Lec 9: Impression

Question 1

Negative impression or imprint made placing some soft, semifluid material in the mouth and allowing the material to set.

Answer 1

Impression

Question 2

Ideal impression material:
1. Complete _____ before cure
2. Sufficient ______ to record define detail.
3. Ability to ____ oral cavity.
4. Dimensional ____
5. Dimensional ____
6. Complete _____ after cure
7. Optional ____-

Answer 2

Plasticity
Fluidity
Wet
Accuracy
Stability
Elasticity
Optional Utilities

Question 3

• Set to rigid consistency.
• Cannot be removed from the mouth without fracture and reassembly.

Answer 3

Rigid impression materials

Question 4

2 Rigid impression materials?

Answer 4

Plaster of Paris
Metallic oxide paste

Question 5

Used in dentistry for over 200 years; was once the only material available.

Answer 5

Plaster of Paris / impression plaster

Question 6

• Usually made of some form of zinc oxide-eugenol combination.
• Not used as primary material, never for impressions with remaining natural teeth.
• Not used on stock impression trays.

Answer 6

Metallic oxide paste

Question 7

2 thermoplastic impression materials?

Answer 7

Modelling plastic
Impression waxes & natural resins

Question 8

Oldest impression material used in prosthodontic dentistry.

Answer 8

Modelling plastic / compound

Question 9

Most often for border correction of custom trays for distal free-end classifications.

Answer 9

Modelling plastic / compound

Question 10

Most commonly used modelling plastic for corrected impressions for extension base areas (softens at 132 F / 55.55 C).

Answer 10

Red-brown material

Question 11

Obtainable in stick form for border molding.

Answer 11

Red, gray, green (modelling compound)

Question 12

• Used to record non-undercut edentulous portions and are usually used in combination with other impression materials.
• To correct borders of impression = establish optimum contact at border.
• To record border detail accurately and include correct width.

Answer 12

Impression waxes and natural resins

Question 13

Used for:
> Corrective impression
> Bite registration

Answer 13

Impression waxes and natural resins

Question 14

Iowa wax & Korecta wax are examples of this.

Answer 14

impression waxes

Question 15

• Those that remain in an elastic or flexible state after they have set and removed form the mouth.
• Only ones that can be withdrawn without permanent deformation.
• For making impressions and when tooth undercuts and surface detail must be recorded with accuracy.

Answer 15

Elastic impression materials

Question 16

Example of reversible hydrocolloid

Answer 16

Agar-agar

Question 17

• Fluid at higher temperatures, gel on reduction in temperature.
• For fixed restorations
• Acceptable accuracy

Answer 17

Reversible hydrocolloid

Question 18

• Used for removable partial denture impressions and especially for secondary corrected or altered cast impressions.
• Unpleasant odor, can stain clothes.
• Moderately inepensive
• High tear strength
• Long working and setting times (8-10 mins)
• Can be disinfected in liquid, cold-sterilizing solutions

Answer 18

Mercaptan Rubber-base / Polysulfide polymer (thiokol) impression material

Question 19

• An elastic-type of material
• Demonstrated good accuracy and thixotropic, which provides good surface detail and makes them useful for border molding material
• Also hydrophilic, produces good wettability for easy cast forming.
• Low to moderate tear strength and much shorter working and setting times.

Answer 19

Polyether impression materials

Question 20

• More accurate and easier to use
• Pleasant odor, moderately high tear strength, and excellent recovery from deformation
• Hydrophobic, which can make cast formation a problem
• Can be disinfected with no alteration in accuracy
• Poured within 1 hour.

Answer 20

Silicone based materials

Question 21

Possible causes of an inacurate / weak cast (7)

Answer 21

1. Distortion of hydrocolloid material
2. Ratio of water to powder is too high = weak
3. Improper mixing = weak / chalky
4. Trapping of air (due to insufficient vibration)
5. Soft or chalky surface from retarding action of the hydrocolloid or absorption of necessary water for crystallization by dehydrating hydrocolloid.
6. Premature separation of the cast from the impression.
7. Failure to separate the cast from the impression for an extended period.

Question 22

A stock tray should?
* Be sufficiently _______ to avoid distortion.
* Fit in the mouth about _______ clearance.
* Permits _____ and _____ to fit the mouth.

Answer 22

• Rigid
• 4-5mm clearance
• Trimming & Shaping