Denture Base materials Flashcards by Lucy Currie

Function of dentures?

Return form and function when teeth missing

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Requirements of denture base - biocompatibility

Dentures worn for long time - must maintain health of soft tissue = non-toxic and non-irritant

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Requirements of denture base - manufacturing

Mouldable to individual shape (ill fit may effect comfort and performance)
Must be cheap to make

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Mechanical requirements of denture base

a) strength
b) toughness
c) stiffness
d) deformation
e) hardness

a) sufficient strength to resist fracture during everyday life
b) sufficient toughness to withstand rapid energy transfer e.g. dropped (older people have bad manual dexterity)
c) sufficient stiffness so it doesn’t deform during everyday activities
d) resist permanent deformation - if any deformation it must be elastic = high proportional limit
e) sufficient hardness so resistant scratches that cause weakness or abrasive cleaners

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Why do denture bases need a high proportional limit?

To ensure that any deformation that does occur is elastic

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Ideally should denture bases have high density or low density?

Low density

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What is the problem with a denture base with low density?

Strength and density are proportional so low density material will not be strong

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Why do denture bases need to be a conductor of heat?

To maintain health of underlying soft tissues

- don’t scold tissues without noticing

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Why do denture bases need to be radio-opaque?

Detect fragments if swallowed or inhaled

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List the physical requirements of denture bases

low density - but compromise for high strength 
Thermal diffusivity and conductivity
Radio-opaque
Accurate and dimensionally stable
Able to be cleaned easily

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What are the 2 main types of acrylic denture base materials?

Heat curing denture acrylic or self curing denture acrylic

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How are acrylic denture bases set?

Free radical addition polymerisation

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What are the 4 stages of free-radical addition polymerisation?

Activation
Initiation
Propagation (cross-linking to improve mechanical properties)
Termination

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What is the dough moulding process?

The process when the powder and liquid are mixed together - occurs before polymerisation

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What are the series of stages in dough moulding process?

Creamy
Sandy - monomer starts to stack into PMMA
Stringy - beads joined together by vanderwalls forces
Dough - strings bind together

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Describe the dough moulded denture production?

Dough placed into gypsum mould
Mould sealed and pressurised - force dough into all of the mould
heat cure - place into water bath
Cold cure - will cure on its own - no need for bath
Remove and de-flask after setting time

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List 3 alternative production methods to dough moulded denture production?

Injectable - inject dough into mould
Pourable
Light cured

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Describe set acrylic denture structure

Composite structure

continuous phase - fresh resin
discrete phase - original resin beads

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Effect of curing process on properties -

a) 2 main processes used to make dentures
b) difference between the 2

a) heat cure and cold (self-cure)
b) heat = activated above 65 degrees, takes hours to make denture
Cold = activation at RT, less than an hour

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Advantages of heat cure process?

Dough formed at RT strait closure is possible

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At what temperature is the initiator activated in heat cure?

65-70 degrees

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What type of reaction is the heat cure activation? What is the consequence of this?

Exothermic
Rapid temp rise = monomer boils and turns to gas = gaseous porosity = weakness especially in areas with greater volume = needs to be the strongest

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Where does gaseous porosity tend to form?

areas with most volume = needs to be the strongest

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What method is used to combat gaseous porosity in heat curing process?

Different heating cycles

Describe different heating cycles to reduce gaseous porosity during heat cure

Slow heating rise up to 70 degrees (can take up to an hour) this means you are not likely to reach 100 degrees in exothermic reaction Can't leave this here because high residual monomer found Add a final heating phase to 100 degrees to maximise polymerisation, reduce residual monomer, reduce gaseous porosity (because vast percentage of polymerisation occurred at 70 degrees)

Why can you not just slowly increase the temperature to 70 degrees?

high residual monomer

Why do you add final heating phase at the end of heat cure process?

Maximise polymerisation reduce residual monomer reduce gaseous porosity

Why does the final heating phase at the end of heat cure process not cause gaseous porosity?

