Impression Materials Flashcards Preview

2 - Dental Material Science > Impression Materials > Flashcards

Flashcards in Impression Materials Deck (20)
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1
Q

What is the function of impression materials (IM)?

A

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

2
Q

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

A

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

3
Q

What are clinical classfications of IM?

A
  • mucostatic
  • mucocompressive
4
Q

What is mucostatic and what are examples?

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

eg. zinc oxide eugenol, low viscosity alginates

5
Q

What is mucocompressive and what are examples?

A
  • 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

6
Q

What decreases permanent strain in viscoelastic IM?

A

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)

7
Q

What are examples of elastic materials?

A

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

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

8
Q

What happens to non-elastic IM when removing?

A

either deformed or fractured

9
Q

What are the ideal properties that ensure accuracy?

A

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)

10
Q

What is the ideal expansion coefficient for IM?

A

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

11
Q

What are the ideal properties for patient comfort?

A
  • non-toxic, non-irritant
  • acceptable taste and smell
  • short setting time
  • removable without damage to oral tissues
12
Q

What are the ideal properties for operator convenience?

A
  • quick, simple technique
  • convenient working/ setting times
  • must be able to be decontaminated
  • cost (inexpensive) is a consideration BUT not a property)
13
Q

What are examples of non-elastic IM?

A

Impression compound
Impression paste

14
Q

What is alginate made of?

what do the components do

A

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

15
Q

How does trisodium phosphate work and why?

A

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

16
Q

What are the phases/states of hydrocolloids?

A

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.

17
Q

How does alginate set?

A

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

18
Q

What are the steps taken for the ideal alginate impression?

A
  • 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
19
Q

How is alginate distortion and tearing slightly reduced?

A

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

20
Q

How should alginate be stored?

A

avoid moisture

*prepare cast ASAP to avoid dimensional change

syneresis - release of water
imbibition - uptake of water