Impression Materials Flashcards

1
Q

what is the function of an impression material?

A

to produce an accurate replica of the surface and shape of hard and soft oral tissues

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2
Q

what kind of reproduction does an impression give of the soft tissue?

A

negative

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3
Q

what is dental stone used for?

A

to fill the impression and produce a positive replica (stone cast) which represents the position, shape, size and orientation of each tooth

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4
Q

what are casts used for?

A

evaluating dentition when orthodontic, occlusal or other problems involved, in laboratory fabrication of restorations and prostheses

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5
Q

what are the classifications of impression materials?

A

mucostatic, mucocompressive and elastic/non-elastic

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6
Q

what are mucostatic materials?

A

fluid materials that displaces the soft tissues slightly and give impression of the undisplaced mucosa

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7
Q

what are mucocompressive materials?

A

viscous materials that record an impression of the mucosa under load i.e. give impression of displaced soft tissues

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8
Q

what is the ideal elastic behaviour for an impression material?

A

once the strain is taken off of the material i.e. removing impression material from the tooth, it should go back to the way it was when it covered the tooth making an exact impression

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9
Q

what term is used for the elastic behaviour of impression materials?

A

viscoelastic

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10
Q

what does the term viscoelastic behaviour mean?

A

when the impression material is removed from the tooth it never fully recovers and so it has permanent strain

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11
Q

what helps to decrease permanent strain?

A

if the load time with the impression material on the tooth is less

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12
Q

name a hydrocolloid

A

alginate

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13
Q

name 3 elastomers

A

polysulphides, polyethers, silicones

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14
Q

how do you prepare impression materials

A

mixing 2 pastes either a paste and liquid or powder and liquid which produces a chemical reaction

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15
Q

what are the ways in which an impression material sets?

A

chemical, polymerisation, cooling

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16
Q

what are the ideal properties for accuracy for an impression material?

A

surface detail recorded, no dimensional change, no effect on oral tissues, no change dimensionally, complete elastic recovery, does not change dimensionally over time, dimensions and surfaces unaltered, compatible with cast material

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17
Q

what type of factors affect accuracy?

A

viscosity, setting mechanism, thermal expansion coefficient, hydrophobic, elasticity, tear strength

18
Q

what should the thermal expansion coefficient be?

19
Q

what does viscosity determine?

A

the ability to flow over the surface and so the level of detail recorded

20
Q

what are the ideal properties for patient comfort?

A

non-toxic, non-irritant, acceptable taste and smell, short setting time, removable without damage to oral tissues

21
Q

what are the ideal properties for operator convenience?

A

quick, simple technique, convenient working/setting times, must be able to be decontaminated, cost is a consideration

22
Q

what are the 2 main types of elastic impression materials?

A

hydrocolloids and elastomers

23
Q

what are the 2 types of non-elastic impression materials?

A

impression compound, impression paste

24
Q

what is a hydrocolloid?

A

a colloid is a two phase system of fine particles of one phase dispersed in another phase - the dispersing medium is water

25
what are the two states of hydrocolloid?
sol and gel
26
what is the sol state of hydrocolloid?
viscous liquid state
27
what is the gel state of hydrocolloid?
jelly like consistency, agglomeration of particles, entangled framework of solid particles enclosing liquid by capillary forces
28
in alginate, what does trisodium phosphate do?
delays gel formation so varying amount of this varies the clinicians working time
29
what is the basic setting reaction of alginate?
sodium alginate to calcium alginate
30
what occurs in the setting reaction of alginate?
sodium molecules are swapped over for divalent calcium ions which allow crosslinking to increase viscosity and strength
31
how does trisodium phosphate work in the setting reaction of alginate?
trisodium preferentially reacts with Ca ions in CaSO4 and then sodium alginate reacts with calcium ions
32
how can you manipulate alginate?
use correct powder/liquid ratio, use water at 18-24 degrees, use perforated tray and adhesive, remove impression with a sharp pull
33
what does increased temperature do to alginate setting?
speeds up setting
34
how do you get greater elastic properties with alginate?
wait further minute or two before removing from mouth
35
how do you reduce permanent distortion and tearing?
using large bulk of materials
36
how do you store alginate?
avoid moisture, prepare cast ASAP to avoid dimensional change
37
what is syneresis?
release of water
38
what is imbibition
uptake of water
39
what is the tear strength of alginate?
poor so avoid deep undercuts
40
what does movement of alginate cause?
internal stresses and distortion