What are the functions of a denture base?
- replaces the function of natural teeth
- fits properly in the patient’s mouth
- has appropriate aesthetics
What are the ideal properties of a denture base?
- 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
What are the ideal mechanical properties of a denture base?
- High Young’s (elastic) modulus
- high proportional limit
- high transverse/flexural strength
- high fatigue strength
- high impact strength
- high hardness/abrasion resistance
Why must a denture base have good transverse strength?
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
Why must a denture have good impact strength?
There is a high likelihood that dentures will be dropped so it is advantageous to have a material which can withstand impacts.
Discuss the impact strength of acrylic resin as a denture base
- 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
Through what process does PMMA polymerise?
Free radical addition polymerisation
What is free radical addition polymerisation?
the chemical union of two molecules, either the same or different, to form a larger molecule without the elimination of a smaller molecule
What type of molecules are necessary for free radical addition polymerisation?
molecules with C=C bonds
What is the methacrylate monomer?
CH3
|
CH2 = C
|
COOCH3
What are the 4 stages of polymerisation?
- 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
What are the components of heat cure acrylic powder?
- 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
What are the components of heat cure acrylic liquid?
- 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
Describe the process by which heat cure acrylic is shaped and cured
- 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
Why is effective polymerisation important in heat cure acrylics?
Effective polymerisation creates a high molecular weight polymer. This provides good mechanical properties
What are the properties of acrylic?
- 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
Discuss the contraction and expansion experienced by heat cured acrylic
- 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
How does polymerisation differ in self curing acrylic from heat cure?
benzoyl peroxide is activated by the promoter in the liquid
- promotor is a tertiary amine
- tertiary amine found in liquid activated inhibitor
What are the potential advantages of self cure acrylics?
- self cure acrylics experience less thermal contraction
How does chemical activation compare to heat activation? What is the impact of this on the cured material?
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
What is the impact of unreacted monomer in self cured acrylic?
- acts as a plasticiser
- softens denture base
- reduction in transverse strength
- more vulnerable to failure
- potential tissue irritant
- compromised biocompatibility
How much unreacted monomer is present in self cured acrylic?
3-5%
How much unreacted monomer is present in heat cured acrylic?
0.2-0.5%
Discuss the dimensional accuracy of self cured versus heat cured acrylic
- 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
