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BDS3 DMS > PMMA > Flashcards

PMMA Flashcards

1
Q

What are the functions of a denture base?

A
  • replaces the function of natural teeth
  • fits properly in the patient’s mouth
  • has appropriate aesthetics
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2
Q

What are the ideal properties of a denture base?

A
  • dimensionally accurate and stable in use
  • high softening temperature
  • unaffected by oral fluids
  • thermal expansion
  • low density
  • high thermal conductivity
  • radiopaque
  • non toxic, non-irritant
  • appropriate colour/translucency
  • easy and inexpensive to manufacture
  • easy to repair
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3
Q

What are the ideal mechanical properties of a denture base?

A
  • High Young’s (elastic) modulus
  • high proportional limit
  • high transverse/flexural strength
  • high fatigue strength
  • high impact strength
  • high hardness/abrasion resistance
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4
Q

Why must a denture base have good transverse strength?

A

The acrylic resin experiences flex in the mouth with the hard palate as a pivot point. There is a risk of fracture at the pivot point and this must be avoided

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5
Q

Why must a denture have good impact strength?

A

There is a high likelihood that dentures will be dropped so it is advantageous to have a material which can withstand impacts.

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6
Q

Discuss the impact strength of acrylic resin as a denture base

A
  • acrylic resin has poor impact strength
  • dentures often break when dropped
  • if there is not initial fracture, cracks may have formed
  • denture fails at a later stage, often on try in
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7
Q

Through what process does PMMA polymerise?

A

Free radical addition polymerisation

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8
Q

What is free radical addition polymerisation?

A

the chemical union of two molecules, either the same or different, to form a larger molecule without the elimination of a smaller molecule

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9
Q

What type of molecules are necessary for free radical addition polymerisation?

A

molecules with C=C bonds

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10
Q

What is the methacrylate monomer?

A

CH3
|
CH2 = C
|
COOCH3

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11
Q

What are the 4 stages of polymerisation?

A
  • activation
    • initiator activated to provide free radicals
  • initiation
    • free radicals break C=C bond in monomer
    • free radical transferred
  • propagation
    • polymer chain grows
  • termination
    • polymerisation ends
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12
Q

What are the components of heat cure acrylic powder?

A
  • initiator
    • benzoyl peroxide (0.2-0.5%)
  • PMMA particles
    • pre-polymerised beads
  • plasticiser
    • dibutyl phthalate
    • allows quicker dissolving in monomer liquid
  • pigments
    • provide natural looking colour
  • co-polymers
    • ethylene glycol dimethacrylate
    • improve mechanical properties
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13
Q

What are the components of heat cure acrylic liquid?

A
  • methacrylate monomer
    • dissolves PMMA particles
    • polymerises
  • inhibitor
    • hydroquinone (0.006%)
    • prolongs shelf life
    • reacts with free radicals produced by heat or UV light
  • co-polymers
    • improve mechanical properties
    • improves cross-linking of polymers
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14
Q

Describe the process by which heat cure acrylic is shaped and cured

A
  • vessel made containing mould material with impression of denture
  • artificial teeth placed into spaces in mould
  • acrylic resin in dough form placed into mould recess
  • vessel calmed together
  • vessel subjected to heating cycle to cause polymerisation of acrylic resin
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15
Q

Why is effective polymerisation important in heat cure acrylics?

A

Effective polymerisation creates a high molecular weight polymer. This provides good mechanical properties

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16
Q

What are the properties of acrylic?

A
  • non toxic
  • non irritant
    • if no monomer is released (fully polymerised)
    • few patients are allergic
  • unaffected by oral fluids
    • acrylic redin is insoluble
  • poor mechanical properties
    • bulk increased to compensate
  • reasonable fatigue strength/impact strength
    • reasonable
    • impact often cause of failure
    • can withstand repeat application of low level stresses
  • high hardness /abrasion resistance
    • retains polished finish
    • some wear apparent over time (feels rough)
  • thermal expansion
    • no issue if acrylic teeth are used
    • expansion much greater than that of porcelain
  • low thermal conductivity
    • poor
    • risk of soft tissue burns in throat
  • low density
    • increased in bulk to improve mechanical properties
    • advantage not experienced
  • high softening temperature
    • 75 degrees
    • ok for ingestion of hot fluids
    • boiling water should not be used for cleaning
  • dimensionally accurate and stable in use
    • contraction of 0.5%
    • considered acceptable
    • water absorption can occur
17
Q

Discuss the contraction and expansion experienced by heat cured acrylic

A
  • during manufacture there is 0.5% liner contraction
  • during usage there is 0.4% expansion (water absorbed)

overall 0.1% contraction experienced by the denture

18
Q

How does polymerisation differ in self curing acrylic from heat cure?

