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3rd yr- CSB- Partial dentures > Denture base polymers > Flashcards

Denture base polymers Flashcards

1
Q

What is the denture base for a denture

A

The part of the denture that the prosthetic teeth are supported by and attached to

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2
Q

What does the denture base act as in a denture

A

Acts as a replacement for hard and soft tissues

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3
Q

More specifically what does the denture base replace

A

Replaces the gingivae and alveolar support for the teeth

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4
Q

What requirements do denture base material sneed to meet?

A
  1. Needs to bond to teeth
  2. Be biocompatible
  3. High polish
  4. Translucent
  5. Colour/ colour stability
  6. No porosity
  7. Low residual monomer
  8. Low sorption/ solubility
  9. Flexural stretch > 65MPa
  10. Flexural modules > 2GPa
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5
Q

What is resilience

A

The amount of energy absorbed by a material up until the point at which it undergoes permanent deformation

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6
Q

What is toughness

A

The amount of energy absorbed by a material until it breaks

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7
Q

Why do dentures need to be strong

A

As they are frequently dropped

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8
Q

List some DESIRABLE features for dentures

A
  1. Resilient
  2. Tough
  3. High impact strength
  4. Dimensional stability
  5. Repairable if breaks
  6. Good thermal conductor
  7. Radio opaque
  8. Cheap
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9
Q

What materials can denture bases be made from

A
  1. Poly (methyl methacrylate) (PMMA)
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10
Q

When was Poly (methyl methacrylate)(PMMA) developed

A

1932

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11
Q

What reaction does pMMA undergo

A

Addition polymerisation

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12
Q

List the 4 stages involved in the additional polymerisation of PMMA

A
  1. Activation
  2. Initiator
  3. Propagation
  4. Termination
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13
Q

What happens in the activation stage of pMMA

A

A chemical us made to produce a free radical

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14
Q

What is the Initiator stage of PMMA

A

Where the free radical starts the polymerisation process by connecting with a monomer molecule

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15
Q

What happens in the Propagation stage of PMMA

A

One monomer attached to another

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16
Q

What happens in the Termination stage of PMMA

A

As one monomer attaches to another the reaction is said to propagate, and finally if the growing chain meets another and joins with it, or it doesn’t encounter any more monomer molecules, the reaction terminates.

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17
Q

What do denture base resins contain

A

benzoyl peroxide

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18
Q

What happens to denture based resin when it is heated above 60 degrees

A

Molecules of benzoyl peroxide decompose to yield electrically neutral specials containing unpaired electrons, free radicals

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19
Q

What role does benzoyl peroxide have in denture base resin

A

It is an initiator

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20
Q

What is the activator for denture base resin

A

Heat

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21
Q

What happens to benzoyl peroxide when it is heated

A

It breaks down to give 2 free radicals

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22
Q

What do free radicals do

A

They attack double bonds on the monomer molecule opening them up and exposing a site for reaction

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23
Q

What happens as monomer molecules join together?

A

They form a polymer chain and the space between them is reduced

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24
Q

What is the shrinkage for PMMA

A

21% by volume

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25
Q

What do resin manufactures do to try and minimise dimensional changes

A

Resin manufactures prepolymerise a significant a significant fraction of the denture based resin

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26
Q

What can we add to MMA to reduce shrinkage

A

Mix it with PMMA beads that are already polymerised

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27
Q

How much shrinkage does MMA occur with mixed with PMMA

A

6%

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28
Q

How is heat cured acrylic resin usually stored

A

As a powder and liquid

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29
Q

What is the powder part of heat cured acrylic resin made up of

A

Beads of granules of polymathy methacrylate

Can also include pigments, dyes, optical opacifiers, Plasticizers, synthetic and coloured fibres

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30
Q

What is the liquid part of heat cured acrylic resin made up of

A

Methyl methacrylate monomer

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31
Q

What happens when we mix the polymathy methacrylate powder with the methyl methacrylate monomer liquid on a molecular level

A

The old and new polymer chains intertwined forming an acrylic dough
This results in a polymer

32
Q

Describe the setting reaction that occurs when we mix the polymathy methacrylate powder with the methyl methacrylate monomer liquid

A

Mixing of powder and liquid cause monomer diffusion and softening of the surface of the powder producing gelling stages:

33
Q

Name some of the gelling stages the polymathy methacrylate powder with the methyl methacrylate monomer liquid go through

A
  1. Sandy
  2. Stringy/ sticky
  3. Dough
  4. Rubbery
34
Q

Describe the sandy stage

A

It is the initial melting of the beads

35
Q

Describe the stringy stage

A

Enlargements with swollen beads and thickened interstitial monomer

36
Q

Describe the dough stage

A

This is when gelation occurs

37
Q

Describe the rubbery phase

A

The monomer has penetrated to the core of th beads plasticizing them,

38
Q

Out of the 4 setting stages which is the most useful

A

The dough stage

39
Q

Name the different processing stages we follow when we are making a wax mould for our dentures

