Composite 2 Flashcards
Acrylic resin filling materials based on …
methyl methacrylate/MMA
Features of MMA
- 100.1g/mol molar mass so small
- large polymer shrinkage around 21%
- low viscosity (0.6mPa at 20 degrees)
- monofunctional - linear polymer making
What does the low viscosity of MMA mean?
- easy to mix
- difficult to transfer to cavity
How to fix the shrinkage of MMA?
- add PMMA powder
- reduces shrinkage to 5-6%
- still a clinical problem
Acrylic resin fillings have good/poor durability
Explain
- poor
- marginal leakage and staining
- poor colour stability
- low strength, stiffness, hardness
- high coeffiicent of thermal expansion
- no adhesion to enamel/dentine
What is Bowen’s resin made in 1962?
- bisphenol A-glycidyl methacrylate or BisGMA
- matrix phase
Features of BisGMA
- big monomer (513g/mol)
- lower polymerisation shrinkage (2-3%)
- high viscosity (1200Pas)
- difunctional -crosslinked polymers
- more rigid monomer
- good potential but too viscous
Explain the high viscosity of BisGMA
- 1 million times more than water
- hard to mix
- increased with filler addition makes it even harder to mix
Why is it good that BisGMA is rigid?
- steric hinderance of aromatic groups
- creates stiffer and stronger polymers
Why do we make copolymers of BisGMA?
- to reduce viscosity
- dilute the BisGMA
Most common diluent monomer used with BisGMA
triethylene glycol dimethacrylate
- TEGDMA
Properties of TEGDMA
-286 g/mol molar mass
- 0.01Pas viscosity - closer to water
- difunctional - crosslinked polymers
- flexible monomer
TEGDMA can be mixed with BisGMA from … to …
1 to 100%
With more TEGDMA, BisGMA is …
Even more …
And even more …
- easier to mix, lower viscosity
- weaker copolymer
- more shrinkage/lower molar mass
List problems that cause shrinkage
- debonding
- marginal staining
- microleakage
- secondary caries
- enamel micro-cracks
- post-operative sensitivity
Link of high molar mass to shrinkage
- low polymerisation shrinkage
- high viscosity
Link of low molar mass to shrinkage
- low viscosity
- but high polymerisation shrinkage
How to compromise between high and low molar mass to balance shrinkage and viscosity
- balance mixing and placing with shrinkage
- leads to different products
- different types like packable, flowable or universal composites
Universal composite is good for …
general work
Properties of flowable composite
- lower viscosity (low filler conc or increased diluent)
- weaker
- less wear resistant
- more shrinkage than universal
Uses of flowable composite
- non-carious tooth surface loss repair
- fissure sealant
- cavity lining
Properties of packable composite
- higher viscosity (higher filler conc)
- similar feel to amalgam
- increased risk of porosity
Packable composite is used for …
posterior restorations
2 reasons we’re looking for alternative monomers than BisGMA
- reduce and remove bisphenol A (BPA) use
- better clinical properties
Why are we trying to reduce BPA use?
- health issues related to BPA
- anti-BPA public campaigns
Why are we looking for better clinical outcomes than BisGMA?
- better handling, mixing, placement
- lower polymerisation shrinkage
- better mechanical properties
Lots of papers are produced each year about alternatives. What’s the problem?
- hard to translate lab reasearch to clinic
3 recently developed monomers
- urethane dimetacrylate (UDMA)
- ethoxylated BisGMA (BisEMA)
- silorane
Explain features of UDMA
- high molecular weight
- low viscosity
- no BPA
Features of BisEMA
- high molecular weight
- low viscosity
- low BPA
Features of silorane
Is it available now?
- polymerised by different route (ring-opening), claimed to be low shrinkage
- clinical results didn’t match lab results so withdrawn from market
Why are coupling agents needed?
- acrylics don’t bond to silicates
- heat and stress cause pores to form and these pores cause clinical failure
How do coupling agents work?
- silanes have carbon carbon double bonds and SiO groups
- that bond acrylics to silicates (matrix to filler)
- increases mechanical properties of composites
Which coupling agent is most commonly used?
lambda-methacryloxypropyltrimethoxysilane
Issue with silanes
- they hydrolyse
- and eventually break down
- effects durability
Conventional composites have a … system
- two pastes
- base and catalyst
Explain hand mixed two paste system
- inexpensive - spatula and pad in pack
- technique sensitive
- mixing can cause porosity
- short working time, long setting time
Issues with a short working time, long setting time
- increases problems with technique sensitivity
- patient comfort decreases over long setting time
How does chemically activated polymerisation work with composite?
- benzyl peroxide used as initiator
- tertiary amine used as activator
- uniform degree of polymerisation - but low levels, high residual monomer
Do you still need 2 pastes with light activation?
no
- no mixing
Advantages of light activation rather than hand mixing
- no mixing so reduced porosity
- unlimited working time ajnd command setting/only when light applied
- higher degree of polymerisation - better mechanical properties
Disadvantages of light activation of composite rather than hand mixing
- require specialist light-activation unit - more expensive for purchase and maintenance
- more expensive
- limited depth of care
- higher marginal stress in curing - faster rate leads to faster development of stress
Explain process of UV light activation
- light supplied by hand unit
- light in UV range
- used benzoin methyl ether as initiator
- 2mm max depth of cure with 100% illumination (not often achieved)
Disads of UV light curing
Consequence
- health risks
- corneal burns/melanoma
- stopped in UK - no UV light used at all in UK dentistry
Explain visible light activation
- light units similar to UV - visible spectrum range (400-700nm)
- camphoroquinone initiator - diff to UV
- needs tertiary amine for activation
- very yellow - colour stablility issues
Compare visible light activation to UV
- visible light has
- improved depth of cure
- improved degree of polymerisation
- reduced health concerns
Different designs of light activation units
- originally fibre-optic cable
- had low light intensity and optical fibres broke easily and needed replacing
- so gun type - had a solid ‘light guide’ and was less prone to breaking
List light source types
- quartz, tungsten, halogen
- plasma
- LED
Features of quartz, tungsten, halogen/QTH light
- normal light bulb - broad spectrum light
- needs filters to remove red light
Is plasma lighting used?
- proved unsuccessful
- phased out
Features of LED light
- tuned to the initiator wavelength
- more powerful LEDs being developed
What is radiant energy?
- light intensity x time
- links the 2 and sees how much light exposure is needed for successful polymerisation
Whats the optimum curing?
- 16J/cm2
- 10-20s of illumination
Might curing time shorten?
- as more intense LEDs are made hopefully
- but no link found in dental composites
- increased intensity did not allow shorter curing times
Light intensity relates to …
- LED power output
- distance of light from surface (intensity dissipates over distance)
- correct alignment of light (full coverage of restoration)
Chemically activated composites and setting
- uniform degree of conversion
- placed in one procedure - bulk fill procedure
Light activated composites and curing
- light reduces over distance
- non-uniform degree of conversion
- requires placement in increments - depending on depth of cure, time consuming and technique sensitiven
Bulk-fill materials and depth of cure
- larger increments possible with recent developments
- matchrefractive index of filler and matrix
- increments max 4mm
- products with higher TEGDMA concentration
- weak clinical evidence
Alternative initiators intend to improve …
- curing time
- depth of cure
- colour stability
Alternatives to CQ at the minute
- PPD
- TPO
- BAPO
Differences of alternative initiators to CQ
- different excitation wavelengths
- specialised light curing units required
- very few products - limited clinical evidence
