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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
Impression setting process (2)
Chemical, polymerisation, cooling | Forms solid replica
26
Ideal properties of an impression material (3)
Accuracy Patient factors Operator factors
27
Features of impression material accuracy (6)
``` Flow Setting changes Removal Storage Decontamination Compatible with cast material ```
28
Effect of accurate flow
Surface detail recorded
29
Effect of accurate setting changes
No dimensional change
30
Effect of accurate removal (3)
No effect on oral tissues No change dimensionally (no thermal contraction, etc.) Complete elastic recovery
31
Effect of accurate storage
Does not change dimensionally over time (moisture control)
32
Effect of accurate decontamination
Dimensions and surfaces unaltered
33
Properties that affect accuracy (6)
``` Viscosity Setting mechanism Thermal expansion coefficient Hydrophilic/hydrophobic Elasticity Tear strength ```
34
Effect of viscosity on accuracy
Determines ability to flow over surface and so therefore, the level of detail recorded
35
Effect of eating mechanism on accuracy
Some physical or chemical processes may cause dimensional contraction
36
Ideal thermal expansion coefficient of impression materials
Ideally low (to cope with mouth to room temperature transition)
37
Effect of hydrophilic/hydrophobic on accuracy
Affects surface contact
38
Effect of elasticity on accuracy
Elastic/viscoelastic behaviour
39
Effect of tear strength on accuracy
Withstand tearing when remove, especially from undercuts
40
Ideal patient properties (5)
``` Comfortable Non-toxic, non-irritant Acceptable taste and smell Short setting time Removable without damage to oral tissues ```
41
Ideal operator properties (3)
Quick, simple technique Convenient working/setting times Must be able to be decontaminated
42
Definition of colloid
A two-phase system of fine particles (1-200um) of one phase dispersed in another phase (water, etc.)
43
Definition of hydrocolloid
A colloid dispersed in water
44
Hydrocolloids can be in two states (2)
SOL - viscous liquid state | GEL - jelly-like consistency
45
Features of GEL state hydrocolloids (2)
Agglomeration of particles | Entangled framework of solid particles enclosing liquid by capillary forces
46
Definition of alginate
Irreversible hydrocolloid
47
Composition of alginate (5)
``` Salt of alginic acid (Na alginate) CaSO4 Trisodium PO4 Filler Modifiers, flavourings, chemical indicators ```
48
Function of salt of alginic acid (Na alginate) in alginate
Reacts with Ca ions
49
Function of CaSO4 in alginate
Provides Ca ions
50
Function of trisodium PO4 in alginate
Delays gel formation
51
Function of filler in alginate
Cohesion, strength
52
Function of modifiers, flavourings and chemical indicators in alginate
Improve surface, taste, pH colour change
53
Features of alginate setting
Involves long cross-linked fibrils entangling undissolved particles
54
Features of alginate delay
Trisodium phosphate preferentially reacts with calcium ions in calcium sulfate
55
Setting reaction of alginate (2)
2Na3PO4 + 3CaSO4 --> Ca3(PO4) + 3Na2SO4 | Sodium alginate then reacts with Ca ions
56
Components of accurate alginate manipulation (4)
Use correct powder/liquid ratio Use water 18-24C Use perforated tray and adhesive Remove impression with a sharp pull
57
Relationship between temperature and setting time of alginate
Increased temperature speeds up setting (sets nearest tissue first)
58
Importance of removing impression with a sharp pull (2)
Elastic recovery; maximum tear strength | Crosslinking continues after apparent set – greater elastic properties if you wait a further minute or two
59
How is permanent distortion and tearing of alginate reduced
Slightly reduced by using a large bulk of material (typically 0.5mm)
60
Properties of alginate (9)
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
Factors that affect dimensions and/or accuracy (2)
Syneresis | Imbibition
62
Definition of syneresis
Release of water
63
Definition of imbibition
Uptake of water

BDS2 CDS DMS (15 decks)

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