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Dental Material Science > Impression Materials > Flashcards

Impression Materials Flashcards

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1
Q

State the function of impression materials

A

Produce an accurate replica of the surfaces/shape of hard + soft oral tissues

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2
Q

What is an impression

A

A negative replica of tissues

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3
Q

What is the function of gypsum (dental stone)

A

Used to fill the impression and produce a positive replica of the tissues (stone cast)

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4
Q

What is the function of the dental stone cast formed by gypsum? (2)

A
  1. Evaluating dentition when orthodontic/occlusal etc problems are involved
  2. In laboratory fabrication of restorations and prostheses
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5
Q

State the clinical classification of impression material (2)

A
  1. Mucostatic
    - Zinc oxide eugenol
    - Low viscosity alginates
  2. Mucocompressive
    - Impression compound
    - High viscosity alginates/elastomers
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6
Q

Give some property classifications of impression materials (5)

A

Elastic Materials
1. NON RIGID MATERIALS:
> Reversible hydrocolloids (agar)
> Irreversible hydrocolloids (sodium alginate)

  1. ELASTOMERS
    - Polysulphides
    - Polyethers
    - Silicones

Non Elastic Materials
3. RIGID MATERIALS:
> Impression compound
> Zinc oxide eugenol based impression paste

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7
Q

Define the term mucostatic impression materials and state a disadvantage

A
  • Give an impression of the undisplaced mucosa in resting position

DISADVANTAGE:
- Will lead to a denture with a good fit during rest, but on function the denture will pivot around incompressible areas (torus palatines) and dig into compressive areas

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8
Q

Define the term mucocompressive impression materials and state an advantage

A
  • Record displaced mucosa under load

ADVANTAGE:
- Will lead to a more stable denture during function, but not at rest

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9
Q

Describe ideal elastic behaviour (4)

A
  1. Load applied
  2. Instantly reaches maximum strain
  3. Strength maintained
  4. Returns to initial dimensions with minimum change in shape if you take impression fast
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10
Q

Define the term viscoelastic impression material

A

Nearly elastic but not quite

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11
Q

Describe the viscoelastic behaviour of impression materials (4)

A
  1. Load applied
  2. Gradually reaches maximum strain
  3. Gradually returns to initial dimensions
  4. Doesn’t quite go back to initial dimensions so has a permanent strain
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12
Q

Why is it advantageous to wait for a time after removing the impression tray before you pour the cast? (2)

A
  • Allows permanent strain to be minimised

- More time to return to initial dimensions

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13
Q

How can the permanent strain of an impression material be minimised?

A

If the load time is less

Remove the impression with a sharp pull to reduce the overall permanent strain (lower deformation)

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14
Q

Give some different curing methods of silicones (2)

A
  1. Conventional

2. Addition cured

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15
Q

List setting reactions involved in the impression process (3)

A
  1. Chemical
  2. Polymerisation
  3. Cooling
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16
Q

List ideal properties for accuracy (7)

A
  1. Flow
  2. Setting changes
  3. Removal
  4. Decontamination
    - dimensions and surfaces unaltered
  5. Poor tear strength
    - Avoid deep undercuts
  6. Storage
  7. Compatible with cast material
17
Q

List the ideal properties of an impression material in relation to REMOVAL accuracy (3)

A
  1. No effect on oral tissues
  2. No change dimensionally (i.e. no thermal contraction)
  3. Complete elastic recovery
18
Q

List some properties of impression materials that AFFECT accuracy (6)

A
  1. Viscosity
    - Determines the ability to flow over the surface and so level of detail recorded
  2. Setting mechanism
    - Potential dimensional contraction due to physical or chemical process
  3. Thermal expansion
    - Should ideally be low to cope with the mouth to room temperature transition
  4. Hydrophobic / hydrophilic
    - Affects surface contact
  5. Elasticity
  6. Tear strength
    - Allows it to withstand tearing when removing- especially from undercuts
19
Q

List the ideal properties of an impression material with relation to patient comfort (4)

A
  1. Non-toxic, non-irritant
  2. Acceptable taste and smell
  3. Short setting time
  4. Removable without damage to oral tissues
20
Q

List the ideal properties of an impression material with relation to operator convenience (4)

A
  1. Quick simple technique
  2. Ok setting time
  3. Must be able to be decontaminated
  4. Convenient working time
21
Q

Define a colloid

A

A two phase system of fine particles (1-200nm) of one phase dispersed in another phase

22
Q

Define a hydrocolloid

A

Colloid in water

23
Q

List the ideal properties of an impression material for patient comfort (3)

A
  1. Non toxic
  2. Non irritant
  3. Acceptable taste + smell
24
Q

Give the different states of hydrocolloids (2)

A
  1. SOL
    - Viscous liquid state
  2. GEL
    - Jelly like consistency
    - Agglomeration of particles
    - Entangled framework of solid particles enclosing liquid by capillary forces
25
Q

State the components of Alginate (4)

A
  1. Salt of alginic acid (12%)
    Reacts with Ca ions
  2. Calcium Sulphate (12%)
    - Provides Ca ions
  3. Trisodium Phosphate (2%)
    - Delays gel formation
  4. Filler (70%)
    - Cohesion + strength
26
Q

Setting reaction for alginate

A

2NanAlg + nCaSO4 —> nNa2SO4 + CanAlg

27
Q

What happens in the alginate setting reaction?

A

Long cross-linked fibril entangling undissolved particles

28
Q

Describe the crosslinking reaction in the alginate setting reaction

A

Sodium alginate –> calcium alginate

Crosslinking due to calcium results in better mechanical properties

29
Q

Describe the alginate delay

A

Trisodium phosphate preferentially react with Ca ions in CaSO4
2Na3PO4 + 3CaSO4 –> Ca3(PO4)2 + 3Na2SO4
Then 2NanAlg reacts with calcium

30
Q

How is alginate manipulated for use? (5)

A
  1. Use correct powder / liquid ratio (shake container)
  2. Use water at 18-24 degrees
  3. Use perforated tray and adhesive
  4. Remove impression with a sharp pull (for elastic recovery and maximum tear strength)
  5. Increased temperature speeds up setting reaction
    Crosslinking continues after apparent set
31
Q

What is the significance of increased temperature speeding up the setting reaction of alginate?

A
  • Sets nearest tissue first

- Using warmer water allows alginate to set quicker

32
Q

What is the significance of the crosslinking in alginate continuing after the apparent set?

A

There are greater elastic properties if you wait a few more minutes

33
Q

How can permanent distortion and tearing of alginate be slightly reduced?

A

By using large bulk of materials (typically 5mm)

34
Q

Why should excessive moisture be avoided when storing impressions? (2)

A

Can lead to:

  1. Syneresis
    - Release of water
  2. Imbibition
    - Uptake of water

Dental Material Science (7 decks)

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