Most of the polymerisation occurred at 70 degrees

Disadvantage of cold cure process?

activation occurs during dough formation - no trail closure possible. Polymerisation occurs during dough formation so shorter WT

Advantages of cold cure process?

exothermic reaction never reaches 100 degrees so no gaseous porosity

Describe the residual monomer content of cold cure process?

Related to curing temperature - high temp = lower residual monomer

Which process produces higher degree of polymerisation? a) heat curing b) cold curing

Why does heat curing process have higher degree of polymerisation than cold curing?

PMMA glass transition temp = 100 degrees In chemical activation - temperature always below Tg - polymer always glassy so monomer finds it hard to now to active chain In heat activation - temp close to Tg - monomer finds it easier to flow to active chains Beware that Tg is close to monomer vaporisation temperature

What is glass transition temperature?

The temperature at which a material goes from being rigid (glassy) to more rubbery

What provides better molecular properties? a) high molecular weight b) low molecular weight Why?

``` a Decreased polymerisation shrinkage More crosslinking Stronger Stiffer ```

What method produces a high average molecular weight and low residual monomer content? a) heat cure b) cold cure

What controls glass transition temperate?

Molecular weight and residual monomer | residual monomer acts as plasticiser = lower tg

What is the consequence is Tg is too low (close to mouth temperature)

Denture may be dimensionally unstable may go rubbery in the mouth so when the patient bites into it it can be deformed Quality of fit over time

Affect of low molecular weight monomer on polymerisation shrinkage?

Increased shrinkage

name monomer in denture and polymer it forms?

Methyl methacrylate to Polymethyl methacrylate

Does methyl methacrylate have high or low molecular weight?

Low

What is added to methyl methacrylate to decrease shrinkage?

Add powder

Adding powder to methyl methacrylate decreases shrinkage but doesn't stop it, contraction porosity still occurs, how is this managed?

Trail closure - check sufficient material is added

what process allows trial closure?

heat cure

PMMA physical properties? - strength - flexibility - toughness - fatigue - hardness

Weak and flexible Just does well enough to resist masticatory forces Increasing thickness would increase strength but limited by patient comfort Low toughness - shatters when dropped easy to break when scratched Low fatigue strength - 'flex' during biting Soft - low abrasion resistance, scratches easily

2 types of cleaning regimes available for cleaning dentures

mechanical - brush and paste | chemical - soaking, oxygenating tablet, sodium hypochlorite solution

Why must care be taken when brushing dentures?

PMMA is soft - scratches easily | Cleaning pastes are abrasive

Effect of scratches on dentures?

Effect comfort of denture | Microbial colonisation - may lead to stomatitis

Why must care be taken when soaking dentures ?

Soaking requires warm water - if too warm = too close to Tg = distortion Change in pigmentation can occur 'denture bleaching' = water too warm = change in refractive index

biocompatibility of PMMA? - residual monomer - pigments

Residual monomer is irritant and cytotoxic some people may be allergic to monomer some people can be allergic to the pigments used

Appearance of PMMA?

Good colour match with pigments Fibres added to look like BV Can stain cleaning can cause bleaching

Why are dentures radio-opaque?

incase it shatters and is swallowed - can be seen on X-ray

What is radio-opacity dependant on?

Atomic number - lower atomic number (like PMMA) = radio-lucent

What is added to PMMA to make it radio-opaque, what is the effect of this?

Radio-opacifier Reduce strength Compromise needed

Requirements for artificial teeth added to denture base?

Good mechanical and physical properties - similar to denture base Strong bond to denture base

Name 2 types of material used to make artificial teeth?

Porcelain - traditional | Acrylic - modern

Porcelain teeth a) advantages b) disadvantages

``` a) mass produced hard, rigid - abrasion resistant natural appearance b) bad bonding abrasive brittle unnatural sound difficult to adjust ```

Acrylic teeth a) advantages b) disadvatages

``` a) mass produced standardised metal moulds made bonding achieved easily properties similar to acrylic base b) poor abrasion resistance strength only just good enough - deformation (properties similar to acrylic base) ```