Dentures

Question 1

When impression materials are in an elastic and plastic state

Answer 1

-Low viscosity to flow around tissues to record details (plastic)
-Undergoes setting where it decreases fluidity to become viscoelastic then elastic
-When removing from mouth it needs to be elastic so doesn’t distort, and not too rigid or stiff so easy to remove
-Over time it needs to be dimensionally stable so little expansion and contraction

Question 2

Name materials that are thermoplastics, hydrocolloids, elastomers

Answer 2

-thermoplastic=compound
-hydrocolloids= alginate, agar
-elastomers= polysulphide (not clinically used in the UK), silicone, polyether

Question 3

Ideal space between impression tray and teeth

Answer 3

5-7mm

Question 4

What factors affect accuracy of impression material

Answer 4

-viscosity (mucocompressive, mucostatic)
-wetting (droplets form on hydrophobic materials which cause blows)
-dimensional changes (polymerisation shrinkage, crystal expansion, thermal contraction, warping during warming/cooling)
-adhesion to tray/ distortion on removal or setting
-undercuts present in the tissues (therefore need elastic)
-mixing, long enough WT
-tearing due to thin areas

Question 5

Average temperature of mouth compared to room temperature

Answer 5

mouth= 32 degrees celsius
room= 23

Question 6

what is working and setting time

Answer 6

WT = from mixing to time it needs to be seated in mouth
ST = from seated in mouth to sufficient stiffness or elasticity

Question 7

Name non-elastic materials. And elastic materials (hydrocolloid and elastomers)

Answer 7

-non-elastic= compound, ZOE, plaster [plastically deforms when removed past undercuts]
-hydrocolloids= agar, alginate
-elastomers polysulfide, silicone, polyethers

Question 8

What is a hydrocolloid. Which is reversible: agar or alginate

Answer 8

-water-based. A colloid with water.
-A sol when initially mixed, then forms a gel.
-colloid= heterogenous (particles in a liquid). 2 phases that cannot be readily differentiated.
-between a solution and suspension. Particles dispersed in another material (continuous phases) but form a homogonous solution. Particles don’t settle like in a suspension. If continuous phase is water then the material is a hydrocolloid

agar= reversible
alginate=irreversible, cannot return to sol state

-low viscosity, elastic, can go in undercuts, hydrophilic, doesn’t require dry field, poor dimensional stability (imbibition and syneresis)

Question 9

Components of alginate and its reaction with water

Answer 9

-sodium alginate, calcium sulphate and other stuff
-when mixed with water, Soluble alginate reacts with calcium sulfate to produce calcium alginate gel
-Ca replaces Na ions and crosslinking and ionic bonding is possible

Question 10

Difference in edentulous and dentate stock trays

Answer 10

-dentulous= square, wide and flat floor, longer flanges,
-edentulous=oval, short flanges, narrow floor

Question 11

What is a sol and gel

Answer 11

-types of colloid solutions

sol= viscous liquid. Random arrangement of fibrils
gel= elastic solid. Crosslinks form between fibrils, which are either reversible (agar) or irreversible (alginate)

Question 12

What is gypsum. Requirements

Answer 12

-calcium sulphate dihydrate which is dried down to a calcium sulphate hemihydrate powder
-water is added when needed
-Used for models, casts, impression material, investment material

-Requirements: minimal dimensional change on setting and over time for good accuracy, compatible with impression materials, color contrast so easy to read, easy to use, not costly, initially low viscosity (mucostatic), when set it is rigid (and brittle)

Question 13

What anti-expansion agents are added to gypsum

Answer 13

-K2SO4= reduces expansion but accelerates setting
-Borax= slows down reaction

so concentrations need to be carefully controlled. May be added to the powder or supply it as a solution so read the instructions before use

Question 14

How stone and plaster differs

Answer 14

-0.3mm expansion for plaster. 0.2mm for stone. Stone has smaller particles, more regular, more dense, less porous.