A
  1. Make wax pattern
  2. Flasking
  3. Boiling out
  4. Dough moulding
  5. Boiling
  6. Deflasking
40
Q

How long can the polymerisation (boiling) stage of processing dentures take

A

8 hours

41
Q

What problems can arise if processing is done incorrectly

A
  1. Increased porosity
42
Q

Why can porosity be a problem In dentures

A

Can become a plaque retentive factor

43
Q

Name the 3 different types of porosity

A
  1. Gaseous
  2. Contraciton
  3. Granular
44
Q

When does gaseous porosity occur

A

When MMA liquid boils in the middle of the polymerisation process

45
Q

What is the boiling point of MMA liquid

A

100.3 degrees

46
Q

How can we prevent gaseous porosity from occuring?

A

The MMA must be heated very slowly

47
Q

Why should we heat the MMA very slowly

A

So that the denture fully solidifies before the boiling point is reached
This prevents gaseous porosity

48
Q

Why does contraction porosity occur

A

Due to the 6% shrinkage that is inherit in the PMMA production

49
Q

How can we avoid contraction porosity?

A

By adding a little extra PMMA dough to the mould before heating
Keep the mould under very high pressure throughout the process to prevent bubbles from growing in size

50
Q

What is granular porosity due to?

A

Due to dough becoming dry before processing

51
Q

Which of the 3 types of porosity is the rarest?

A

Granular porosity

52
Q

What are the advantages of PMMA

A
  1. Reasonably good physical properties
  2. Good aesthetics
  3. Easily repaired
  4. Highly polishable
  5. Cheap and easily manufactured
  6. Bonds to denture teeth
  7. Light and ride
  8. Low water sorption
  9. High glass transition temperature
53
Q

What is the glass transition temperature

A

The temperature above which a solid polymer becomes soft and rubbery

54
Q

What is another name for the glass transition temperature

A

Glass to rubber transition

55
Q

What factors can affect the glass transition temperature

A

Molecular structure

56
Q

Why does the glass transition temperature need to be high for dentures

A

As liquids and food as hot as 70 degrees may be consumed and if the glass transition temperature is lower than 70 the denture will melt/ go soft in the patients mouth

57
Q

How can molecular structure affect glass transition temperature

A

Big pendant groups limit how closely the polymer chains can pack together
The further away the groups are from each one the more easily they move around and melt (so lower glass transition temperature )

58
Q

What are some disadvantages of PMMA

A
  1. Poor impact strength

2. Poor thermal conductor

59
Q

How much does repairing dentures cost the eNHS

A

9 million a year

60
Q

What sort of dentures fracture the most

A

Upper partial dentures

61
Q

Why are upper partial dentures most likely to fracture

A

As they are often weak areas where small saddles are connected to major connectors

62
Q

Where on the denture do most fractures occur

A

50% of all complete denture fractures occur in the midline

63
Q

What are upper denture fractures associated with

A
  1. Deep frenal notches
  2. Sharp fraenal notches
  3. Diastema
  4. Worn teeth
  5. Ill fitting dentures
  6. Hard bony suture in palatal midline
  7. Palatine tori
64
Q

how can Deep or sharpfrenal notches and Diastema cause denture fracrtie

A

As they are stress concentrators

65
Q

How can we improve PMMA

A
  1. By adding metal strengtheners
  2. Reinforcement
  3. Co-polymerization
  4. Injection moulding
66
Q

Why can’t we carry out injection moulding

A

As it is expensive and very high tech

67
Q

Give examples of some metal strengtheners

A
  1. Bars

2. Mesh

68
Q

Are metal strengtheners effective

A

there is little evidence that they actually strengthen a denture and may in fact act as a stress concentrator

69
Q

Out of all the options which is the best one to improve PMMA

A

Co polymers

70
Q

What do co polymerise PMMA with

A

Butadiene styrene

71
Q

What is Butadiene styrene

A

A rubber like material

72
Q

What does Butadiene styrene do to PMMA

A

toughness is greatly increased, however the modulus is reduced in the process and the dentures are more flexible

73
Q

What type of teeth do we use on a denture

A

Plastic acrylic teeth

74
Q

What are some qualities our denture teeth should have

A
  1. Bind chemically to the denture
  2. Can be adjusted
  3. Not cause wear to opposing teeth
  4. Good colour match
  5. minor resilience
  6. May stain with time
75
Q

Other than plastic acrylic what other materials can denture teeth be made of

A
  1. Porcelain

2, Experimental fiber-reinforced teeth

3rd yr- CSB- Partial dentures (5 decks)

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