Impression Materials

Question 1

What is the function of impression materials?

Answer 1

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

Question 2

What is an impression?

Answer 2

A negative reproduction of tissues (hard and soft)

Question 3

How can impression materials be classified clinically?

Answer 3

• mucostatic

- mucocompressive

Question 4

What does mucostatic mean?

Answer 4

-fluid materials that displace the soft tissues slightly i.e. give an impression of the undisplaced mucosa

Question 5

What does mucocompressive mean?

Answer 5

-viscous materials that record an impression of the mucosa under load i.e. give impression of displaced soft tissue

Question 6

What are some examples of mucostatic materials?

Answer 6

• zinc oxide eugenol

- low viscosity alginates

Question 7

What are some examples of mucocompressives ?

Answer 7

• impression compound

- high viscosity alginates/elastomers

Question 8

How can impression materials be classes based on their properties?

Answer 8

• elastic

- non-elastic

Question 9

During removal what does the impression material undergo?

Answer 9

Elastic strain as it is coming over the most bulbous part of the tooth

Question 10

After removal, what does the impression material have to undergo?

Answer 10

Elastic recovery

Question 11

What are the 2 categories of ‘elastic’ impression materials? (not perfectly elastic)

Answer 11

• hydrocolloids

- elastomers

Question 12

What are some examples of hydrocolloids?

Answer 12

• agar (no longer used)

- alginate

Question 13

What are some examples of elastomers?

Answer 13

-polyethers -silicones (conventional and addition cured)

Question 14

What would happen to materials that are non-elastic when taking an impression?

Answer 14

Either be deformed or become fractured

Question 15

Summarise the setting reaction for IM materials?

Answer 15

• chemical reaction
• polymerisation
• cooling
• forms solid replica

Question 16

After the impression material has set, what happens next?

Answer 16

• tray removed from mouth
• disinfected
• stored
• cast prepared

Question 17

What needs to be considered when storing the impression?

Answer 17

• how long can it be stored before dimensional changes become significant
• should you wait to allow elastic recovery?

Question 18

Why do you need flow of an impression material?

Answer 18

so the surface detail can be recorded

Question 19

What are the ideal properties for setting changes in an impression material?

Answer 19

NO dimensional change (dont want it to shrink or expand when removed)

Question 20

What are the ideal properties of an impression material on removal?

Answer 20

• NO effect on oral tissues (irritate them or pull off etc)
• NO change dimensionally (ie no thermal contraction)
• complete elastic recovery

Question 21

What are the ideal properties for decontamination of an impression material?

Answer 21

Dimensions and surfaces unaltered by decontamination

Question 22

What properties afect the accuracy of an impression material?

Answer 22

• viscosity
• setting mechanism (does it cause contraction)
• thermal expansion coefficient
• hydrophobi/hydrophilic
• elasticity
• tear strength

Question 23

How does viscosity affect accuracy?

Answer 23

-determines the ability of the material to flow over surface and so level of detail recorded

Question 24

How does whether a material is hydrophobic or hydrophilic affect accuracy?

Answer 24

Can affect the surface contact Might need to remove saliva for good impression

Question 25

How does tear strength affect accuracy?

Answer 25

-if the material isnt strong enough to withstand tearing when removed will have to take another impression

Question 26

What aspect in the mouth can increase the likelihood of tearing?

Answer 26

undercuts

Question 27

What are the ideal properties of impression materials for patient comfort?

Answer 27

-non-toxic, non-irritant -acceptable taste and smell -short setting time -removable wihtout damage to oral tissues

Question 28

What are some non-elastic impression materials?

Answer 28

Impression compound

Question 29

What are some elastic impression materials?

Answer 29

• hydrocolloids

- elastomers

Question 30

What two states do hydrocolloids exist in?

Answer 30

SOL state

GEL state

Question 31

Describe the SOL state of hydrocolloids.

Answer 31

Viscous liquid state

Question 32

Describe the GEL state of hydrocolloids

Answer 32

-jelly-like consistenct (when the material undergoes reaction becomes firmer) -agglomeration of particles -entangles framework of solid particles enclsoing liquid by capillary forces

Question 33

What are the components of alginate?

Answer 33

• salt of alginic acid (e.g. Na alginate)
• calcium sulphate
• trisodium phosphate
• filler
• modifiers, flavourings and chemical indicators

Question 34

Function of the salt of alginic acid?

Answer 34

react with Ca ions provided by calcium sulpate

Question 35

What happens before the salt of alginic acid reacts with the calcium ions?

Answer 35

The calcium ions react with the trisodium phosphate

Question 36

What is the function of the trisodium phosphate?

Answer 36

To delay the gel formation

Question 37

What is the function of the filler?

Answer 37

• cohesion

- strength

Question 38

Setting reaction for alginate?

Answer 38

2NanAlg + nCaSo4 —> nNa2SO4 + CanAlg