Question 15

Requirements for dental base

Answer 15

natural appearance, easy processing, easy to clean, easy to repair, inexpensive, long shelf life, biocompatible, resistant to bacterial contamination, radio-opaque (8% BaSO4 so can see in x-rays if in airways), heat transfer (thermal diffusivity and conductivity to maintain health of soft tissues), accurate reproduction of surface detail, dimensional stability, good fit, resists distortion, no water or saliva absorption, corrosion resistant, adequate mechanical properties (strength, stiffness, hardness, toughness),

Question 16

Impression compound properties

Answer 16

-Thermoplastic= hard when cool, soft when warm (55-60 degrees)
-non-elastic. Too rigid to be removed past undercuts. It plastically deforms
-high viscosity so mucocompressive, poor detail reproduction
-thermal contraction when softened
-stress relief can occur after removal

Question 17

Zinc oxide eugenol properties

Answer 17

-Setting via acid-base reaction- chelation and ionic reaction, accelerated by water
-very low viscosity so mucostatic and can record fine detail.
-Used in thin section (1mm thick) as low viscosity makes it difficult to produce thick films
-Good accuracy and dimensional stability
-Rigid and non-elastic so cannot record undercuts
-pseudoplastic=viscosity reduces under stress
-hydrophilic

Question 18

What is an elastomer. name impression materials

Answer 18

-a polymer that displays rubber-like properties. Either natural or synthetic. Synthetic used for impression materials
-Flexible repeating units (Si-O and C-O)
-better accuracy, dimensional stability and tear resistance than alginate
-used for crown and bridge work

Question 19

How does filler affect viscosity and shrinkage

Answer 19

-higher= lower shrinkage (as doesn’t polymerise)
-higher= more viscous

Question 20

What are the properties of addition and condensation type Silicones.

Answer 20

-condensation type= initially fluid, becomes elastic, alcohol formed as by-product so significant dimensional change over time

-Addition type (polyvinylsiloxanes)= no by-product but hydrogen may be evolved so need to wait to pour model.
Available in different viscosities – ultra-light, light, regular, heavy and putty

Question 21

Properties of polyvinylsiloxanes. Are they addition or condensation silicones

Answer 21

Addition silicones (elastomers)
-good dimensional stability
-elastic when set
-resist tearing
-hydrophobic so needs a dry field. Surface treatment sprays available to avoid wetting
-latex gloves can ‘pollute’ catalyst so prevent polymerization

Question 22

Which out of light body, regular and heavy body/putty polyvinylsiloxanes (addition silicones) have highest shrinkage. Which are used for high accuracy impressions

Answer 22

-Light body =lowest viscosity so produces finest detail
BUT high polymerization shrinkage due to low filler
-Regular=medium shrinkage
-Heavy body/ putty = least shrinkage, less thermal contraction, more stable, easier to handle in bulk

-General work= regular
-High accuracy= Putty placed into impression tray then syringe light onto teeth for high accuracy

Question 23

Properties of a polyether (elastomer) Are they addition or condensation polymerisation

Answer 23

-Addition. No by-product
-light, regular and heavy viscosities available
-Quite rigid when set so difficult to remove from mouth and cast, and can remove mobile teeth!
-Before setting are fluid and pseudoplastic so good detail reproduction
-Elastic when set, suitable for crowns and bridge work. May be only choice for implants
-Hydrophilic so doesn’t require a dry field
-Dimensionally stable but can absorb water and swell.
-Difficult to decontaminate
-Can tear in thin sections

Question 24

Which materials are hydrophobic so need a dry field, and which are hydrophilic

Answer 24

-hydrophobic= silicone
-hydrophilic= alginate, compound, ZOE, polyether

Question 25

Requirements for denture bases

Answer 25

-biocompatible, non-toxic, non-irritant to maintain health of soft tissues.
-Moldable to individual shape for good comfort, fit, performance.
-Cheap to make.
-Strong so doesn’t easily fracture. Tough to survive being dropped. Stiff so doesn’t deform during everyday activities. Resist permanent deformation so doesn’t change the fit.
-Good hardness so scratched don’t weaken it and so it allows cleaning with abrasive cleaners.
-Ideally low density for high strength.
-Ideal heat transfer to prevent burns but then don’t want to take away sensations of eating and drinking.
-Radio-opacity to detect fragments if swallowed or inhaled.
-dimensionally stable. Should not absorb water or be affected by dietary agents and cleaning products.

