Non-metallic denture bases Flashcards
What does a denture consist of?
- denture base
- artificial teeth
What are the types of artificial teeth?
- acrylic resin
- porcelain
What are the types of denture bases?
- metallic
- non-metallic
What are the types of non-metallic denture bases?
- heat cured
- self (cold) cured
What are the types of metallic denture bases?
- gold alloy
- chromium-cobalt
What is the criteria of an ideal denture base material?
- natural appearance
- high mechanical properties
- good thermal conductivity to maintain healthy tissues
- low density to help denture retention
- dimensionally stable and reproduce surface details
- absence of odour, taste or toxic products
- good retention to other polymers, porcelain and metals
- radio-opaque to be seen in x-ray if swallowed
- easy to repair
- easy to manipulate
- good shelf life
- inexpensive
What is the most popular denture base material?
acrylic resin
What are the types of acrylic resin?
- heat cured
- cold cured
- light cured
What is the process of making acrylic resin?
free radical addition polymerisation to form polymethylmethacrylate (PMMA) and the monomer is methyl methacrylate (MMA)
What does heat cured acrylic resin consist of?
powder and liquid that form a rigid solid upon mixing and subsequent heating
Why do we use powder/liquid system?
decrease polymerisation shrinkage and to process it by dough technique
What is in the powder of heat cured acrylic resin?
- granules of PMMA
- benzoyl peroxide initiator
- dibutyl phthalate plasticiser t produce a less brittle polymer
- pigments to give the natural tissue like appearance
- synthetic fibers to stimulate blood vessels of oral mucosa
- bismuth salts opacifiers to be seen in x-ray if swallowed
What is in the liquid of heat cured acrylic resin?
- MMA
- hydroquinone as inhibitor to react with any free radical that may form in liquid and to prevent polymerisation of monomer during storage
- ethylene glycol dimethacrylate as a cross-linking agent to improve mechanical properties and increase resistance of denture base to craze
What is the polymer/monomer ratio?
- 3:1 by volume
- 2.5:1 by weight
Why is the ratio 3:1?
polymerisation shrinkage will be decreased from 21% to 6-7%
What happens if less monomer is used than the ratio?
- insufficient wetting of polymer
- weaker and granular denture is obtained
What happens if more monomer is used than the ratio?
- increase in polymerisation shrinkage
- possibility of porosit
What are the stages that the powder and liquid mixing?
- sandy stage
- sticky or fibrous stage: polymer begins to dissolve in monomer
- dough stage: material becomes smooth and does not stick to the mixing jar or hands and it is easily manipulated (ideal stage for packing)
- rubbery stage: material becomes rubbery (soft but can’t be shaped)
- stiff stage
When is packing stage reached?
when material reaches dough stage
What happens if material is packed in sandy stage?
material will be of high fluidity and will flow out of the flask resulting in denture porosity
What happens if material is packed in rubbery or stiff stage?
- material will not flow under pressure manifested by absence of metal to metal contact of flask parts
- this results in lack of denture details and usually with moved or fractured teeth
Why should adequate pressure be applied on the mould after packing?
inadequate pressure will lead to irregular voids on the surface of dentre (shrinkage porosity)
Why should acrylic dough be packed in excess?
to decrease polymerisation shrinkage from 7% to 0.2%
Does the acrylic dough need to be packed in excess?
yes
When do we add a separating medium to the material?
before packing the dough, the separating medium should be applied to all parts except the surface of the teeth
Why should we not add separating medium at the surface of teeth?
to allow the teeth to stick to the denture base material
Why do we add separating medium?
- to prevent the passage of water from gypsum to the resin because it may cause crazing
- to minimize the passage of the monomer to plaster
At which temperature does benzoyl peroxide initiator decomposes?
above 65 degrees
What is the boiling point of monomer?
100.3 degrees
What happens if the temperature o the dough is raised above 100.3 degrees?
gaseous porosity
Is the polymerisation reaction highly exothermic?
yes
What happens during curing?
the flask is placed in a heated water bath under pressure at 72 degrees
What are the 2 cycles for curing?
- long cycle: heating at 72 degrees for 8-16 hours
- short cycle: heating at 72 degrees for 1 hour then temperature is raised to 100 degrees for another one hour
What happens during under curing (less time and/or temperature)?
results in a denture with residual monomer (irritant to tissues and of inferior properties)
What happens during over curing (more time and/or temperature)?
result in boiling of monomer due to added heat of polymerisation resulting in vaporisation of monomer leaving pores inside the denture (gaseous porosity)
What happens during cooling stage?
- flask should be slowly cooled because there is a difference between coefficient of thermal contraction between the mould material and acrylic resin
- this difference induces stresses in the denture
- slow cooling allows the release of these stresses
What happens after packing?
curing
What happens after curing?
cooling
What happens after cooling?
deflasking
What happens in deflasking?
should be done with care to avoid breaking of denture
What happens after secondary impression?
wax bite block is constructed
What does the wax bite block does?
simulates space provided by height of teeth then the jaw relation is done then it is mounted on the articulator followed by setting up of artificial teeth
What is a flask?
a device in which the waxed up denture is inserted and the final acrylic denture is cured
What are the steps of flasking?
