Dentures: impression, base and lining materials Flashcards
What are the 5 chemical classes of impression materials?
- Pastes e.g. Zinc Oxides
- Gypsum e.g. impression plaster
- Thermoplastics e.g. Compound and Wax
- Hydrocolloids e.g. Alginate
- Elastomers e.g. Polyether, Silicones, Polysulphides
What is meant by the rheological properties of impression materials?
The viscosity of the unset material
What viscosity of impression material provides higher accuracy?
Low viscosity e.g. light-bodied
Define mucostatic
Low viscosity materials that do not displace tissue. Shows tissue at rest
Define mucocompressive
Very viscous materials displaces soft tissues - not as representative as tissues at rest but shows sulcus depth and without denture may be too big
What is meant by the mechanical properties of impression materials?
The properties of the set material e.g. stiffness, elasticity
What properties of impression materials will mean it distorts around undercuts?
Plastic and viscoelastic materials
What does rigidity of an impression material mean?
How easy it is to deform the material - need low stiffness to get past undercuts
What impression materials are rigid and clinically can only be used in areas without undercuts?
- Thermoplastics (compound and wax)
- Gypsum (impression plaster)
- Pastes (Zinc Oxide)
What impression materials are elastic and clinically can be used to record undercuts?
- Hydrocolloids (alginate, agar)
- Elastomers (Polyether, silicones, polysulphides)
If there is not enough space between the tray and the tissue what does this do to the viscosity of the impression material?
Increases viscosity as less flow possible
What are stock trays made out of?
How does the material attach in the tray?
- Polymers = disposable, alloys = reusable
- Perforations or tray adhesives or rim lock (lip on tray)
How can special trays be constructed?
- On a model in acrylic
- Using putty/wash systems e.g. elastomers
What 6 things affect the accuracy of the impression material?
- Viscosity
- Wetting (how well the material spreads on moist tissues)
- Setting reaction (dimensional changes)
- Thermal contraction (from temp change in mouth)
- Adhesive to tray
- Undercuts
If an impression material is hydrophobic what does this mean clinically?
Water forms droplets on the surface which leads to bubbles/blows appearing on the surface - so must have a dry field
What will occur if the impression material undergoes polymerisation shrinkage?
Bonded to tray so shrinks away from tooth leading to oversized die (crown too big)
What will occur if the impression material undergoes crystal growth?
Expansion so leads to an undersized die (crown too small)
What temperature changes can occur during setting of impression material?
- Thermal contraction - higher the coefficient of thermal expansion the more contraction. Reduction in size leads to crown not fitting
- Thermoplastic warming/cooling - compound softened at 55 degrees then put in mouth. Temp drop can cause warping and distortion
What factors affect the accuracy of the impression when undercuts are present?
- Depth of undercuts (deeper = more distortion)
- Proper setting of the material (allow to become fully elastic not viscoelastic)
- Removal technique
- Thickness of impression material (tearing of thin sections)
What can dimensional changes of impression materials when they are sent to the lab be down to?
- Continues setting i.e. continuation of polymerisation or crystallisation
- Loss of volatiles such as water or ethanol evaporating
- Stress relief
What is gypsum?
Plaster of paris i.e. beta-calcium sulphate
Why does gypsum expand?
Setting reaction leads to crystallisation
What is added to gypsum as
a) anti-expansion agent (but accelerates setting)
b) Retarder to slow setting time
a) K2SO4
b) Borax
How much of powder and water do you need for gypsum to be a fluid mix?
100g powder to 50-60ml water
Name the properties of impression plaster
- Mucostatic as low viscosity
- Very rigid and brittle when set
- Small expansion on setting less than 0.1%
- Dimensionally stable
- Non-toxic
- Compatible with model materials
What is the composition of impression compound?
- Thermoplastic material e.g. natural or synthetic resins and waxes
- Filler e.g. Talc
- Plasticiser e.g. stearic acid
- Pigment
What is the role of a)filler and b)plasticiser in impression compound?
a) Controls viscosity, rigidity, thermal contraction
b) Improves flow of material in and around soft tissues
What is Type 1 impression compound?