Question 26

For PMMA acrylic, what is in the powder and the liquid, before being mixed

Answer 26

-Powder= PMMA, Benzyl peroxide initiator, Pigment
-Liquid= MMA monomer, ethylene glycol di-methacrylate (cross-linker), hydroquinone (stabiliser), amine compound (activator for self curing only, not heat cure)

Question 27

What stages does acrylic go through when being mixed. When placed in gypsum mould, what do you do next

Answer 27

1. Slurry/creamy
2. Sandy
3. Stringy
4. Dough - polymerisation occurs
   Dough is then placed into gypsum mould contained in metal flask. Mould is sealed and pressurised. Heat curing acrylic placed into oven/bath, cold curing don’t need a bath but may be put in pressure pot. Remove and de-flask after setting time. Then requires grinding and polishing

Question 28

What are the 4 stages of an addition polymerisation reaction

Answer 28

1. Activation: breakdown of initiator, production of free radicals
2. Initiation: radicals react with monomer
3. Propagation: chains lengthen, includes cross-linking
4. Termination: reaction ends, residual monomer remains

Question 29

How molar mass of monomer affects shrinkage and viscosity

Answer 29

high= low shrinkage, high viscosity

Question 30

Why polymerisation shrinkage occurs

Answer 30

monomer is less dense than polymer
occurs if low monomer mass, low filler, high c factor (more bonded surfaces)

Question 31

Explain the structure of acrylic when set

Answer 31

-has a composite structure: continuous phase (newly polymerised resin, containing residual monomer) and discrete phase (original resin beads, spherical structure)

Question 32

Properties of acrylic that are not ideal

Answer 32

-low toughness as can break quite easily when dropped. Shatters into many pieces when dropped. Notches and scratches increase fracture. [More toughness = more expensive]
- weakens when scratched. So good to polish denture to extend its lifetime
-an insulator so warn against potential scalding
-porous so candida can infiltrate and cause continuous infection
- Weak and flexible (Thicker denture = strong and better at resisting forces. But thickness limited as patient would have problems with biting etc. so needs compromise)
-Low fatigue life. Repeated stress. Denture bases flex during biting so more deformation occurs in the mid-line and prone to breaking
-soft material so low abrasion resistance and scratches easily
-biocompatity- residual monomer can be an irritant and cytotoxic

Question 33

Difference between heat and chemical activation acrylic. Which has higher degree of polynerisaiotn and therefore used more

Answer 33

-Heat= initiator breaks down above 65 degrees, so takes hours to make denture
-Chemical=initiator breaks down at room temperature so quicker to make. Shorter WT. Temp never reached 100.3 so no gaseous porosity

But cold cure= 3-5 % residual monomer, due to less polymerisation=bad
-heat cure= 0.5% - 1.5% so greater degree of polymerization. So more ideal and so used more

Question 34

What temperature does monomer boil

Answer 34

Monomer boils at 100.3 so can turn to gas before polymerizing = gaseous prosity = leads to weakness.
-Rapid heating = lots of porosity

Question 35

Where does greatest gaseous porosity occur in acrylic dentures and why. How to reduce gaseous porosity

Answer 35

-Greater volume = greater temperature rise so more gaseous porosity. Area of greatest volume on dentures is just below teeth so greatest gaseous porosity likely to happen. This is where we want good mechanical properties so not good if porosity occurs here.

-To combat gaseous porosity, different heating cycles are possible. Slow heating using water bath = Maximises polymerisation and reduce residual monomer. So reduces gaseous porosity as less gas forming

Question 36

What is the Transition temperature. What is the Tg of PMMA

Answer 36

Tg of PMMA is ~100 degrees
The temperature at which it changes from glass/stiff state to ductile/rubber state.

-Tg below mouth temp means ductile in mouth. Above mouth temp means stiff in mouth

-In chemical activation, temperature is always below Tg
Polymer is always stiff so monomer finds it hard to flow to active chains
-In heat activation, temperature can be close to or above Tg
Monomer finds it easier to flow to active chains. Beware Tg is close to the monomer vaporisation temperature

Question 37

how molecular weight, residual monomer, cross-linking, increased initiator, C=C bonds affects Tg

Answer 37

-High molecular weight=higher Tg
-low residual monomer = higher Tg (as more polymerisation has occurred so higher weight)
-More C=C is more rigid so higher Tg
-More Si-O makes it more flexible so lower Tg
-more cross-linking= higher Tg (as harder to deform)
-increased initiator= lower Tg (more chains with smaller length)

