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Dental Materials Y3 > Impression Materials for Removable Pros > Flashcards

Impression Materials for Removable Pros Flashcards

1
Q

What are the 3 main branches of impression materials

A
  • Hydrocolloids
  • Elastomers
  • Non-elastic materials
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2
Q

What types of Hydrocolloid are there

A
  • reversible (agar)

- irreversible (alginate)

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3
Q

What types of elastomer are there

A
  • Polyethers
  • Polysulphides
  • Cond. and Add. Silicones
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4
Q

What types of non elastic impression materials are there

A
  • Impression compound
  • Zinc Oxide Eugenol (ZOE)
  • Impression Plaster
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5
Q

What do mucodisplacive (muco-compressive) impression materials do

A

These are viscous and record an impression of the mucosa under load
Resulting removable prosthesis e.g. dentures, result in a distribution of load during function

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6
Q

How does taking an impression using mucodisplacive impression materials reduce fracture risk

A

The record of the mucosa under load makes a prosthesis that compensates for the differing compressibility of the denture bearing area, reducing the risk of a fracture due to flexion.

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7
Q

Why might the retention of the prosthesis made from mucodisplacive impression materials be compromised

A

As the tissues recorded in the impression will return to their original position at rest

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8
Q

Give some examples of mucodisplacive impression materials

A
  • Impression compound
  • High viscosity alginates
  • High viscosity elastomers (e.g. polyether)
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9
Q

What do mucostatic impression materials take an impression of

A

These displace the tissues less and record an impression of the un-displaced mucosa

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10
Q

What might cause instability of the denture produced from mucostatic impression materials

A

During function the tissues may distort and cause instability

NOTE: mucostatic dentures generally have better retentions than mucodisplacive as there is a closer adaptation to the mucosa at rest

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11
Q

Give some examples of mucostatic impression materials

A
  • Impression plaster
  • Zinc Oxide Eugenol (ZOE)
  • Low viscosity alginates
  • Light body addition silicone
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12
Q

Where are non-elastic impression materials used

A

Where there are no undercuts and in edentulous patients

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13
Q

AY BAWS CAN I HABE DE NOTE PLEASSE

A

Any significant deformation produces a permanent deformation

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14
Q

Describe the composition of impression plaster (mucostatic - non-elastic)

A
  • beta hemihydrate of calcium sulphate
  • potassium sulfate - anti-expansion agent
  • Borax - counteracts fast set caused by K2SO4
  • Colouring agents - contrast with model plaster
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15
Q

What happens to impression plaster (mucostatic - non-elastic) when you add water

A

the beta-hemihydrate converts to the dihydrate and there is expansion on setting (controlled by anti-expansion agent K2SO4)

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16
Q

When is the only time impression plasters are used

A

Only in edentulous cases

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17
Q

AY BAWS CAN I HABE DE NOTE PLEZ

A
  • Mucostatic non-elastic impression material so doesn’t displace soft tissue
  • Material sets hard
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18
Q

What happens if the impression plaster fractures when you try to remove it from the mouth after an impression

A

You should retrieve the pieces and glue them back before casting

NOTE: a separating medium should be used

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19
Q

Describe the composition of impression compound (mucocompressive non-elastic material)

A
  • resins and waxes
  • talc as a filler
  • Stearic acid as a lubricant

Talc = hydrated magnesium silicate

20
Q

How do you used impression compound (mucocompressive non-elastic material) in clinics

A
  • Thermoplastic material
  • Softened by heating the sheet in water at ~60C
  • Placed in a stock tray which is then positioned in the mouth
21
Q

Describe the thermal conductivity of impression compound

A
  • Poor thermal conductivity/flow properties
22
Q

What won’t impression compound reproduce in the mouth

A

Undercut areas

23
Q

What is the advantage of impression compound having a high viscosity

A
  • If full depth of the sulcus is required to support the subsequent denture you can do it with impression compound (border moulding)
24
Q

Describe the contents of the pastes used in ZOE mucostatic non-elastic impression materials

A

Paste 1:

  • Zinc Oxide
  • Zinc acetate

Paste 2:

  • Eugenol
  • Inert filler (e.g. talc or kaloin)

Kaolin = aluminium silicate

25
Q

When are ZOE mucostatic non-elastic impression materials used

A
  • Edentulous cases

- Often when relining a denture

26
Q

Describe some of the properties of ZOE mucostatic non-elastic impression materials

A
  • Brittle material when set - bits can fracture off
  • Accurate in thin section
  • Initial low viscosity and pseudo plasticity
27
Q

Describe the uses of hydrocolloid elastic impression materials

A
  • Complete/partial dentures
  • Orthodontics - base plate
  • Study models and working casts
  • Duplicating models
28
Q

Is a high viscosity alginate (hydrocolloid) mucocompressive or mucostatic

A

Mucocompressive

NOTE: Low viscosity alginate = mucostatic

29
Q

What are the advantages of using alginates (hydrocolloids)