A

benzoyl peroxide is activated by the promoter in the liquid

  • promotor is a tertiary amine
  • tertiary amine found in liquid activated inhibitor
19
Q

What are the potential advantages of self cure acrylics?

A
  • self cure acrylics experience less thermal contraction
20
Q

How does chemical activation compare to heat activation? What is the impact of this on the cured material?

A

chemical activation is less efficient than heat activation resulting in less efficient polymerisation and more unreacted monomer present

material has a lower molecular weight which results in poorer mechanical properties and a lower softening temperature

21
Q

What is the impact of unreacted monomer in self cured acrylic?

A
  • acts as a plasticiser
  • softens denture base
  • reduction in transverse strength
  • more vulnerable to failure
  • potential tissue irritant
  • compromised biocompatibility
22
Q

How much unreacted monomer is present in self cured acrylic?

A

3-5%

23
Q

How much unreacted monomer is present in heat cured acrylic?

A

0.2-0.5%

24
Q

Discuss the dimensional accuracy of self cured versus heat cured acrylic

A
  • self cured
    • experiences less thermal contraction
    • initially fits better
    • expands during use due to water absorption
    • denture ends up oversized
  • heat cured
    • contracts during manufacture
    • absorbs some water while in use
    • results in an undersized denture
    • better tolerated by the patient
25
Q

Compare the properties of heat cured and self cured acrylic

A
  • heat cured
    • higher molecular weight, stronger
    • lasts longer, more convenient for patients
    • curing may cause porosity (technician skill reduces)
  • self cured
    • higher monomer levels, can act as irritant
    • absorbs water resulting in oversized denture
    • poor colour stability (tertiary amines may oxidise)

neither have ideal properties

26
Q

What is the fracture rate of acrylic resin dentures in 3 years?

A

10%

27
Q

What are the two ways in which it has been attempted to strengthen acrylic resin?

A
  • high impact resistance materials
    • rubber toughening agent incorporated
      - (butadiene styrene)
    • stops crack propagation
    • long term fatigue problems
  • incorporation of fibres
    • carbon
    • UHMPE (ultra-high molecular weight polyethylene)
    • glass
    • difficult to manufacture
28
Q

What is Ultra-Hi

A

high impact heat cure acrylic resin

  • used in GDH labs
29
Q

What are the properties of Ultra-Hi heat cure acrylic resin

A
  • high flexural strength
    • slight bending aspect
  • high fracture toughness
    • more ductile
30
Q

What are pour and cure resins?

A
  • similar to self cure
  • have smaller powder particles
  • fluid mix is poured into a mould
  • fits well but has poor mechanical properties
  • inadequate for use as a denture
31
Q

What are light activated denture resins?

A
  • composed of
    • urethane dimethacrylate matrix (UDMA)
    • acrylic copolymers
    • microfine silica fillers
    • photoinitiator systems
  • adapted to casts and cured in light chambers
  • depth of cure limits thickness of denture
32
Q

What are light activated denture resins used for?

A
  • customised impression tray material
  • repair of fractured dentures
33
Q

Why is it helpful for denture materials to be radiopaque

A

incase of fracture and swallowing or inhalation, denture can be viewed on radiographs

34
Q

What methods have been used to attempt to create radiopaque polymers?

A
  • metal inserts
    • weaken denture base
    • poor aesthetics
  • inorganic salts (barium sulfate)
    • low concentrations were not radiopaque
    • high concentrations resulted in a weak base
  • comonomer containing heavy metals (barium sulfate)
    • results in poor mechanical properties
  • halogen containing comonomer or additives
    • tribromophenylmethacrylate
    • may act as plasticiser
    • expensive
    • no clinical evidence substances are efficient
35
Q

What are the alternative polymers that can be used in case of a proven acrylic allergy?

A
  • nylons
  • vinyl polymers (polyvinyl acetate, polyvinyl chloride, styrene)
  • polycarbonates
36
Q

What are the issues presented by nylon denture bases?

A

nylon absorbs water
- swelling
- softening

37
Q

What are the issues presented by vinyl polymer denture bases?

A
  • made through injection moulding (expensive)
  • low softening temperature (60 degrees)
  • soften during use
38
Q

What are the advantages of polycarbonate denture bases

A
  • good impact strength
  • high softening temperature (150 degrees)
39
Q

What are the issues presented by polycarbonate denture bases

A
  • made through injection moulding (expensive)
  • internal stresses cause distortion
  • results in poor fit

BDS3 DMS (10 decks)

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