Question 38

Why we want as much polymerisation as possible

Answer 38

to get the right properties we need, and no monomers left over.
But will never get full polymerization so will be some monomer left. Left over residual plasticizer means it can be more flexible than we want it to be for example

Question 39

How to clean denture

Answer 39

-Regular cleaning vital for hygiene.
Mechanical: Soft brush (to avoid scratches) and paste
Chemical: Soaking in cold water (otherwise causes distortion and white appearance) with oxygenating tablet. Or Sodium hydrochloride solution
-Leave out of mouth over night

Question 40

Requirements of artificial teeth on dentures

Answer 40

need to have good mechanical and physical properties. Need to be similar to bases. Need to withstand chewing forces. Need to have a strong bond to the base to prevent debonding which can be dangerous (swallowing).

Question 41

Properties of porcelain and acrylic artificial teeth on dentures. Which is more commonly used

Answer 41

-Porcelain= a ceramic. Can be mass produced by standard shapes/ sizes. Bonding to denture is mechanical so uses pins and holes. Bonding not as good as acrylic. It is hard, rigid and brittle. Abrasion resistant. Abrasive. Can chip easily. Have a natural appearance but make an unnatural clicking sound when biting together.

Acrylic= same material as the base. Produced in standard shapes/ sizes. Polymerized in metal molds using heat and pressure. Bonding is very good due to being same material as base. Properties similar to acrylic bases. Down side is that is has poor abrasion resistance

Question 42

Uses of soft liners (tissue conditioners, and temporary and permanent soft liners)

Answer 42

Permanent:
-to cover surface of existing denture to provide alleviation to patients struggling to wear them comfortably. Provides cushioning over thin oral mucosa or severe alveolar resorption.
-Aids retention by engaging in hard tissue undercuts

Temporary:
-Improves comfort. Aid traumatized tissues, wound healing after extraction, improves ill-fitting dentures while new ones are made. A good diagnostic aid as it can be used between short regular appointments to identify what and where is causing the issue. cannot be used permanently as distorts and changes fit.

Question 43

Difference between tissue conditioners, soft liners and permanent denture soft liners

Answer 43

-tissue conditioners= useful over a few days. cannot be used permanently as distorts and changes fit
-temporary liners=useful over a few weeks. cannot be used permanently as distorts and changes fit
-permanent= a long-term solution over months up to years. Should last lifetime of denture

Question 44

Requirements for soft liners

Answer 44

-cover denture surface
-soft when set (low Tg)
-elastic for cushioning,
-high resilience
-plastic for functional impression
-biocompatible with denture
-permanently soft
-durable
-easy to clean

Permanent= -adheres to denture base very well

Question 45

Properties of temporary soft liners

Answer 45

-initially very soft (few weeks maximum)
-Viscoelastic – not fully elastic so will flow under load to act as functional impression as will flow away from points of high load. The new denture can be designed to compensate for these high load points.
-Hardens after few days/ weeks- due to solvent & plasticizer leaching out.
-Separates from base. So needs replacement material regularly or new denture fitted
-cannot be used permanently as distorts and changes fit

Question 46

Properties of permanent liners

Answer 46

-acrylic or silicone based
-initial softness (light activated too hard so not used. Other materials are soft, but harder than temporary)
-softness on ageing (silicones remain soft but acrylics harden due to loss of plasticizer)
-Difficult to clean. Oxygenating cleansers cause surface pitting. Care required when brushing with toothpastes due to abrasion and can cause de-ponding. Soaking in dilute hypochlorite may help, but too strong degrades lining, and too weak is ineffective at cleaning
-Adding liners requires removing some of denture to make room for it. Becomes thinner so strength decreases. Fractures more likely to occur. So a strip placed at back so it is longer lasting

Question 47

How to bond acrylic and silicone permanent soft liners to denture

Answer 47

-acrylic: bond ‘like to like’ with acrylic denture
-silicones: self cure require adhesive
heat cure bond through copolymerisation

Question 48

Which is elastic and which is viscoelastic when set: acrylic or silicone soft liners

Answer 48

-silicone=elastic. Deformation is not permanent
-acrylic=viscoelastic. Recovers over time