Dental Materials - impression, trays, models Flashcards

1
Q

Define impression

A

Negative imprint from which a positive reproduction or model can be made

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2
Q

Impression materials can be classified by viscosity. describe

A

High viscosity - impression compound

Medium viscosity = Medium bodied elastomers

Low viscosity = hydrocolloids (alginate), light bodied elastomers

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3
Q

Impression can be classified by elasticity. Describe

A

Non-elastic (rigid) - compound, zinc oxide eugenol

Elastic = elastomers or hydrocolloids
Elastomers = polyether, silicones
Hydrocolloids = aliginate
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4
Q

Differences in uses of elastic vs non-elastic impression materials?

A

Non-elastic cannot cover undercuts

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5
Q

Impression materials used for complete dentures

a) primary impression
b) major impression

A

a) compound

b) zinc oxide eugenol

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6
Q

What material should be used for major impressions if patient is allergic to zinc oxide eugenol

A

Silicones

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7
Q

How should an impression be taken of a patient with flabby ridge?

A

Tray is modified to have a window in area of flabby ridge. Impression taken in areas without flabby ridge

Whilst tray is still in mouth - plater is applied to flabby area by brush

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8
Q

Ideal impression material requirements (14)

A

Biocompatible

Sufficiently fluid on insertion, good wettability

Flexible

Pleasant taste, smell and aesthetic colour

Dimensionally stable

Capable of compressing soft tissues to desired degree without distortion

Set or harden at or near mouth temperature

Adequate WT and ST

Capable of additions

Simple to use

Compatible with model materials

Disinfected without loss of accuracy

Elastic properties

Tear resistant

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9
Q

What is wettability?

A

Preference of a liquid to be in contact with a solid surrounded by another fluid

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10
Q

Advantages of non-elastic impression compound

A

Muco-compressive
Can be used to make a special tray which helps make an accurate major impression
Easily trimmed, added to and re-adapted
Well-extended

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11
Q

Disadvantages of impression compoun

A
Cannot record undercuts
Technique sensitive
High viscosity = surface detail not accurate if not softened enough
Needs water bath
Shrinkage (poor dimensional stability)
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12
Q

Advantages of zinc oxide eugenol

A
Accurate detail
Dimensionally stable
Compatible with wet mouth (wettability)
Can be added to 
Muco-static (but not muco-compressive)
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13
Q

Disadvantages of zinc oxide eugenol?

A
Cannot be used on undercuts
Only sets rapidly in a thin layer
Doesn't accurately record the periphery
Allergies to eugenol
Adhere to skins
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14
Q

Advantages of alginate?

A
Cheap
Easy to use
Non-toxic and non-irritant
Can be used in stock/special trays
Records fine detail and undercuts
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15
Q

Disadvantages of alignate?

A

Dimensionally unstable
Surface reproducibility not as good as elastomers
Poor tear resistance and strength

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16
Q

Main causes of impression inaccuracy

A

Polymerization shrinkage
Loss of condensation reaction by product (water)
Thermal contraction (oral-room)
Absorption of water/disinfectant over time
Taken out before fully setting
Inappropriate mixing
Technique

17
Q

What type of special tray should be used for zinc oxide eugenol paste?

A

Close fitting tray

18
Q

What type of special tray is used for elastomeric impression materials?

A

Spaced special tray

19
Q

Why should special trays for alginate be perforated?

A

For excess to be pushed out and for retention

20
Q

Why should special trays for elastomers not be perforated?

A

It adheres well to the tray

21
Q

Intra-oral vs extra-oral special tray handles

A

Intra-oral for close fitting trays

Extra-oral for spaced

22
Q

What material is used to construct special trays?

A

Light-cure acrylic resin - easily adaptable and uniform in thickness