Impression Materials

Question 1

What is the function of impression materials (IM)?

Answer 1

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

Question 2

What is an impression and what is dental stone used for?

Answer 2

a negative reproduction of tissues
used to fill the impression and produce a positive replica (stone cast)

Question 3

What are clinical classfications of IM?

Answer 3

• mucostatic
• mucocompressive

Question 4

What is mucostatic and what are examples?

Answer 4

• fluid materials that displace the soft tissues slightly
• ie give an impression of the undisplaced mucosa.

eg. zinc oxide eugenol, low viscosity alginates

Question 5

What is mucocompressive and what are examples?

Answer 5

• viscous materials that record an impression of the mucosa under load
• ie give impression of displaced soft tissue.

eg. impression compound, high viscosity alginates/elastomers

Question 6

What decreases permanent strain in viscoelastic IM?

Answer 6

If the LOAD time is LESS…ie the impression tray is removed with a sharp pull, there is LESS overall PERMANENT STRAIN (ie permanent deformation)

wait for a time after removing the tray before you pour the cast – so as to minimise permanent strain (deformation)

Question 7

What are examples of elastic materials?

Answer 7

Hydrocolloids
* Agar - reversible (lab duplicating)…..no longer used!
* Alginate – irreversible

Elastomers
– Polysulphides
– Polyethers
– Silicones (conventional & addition cured)

Question 8

What happens to non-elastic IM when removing?

Answer 8

either deformed or fractured

Question 9

What are the ideal properties that ensure accuracy?

Answer 9

flow
- surface detail recorded

setting changes
* NO dimensional change

removal
* NO effect on oral tissues
* NO change dimensionally (ie no thermal contraction)
* COMPLETE ELASTIC RECOVERY

storage
* does not change dimensionally over time (moisture)

decontamination
* dimensions and surfaces unaltered

high tear strength (some elascity required)

Question 10

What is the ideal expansion coefficient for IM?

Answer 10

ideally low (to cope with mouth to room temperature transition)

Question 11

What are the ideal properties for patient comfort?

Answer 11

• non-toxic, non-irritant
• acceptable taste and smell
• short setting time
• removable without damage to oral tissues

Question 12

What are the ideal properties for operator convenience?

Answer 12

• quick, simple technique
• convenient working/ setting times
• must be able to be decontaminated
• cost (inexpensive) is a consideration BUT not a property)

Question 13

What are examples of non-elastic IM?

Answer 13

Impression compound
Impression paste

Question 14

What is alginate made of?

what do the components do

Answer 14

sodium alginate (reacts with ca ions)
calcium sulfate (provides ca ions)
trisodium phosphate (delays gel formation)
filler, and modifiers (improve surface, ph, taste)

Question 15

How does trisodium phosphate work and why?

Answer 15

trisodium phosphate acts as a retarder for the gel formation of alginate by reacting with and depleting the calcium ions in calcium sulfate first, before allowing the alginate crosslinking reaction to occur.

this is to increase mixing and loading time before setting and allow working time

Question 16

What are the phases/states of hydrocolloids?

Answer 16

A colloid is a two-phase system of fine particles (1-200nm) of one phase dispersed in another phase. If the dispersing medium is water, it is called a hydrocolloid

SOL
viscous mix of alginate powder and water before the setting reaction occurs.

GEL
alginate polymers have cross-linked and formed an entangled network trapping the undissolved filler particles and enclosed liquid.

Question 17

How does alginate set?

Answer 17

the cross-linking of sodium alginate with calcium ions released from calcium sulfate, forming a fibril entanglement that traps undissolved particles and liquid.

Question 18

What are the steps taken for the ideal alginate impression?

Answer 18

• use correct powder/ liquid ratio
• shake container
• use water at 18 - 24°C
• use perforated tray and adhesive
• remove impression with a sharp pull (elastic recovery and maximum tear strength)
• wait a minute or two after as crosslining continutes after apparent set

Question 19

How is alginate distortion and tearing slightly reduced?

Answer 19

by using large bulk of materials (Typ. 5mm)

Question 20

How should alginate be stored?

Answer 20

avoid moisture

*prepare cast ASAP to avoid dimensional change

syneresis - release of water
imbibition - uptake of water