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BDS3 CLINICAL DMS > PMMA > Flashcards

PMMA Flashcards

1
Q

What are the ideal features of a denture

A

it replaces the function of natural teeth
goes into the px mouth and fits properly
has good aesthetics as its seen by there other people
good value

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2
Q

What are ideal mechanical properties of a denture material

A
  • Dimensionally accurate and stable in use
    • High softening temperature (Tg)
    • Unaffected by oral fluids
    • Thermal expansion (not expand or contract too much)
    • Low density
    • High thermal conductivity
    • Radiopaque
    • Non toxic/irritant
    • Colour/translucency is suitable
    • Easy and inexpensive to manufacture
    • Easy to repair
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3
Q

What are the ideal properties of a denture material

A 
• High YM
	• High proportional limit
	• High transverse strength
	• High fatigue strength
	• High impact strength
High hardness/abrasion resistance
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4
Q

What is transverse strength

A
  • Flexural strength

* How well does the denture cope with stresses that cause deflection

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5
Q

What is the pivot point of denture

A

• The pivot point is the palate, forces are being applied on either side and the denture may fracture at the pivot point and this is the worst case scenario for acrylic resin

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6
Q

What is the insidious failure of a denture

A

where the acrylic resin withstands the impact created and microcracks appear in the subsurface so it may fail later

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7
Q

What is free radical addition polymerization

A
  • Chemical union of two molecules either the same or different to form a larger molecule without the elimination of a smaller molecule
    • Involves molecules with C=C bonds
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8
Q

What is the monomer

A

methacrylate monomer

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9
Q

What are the stages of polymerization

A

○ Activation - of initiator to provide free radicals
○ Initiation - free radicals break C=C bond in monomer and transfer free radicals
○ Propagation - growing polymer chain
○ Termination - of polymerisation

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10
Q

What is the composition of the powder in heat cured acrylic

A 
initiator 
PMMA particles
plasticizer 
pigments
copolymer
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11
Q

What is the ignitor

A

benzoyl peroxide 0.2-0.5%

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12
Q

What is PMMA particles

A

prepolymerised beads

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13
Q

What is plasticizer for

A

allows for quicker dissolving in monomer liquid e.g dibutyl phthalate

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14
Q

What is the pigment for

A

to give ‘natural’ colour

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15
Q

What is the copolymer for

A

to improve mechanical properties e.g ethylene glycol dimethacrylate

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16
Q

What is the composition of the liquid component

A

○ Methacrylate monomer - dissolves PMMA particles (polymerises)
○ Inhibitor
○ Co-polymers - improve mechanical properties (particularly cross-linking of polymers)

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17
Q

What is the inhibitor

A

(hydroquinone, 0.006%)

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18
Q

What is the technique for heat cured acrylic

A
  • A vessel containing the mould material and artificial teeth is used
    • Dough form acrylic is packed in
    • Mould is pressed and takes up the shape of the patient’s dentition
    • The acrylic needs to be cured to form a strong solid denture base
    • It is subjected to a heat cycle to cause polymerisation
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19
Q

Why is effect polymerization important

A

• Efficient polymerisation is needed to give a high molecular weight polymer i.e good mechanical properties hence a high temperature is required but gaseous porosity limits

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20
Q

Is acrylic non toxic

A

yes

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21
Q

Is acrylic non irritant

A

if no monomer released, only few patients are allergic

22
Q

Is acrylic unaffected by oral fluids

A

water absorption is possible which can cause distortion but acrylic resin is fortunately insoluble in fluids taken orally

23
Q

How are the mechanical properties of acrylic

A

poor but we increase in bulk to compensate

24
Q

How is the fatigue/impact strength of acrylic

A

fairly resistant but can be a cause of failure

25
Q

What is the high hardness/abrasion resistance

A

retains good polish but there is some wear overtime which can feel rough to the patients tongue

26
Q

How is the thermal expansion of acrylic

A

equivalent to that of artificial tooth - okay if acrylic teeth are used but it is significantly higher than porcelain teeth

27
Q

How is the thermal conductivity of acrylic

A

low/poor

28
Q

What is the density of acrylic

A

low which is good but the increase in bulk to overcome the poor mechanical properties offsets this advantage

