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Dental Materials > Impression Materials > Flashcards

Impression Materials Flashcards

1
Q

What is the function of IMs

A

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

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2
Q

How can IMs be classified

A

Clinical - mucostatic or mucocompressive

Properties - elastic or non-elastic

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3
Q

What is a mucostatic material and give examples

A

Fluid material that displaces soft tissues slightly so it gives an impression of the undisplaced mucosa
eg - zinc oxide eugenol and low viscosity alginates

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4
Q

What is a mucocompressive material and give examples

A

Viscous material that records an impression of the mucosa under load - gives impression of displaced soft tissue
eg - high viscosity alginates/elastomers

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5
Q

Describe the shape of IMs are they are removed from teeth

A

The IM experiences strain to overcome the bulbous aspect of the tooth and temporarily changes shape
It then undergoes elastic recovery to revert to very close to the original shape

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6
Q

What describes the elastic behaviour of IMs

A

Viscoelastic - somewhere between elastic and non-elastic

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7
Q

What happens when a load is applied to a viscoelastic material

A

Change in dimension is not immediate and it doesn’t reach its maximum strain level instantly
There is a rapid increase in strain initially, which tapers off
When the load is released, it gradually returns towards its original dimensions but never achieves full elastic recovery

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8
Q

How can clinical technique minimise permanent strain

A

A short, sharp pulling force can help mitigate some of the deformation a viscoelastic material experiences

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9
Q

What are the different types of elastic IMs

A

Hydrocolloids - agar and alginate

Elastomers - polysulphides, polyethers and silicones

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10
Q

What differentiates an elastic from a non-elastic material

A

Elastic material recovers its original shape whereas the non-elastic IM ends up deformed, either giving a poor fit or fracturing

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11
Q

What are the stages of the impression process

A 
Preparation
Insertion
Setting
Removal
Storage
Cast preparation
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12
Q

What happens in the preparation of impressions

A

Material needs to be prepared either by mixing a powder with water (alginate) or mixing two pastes

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13
Q

How are pastes mixed for impressions and why

A

Using a mixing tip to produce a homogenous paste without air bubbles and with a uniform colour

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14
Q

What should be considered for insertion of the IM

A

The type of tray needed

Whether an adhesive is required

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15
Q

What properties may affect the accuracy of the IM (13)

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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16
Q

What are ideal properties of an IM considering patient comfort

A

Non-toxic and non-irritant
Acceptable taste and smell
A short setting time
Removable without damage to the oral tissues

17
Q

What are ideal properties of an IM considering operator convenience

A

Quick, simple technique
Convenient working and setting times
Must be able to be decontaminated
Cost - consideration but not a property

18
Q

What is a colloid

A

A 2 phase system where fine particles (1-200 nm) are dispersed in another phase

19
Q

What is a hydrocolloid

A

A colloid where the dispersing medium is water

Consists of fine particles suspended in water

20
Q

What states can hydrocolloids exist in

A

Sol - a viscous liquid state

Gel - jelly like consistency whereby fine particles form a framework which encapsulate a liquid through capillary forces

21
Q

What are the components of alginate

A 
Salt of alginic acid (Na alginate)
Calcium sulphate
Tri sodium phosphate
Filler
Modifiers, flavourings and chemical indicators
22
Q

Describe the composition and role of Na alginate in alginate

A

12%

Reacts with Ca ions released by calcium sulphate

23
Q

Describe the composition and role of calcium sulphate in alginate

A

12%

Provides Ca ions for Na alginate to react with

24
Q

Describe the composition and role of trisodium phosphate in alginate

A

2 %

Delays gel formation

25
Q

Describe the composition and role of filler in sodium alginate

A

70%

Provides cohesion and strength to the alginate

26
Q

What role do modifiers, flavourings and chemical indicators play in alginate

A

Improve surface quality
Control the taste
Highlight changes in pH via a colour change

27
Q

Describe the alginate setting reaction

A

Sodium alginate reacts with calcium sulphate to form calcium alginate
The calcium alginate formed consists of long cross-linked molecules surrounding undissolved particles

28
Q

Describe why alginate delay occurs in the alginate setting reaction

A

Trisodium phosphate preferentially reacts with the Ca ions, delaying the key reaction of the sodium alginate and Ca ions

29
Q

What are the key points when using alginate

A

Mix the powder-water using the designated ratio
Use water between 18-24*C
Use a perforated tray and adhesive
Remove impression with a sharp pull to maximise elastic recovery

30
Q

How does temperature of the mouth affect the setting of alginate

A

Sets at higher temperature quicker - sets nearest tissue first

31
Q

Why should you wait a few minutes even when alginate is firm to touch

A

Cross-linking of alginate is ongoing and it is best to wait i until this is complete and the alginate has developed all of its elastic properties

32
Q

Why is a large bulk of alginate used

A

This minimises the risk of permanent distortion and tearing

33
Q

Why should a gypsum cast be prepared ASAP

A

During storage, alginate can experience either:
Syneresis - the release of water
Imbibition - the uptake of water
If either of these occurs, the alginate will be distorted and will be of no use

Dental Materials (19 decks)

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