Dental Materials > Composite 2 > Flashcards

Composite 2 Flashcards

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1
Q

Acrylic resin filling materials based on …

A

methyl methacrylate/MMA

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2
Q

Features of MMA

A
  • 100.1g/mol molar mass so small
  • large polymer shrinkage around 21%
  • low viscosity (0.6mPa at 20 degrees)
  • monofunctional - linear polymer making
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3
Q

What does the low viscosity of MMA mean?

A
  • easy to mix
  • difficult to transfer to cavity
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4
Q

How to fix the shrinkage of MMA?

A
  • add PMMA powder
  • reduces shrinkage to 5-6%
  • still a clinical problem
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5
Q

Acrylic resin fillings have good/poor durability
Explain

A
  • poor
  • marginal leakage and staining
  • poor colour stability
  • low strength, stiffness, hardness
  • high coeffiicent of thermal expansion
  • no adhesion to enamel/dentine
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6
Q

What is Bowen’s resin made in 1962?

A
  • bisphenol A-glycidyl methacrylate or BisGMA
  • matrix phase
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7
Q

Features of BisGMA

A
  • big monomer (513g/mol)
  • lower polymerisation shrinkage (2-3%)
  • high viscosity (1200Pas)
  • difunctional -crosslinked polymers
  • more rigid monomer
  • good potential but too viscous
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8
Q

Explain the high viscosity of BisGMA

A
  • 1 million times more than water
  • hard to mix
  • increased with filler addition makes it even harder to mix
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9
Q

Why is it good that BisGMA is rigid?

A
  • steric hinderance of aromatic groups
  • creates stiffer and stronger polymers
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10
Q

Why do we make copolymers of BisGMA?

A
  • to reduce viscosity
  • dilute the BisGMA
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11
Q

Most common diluent monomer used with BisGMA

A

triethylene glycol dimethacrylate
- TEGDMA

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12
Q

Properties of TEGDMA

A

-286 g/mol molar mass
- 0.01Pas viscosity - closer to water
- difunctional - crosslinked polymers
- flexible monomer

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13
Q

TEGDMA can be mixed with BisGMA from … to …

A

1 to 100%

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14
Q

With more TEGDMA, BisGMA is …
Even more …
And even more …

A
  • easier to mix, lower viscosity
  • weaker copolymer
  • more shrinkage/lower molar mass
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15
Q

List problems that cause shrinkage

A
  • debonding
  • marginal staining
  • microleakage
  • secondary caries
  • enamel micro-cracks
  • post-operative sensitivity
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16
Q

Link of high molar mass to shrinkage

A
  • low polymerisation shrinkage
  • high viscosity
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17
Q

Link of low molar mass to shrinkage

A
  • low viscosity
  • but high polymerisation shrinkage
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18
Q

How to compromise between high and low molar mass to balance shrinkage and viscosity

A
  • balance mixing and placing with shrinkage
  • leads to different products
  • different types like packable, flowable or universal composites
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19
Q

Universal composite is good for …

A

general work

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20
Q

Properties of flowable composite

A
  • lower viscosity (low filler conc or increased diluent)
  • weaker
  • less wear resistant
  • more shrinkage than universal
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21
Q

Uses of flowable composite

A
  • non-carious tooth surface loss repair
  • fissure sealant
  • cavity lining
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22
Q

Properties of packable composite

A
  • higher viscosity (higher filler conc)
  • similar feel to amalgam
  • increased risk of porosity
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23
Q

Packable composite is used for …

A

posterior restorations

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24
Q

2 reasons we’re looking for alternative monomers than BisGMA

A
  • reduce and remove bisphenol A (BPA) use
  • better clinical properties
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25
Why are we trying to reduce BPA use?
- health issues related to BPA - anti-BPA public campaigns
26
Why are we looking for better clinical outcomes than BisGMA?
- better handling, mixing, placement - lower polymerisation shrinkage - better mechanical properties
27
Lots of papers are produced each year about alternatives. What's the problem?
- hard to translate lab reasearch to clinic
28
3 recently developed monomers
- urethane dimetacrylate (UDMA) - ethoxylated BisGMA (BisEMA) - silorane
29
Explain features of UDMA
- high molecular weight - low viscosity - no BPA
30
Features of BisEMA
- high molecular weight - low viscosity - low BPA
31
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
32
Why are coupling agents needed?
- acrylics don't bond to silicates - heat and stress cause pores to form and these pores cause clinical failure
33
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
34
Which coupling agent is most commonly used?
lambda-methacryloxypropyltrimethoxysilane
35
Issue with silanes
- they hydrolyse - and eventually break down - effects durability
36
Conventional composites have a ... system
- two pastes - base and catalyst
37
Explain hand mixed two paste system
- inexpensive - spatula and pad in pack - technique sensitive - mixing can cause porosity - short working time, long setting time
38
Issues with a short working time, long setting time
- increases problems with technique sensitivity - patient comfort decreases over long setting time
39
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
40
Do you still need 2 pastes with light activation?
no - no mixing
41
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
42
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
43
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)
44
Disads of UV light curing Consequence
- health risks - corneal burns/melanoma - stopped in UK - no UV light used at all in UK dentistry
45
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
46
Compare visible light activation to UV
- visible light has - improved depth of cure - improved degree of polymerisation - reduced health concerns
47
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
48
List light source types
- quartz, tungsten, halogen - plasma - LED
49
Features of quartz, tungsten, halogen/QTH light
- normal light bulb - broad spectrum light - needs filters to remove red light
50
Is plasma lighting used?
- proved unsuccessful - phased out
51
Features of LED light
- tuned to the initiator wavelength - more powerful LEDs being developed
52
What is radiant energy?
- light intensity x time - links the 2 and sees how much light exposure is needed for successful polymerisation
53
Whats the optimum curing?
- 16J/cm2 - 10-20s of illumination
54
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
55
Light intensity relates to ...
- LED power output - distance of light from surface (intensity dissipates over distance) - correct alignment of light (full coverage of restoration)
56
Chemically activated composites and setting
- uniform degree of conversion - placed in one procedure - bulk fill procedure
57
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
58
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
59
Alternative initiators intend to improve ...
- curing time - depth of cure - colour stability
60
Alternatives to CQ at the minute
- PPD - TPO - BAPO
61
Differences of alternative initiators to CQ
- different excitation wavelengths - specialised light curing units required - very few products - limited clinical evidence
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