A
  • Material is cheap and reliable
    Setting behaviour is v good:
  • Na3PO4 suppresses setting so material is a viscous paste while impression tray is seated in the mouth
  • When setting reaction begins its quick af so impression taking time is short
30
Q

What are the disadvantages of using alginates

A

Water loss:
- Shrinkage after setting so poor dimensional stability
- Impression covered with damp napkin/sealed inelastic bag
Alginate immersed in disinfecting solution:
- swells
- then shrinks
- prolonged immersion in disinfecting solution impractical
Poor Tear strength - large undercuts can’t be reporduced
Highly viscoelastic material:
- snap removal technique needs to be employed
- permanent deformation up to 1.5%

31
Q

What are the advantages of using agar (hydrocolloid)

A
  • Once equipment is set up its easy to use
  • Material is cheap
    Impressions have good surface detail:
  • fine detail recorded due to setting behaviour
32
Q

What are the disadvantages of using agar (hydrocolloid)

A
  • Impressions must be cast up immediately due to water loss
  • Also absorbs water
  • Compatibility with model materials
  • Agar has a poor tear strength (but better than alginates)
  • Highly viscoelastic material: snap-removal technique needed, permanent deformation up to 1%
33
Q

Are high viscosity elastomer mucocompressive or mucostatic

A

Mucocompressive

NOTE: Low viscosity elastomers = mucostatic

34
Q

What are elastomers used for and why

A

Due to their strength and dimensional stability they’re used to:

  • Produce accurate replicas of teeth and tissues to make full and partial dentures, crowns, bridges and inlays
  • Border moulding of special trays, duplicating materials for refractory casts, bite registration
35
Q

What is a disadvantage of elastomers

A
  • Costs more than alginates/agar
36
Q

What are the advantages of condensation silicones (Poly-dimethyl siloxanes = elastomer?)

A
  • More elastic than polysulfides and polyethers

- Tear strength and elongation at break are adequate - undercuts can be reproduced

37
Q

What are the disadvantages of condensation silicones (Poly-dimethyl siloxanes = elastomer?)

A
  • Shrinks in air (affects dimensional stability)

- Hydrophobic material (contain some detergent for wettability)

38
Q

What are the advantages of using addition silicones (poly-vinyl dimethyl siloxanes = elastomers?)

A
  • Best dimensional stability compared to all other impression materials (v little shrinkage)
  • Elastic recovery is v good
39
Q

What are the disadvantages of using addition silicones (poly-vinyl dimethyl siloxanes = elastomers?)

A
  • Free water in plaster reacts with unreacted Si-H groups and releases hydrogen, resulting in a porous model - wait at least 20-30mins before casting
  • %elongation and tear strength are generally less than that of Condensation silicones
  • Hydrophobic material (detergents added for wettability)
  • Don’t use natural rubber gloves - setting can be v impaired (S poisons Pt catalyst)
40
Q

What are the advantages of polyether (impregum) mucodisplacive (due to their stiffness)

A
  • Good dimensional stability in AIR

- Quick setting compared to polysulfide and reliable

41
Q

What are the disadvantages of polyether (impregum) mucodisplacive (due to their stiffness)

A
  • Original impregum = very stiff material
  • Original impregum = 50% elongation + high stiffness = tearing on withdrawal
  • Permanent deformation
  • Dimensional stability on prolonged contact with water or water vapour (don’t wrap in damp napkin) - disinfection is a problem
42
Q

What are the advantages of using polysulfide elastomer impression materials

A
  • Its the strongest of all impression rubbers (elongation at break ~500%)
43
Q

What are the disadvantages of using polysulfide elastomer impression materials

A
  • Dimensional stability (shrinks by 0.1-0.2%)
  • slow setting
  • Dirty to handle and has an unpleasant smell
  • plastic recovery is not as good as the silicone and polyether materials
44
Q

Are elastomers or hydrocolloids preferred for producing duplicating materials

A
  • Elastomers are recommended as duplicating materials for fixed restorations
  • Agar and alginates can be readily used in the preparation of partial dentures
45
Q

What materials are most commonly used for dentate patients

A
  • Alginate
  • Agar
  • Non-aqueous elastomers
46
Q

What materials are most commonly used for edentulous patients

A
  • Impression compound
  • Impression plaster
  • ZOE
  • Alginates
  • Non-aqueous elastomers
47
Q

List as many desirable qualities of an ideal impression material as possible

A
  • Pleasant odour, taste and aesthetic colour
  • Adequate shelf life for requirements of storage and dsitribution
  • Free from toxic irritant constituents
  • Economically commensurate with the results obtained
  • Easy to use
  • Setting characteristics that meet clinical requirements
  • readily wets oral tissues
  • Elastic properties free from permanent deformation after strain
  • Adequate strength to not tear upon removal from mouth
  • Dimensional stability over temp and humidity ranges
  • readily disinfects without loss of accuracy

Dental Materials Y3 (9 decks)

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