- a plaster mixture is poured in the bottom section of the flask then after the setting it is coated with a separating medium
- the upper half of flask is put in place and a plaster mixture is poured up to the occlusal surface of teeth
- the upper part of the flask is filled with stone to coat the occlusal surfaces of artificial teeth and fill the flask to the top and the cover is put in place
How to eliminate wax from mould?
- flask is heated in boiling water to soften the wax
- after cooling, the two halves of the flask are separated and the teeth remain in the upper half embedded in he plaster while the cast is retained to the lower half
- apply a separating medium on the plaster surface
What happens during packing?
mould is filled with resin in the dough stage then the flask is placed in the upper half of the flask then in a bench press to apply pressure on it
What happens after flasking?
elimination of wax
What happens after elimination of wax?
packing
What are the two cycles for curing?
- long cycle: heating at 72 degrees for 8-16 hours
- short cycle: heating at 72 degrees for 1.5 hours then raised to 100 degrees for an additional hour
Where is the flask placed during curing?
in a water bath
What happens during deflasking?
flask is removed from water bath and is allowed to cool to room temperature
What happens after curing?
deflasking
What happens after deflasking?
finishing and polishing and delivery to patient
Why is polishing done?
to obtain a smooth surface of the denture
Should the the denture b kept wet until delivery to the patient?
yes
What are the properties of heat cured acrylic resin?
- residual monomer
- porosity
- water sorption
- crazing
- mechanical properties
- thermal conductivity
- coefficient of thermal expansion
How much % of the monomer remains (residual monomer)?
0.2-0.5%
What happens if curing was at too low temperature or too short time?
higher % of residual monomer
Why should increased residual monomer presence should be avoided?
- free monomer may be released from the denture and irritate oral tissues
- residual monomer will act as plasticiser and make the denture weaker and more flexible
What is the site and shape of contraction/shrinkage porosity?
- site: on the surface of denture
- shape: irregular voids
What is the cause of contraction/shrinkage porosity?
- underpacking
- packing before dough stage: because before this it can cause rapid loss of pressure
What is the site and shape of gaseous/internal porosity?
- site: inside denture (especially in thick parts)
- shape: appears as fine uniform rounded voids
What is the cause of gaseous/internal porosity?
- over or prolonged curing temperature
What happens during water sorption of denture?
- acrylic denture absorbs water by 2% of its weight
- each 1% increase in weight causes linear expansion of 0.22%
- similarly, drying out the acrylic denture is associated with shrinkage
Why is water sorption an advantage?
to compensate for processing shrinkage which is 0.44% (polymerisation shrinkage and thermal shrinkage occurring when cooling from 72 degrees to 20 degrees) and that’s why dentures should be kept wet at all times when not in service
Why might cracks appear on the surface of denture?
- tensile stresses that cause separation of polymer chains
- crazing has a weakening effect on the denture
What are the causes of crazing?
- continuous stresses due to repeated drying and wetting of denture which causes alternate contraction and expansion
- action of solvents like alcohol, acetone, chloroform or monomer when denture is repaired, some monomer comes in contact with the resin and may cause crazing
- differences in coefficient of thermal expansion between acrylic denture base and porcelain artificial teeth
What makes crazing less likely?
cross-linking
What is usually associated with fractures?
- sudden impacts e.g. if dropped on the floor
- fatigue from repeated bending of denture in service
How are mechanical properties determined?
transverse strength
What do mechanical properties depend on?
- molecular weight
- spatial structure
- cross-linking
- porosity
- degree of polymerisation
- residual monomer
- crazing
What is the thermal conductivity in heat cured denture base?
- very low
- may cause burning of the back of the throat since the oral tissues are isolated and the patient won’t know if the drink’s temperature
What is the problem of coefficient of thermal expansion for heat cured acrylic denture?
- very high
- porcelain teeth in the denture base may gradually loosen and be lost
What is the difference between self-cured and heat-cured acrylic denture base?
self cured has a chemical activator but heat cured has heat activator
What is the chemical activator of self-cured acrylic resin?
dimethyl para toluidine
What is the percentage of residual monomer for self-cured acrylic denture base?
5%
Which has a higher residual monomer: self cured or heat-cured?
self-cured
Which denture base causes more tissue irritation?
self-cured
Which denture base has higher molecular weight?
heat-cured
Which denture base has more degree of polymerisation?
heat-cured
Which denture base has more strength?
heat-cured
Which denture base has more colour stability?
heat-cured
Why is the colour stability less in self-cured acrylic denture base?
due to the presence of tertiary amine activator
What are the uses of heat-cured acrylic resin?
- denture base
- artificial teeth
What are the uses of self-cured acrylic resin?
- special trays
- repairing
- relining
- rebasing
Where does the dough polymerise in self-cured acrylic denture base?
in room temperature rather than water bath (heat cured)
