Polyalkenoate Cements Flashcards
Polyalkenoate cements subsets
-Glass ionomer cements and resin-modified glass ionomer cements
GICs definition and general properties
-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
Applications of GIC
- 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
Composition of GICs
- 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
Ions released from the base and roles
- 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
Examples of the acid parts
- Poly-alkenoate acids
- Polyacrylic acid
- Polymaleic acid
- Polyitaconic acid
OR
-Poly acrylic co maleic acid
Poly acrylic co itaconic acid
Role of tartaric acid
- Improves handling
- Extends the working time
- Sharpens the set
Chemical Reaction of GICs
-Acid-base neutralisation reaction
Why is water required for the reaction
-Acids dont behave like acids in the absence of water
Stage 1 of the setting reaction
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
Stage 2 of the setting reaction
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
Stage 3 of setting reaction
Hydration
- Hardening can take over a week
- Aluminium ions become involved and aluminium polyacrylates are formed
- 4-6 months
- Improvement of properties
How does GIC appear on dental clinic
- 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
Benefits of the aqueous system in GIC
- Provides ion transport
- Acid base setting reaction
- Fluoride release
- Bound into the set cement
- Affects ultimate stability
Water balance in GICs
- 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
Adhesion between GIC and enamel
- 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
Adhesion between GIC and dentine
-Hydrogen bond formation or metallic ion bridging between carboxyl groups of the polyacid and collagen
Specific properties of GICs
- 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
Advantages of GICs
- 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
Disadvantages of GICs
- Low fracture resistance
- Susceptible to early water contamination
- Short working time
- Long setting time
Dentine conditioning and reasoning
-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
Resin-Modified Glass Ionomer Cements definition and composition
-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
Setting in RMGICs
- 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
Properties of RMGICs and clinical considerations
- 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
