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BDS2 CDS DMS > 10. Impression Materials > Flashcards

10. Impression Materials Flashcards

1
Q

Definition of an impression

A

A negative reproduction of tissues

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2
Q

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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3
Q

Function of dental stone

A

Used to fill the impression and produce a positive replica (stone cast)

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4
Q

Function of dental stone cast (2)

A

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

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5
Q

Impression materials can be used for (3)

A

Single tooth
Whole dentition
Edentulous mouth

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6
Q

Impressions are important, as treatment outcome hinges on (3)

A

Quality and accuracy of the initial impression
Requires a material with the correct properties
Requires a clinician with a good technique

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7
Q

Impression materials can be classified as (2)

A

Clinically

Properties

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8
Q

Clinical classification of impression materials (2)

A

Mucostatic

Mucocompressive

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9
Q

Definition of mucostatic impression material

A

Fluid materials that displace the soft tissues slightly (give an impression of the undisplaced mucosa)

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10
Q

Examples of mucostatic impression materials (2)

A

Zinc oxide eugenol

Low viscosity alginates

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11
Q

Definition of mucocompressive impression material

A

Viscous materials that record an impression of the mucosa under load (give an impression of displaced soft tissues)

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12
Q

Examples of mucocompressive impression materials (2)

A

Impression compound

High viscosity alginates and elastomers

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13
Q

Classification of impression materials based on properties (2)

A

Elastic

Non-elastic

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14
Q

Ideally, elastic strain during removal leads to

A

Elastic recovery after removal

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15
Q

Ideal elastic behaviour (3)

A

Upon removal, the material reaches the maximum amount of strain almost instantly
The maximum strain is ‘held’ during removal
When fully removed, the material instantly returns to the original strain and returns to its pre-removal shape

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16
Q

Actual elastic behaviour (3)

A

Upon and during removal, the material gradually increases to just below the maximum amount of strain
When fully removed, the material quickly (instantly) returns to almost the initial strain
This results in a permanent strain/deformation and a permanent change in dimension

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17
Q

Types of elastic materials (2)

A

Hydrocolloids

Elastomers

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18
Q

Types of hydrocolloids (2)

A

Reversible (agar)

Irreversible (alginate)

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19
Q

Types of elastomers (3)

A

Polysulphides
Polyethers
Silicones (conventional and addition cured)

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20
Q

Types of non-elastic impression materials (2)

A

Impression compound

Impression paste

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21
Q

Actual non-elastic behaviour

A

Upon removal, if undercuts are present, non-elastic impression materials may become deformed (permanent strain) or fracture

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22
Q

Impression process (6)

A 
Preparation
Insertion
Setting
Removal
Storage
Cast preparation
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23
Q

Impression preparation process (3)

A

Mixing (2 pastes/paste and liquid/powder and liquid, etc.) – chemical reaction
Heat
A material that flows and takes up shape is required

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24
Q

Impression insertion process (2)

A

Into standard/custom tray

Need for adhesive

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25
Q

Impression setting process (2)

A

Chemical, polymerisation, cooling

Forms solid replica

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26
Q

Ideal properties of an impression material (3)

A

Accuracy
Patient factors
Operator factors

27
Q

Features of impression material accuracy (6)

A 
Flow
Setting changes
Removal
Storage
Decontamination
Compatible with cast material
28
Q

Effect of accurate flow

A

Surface detail recorded

29
Q

Effect of accurate setting changes

A

No dimensional change

30
Q

Effect of accurate removal (3)

A

No effect on oral tissues
No change dimensionally (no thermal contraction, etc.)
Complete elastic recovery

31
Q

Effect of accurate storage

A

Does not change dimensionally over time (moisture control)

32
Q

Effect of accurate decontamination

A

Dimensions and surfaces unaltered

33
Q

Properties that affect accuracy (6)

A 
Viscosity
Setting mechanism 
Thermal expansion coefficient
Hydrophilic/hydrophobic
Elasticity
Tear strength
34
Q

Effect of viscosity on accuracy

A

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

35
Q

Effect of eating mechanism on accuracy

A

Some physical or chemical processes may cause dimensional contraction

36
Q

Ideal thermal expansion coefficient of impression materials

A

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

37
Q

Effect of hydrophilic/hydrophobic on accuracy

A

Affects surface contact

38
Q

Effect of elasticity on accuracy

A

Elastic/viscoelastic behaviour

39
Q

Effect of tear strength on accuracy

A

Withstand tearing when remove, especially from undercuts

40
Q

Ideal patient properties (5)

A 
Comfortable
Non-toxic, non-irritant
Acceptable taste and smell
Short setting time
Removable without damage to oral tissues
41
Q

Ideal operator properties (3)

A

Quick, simple technique
Convenient working/setting times
Must be able to be decontaminated

42
Q

Definition of colloid

A

A two-phase system of fine particles (1-200um) of one phase dispersed in another phase (water, etc.)

43
Q

Definition of hydrocolloid

A

A colloid dispersed in water

44
Q

Hydrocolloids can be in two states (2)

A

SOL - viscous liquid state

GEL - jelly-like consistency

45
Q

Features of GEL state hydrocolloids (2)

A

Agglomeration of particles

Entangled framework of solid particles enclosing liquid by capillary forces

46
Q

Definition of alginate

A

Irreversible hydrocolloid

47
Q

Composition of alginate (5)

A 
Salt of alginic acid (Na alginate)
CaSO4
Trisodium PO4
Filler
Modifiers, flavourings, chemical indicators
48
Q

Function of salt of alginic acid (Na alginate) in alginate

A

Reacts with Ca ions

49
Q

Function of CaSO4 in alginate

A

Provides Ca ions

50
Q

Function of trisodium PO4 in alginate

A

Delays gel formation

51
Q

Function of filler in alginate

A

Cohesion, strength

52
Q

Function of modifiers, flavourings and chemical indicators in alginate

A

Improve surface, taste, pH colour change

53
Q

Features of alginate setting

A

Involves long cross-linked fibrils entangling undissolved particles

54
Q

Features of alginate delay

A

Trisodium phosphate preferentially reacts with calcium ions in calcium sulfate

55
Q

Setting reaction of alginate (2)

A

2Na3PO4 + 3CaSO4 –> Ca3(PO4) + 3Na2SO4

Sodium alginate then reacts with Ca ions

56
Q

Components of accurate alginate manipulation (4)

A

Use correct powder/liquid ratio
Use water 18-24C
Use perforated tray and adhesive
Remove impression with a sharp pull

57
Q

Relationship between temperature and setting time of alginate

A

Increased temperature speeds up setting (sets nearest tissue first)

58
Q

Importance of removing impression with a sharp pull (2)

A

Elastic recovery; maximum tear strength

Crosslinking continues after apparent set – greater elastic properties if you wait a further minute or two

59
Q

How is permanent distortion and tearing of alginate reduced

A

Slightly reduced by using a large bulk of material (typically 0.5mm)

60
Q

Properties of alginate (9)

A

Non-toxic, non-irritant
Acceptable taste, smell
Adequate setting time
Relatively easy to use
Storage - avoid moisture, prepare cast ASAP to avoid dimensional change
Flow – fine detail - adequate
Setting changes - adequate but movement leads to stresses and distortion
Elastic - nearly elastic but not 100% elastic recovery
Tear strength - poor – avoid deep undercuts

61
Q

Factors that affect dimensions and/or accuracy (2)

A

Syneresis

Imbibition

62
Q

Definition of syneresis

A

Release of water

63
Q

Definition of imbibition

A

Uptake of water

BDS2 CDS DMS (15 decks)

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