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BDS2 CLINICAL Dental Materials > Impression Materials > Flashcards

Impression Materials Flashcards

1
Q

What is the function of impression materials

A

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

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2
Q

What is an impression

A

a negative reproduction of tissues

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3
Q

What is dental stone used to do

A

fill impressions to 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 dental stone 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 does the treatment properties hinge on

A

the quality and accuracy of the initial impression

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6
Q

What does the impression material require

A

Impression materials with the right properties

clinician with good technique

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7
Q

What are the classifications of impression materials based on

A

Clinical

Properties

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8
Q

What are the clinical classification

A

mucostatic

mucocompressive

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9
Q

What are the properties

A

elastic

non elastic

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10
Q

What are mucostatic impression materials

A

zinc oxide eugenol, low viscosity alginates

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11
Q

What do mucostatic impression materials do

A

fluid materials that displace the soft tissues slightly

give an impression of the undisplased mucosa

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12
Q

What are mucocompressive impression materials

A

impression compound
high viscosity alginates
elastomers

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13
Q

What do mucocompressive materials do

A

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

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14
Q

What happens to the impression material during removal

A

it undergoes elastic strain and has to change shape to overcome the bulbosity of the tooth

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15
Q

What do we want the impression material to do after removal

A

change back to its original shape making it truly elastic

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16
Q

What is it called when the impression material never returns to its original shape

A

permanent strain
it does not recover
making the impression inaccurate

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17
Q

What should be done to try and decrease the permanent deformation

A

keep the load time less by removing with a sharp pull so there is less overall permanent strain

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18
Q

What are the two types of elastic materials

A

hydrocolloids

elastomers

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19
Q

What are the elastomers

A

polysulphides
polyethers
silicones

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20
Q

What are the hydrocolloids

A

agar (reversible (lab duplicating) but no longer used

alginate - irreversible

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21
Q

What are the elastomers

A

polysulphides
polyethers
silicones (conventional and addition cured)

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22
Q

What happens to non elastic material on removal

A

don’t recover well

either deform/fracture

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23
Q

What is the impression process

A 
preparation
insertion
setting 
removal
storage 
cast prep
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24
Q

What happens in preparation

A

mixing
chemical reaction
heat
to result in a material that slows and takes up shape/space available

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25
Q

What happens in insertion

A

pick a tray

need adhesive maybe

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26
Q

What happens in setting

A

chemical
polymerisation
cooling (only some)
to form a solid replica

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27
Q

What happens in storage

A

need to see how quickly the impression needs to be poured before the dimensional changes become significant
but should wait a bit to allow elastic recovery

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28
Q

What are the ideal properties needed for

A

accuracy
patient
operator

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29
Q

What are the properties for accuracy

A 
flow
setting changes
removal
storage
decontamination 
compatible with cast material
viscosity 
setting mechanism
thermal expansion coefficient 
hydrophobic/hydrophilic
elasticity 
tear strength
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30
Q

What is the ideal property for flow

A

that it can flow over the dentition and make intimate contact with the surfaces and record surface detail

31
Q

What is the ideal property regarding setting changes

A

we don’t want it to shrink nor expand, we want it to remain the same dimension

32
Q

What is the ideal property for removal

A

no effect on oral tissues
no change dimensionally
complete elastic recovery

33
Q

What are the ideal properties for storage

A

does not change dimensionally over time (moisture) - a weak point for alginates

34
Q

What are the ideal properties for decontamination

A

dimensions and surfaces unaltered

35
Q

Why is viscosity important

A

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

36
Q

What is the ideal property for setting mechanism

A

does physical/chemical process = dimensional contraction

37
Q

What is the ideal property for thermal expansion coefficient

A

ideally low

to cope with mouth to room temperature transition

38
Q

What is the ideal property for hydrophobic/hydrophilic

A

affects surface contact because saliva will be present on the tooth

39
Q

What is the ideal property for elasticity

A

elastic/viscoleastic behavior

for undercuts so that it won’t fracture

40
Q

What is the ideal property of tear strength

A

withstand tearing when removed especially from undercuts

41
Q

What are the ideal properties for patient comfort

A

non-toxic
acceptable taste and smell
short setting time
removable with damage to oral tissues

42
Q

What are the ideal properties for operator convenience

A

quick, simple technique
convenient working/setting times
must be able to be decontaminated
cost (inexpensive) is a consideration BUT not a property

43
Q

What is a colloid

A

two phase system of fine particles (1-200nm) of one phase dispersed in another phase

44
Q

What is a hydrocolloid

A

colloid dispersed in water

45
Q

What are the two states of a hydrocolloid

A

sol

gel

46
Q

What is the sol phase

A

viscous liquid state

47
Q

What is the gel phase

A

jelly like consistency

48
Q

What is the gel phase due to

A

agglomeration of particles

entangled framework of solid particles enclosing liquid by capillary forces

49
Q

What is the composition of alginate

A 
salt of alginic acid
calcium sulphate
trisodium phosphate
filler
modifiers 
flavourings
chemical indicators
50
Q

What is the function of the salt of alginic acid

A

it reacts with calcium ions

51
Q

What is the function of calcium sulphate

A

provides calcium ions

52
Q

What is the function of trisodium phosphate

A

delays gel formation

53
Q

What is the function of filler

A

for cohesion and strength

54
Q

What is the function of modifiers, flavourings and chemical indicators

A

improve the surface
taste
pH color change (to show transition from sol to gel)

55
Q

How does trisodium phosphate delay the setting reaction

A

calcium ions interact with the phosphate ions and it is only when the phosphate ions are used up that the remaining calcium ions react with sodium alginate

56
Q

What happens in the setting reaction

A

there is long cross linked fibril entangling undissolved particles

57
Q

What is the setting reaction

A

2NanAlg + nCaSO4 –> nNa2SO4 + CanALg

58
Q

How does calcium ions result in cross linking

A

because they’re divalent

59
Q

What is the alginate delay

A

trisodium phosphate preferentially reacts with Ca ions in CaSO4 then 2NanAlg reacts with Ca ions

60
Q

How can alginate be manipulated

A 
use correct powder/liquid ratio 
use water at 18-24 degrees
use perforated tray and adhesive
remove impression with a sharp pull 
use in bulk
61
Q

Why does alginate have greater elastic properties if a minute or two is waited

A

cross linking continues after apparent set

62
Q

Why does alginate set quicker on soft tissues

A

because an increased temp speeds up setting

63
Q

Why should a bigger bulk of alginate be used (5mm)

A

as permanent distortion and tearing is slightly reduced

64
Q

What features of alginate allow for patients comfort

A

non toxic, non irritant
acceptable taste and smell
setting time is ok

65
Q

What features of alginate allow for operator convenience

A

easy to use
okay setting time
storage - need to avoid moisture but should be left with damp gauze. prepare cast ASAP

66
Q

what is syneresis

A

release of water

67
Q

what is imbibition

A

uptake of water

68
Q

What is the issue with storage of alginate

A

it can release water but also take on water

could distort it

69
Q

How is the flow for alginate

A

okay

70
Q

How is the setting changes for alginate

A

ok

71
Q

What happens if the patient is moving during setting

A

can result in internal stresses that lead to distortion

72
Q

How is the elastic property of alginate

A

not 100% elastic recovery

73
Q

How is the tear strength of alginate

A

poor
avoid deep undercuts
use in bulk

74
Q

How is the storage of alginate

A

syneresis or imbibition may effect dimensions/accuracy

BDS2 CLINICAL Dental Materials (15 decks)

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