Dental Material Science > Impression Materials > Flashcards

Impression Materials Flashcards

(34 cards)

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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)

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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)

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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)

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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)

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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)

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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

Study These Flashcards
A

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

22
Q

Define a hydrocolloid

Study These Flashcards
A

Colloid in water

23
Q

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

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

Give the different states of hydrocolloids (2)

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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
State the components of Alginate (4)
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
Setting reaction for alginate
2NanAlg + nCaSO4 ---> nNa2SO4 + CanAlg
27
What happens in the alginate setting reaction?
Long cross-linked fibril entangling undissolved particles
28
Describe the crosslinking reaction in the alginate setting reaction
Sodium alginate --> calcium alginate | Crosslinking due to calcium results in better mechanical properties
29
Describe the alginate delay
Trisodium phosphate preferentially react with Ca ions in CaSO4 2Na3PO4 + 3CaSO4 --> Ca3(PO4)2 + 3Na2SO4 Then 2NanAlg reacts with calcium
30
How is alginate manipulated for use? (5)
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
What is the significance of increased temperature speeding up the setting reaction of alginate?
- Sets nearest tissue first | - Using warmer water allows alginate to set quicker
32
What is the significance of the crosslinking in alginate continuing after the apparent set?
There are greater elastic properties if you wait a few more minutes
33
How can permanent distortion and tearing of alginate be slightly reduced?
By using large bulk of materials (typically 5mm)
34
Why should excessive moisture be avoided when storing impressions? (2)
Can lead to: 1. Syneresis - Release of water 2. Imbibition - Uptake of water
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