29
Q

How is the softening temperature of acrylic

A

okay for ingested hot fluids but don’t use boiling water for cleaning

30
Q

Is acrylic dimensionally stable

A

okay, linear contraction 0.5% - acceptable

31
Q

How is the expansion of acrylic

A
  • Manufacture: 0.5% linear contraction

* Usage - 0.4% expansion

32
Q

What is the composition fo self cure

A

○ The same as heat cured except benzoyl peroxide is activated by promoter (tertiary amine e.g dimethyl-para-toluidine) in liquid

33
Q

Why self cure

A

• Lower temperature since not required to initiate polymerisation so less thermal contraction so should give a better dimensional accuracy

34
Q

What are the disadvantages of self cure

A
  • Chemical activation is less efficient so there is a lower molecular weight and poorer mechanical properties as well as the Tg being lower
    • There is more unreacted monomer which acts as a plasticiser, softening the denture base and reducing its transverse strength
    • Its also a potential tissue irritant, compromising its biocompatibility
    • Chemical cure leaves 3-5% unreacted monomer compared to 0.2-0.5% in heat cure
35
Q

What is the issue of dimensional accuracy for self cure

A
  • Fits the original cast better than heat cured but water absorption gives expansion
    • Self cured is oversized and heat cured is undersized but undersized is better tolerated
36
Q

Compare self cure with heat cure

A
  • HC - higher molecular weight so it’s stronger
    • HC - curing process may cause porosity
    • SC - higher monomer levels which can cause irritation and have a significant impact on the patient’s tissue
    • SC - fits cast better but water absorption in mouth makes it oversized
    • SC - poorer colour stability as the tertiary amines are susceptible to oxidation
    • Neither are ideal
37
Q

Why were variations of acrylic made

A

• Acrylic resin and dentures have poor strength and toughness with 10% fracturing within 3 years

38
Q

What are the attempt to strengthen acrylic

A

○ High impact resistance materials which was done by incorporating rubber toughening agent (butadien styrene) to stop crack propagation and long term fatigue problems
○ Incorporate fibres (carbon, UHMPE - ultra high molecular weight polyethylene, glass)
§ Difficult processing and manufacturing

39
Q

Describe ultra hi (used in GDH)

A
  • High impact heat cure acrylic resin that exudes quality and gives the technician confidence
    • It has been formulated with flexural strength and superior fracture toughness (ductility)
    • The two features together give it a slight bending aspect which keep the material from being brittle and subject to cracking and breaking
40
Q

Describe pour’n’cure resins

A
  • Similar to self cure
    • Smaller powder particles to produce a fluid mix not a dough like form
    • The fluid mix is poured into a mould
    • A good fitting denture can be produced but the mechanical properties are inadequate
41
Q

What are light activated denture resins

A
  • Urethane dimethacrylate matrix plus acrylic copolymers
    • Microfine silica fillers - small amounts to control rheology
    • Photo initiator system
    • Adapted to cast
    • Cured in a light chamber
    • Used mostly as customised impression tray material and for repair of fractured dentures
42
Q

why should a denture material be radiopaque

A

so if any fragments broke off and there is a risk of it being swallowed then a radiograph can confirm

43
Q

What are alternative materials that have been developed for radiopacity

A

metal inserts
inorganic solts
comonomer containing heavy metails
halogens containing comonomer or additives

44
Q

Why are metal inserts an issue

A

they weaken the material and result in poor aesthetics

45
Q

Why are inorganic salts not ideal

A

§ In low concentrations it is not radiopaque

§ In high concentrations it is a weak base

46
Q

Why are comonomer containing heavy metals not ideal

A

○ Comonomers containing heavy metals e.g barium sulphate which results in poor mechanical properties

47
Q

Why are halogen containing comonomer or additives not ideal

A

§ May act as plasticiser
§ Expensive
§ Promising

48
Q

For those with a proven allergy to acrylic what can be tried instead

A

nylons
vinyl polymers
polycarbonates

49
Q

What is the issue with nylons

A

§ Water absorption leading to swelling and softening

50
Q

What is the issue with vinyl polymers

A

§ E.g polyvinyl acetate, polyvinylchloride, styrene
§ Injection moulding (expensive)
§ Softening temperature of 60 degrees celcius

51
Q

What are the issue with polycarbonates

A

§ Injection moulded
§ Softening temperature of 150 degrees
§ They develop internal stresses during use and can result in distortion and poor fit

BDS3 CLINICAL DMS (9 decks)

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