What is a denture base made of
Acrylic polymers or metal alloys
Process of making a denture
-Impressions of upper and lower dental arches are taken and bite registration is recorded
- Mock up denture in wax is fabricated in the dental laboratory
- Waxed denture is tried in the patient’s mouth and adjustment is made accordingly
- Denture is processed and finished in the dental laboratory
Ideal Requirements of denture base materials
Appearance:
-Must match oral tissues
Low specific gravity:
- Needs to be light
- Comfort
Thermal properties:
- High Tg
- High thermal diffusivity (so patient knows when he is havign a hot drink so can avoid burns)
- Low coefficient of thermal expansion
- Match aritficial toooth for COTE
- Non toxic
- Non irritant
- High modulus of elasticity
- Adequate flexural and impact strength (don’t want pt to swallow pieces of denture)
- Fatigue strength due to intermittent loading
- High elastic limit
- Denture base material must be inert
- Radioopaque
- Abrasion resistance
- Repairable
- Long shelf life
- Inexpensive
Producing acrylic denture base material
- 2 phase system
- Powder: Liquid phases
Powder phase:
- Polymethylmethacrylate in the form of beads
- Initiator is benzoyl peroxide which is incorporated into the beads
Liquid phase:
- Methylmethacrylate
- Activator: tertiary amine (N,N dimethyl p toulidine)
- Stabilisers
- Inhibitors: hydroquinone
- Cross linking agents
Polymerisation of the denture base mix process
- PMMA powder (beads) is mixed with methylmethacrylate monomer
- Firstly a sandy mix is obtained whilst the MMA dissolves the PMMA beads and leaching the initiator, benzoyl peroxide
- Soon acquires the consistency of a dough, due to the onset of polymerisation of MMA
- Dough then needs to be packed in flasks
- As the polymerisation progresses, the temperature rises and major part of the reaction takes place within 10-15 minutes to yield a hard polymer
- Slow curing is carried out in the flask to get maximum polymerisation
Polymerisation reaction of acrylics
Free radical addition polymerisation
Cross linking agent
Ethylene glycol dimethacrylate
Manipulation of the polymerisation reaction and subsequent acrylic formed
Polymer: Monomer ration of 2.5:1
Powder:Liquid
Excess monomer (liquid) leads to a higher polymerisation shrinkage
Excess powder leads to granular porosity
Mixing: adequate mixing is required, air bubbles can be incorporated
Wetting of polymer: proper wetting should be ensured
Methods of curing denture bases
Heat
Chemical
Microwave
Light (not usually done)
Methods of fabricating dentures
Compression mounding or dough moulding
Injection moulding
Pour and cure resins
Heat curing acrylics
- Polymerisation of MMA is an exothermic reaction
- Gradual increase in temperature is necessary
- Loss of monomer via evaporation
- Gradual cooling of flask to avoid warpage
- Gaseous porosity: Polymerisation rxn is exothermic however also requires heat for the heat cured resins, thus temperatures above 100 degrees celcius can cause the monomer to evaporate leading to porosity
- Precautions: temperature should be raised gradually
-Ensure that the temperature does not get greater than 60-70 initially, allowing all the polymerisation to take place before evaporation can
Importance of heating slowly
-Monomer can evaporate if increase heat too drastically
Chemical curing of acrylics
Powder consists of the initiator: generally benzoyl peroxide
- Liquid consists of the tertiary amine
- Amine + peroxide leads to free radicles
- Free radical polymerisation
- Room temperature polymerisation
- Short working and dough time
- Packing in flasks at the dough stage
- Used for repairs
Light curing of acrylics
- Not popular
- Urethane dimethacrylate and microfine silica and camphorquinone as light initiator
- Available in sheets, rope or single component
Microwaving of acrylics
- Special containers required
- Speed of set
- Properties comparable with heat cured ones
- NATURE-CRYL is a denture acrylic resin specially created for microwave curing
- High precision dentures can be cured in just 3 mins in a hoursehold microwave oven
General properties of PMMA based denture bases
Molecular weight
- High molecular weight
- Good mechanical properties
Residual monomer
- Can be kept low in PMMA polymerisation by adequate curing
- Excess monomer may cause plasticisation to weaken the material
Porosity
-Shrinkage and gaseous porosity may compromise strength and aesthetics
Water Uptake
- Fatigue life
- May cause dimensional changes
- Complete drying of dentures may lead to shrinkage and distortion
- Patients told to keep dentures wet at all times
Thermal properties of PMMA
Crazing:
-Repeated cycles of drying and wetting cause tensile stresses
Mismatch between base and tooth material of COTE may cause crazing and during denture repair
Softening may occur if washed with boiling water (Tg of 105)
Low thermal diffusivity and conductivity means denture base is an insulator
PAtient needs to make sure their tea isnt too hot to avoid burns
Mechanical properties of PMMA
-Weak in comparison to alloys but adequate thickness gives appropriate mechanical properties
- Fatigue and impact fractures may happen due to its brittle nature
- Fatigue life diminishes with water uptake
-Modulus of elasticity adequate
Brittle material with a low impact strength
Mechanical properties compared to stainless steal/cocr
Modulus of elasticity, tensile strength and hardness all much lower than stainless steel and cocr
Stainless steel denture bases
- Denture base and for partial dentures
- Light thin sections
- Adequate thermal conductivity allows to maintain a healthy response to thermal stimuli
- Corrosion resistant
Denture lining materials
- Hard reline materials
- Tissue conditioners
- Soft lining materials
used for relining or repairing of dentures
-As soft lining for use over traumatized tissue
Types and composition of typical hard reline materials
Properties
Type I:
- PMMA beads
- BPO as initiator
- MMA monomer
- Plasticizer
- Tertiary amine
- High polymerisation ecotherm
- Monomer is an irritant
- Leach of plasticizer
- Higher monomer leach
Type II: -Poly(ethylmethacrylate) -BPO -Pigments Tertiary amine
Low polymerisation exotherm
-Low tg
Ductile as higher methacrylates are used
Larger monomer molecule less prone to leaching
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Biocompatibility of materials for use in the body21
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Failure of Materials54
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Polymers in Dentistry33
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Ceramics59
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Polyalkenoate Cements27
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Composites39
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Metals in Dentistry56
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Adhesion to Enamel and Dentine40
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Amalgam39
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Impression Materials36
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Elastomeric Impression Materials36
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Acrylic Materials22
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Cements27
