Impression Materials

Question 0

What different situations would result in different ideal properties for an impression material?

Answer 0

Single tooth
Whole dentition
Edentulous mouth

Question 1

What is the function of an impression?

Answer 1

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

Question 2

What are the classifications of impression materials?

Answer 2

Clinical - mucostatic, mucocompressive

Properties - elastic, non elastic

Question 3

What is a mucostatic impression material?

Answer 3

Fluid material that displaces the soft tissue slightly - giving an impression of the undisplaced mucosa.
e.g. zinc oxide eugenol, low viscosity alginates

Question 4

What is a mucocompressive impression material?

Answer 4

Viscous materials that record an impression of the mucosa under load - giving an impression of displaced mucosa.
e.g. impression compound, high viscosity alginates/elastomers

Question 5

What are the elastic impression materials?

Answer 5

Hydrocolloids - alginate (irreversible)

Elastomers - polyethers, silicones (conventional & addition cured)

Question 6

What are the non-elastic impression materials?

Answer 6

Impression compound

Impression paste

Question 7

What is impression compound?

Answer 7

Combination of resins, wax, stearic acid and filler pigments

Question 8

What are the properties of impression compound?

Answer 8

High thermal expansion
Poor fine detail
Plastic deformation during removal
Stability OK
Non toxic, non irritant
Setting time OK
Sterilisation difficult
Long shelf life
Compatible with die materials

Question 9

What is impression paste?

Answer 9

Two pastes 
- ZnO powder + oil
- eugenol + inert filler
                \+ accelerator
                \+ hydrogenised resin
a.k.a. ZOE paste

Question 10

What are the properties of impression paste?

Answer 10

Setting time OK (3-8 mins)
Fine details OK
Setting changes OK
Not elastic
Storage OK
Compatibility with stone OK
Non toxic, non irritant (eugenol may cause allergies)
Adheres to tissues (must coat lips with petroleum jelly)
Shelf life OK
Expensive

Question 11

What are hydrocolloids?

Answer 11

2 phase system of fine particles (1-200nm) of one phase dispersed in another phase
States - SOL (viscous liquid state), GEL (jelly like consistency)
e.g. alginate & agar (not used anymore)

Question 12

What are the properties of alginate?

Answer 12

Flow fine detail ok
Setting changes ok
Nearly elastic (viscoelastic)
Poor tear strength
Non toxic, non irritant
Acceptable taste and smell
Setting time OK
Relatively easy to use
Must be prepared ASAP as it changes shape

Question 13

What is an elastomer?

Answer 13

A polymer with viscoelasticity & very weak intermolecular forces

Question 14

What properties should you assess in an impression material?

Answer 14

Flow/viscosity
Surface detail reproduction
Contact angle/wettability
Elastic recovery (%)
Stiffness
Tear strength
Mixing time (min)
Working time (min)

Question 15

What are examples of elastomers?

Answer 15

Condensation cured silicone
Addition cured silicone
Polysulphide
Polyether

Question 16

What is the ideal viscosity of an impression material?

Answer 16

Must be able to flow readily

Question 17

What is the ideal surface wetting of an impression material?

Answer 17

Must make intimate contact with teeth/mucosa

Small contact angle

Question 18

When should elastomers be used as an impression material?

Answer 18

When accuracy is paramount

e.g. crown and bridge work and implants

Question 19

What are the two types of hydrocolloids?

Answer 19

Reversible e.g. agar

Irreversible e.g. alginate

Question 20

What is impression compound used for?

Answer 20

Recording edentulous ridges

Useful in denture cases with severe bone resorption

Question 21

How are impression materials disinfected?

Answer 21

Removed from mouth
Rinsed to remove saliva and blood
Submerged in perform (disinfectant) for 10 minutes
Rinse to remove disinfectant
Wrap with wet tissues or gauze
Bag, label & write lab card