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BDS2 - DMS > 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 replica of the hard and soft tissues

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3
Q

What is another name for dental stone?

A

Gypsum

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4
Q

Dental stone is used to fill an impression and produce a positive replica. What does this represent?

A

The patients mouth:

  • represents the position, shape, size and orientation of each tooth
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5
Q

What is a positive replica used for? (2 points)

A
  • Used for evaluating dentition when orthodontic, occlusal or other problems are involved
  • In laboratory fabrication of restorations and prostheses
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6
Q

What can you take an impression of? (3 points)

A
  • Single tooth
  • Whole dentition
  • Edentulous mouth
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7
Q

Treatment outcome hinges on the quality and accuracy of the initial impression. What does this require? (2 points)

A
  • IM with the right properties

- Clinician with a good technique

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8
Q

What can IM be classified into in relation to clinical properties? (2 points)

A
  • Mucostatic

- Mucocompressive

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9
Q

What can IM be classified into in relation to properties? (2 points)

A
  • Elastic

- Non-elastic

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10
Q

What is meant by the term ‘mucostatic’?

A
  • Fluid materials that displace the soft tissues slightly (i.e. give an impression of the undisplaced mucosa)
  • Displace the soft tissues slightly
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11
Q

What are 2 examples of mucostatic IM’s?

A
  • Zinc oxide eugenol

- Low viscosity alginates

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12
Q

What are 2 examples of mucocompressive IM’s?

A
  • Impression compound

- High viscosity alginates/elastonomers

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13
Q

What is a mucocompressive IM?

A
  • A viscous material that records an impression of the mucosa under load (i.e. give impression of displaced soft tissue)
  • Displaces the soft tissue much more
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14
Q

What is the ideal elastic behaviour of an IM?

A

Once the impression material has undergone strain it should snap back to its original shape after the strain has been removed

  • This is what should happen if the material is truly elastic
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15
Q

None of the IM’s are truly elastic so they are not a true representation of the patients dentition. What are two way we can minimise the permanent strain?

A
  • By waiting for a time after removing the tray before the cast is poured
  • If the impression is removed with a sharp pull, there is less overall permanent strain (lower deformation)
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16
Q

What are 2 examples of hydrocolloids?

A
  • Agar (reversible)

- Alginate (irreversible)

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17
Q

What are 3 examples of elastomers?

A
  • Polysulphides
  • Polyethers
  • Silicones (conventional & addition cured)
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18
Q

Are impression materials perfectly elastic?

A
  • No
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19
Q

What are the 2 possible outcomes for impression materials that are non-elastic?

A
  • Deformed

- Fractured

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20
Q

What is the correct process for ‘preparation’ of impression materials? (4 points)

A
  • Mixing (2 pastes; paste & liquid, powder & liquid etc)
  • Chemical reaction
  • Heat
  • Material flows & takes up shape/space available
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21
Q

Do some impression materials require a lining?

A
  • Yes - clinical teacher will help with this
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22
Q

What are the possible setting reactions of IM’s? (3 points)

A
  • Chemical
  • Polymerisation
  • Cooling

(forms solid replica)

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23
Q

What is included in the impression taking process?

A
  • Preparation
  • Insertion (to mouth )
  • Setting
  • Removal
  • Storage
  • Cast impression
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24
Q

One ideal property of an IM is accuracy. How does the flow of an IM affect this?

A
  • Surface detail is recorded

- Needs to flow over the dentition so it can make intimate contact with the surface to record surface detail

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25
Q

One ideal property of an IM is accuracy. How does setting changes of an IM affect this?

A
  • Want NO dimensional change

- Don’t want it to shrink or expand

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26
Q

One ideal property of an IM is accuracy. What are the ideal properties in relation to removal of the IM from the mouth? (3 points)

A

Want:

  • NO effect on oral tissues
  • NO change dimensionally (ie. no thermal contraction)
  • Complete ELASTIC RECOVERY
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27
Q

One ideal property of an IM is accuracy. What are the ideal properties in relation to storage of the IM? (1 points)

A

Does not change dimensionally over time

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28
Q

One ideal property of an IM is accuracy. What are the ideal properties in relation to decontamination of the IM? (1 points)

A

Dimensions and surfaces unaltered

29
Q

Does an impression material have to be compatible with the cast material?

A
  • Yes
30
Q

One ideal property of an IM is accuracy. What are the ideal properties in relation to viscosity of the IM?

A

Determines the ability to flow over surface & so level of detail required

31
Q

One ideal property of an IM is accuracy. What are the ideal properties in relation to the setting mechanism of the IM?

A

Does physical or chemical processes cause dimensional contraction?

32
Q

One ideal property of an IM is accuracy. What are the ideal properties in relation to the thermal expansion coefficient of the IM?