Lower temperature fusing
- High flow value just above mouth temperature
- Sheet form used for preliminary imps for edentulous pts
- Stick form used for marginal additions to stock trays
What is Type 2 impression compound?
High temperature fusing
- Low flow value at or just above mouth temp
- Rigid at mouth temp so used as tray model
- Rarely used
What is the softening temperature of impression compound?
55-60 degrees
What are the properties of compound?
- Mucocompressive (material has high viscosity) so poor detail reproduction
- No setting reaction so final material is rigid and non-elastic
- Thermal contraction - after removal stress relief can occur
What will happen to impression compound if:
a) Water is too cool
b) Water is too hot
c) Softening time too short
d) Softening time too long
a) Compound wont flow sufficiently
b) Compound will be sticky and painful for the patient
c) Wont flow
d) Leaching of plasticiser so won’t flow
Impression paste is typically supplied as two pastes. What is in these pastes?
1) ZnO, Zn-actetate, Oils
2) Eugenol, filler (e.g. kaolin)
What is the setting reaction of ZnO and eugenol in impression paste?
Acid base reaction
- Chelation reaction
- Ionic reaction
What accelerates the setting reaction of ZnO and eugenol in impression paste?
Water
What are the properties of impression pastes?
- Low viscosity (can only be used in thin sections)
- Mucostatic
- Pseudoplastic i.e. viscosity reduces under stress
- Good accuracy and dimensional stability
- Eugenol can be irritant
- Rigid and non-elastic so can’t record undercuts
What is the definition of a colloid material?
Somewhere in between a suspension and a solution. Particles are dispersed in another material (continuous phase) but form a homogenous solution and don’t settle out
In terms of colloid solution types, what are
a) Sols
b) Gels
In impression taking what is the transition?
a) Solid dispersed in liquid, behave like liquids e.g. paint
b) Liquid dispersed in solid, behave like solids
Sol to gel
What is the definition of
a) Solution
b) Suspension
c) Composite
a) Liquid of homogenous mixture of solute and solvent - one phase
b) Liquid continuous phase with solid discrete phase - 2 phases
c) At least 2 phases that can be individually recognised
What are dental hydrocolloids based on?
Polysaccharides
What is the arrangement of the polysaccharides in hydrocolloids when they are in
a) Sol form
b) Gel form
a) Random arrangement of fibrils, material fluid
b) Cross links form between fibrils, hydrogen bonding (weak) - reversible or covalent/ionic bonding (strong)- irreversible
For the hydrocolloid agar
a) Is it reversible or irreversible
b) What material is added to strengthen the gel
c) What material is added to compensate this
d) Why is it heated to 100 degrees, then 65 degrees then 45 degrees
a) Reversible - but not reusable as contamination
b) Borax
c) K2SO4
d) 100= converts to sol
65= maintain as sol
45 = convert to gel and lowers temp before entering mouth
What are the properties of agar?
- Hydrophilic
- Gel form is cross-linked polymer (flexible to move past undercuts)
- Viscoelastic
- Low tear resistance
- Poor dimensional stability
In gel form agar is viscoelastic, so what does distortion depend on?
- Depth of undercut
- Thickness of material - greater impression bulk means more elastic behaviour
- Time under stress - quick snap release means more elastic behaviour ie. rapid removal
Why does agar have a poor dimensional stability?
Initially has a high water content so loses water over time:
- Syneresis = polymer chains collapse in towards each other
- Imbibition = gains water
- Evaporation of water on surface
What does alginate powder contain?
- Sodium or Potassium salt of alginic acid (12%)
- Calcium sulphate/ gypsum (12%)
- Sodium phosphate (2%)
- Filler e.g. diatomaceous earth
- Flavours, dyes, indicators
What is the setting reaction of alginate?
- Water and powder mixed to form sol
- Ca2+ replaces Na+ ions so crosslinking with ionic bonding possible forming a gel
What is the working time and setting time controlled by?
