Impression Materials

Question 1

What is the function of IMs

Answer 1

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

Question 2

How can IMs be classified

Answer 2

Clinical - mucostatic or mucocompressive

Properties - elastic or non-elastic

Question 3

What is a mucostatic material and give examples

Answer 3

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

Question 4

What is a mucocompressive material and give examples

Answer 4

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

Question 5

Describe the shape of IMs are they are removed from teeth

Answer 5

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

Question 6

What describes the elastic behaviour of IMs

Answer 6

Viscoelastic - somewhere between elastic and non-elastic

Question 7

What happens when a load is applied to a viscoelastic material

Answer 7

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

Question 8

How can clinical technique minimise permanent strain

Answer 8

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

Question 9

What are the different types of elastic IMs

Answer 9

Hydrocolloids - agar and alginate

Elastomers - polysulphides, polyethers and silicones

Question 10

What differentiates an elastic from a non-elastic material

Answer 10

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

Question 11

What are the stages of the impression process

Answer 11

Preparation
Insertion
Setting
Removal
Storage
Cast preparation

Question 12

What happens in the preparation of impressions

Answer 12

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

Question 13

How are pastes mixed for impressions and why

Answer 13

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

Question 14

What should be considered for insertion of the IM

Answer 14

The type of tray needed

Whether an adhesive is required

Question 15

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

Answer 15

Flow
Setting changes
Removal
Storage
Decontamination
Compatible with cast material
Viscosity
Setting mechanism
Thermal expansion coefficient
Hydrophobic/hydrophilic
Elasticity
Tear strength

Question 16

What are ideal properties of an IM considering patient comfort

Answer 16

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

Question 17

What are ideal properties of an IM considering operator convenience

Answer 17

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

Question 18

What is a colloid

Answer 18

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

Question 19

What is a hydrocolloid

Answer 19

A colloid where the dispersing medium is water

Consists of fine particles suspended in water

Question 20

What states can hydrocolloids exist in

Answer 20

Sol - a viscous liquid state

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

Question 21

What are the components of alginate

Answer 21

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

Question 22

Describe the composition and role of Na alginate in alginate

Answer 22

12%

Reacts with Ca ions released by calcium sulphate

Question 23

Describe the composition and role of calcium sulphate in alginate

Answer 23

12%

Provides Ca ions for Na alginate to react with

Question 24

Describe the composition and role of trisodium phosphate in alginate

Answer 24

2 %

Delays gel formation

Question 25

Describe the composition and role of filler in sodium alginate

Answer 25

70% | Provides cohesion and strength to the alginate

Question 26

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

Answer 26

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

Question 27

Describe the alginate setting reaction

Answer 27

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

Question 28

Describe why alginate delay occurs in the alginate setting reaction

Answer 28

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

Question 29

What are the key points when using alginate

Answer 29

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

Question 30

How does temperature of the mouth affect the setting of alginate

Answer 30

Sets at higher temperature quicker - sets nearest tissue first

Question 31

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

Answer 31

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

Question 32

Why is a large bulk of alginate used

Answer 32

This minimises the risk of permanent distortion and tearing

Question 33

Why should a gypsum cast be prepared ASAP

Answer 33

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