Denture Base materials Flashcards
Function of dentures?
Return form and function when teeth missing
Requirements of denture base - biocompatibility
Dentures worn for long time - must maintain health of soft tissue = non-toxic and non-irritant
Requirements of denture base - manufacturing
Mouldable to individual shape (ill fit may effect comfort and performance)
Must be cheap to make
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
Why do denture bases need a high proportional limit?
To ensure that any deformation that does occur is elastic
Ideally should denture bases have high density or low density?
Low density
What is the problem with a denture base with low density?
Strength and density are proportional so low density material will not be strong
Why do denture bases need to be a conductor of heat?
To maintain health of underlying soft tissues
- don’t scold tissues without noticing
Why do denture bases need to be radio-opaque?
Detect fragments if swallowed or inhaled
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
What are the 2 main types of acrylic denture base materials?
Heat curing denture acrylic or self curing denture acrylic
How are acrylic denture bases set?
Free radical addition polymerisation
What are the 4 stages of free-radical addition polymerisation?
Activation
Initiation
Propagation (cross-linking to improve mechanical properties)
Termination
What is the dough moulding process?
The process when the powder and liquid are mixed together - occurs before polymerisation
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
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
List 3 alternative production methods to dough moulded denture production?
Injectable - inject dough into mould
Pourable
Light cured
Describe set acrylic denture structure
Composite structure
- continuous phase - fresh resin
- discrete phase - original resin beads
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
Advantages of heat cure process?
Dough formed at RT strait closure is possible
At what temperature is the initiator activated in heat cure?
65-70 degrees
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
Where does gaseous porosity tend to form?
areas with most volume = needs to be the strongest
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
a
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
a
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)
