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GDAN yr 2 - AGEING > impression materials > Flashcards

impression materials Flashcards

1
Q

impression

A

Negative imprint of hard and soft tissues in the mouth from which positive reproduction can be formed

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2
Q

tray

A

container designs to fit over the dental arches

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3
Q

impression materials

A
  • designed to be liquid or semi solid when first mixed

- quickly set to solid to form an imprint in the mouth

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4
Q

things to consider patient perspective

A
  • neutral taste and odour
  • short setting time
  • small tray
  • easy to remove
  • no retakes
  • non toxic
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5
Q

things to consider dentist

A
  • easily mixed
  • short working times
  • good quality impressions
  • low cost
  • easily disinfected
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6
Q

what makes a good impression

A

1) accurate reproduction of surface detail
- viscosity
- wettability
2) Dimensional accuracy and stability
- shrinkage on setting
- cooling contraction
- permanent set
- storage stability
- type of tray

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7
Q

factors involved in choosing application and properties

A

impression material
impression trays (metal/plastic)
areas of application
(full denture, partial dentures, fixed prosthodontics)

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8
Q

what can be used for full dentures

A

plaster of paris
ZnO- eug compo
Zno- eug

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9
Q

partial denture mtierals

A

alginate elastomers

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10
Q

fixed prosthodontics impressiojmaterials

A

comp/wax elastomers

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11
Q

types of impression materials

A

rigid

flexibale

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12
Q

rigid materials

A
  • waxes
  • impression composition
  • Zinc oxide/Eugenol
  • Plaster of Paris
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13
Q

flexible materials

A
  • Hydrocolloids (agar and alginate)

- Elastomers (polysulphide, polyether, silicone polymers)

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14
Q

oligonmer

A

long chain molecule with simple structure

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15
Q

polymer

A

monomer repitiion

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16
Q

sources of wax

A

mineral (paraffin wax)
animal (beeswax)
vegetable (carnauba and candelilla wax)

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17
Q

properties of dental waxes

A

1) softening temperature
- just above mouth temperature
2) high coefficient of thermal expansion
- cooling contraction
3) Rigid
4) Poor thermal conductor
- surface cools faster than inside
5) Low viscosity

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18
Q

what affects dimensional accuracy

A 
Shrinkage
-	bigger model 
-	slightly bigger is good as there is more space for the cement
Expansion
-	smaller model is bad
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19
Q

thermal expansion coefficient

A
  • the change in length, when determined per unit length, for a 1 degree change in temperature
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20
Q

dental waxes thermal expansion coefficient is

A

high TEC
expand a lot
not good as impression materials

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21
Q

use of dental waxes

A

1) Modelling waxes
2) Inlay waxes

3) Sheet casting wax
4) Sticky wax

5) Carding and boxing wax

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22
Q

modelling waxes

A

registration blocks and try in dentures

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23
Q

inlay waxes

A

inlay patterns in the mout

24
Q

sheet casting wax

A

construction of partial dentyres

25
Q

sticky wax

A

joining metal parts prior to soldering

joining dentures prior to repair

26
Q

carding and boxing wax

A

boxing in impressions

mounting teeth in sets

27
Q

composition complex

A

1) Natural or synthetic resins
- Shellac, dammar, Colophony, sandarac
2) Plasticisers
- Stearic acid or gutta percha (avoid brittleness)
3) Fillers
- Talc, calcium carbonate or limestones (helps avoid tackiness)

28
Q

properties of impression composition

A

1) Softening temperature 55-60 degreesC
2) Rigid
3) High co efficient of thermal expansion
4) High viscosity
- mucocompressive
5) Poor thermal conductivity
- subject to stress relief

29
Q

applications of compound

A

1) full or partial impressions
2) sticks for copper band impressions
3) for, adding to the periphery of an impression tray

30
Q

mucocompression

A
  • material is viscous and is able to compress the oral tissues on insertion in the patients mouth
31
Q

mucostatic

A

Impression material is fluid enough to flow and does not displace oral tissues

  • impression plaster
  • Agar impression material
  • ZnEug impression pastes
  • Light body elastomers
32
Q

mucocompression materials

A

Impression compound
Viscous alginate
Some rigid elastomers

33
Q

Rheology

A

study of flow materials

34
Q

how is liquid flow measured

A

viscosity s

35
Q

how to calculate viscosity

A

sheer stress/shear rate

unit Pa.s

36
Q

types of viscous behabiour

A

newtonian
dilatant
pseudoplastic

37
Q

newtonian

A

constant viscosity across all shear rates

38
Q

dilatent

A
  • shear thickening fluids increase in apparent viscosity at higher shear rates (i.e. apply a force and it gets harder)
  • rarely encountered
39
Q

pseudoplastic

A
  • shear thinning fluids have a lower apparent viscosity at higher shear rates eg silicone impression materials (apply force and it gets easier)
40
Q

what is thixotropic behavior

A

time dependant shear thinning property

41
Q

thixotropic behaviour meaning

A

Progressive decrease in viscosity with time for a constant applied shear stress, followed by a gradual recovery when the stress is removed

  • fluids that are viscous under static conditions will flow over time when they are stressed
  • take a fixed time to return to a more viscous state
42
Q

what pastes are found in ZnO Eu and what makes them up

A 
base paste 
- zinc oxide
inert ols
hydrogenated resins
reactor paste
- eugenol
zinc acetate
fillers
43
Q

advantages of ZnO

A

dimensional stability
good surface detail
stable on storage and good shelf lige

44
Q

disadvantages of ZnO

A

cannot be used in very deep undercuts
only sets quick in thin sections
eugenol allergy in some patients

45
Q

properties of ZnO eug

A

1) Mucostatic
2) hydrophilic
- accurate reproduction of surface details
3) dimensionally stable
4) causes burning sensation of lips
5) tendacy to stick to skin
6) stable on storagage/shelf life

46
Q

wetabiltiy

A

ability of a liquid to cover the surface of the substrate completely

47
Q

smaller contact angle (wetaqbility)

A

more hydrophilic

48
Q

why do we want good wetting

A
  • fewer voids
  • less entrapment of oral fluids
  • bubble free dyes and models
  • fewer retakes
49
Q

surface tension what is higher

A

solids have greater than liquids

50
Q

plaster of Paris (gypsum) composition

A

crystalline material of hydrates calcium sulphase

51
Q

gypsum

A
  • form as the partly dehydrated form of plaster of paris
52
Q

composition of impression plaster

A
  • calcium sulphate B- hemihydrate
  • borax (slow setting rate)
  • potassium sulphate (reduces expansion/accelerates setting)
  • starch (aid disintegration of impression on separation from model)
  • liquid (water)
53
Q

what happens upon setting of plaster

A

expands

- results in stress

54
Q

How to prevent expansion of plaster

A

mix with anti expansion solution (potassium sulphate 4% an borax 0.4%)
borax is a retarder
potassium sulphate accelerates

55
Q

advantages of impression plaster

A
  • easy to mix
  • low viscosity (mucostatic)
  • good dimensional stability and accuracy
  • cheap
    Disadvantages
56
Q

disadvantages

A
  • low strength
  • rough surface finish
  • rigid once set
  • dry sensation in mouth
57
Q

applications for impression plaster

A
  • useful for making impressions of patients with excessively mobile soft tissue
  • commonly used as the model material set in impression moulds

GDAN yr 2 - AGEING (6 decks)

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