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16
Q

Adhesion between GIC and enamel

A
  • GIC polyacid displaces phosphate and divalent Calcium ions from the hydroxyapetite and these ions become incorporated into the GIC matrix, which sets
  • pH rises and re-precipitation of the minerals at GIC-tooth surface occurs forming an ion-enriched layer
  • Strongly attached to the tooth
17
Q

Adhesion between GIC and dentine

A

-Hydrogen bond formation or metallic ion bridging between carboxyl groups of the polyacid and collagen

18
Q

Specific properties of GICs

A
  • Bulk placement possible
  • Does not require bonding agents
  • Appears as dense and opaque, due to phase separation of glass and mismatch of refractive index
  • Phase separation can be overcome by changing the Al, Ca and F content but this affects the strength adversely
  • Fluoride release: GICs release fluoride and it is believed that this increases caries resistance
  • Low solubility once set
  • Brittle with low fracture toughness
19
Q

Advantages of GICs

A
  • Adheres to tooth enamel and dentine via an ion exchange mechanism
  • Therapeutic action due to fluoride release
  • Acceptable aesthetics
  • Low solubility after full maturation
  • Can be used in areas of poor moisture control
20
Q

Disadvantages of GICs

A
  • Low fracture resistance
  • Susceptible to early water contamination
  • Short working time
  • Long setting time
21
Q

Dentine conditioning and reasoning

A

-10% polyacrylic acid for 10 seconds

  • Removes smear layer and exposes calcium and phosphate ions on the mineralized surface
  • Increases the surface energy of the tooth surface to allow improved wettability of the GIC as it is placed on the tooth surface
22
Q

Resin-Modified Glass Ionomer Cements definition and composition

A

-Setting via polymerisation and neutralisation
-Same chemistry as a conventinal GIC with addition of HEMA (hydroxyethylmethacrylate- an ampiphilic molecule that can be photopolymerized)
BisGMA
and other photoinitiators

-Command set achieved by the introduction of the resin matrix

23
Q

Setting in RMGICs

A
  • Primarily due to the acid base reaction
  • Rapid set due to polymerisation of 2-HEMA
  • Acid-base reaction continues after the preliminary hardening due to the polymerisation
24
Q

Properties of RMGICs and clinical considerations

A
  • Similiar to GICs
  • Early stage desiccation (removal of water) is reduced
  • Compressive, tensile and shear bond strengths are higher in comparison to GICs

-2-HEMA is cytotoxic, so unreacted HEMA can be cytotoxic to the pulp tissues and osteoblast

  • Adhesion of GIC allows conservative cavity prep
  • Isolation of GIC in early stages is advocated, use of varnishes such as Copal
  • In case of pulpal exposure or a very thin intervening dentine layer, a lining material should be used as it can cause necrosis and inhibit calcific repair
25
Q

Link between GICs and soft drinks

A

-Excessive consumption of soft drinks can put conventional GIC restorations at risk

26
Q

Giomers/Poly-acid modified composites properties and composition

A
  • Primarily resin-based, with some GIC chemistry incorporated
  • Not enough to promote an acid-base setting reaction however
  • Require light activation to promote polymerisation chain reaction
  • Cannot adhere chemically to the tooth structure as conventional GICs do, as the acid-base rxn only occurs over a prolonged time period, after the initial set of the resin component
  • DBAs are therefore required for retention of these materials within a cavity
27
Q

Clinical indications of RMGICs and Poly-acid modified resin composites

A
  • Can be used as provisional or definitive shade matched tooth coloured adhesive restoration
  • RMGIC can be used in a sandwich restoration with an overlying composite completely covering the RMGIC base. However, best to leave GIC/RMGIC for 6 months, allowing for full maturation, prior to veneerign with composite

Dental Material Science (13 decks)

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