CR: Setting and polymerisation Flashcards
What are the steps involved in the setting of Composite Resin?
- Initiation
- Propagation
- Termination
Explain the “initiation” step involved in the setting of Composite Resin.
Initiate through light curing, chemical curing or dual cure
Explain the “propagation” step involved in the setting of Composite Resin.
○ The resin monomers bis-GMA have double carbon to carbon bonds
○ When the monomers are attacked by the initiators, the carbon-to carbon bond splits
○ When the bond splits, the molecule is left with a carbon-carbon single bond and a free radical
Explain the “termination” step involved in the setting of Composite Resin.
○ The now single bonded carbons causes the monomers to bond to other monomers
○ They continuously bond and add to the polymer chain
○ Each linkage leaves a free radical available for further reaction
○ However, not all monomers are converted to polymers
Discuss critically methods to reduce polymerisation shrinkage (4)
- Technique related methods: place a GI base to reduce the volume of composite required. Also, add the composite in 2 mm increments with the oblique layering technique touching one wall at a time and polymerise each increment independently to reduce the next effect of polymerisation
- Adequate curing: use light transmitting wedges/ matrices and ensure light curing is efficient
- Silorane: Silorane has ring- opening monomers. When it polymerises, these ring opens, flatten and extend toward each other. Thus, it has less shrinkage than methacrylate- based (linear monomers which connect by shifting closer together)
- Use composite with higher filler content by volume. Less resin = less shrinkage
List the factors affecting placement of Composite Resin (4)
- Polymerization shrinkage
- Depth of cure
- Oxygen inhibition layer
- Unreacted C=C Bonds
Describe polymerisation shrinkage as a factor affecting placement of Composite Resin
• When the resin matrix is cured/ polymerised, it shrinks away from the cavity walls which creates stress between the composite and tooth structure
• Shrinking composite tends to pull away the bonded walls
* Configuration factor: ratio of bonded surfaces to the unbonded tooth surfaces. The higher the c-factor, the higher the polymerisation shrinkage
Describe depth of cure as a factor affecting placement of Composite Resin
- Filler type and composition: Smaller filler particles scatter the light more than large filler, thus light curing becomes an issue. Also, the higher the filler content, the more difficult it is for light to penetrate
- Shade and translucency: Darker shades require a longer curing time because the light is more readily absorbed by the dark colour and does not transmit through the material as readily as lighter coloured material
- Intensity and duration of light curing: The light should be as close as possible: light intensity drops off with the inverse square law.
- Intensity: 450 nW/cm2
- Wavelength: 400 -500 nm
Describe oxygen inhibition layer as a factor affecting placement of Composite Resin
• Polymerisation is inhibited when a surface is exposed to oxygen in the air which results in a thin, uncured layer forming over the composite
* When another layer of composite is placed over the bonding resin, it will exclude the air. The inhibition layer and the new layer are chemically bonded when the second layer is cured
Describe unreacted C=C Bonds as a factor affecting placement of Composite Resin
• Around 35- 80% of C =C bonds react during polymerisation
• This allows old composite to bond to new composite
* The unreacted C=C bonds in the older material will react with the setting matrix of the new material, though it wouldn’t be as strong
List the physical properties of Composite Resin (6)
- Modulus elasticity
- Compressive strength
- Biocompatibility
- Water sorption and solubility
- Thermal properties
- Polymerisation shrinkage
Describe “modulus elasticity” as a physical property of Composite Resin
• GPa is the measurement. It measures the stiffness or rigidity of material
• Higher values = higher filler
* High filler = high stiffness and wear resistances
Describe “compressive strength” as a physical property of Composite Resin
• Measures the load required to break material/
• Important when occlusal loading is directly applied to restoration
* Heavily filled CR’s = greater compressive strength
Describe “biocompatibility” as a physical property of Composite Resin
CR could potentially irritate pulp tissue by going through dentinal tubules
• Thus, a lining is used to prevent irritation
• Deep cavities are lined with calcium hydroxide and sealed with GIC lining cement
• Irritation to soft tissue are usually related to irregular finishes/ plaque retention
* Very few individuals may be allergic to ingredients in the material “ as a physical property of Composite Resin
Describe “water sorption and solubility” as a physical property of Composite Resin
• Composite expands as a result of water uptake from oral fluids
• But this is isn’t sufficient enough to compensate for polymerisation shrinkage, and it is a slow process which reaches equilibrium
• Water uptake reduces hardness and wear resistance
• If the light curing intensity is not at its highest, proper polymerisation won’t occur and water sorption and solubility becomes an issue
* Water uptake will cause inorganic fillers to leach from the composite, which may manifest as colour instability and other issues