A

Ideally low (to cope with mouth to room temperature transition)

33
Q

One ideal property of an IM is accuracy. What are the ideal properties in relation to hydrophilic/hydrophobic properties of the IM?

A
  • Affects surface contact
  • Do you need to remove saliva or will the impression material still make good contact with the tooth surface where there are remnants of saliva
34
Q

One ideal property of an IM is accuracy. What are the ideal properties in relation to the elasticity of the IM?

A

elastic/viscoelastic behaviour

35
Q

One ideal property of an IM is accuracy. What are the ideal properties in relation to the tear strength of the IM?

A
  • Needs to withstand tearing when removed esp. from undercuts
  • when pour IM into tray in the mouth if there are undercuts then the IM will flow into these regions - when remove impression tray they will be vulnerable to tearing
36
Q

What are the ideal properties for an IM for patient comfort? (4 points)

A
  • Non-toxic, non-irritant
  • Acceptable taste and smell
  • Short setting time
  • Removable without damage to oral tissues
37
Q

What are the ideal properties of an IM for operator convenience? (4 points)

A
  • Quick, simple technique
  • Convenient working/setting times
  • Must be able to be decontaminated
  • Cost (inexpensive) is a consideration BUT not a property
38
Q

What are 2 examples of non-elastic IM’s?

A
  • Impression comound

- Impression paste

39
Q

What are 2 examples of elastic impression materials?

A
  • Hydrocolloids

- Elastomers

40
Q

What is a hydrocolloid? (2 points)

A
  • A COLLOID is a two phase system of fine particles of one phase dispersed in another phase
  • If the dispersing medium (liquid) is water then this will produce a HYDROCOLLOID
41
Q

What are the 2 possible states of a hydrocolloid?

A
  • SOL = viscous liquid state

- Gel = jelly like consistency (when sol undergoes reaction it becomes a gel)

42
Q

What is an example of an irreversible hydrocolloid?

A

Alginate

43
Q

What is the composition of alginate?

A
  • Salt of alginic acid
  • Calcium sulphate
  • Trisodium phosphate
  • Filler
  • Modifiers (flavourings, chemical indicators)
44
Q

What is the % of salt of alginic acid in alginate?

A

12%

45
Q

What is the % of Calcium Sulphate in alginate?

A

12%

46
Q

What is the % of salt of trisodium phosphate in alginate?

A

2%

47
Q

What is the % of salt of filler in alginate?

A

70%

48
Q

What is the % of salt of modifiers in alginate?

A

small amount

49
Q

What is the function of salt of alginic acid in alginate?

A

Reacts with Ca ions (that have reacted with trisodium phosphate)

50
Q

What is the function of calcium sulphate in alginate?

A

Provides calcium ions

51
Q

What is the function of trisodium phosphate in alginate?

A

Delayes gel formation

52
Q

What is the function of filler in alginate?

A

Cohesion, strength

53
Q

What is the function of modifiers in alginate?

A

improves surface, taste, pH colour change

54
Q

What is the setting reaction of alginate?

A
  • 2NanAlg +nCaSO4 -> nNa2SO4 + CanAlg

- Cross-linked fibril entangling undissolved particles

55
Q

Why, even when the material seems set, should you give it an extra few minutes before you remove the IM from the mouth?

A
  • The cross-linking is still going on so don’t remove immediately
56
Q

What temperature of water should be used with alginate?

A

Between 18-24 degrees Celsius

57
Q

What does increased temperature of water do to alginate?

A
  • Speeds up the setting

- Sets nearest tissue first

58
Q

What is permanent distortion and tearing of alginate reduced by?

A

Reduced by using a large bulk of material - typically 5mm

59
Q

What are properties of alginate that are ideal for patient comfort? (3 points)

A
  • Non-toxic, non-irritant
  • Acceptable taste, smell
  • Setting time OK
60
Q

What are properties of alginate that make it convenient for the operator?

A
  • Relatively easy to use
  • Setting time OK
  • Storage (avoid moisture, prepare cast ASAP to avoid dimensional change)
61
Q

What does syneresis mean?

A

Release of water

62
Q

What does imbibition mean?

A
  • Uptake of water
63
Q

How is the flow of alginate?

A
  • fine detail - OK
64
Q

How are the setting changes of alginate?

A

OK

65
Q

How is the accuracy of alginate if the patient moves while it is setting?

A
  • This causes internal stresses and distortion
66
Q

How are the elastic properties of alginate?

A

Nearly elastic - not 100% elasic recovery

67
Q

How is the tear strength of alginate?

A
  • Poor (avoid deep undercuts)
68
Q

How is the storage property of alginate?

A
  • Syneresis or imbibition may affect dimensions and accuracy

BDS2 - DMS (14 decks)

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