Dental Composites Flashcards
What are composites used for in dentistry?
- Anterior and posterior restorations
- Fissure sealants
- Cementation of fixed prosthesis
- Bonding or ceramic veneers
What is a composite?
A material formed from 2 or more materials that cant mix or have different properties.
What are the main 4 reasons as to why we use composites?
1 - Withstand the oral environment
2 - Be easily shaped to cavity
3 - Match the natural tooth colour
4 - Bond directly to the tooth tissue
What are the 2 major constituents in a composite?
Methacrylate monomers Filler particles
Name some of the minor constituents in composites
- Accelerators
- Camphorquinone
- Inhibitors
- Viscosity modifiers
- Silane coupling agents (to enhance resin matrix to filler interaction)
Are resin monomers high or low weight monomers?
High weight monomers
What is the structure of resin monomers? bis GMA
2 methacrylate monomers
Double C=C bond taking part in the polymerisation reaction
Two phenol groups adding rigidity
Hydroxyl groups to allow hydrogen bonding
Other than bis GMA, what is another common methacrylate monomer used?
UDMA monomer
Methacrylate monomers are high weight, how do we reduce the viscosity?
ADD DILUENT MONOMERS!
such as EDMA and TEGDMA.
Other than modifying the viscosity, what else do diluents do?
These monomers also help blend filler particles with the monomer.
They are cross-linking agents to make the composite stronger and lower the fluid uptake,
What are inhibitors added to composties?
Inhibitors prevent premature polymerisation during storage, handling and placement.
What are filler particles commonly made of?
Quartz, silica, glasses such as aluminosilicates
Modern composites use hybrid fillers (different sizes ranging from 5-10 microns) why?
To allow improvement in the efficiency of filler loading.
What are the reasons for using filler particles?
1) Improves the coefficient of thermal expansion (less thermal expansion)
2) Reduces thermal diffusivity
3) Reduces polymerisation shrinkage
4) Improves surface hardness
5) Improves some mechanical properties such as abrasion resistance
6) Improves aesthetics - refractive index matches the tooth
7) Decreases water absorption
Why do we have silane coupling agents?
They improve the adhesion of the filler to the polymer matrix.
They are bi-functional coupling molecules - hydrophilic end bonds with filler and enhances the adhesion between the inorganic and organic phase.
What is the name of the photoinitiator used and what light wave length sets it?
Camphorquinone
470nm
How does light cause polymerisation of the composite?
The light causes camphorquinone to react with amine particles to form free radicals. This initiates free radical addition polymerisation.
How do composites set chemically?
Using two pastes, one with initiator and the other containing amine.
On mixing, they begin reacting and produce free radicals and then polymerisation occurs.
What is the main advantage of light activation rather than chemical setting?
The setting occurs on demand rather than a gradual increase in viscosity.
(chemical needs mixing with incorporates air bubbles)
What are the 3 stages of free radical addition polymerisation?
- Initiation = light activates composite. Polymerisation begins once free radicals are formed. Free radicals attack the double bond of the methacrylate.
- Propagation = monomer free radical attacks another monomer free radical and the chain starts.
- Termination = reaction of two free radicals or the disproportionation of the free radical
What is the main factor during setting that changes the chemical and mechanical properties of the composite?
If cross-linking occurs and its extent.
What happens to the composite viscosity during setting?
Viscosity increases
What affects a composites degree of conversion?
Monomers and fillers.
What is the + and - of a higher degree of conversion?
+ increased strength
- increases shrinkage
What happens if the degree of conversion is too low?
Optimum properties are not achieved.
Residual monomers can be released with fluid uptake - more water can then ingress.
How does cross-linking affect composite physical properties?
- Increase in crosslinking with decrease fluid uptake and reduce interaction with surrounding environment
- More cross linking can increase mechanical properties but too much can make a composite brittle
What does the degree of polymerisation shrinkage depend on?
1 - Type of resin used and degree of filler.
Highly filled composites give lower shrinkage.
2 - Placement, incrementally reduces shrinkage
What can indicate the amount of stress the composite puts on the tooth?
The ratio of bonded to unbonded (C factor).
Less bonded surfaces reduce stress but this also gives a much weaker restoration.
How do we place composite?
Incremental placement
Place in 1-2mm increments
Curing depth depends on the light type, intensity and irradiance
How do composites interact with fluids?
1 - They absorb fluid
2 - They lose ionic species from the fillers
What is the composition of a compomer?
Methacrylate monomers + filler.
Along with aluminosilicate glass (for acid-base reaction).
What are advantages of compomers?
- Easy to place
- No mixing
- Easy to polish
- Good aesthetics
- Excellent handling
- Less susceptible to dehydration
- Radiopaque
What are the disadvantages of compomers?
- Require a bonding agent like composites
- More marginal staining and chipping
- Wears more than composites
- Weaker physical properties than composites
Nanocomposites are a new development.
What is superior about them?
They have non sized filler particles.
- Increases polish retention
- Reduction of the interstitial spacing between filler particles
- Better physical properties
- Increased filler loading
What are the benefits of high polish retention?
- Long lasting aesthetics
- Reduced plaque retention
- No external discolouration
- High oral comfort
Why were flowable composites made?
To overcome the small cavities and allow better adaptation.
they do however give rise to high contraction stress which leads to pain, marginal discolouration and recurrent caries
What makes flowable composites flowable?
They have 20-25% less filler.
does lead to more shrinkage and reduced modulus of elasticity
Why would we pre-warm composites?
This decreases composite viscosity to enhance the flowability.
How does bulk filler composites work?
- Composites do not require incremental curing
- Can be cured 4-5mm in depth
- The modifiers used are more reactive than other composites to allow larger incremental polymerisation
- They have higher translucency to allow the light to penetrate further