- Sodium phosphate and calcium sulphate
- Sodium phosphate added as a retarder to delay the reaction
- Temperature of water added (hot water=faster setting)
What are the properties of alginate?
- Hydrophilic
- Poor dimensional stability, evaportation of water, syneresis, imbibition
- Flexible (good for undercuts)
- Elastic/viscoelastic
- Low strength - easy to tear and needs 3-5mm of material between tray and tissue
What is alginate not used for?
Where great accuracy and dimensional stability is needed e.g. crown and bridge work
What is an elastomer?
What do the elastomeric impression materials usually come as?
A polymer with rubber like properties
Liquid polymers - ‘pre-polymers’ that are not terminated so can still polymerise
What are the flexible repeating units of
a) Silicones
b) Ethers
a) Si-O-Si-O
b) C-O-C-O
For the elastomeric material Silicones, what is the reaction for the condensation type?
Silicone pre-polymer and silica added to make paste
Catalysed by tin
Crosslinking and alcohol formed as by-product
What is the dimensional stability like for the elastomeric condensation type silicones?
- Better than hydrocolloids and polysulphides
- Ethanol evaporates so significant dimensional change over time
What are addition type silicones known as?
Polyvinylsiloxanes
For the elastomeric impression material addition type silicones (Polyvinylsiloxanes), what is in
a) Paste 1
b) Paste 2
a) Liquid silicone polymer with Si-H groups. Filler.
b) Liquid silicone polymer with Si-CH=H2 groups. Filler. Catalyst - chloroplatinic acid
Why should you wait before pouring the model if you are using the addition silicone polyvinylsiloxane?
Hydrogen released from catalyst so will make pits in the model
Why do the light bodied silicones shrink most during polymerisation?
They have less filler
More filler = higher viscosity = lower shrinkage
What are the properties of Polyvinylsiloxane?
- Good dimensional stability
- Elastic when set
- Good tear resistance
- Good accuracy and fine detail
- Hydrophobic - can lead to impression faults so needs a dry field
What is the composition of elastomeric Polyethers?
a) Paste 1
b) Paste 2
a) Imine (triangular group) terminated polyether. Plasticiser. Filler
b) Aromatic sulphonate. Plasticiser. Filler
What is the elastomeric Polyethers setting reaction?
Ring opening cationic polymerisation - addition polymerisation
No by product just chain extension
What are the properties of polyethers?
- Short setting time
- Dimensionally stable but can absorb water and swell
- Elastic when set
- Before setting are hydrophilic and fluid for good detail reproduction
- Very rigid when set and can remove mobile standing teeth
What are the problems with the catalyst in Polyvinylsiloxane?
Latex gloves and freshly placed methacrylate’s pollute catalyst and prevent polymerisation
Why should denture bases be
a) Strong
b) Tough
c) Stiff
d) Elastic deformation
e) Hard
a) Prevents dentrue fracturing
b) Withstand energy transfer from being dropped
c) Shouldn’t deform during everyday activities
d) Maintain shape in function
e) Weakened by surface scratches. Must withstand abrasive cleaners
Why is heat transfer a problem with denture bases?
PMMA is an insulator but need to maintain health of underlying soft tissue so warn of potential scalds
What molecule are acrylic denture bases based on?
What is the difference between the 2 types of acrylic base materials?
Monomer methyl methacrylate (MMA)
Method of polymerisation i.e. heat curing or self curing
What is the composition of the acrylic denture base?
a) In the powder
b) In the liquid
a) Beads of PMMA, peroxide, pigment
b) Methylmethacrylate monomer, dimethacrylate (crosslinker), Hydroquinone (stabiliser), amine compound (activator)
Acrylic denture bases set by free radical addition polymerisation. What are the stages of this setting reaction?
1) Activation
2) Initiation
3) Propagation
4) Termination
Exothermic reaction
When manufacturing acrylic bases what are the distinct physical properties of the dough moulding process?
1) Slurry/ creamy when powder and liquid mix
2) Sandy (monomer soaking into PMMA and beads swell)
3) Stringy (swollen beads join together but are not polymer chains)
4) Doughy (but polymerisation hasn’t occurred)
What occurs during dough moulded denture production?
1) Dough placed into gypsum moulth
2) mould sealed and pressurised - trial closure with heat cured
3) Heat cure (flask into oven), Cold cure (Pressue pot)
4) Remove after ST then grind and polish
What are the other approaches to manufacturing acrylic denture bases?
- Injectable materials
- Pourable materials (poured when sandy, time saving but mechanical properties poor)
- Light cured
What is the structure of set acrylic?
Composite structure: continuous phase = fresh resin, discrete phase = original resin beads
At what temperature in heat cured acrylic is
a) Peroxide activated?
b) Monomer boils and causes gaseous porosities
a) 65-70 degrees
b) 100.3 degrees
Why is a heating cycle of slow rise to 70 degrees and final phase to 100 degrees used?
- Reduces residual monomer
- Maximises polymerisation
- Reduces gaseous porosity
Why does heat curing produce a higher degree of polymerisation?
PMMA glass transition temp (Tg) around 100 degrees, so monomer finds it easier to flow in polymer to active chains
Be aware that Tg is close to monomer vaporisation temperature
What is the glass transition temperature of acrylics controlled by?
So what does high temp cure produce and low temp cure produce?
Molecular weight
Residual monomer
High temp = high Tg - high Mwt and low residual monomer
Low temp = low Tg - low Mwt and high residual monomer
Why does powder have to be added to methyl methacrylate?
Methyl methacrylate has low Mwt so shrinks when converted to polymer. Powder reduces shrinkage.
What is the cold cure process for setting of acrylic?
Activation just by mixing powder and liquid - shorter WT, no gaseous porosity
What are the properties of PMMA?
- Weak and flexible material
- Low toughness (shatters when dropped and notch sensitive)
- Soft material (scratches easily)
- Low fatigue strength (Deformation midline)
Why must care be taken when brushing denture?
- PMMA soft so scratching possible
- Surface made rough - abrasive to soft tissues and site for microbial colonisation
Why must care be taken when soaking dentures?
If low Tg denture may change dimensions in hot water Denture bleaching (change in pigmentation) from too hot water and poor curing
What can cause issues with biocompatibility of PMMA denture base?
Residual monomer irritant and cytotoxic
What material is being used more recently for denture bases?
PUDMA - nylon and light cured acrylics
For porcelain teeth
a) What are they made of
b) How are they bonded to the denture
c) What are its properties
a) Ceramic
b) Mechanical by pins and holes
c) Hard rigid brittle, abrasion resitant but abrasive to natural teeth, natural appearance, unnatural sound, difficult to adjust
For acrylic teeth
a) How are they bonded
b) How are they made
c) What are they poor at resisting
a) Like to like bonding (denture base acrylic)
b) Polymerised using heat and pressure
c) Abrasion
What are the indications for tissue conditioners and temp soft liners?
- Temporary support to aid traumatised tissues, wound healing
- Improve fit of ill fitting denture
- Diagnostic aid / functional impression
What is the composition of tissue conditioners / temporary liners
a) Liquid
b) Powder
a) Plasticiser = benzoate ester, solvent = ethanol
b) Higher methyl methacrylate polymer
Why is the set material soft for a tissue conditioner and temp soft liner?
Plasticiser solvent and low Tg polymer as large pendent groups
What are the properties of temporary soft liners?
- Initially very soft
- Viscoelastic so flows under load making a functional impression
- Oxygenating cleaners cause pitting on surface
- Harden after a few days/weeks as plasticiser leached out
What are the indications for permanent soft liners?
- Pain during mastication
- Aid to retention (engage hard tissue undercuts)
- Obturators - when hard or soft tissues removed to fill space
What are the two types of permanent soft liners?
Acrylic based
Silicone based
For permanent soft liners made of heat curing acrylic
a) What is in the powder
b) What is in the liquid
c) What is the setting reaction
a) Methacrylate polymer, peroxide, pigment
b) Monomer (methacrylate), plasticiser (lower Tg), NO ALCOHOL
c) Polymerisation - dough moulding technique and heat cure
