Lec 9: Impression Flashcards
Negative impression or imprint made placing some soft, semifluid material in the mouth and allowing the material to set.
Impression
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 ____-
Plasticity
Fluidity
Wet
Accuracy
Stability
Elasticity
Optional Utilities
- Set to rigid consistency.
- Cannot be removed from the mouth without fracture and reassembly.
Rigid impression materials
2 Rigid impression materials?
Plaster of Paris
Metallic oxide paste
Used in dentistry for over 200 years; was once the only material available.
Plaster of Paris / impression plaster
- 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.
Metallic oxide paste
2 thermoplastic impression materials?
Modelling plastic
Impression waxes & natural resins
Oldest impression material used in prosthodontic dentistry.
Modelling plastic / compound
Most often for border correction of custom trays for distal free-end classifications.
Modelling plastic / compound
Most commonly used modelling plastic for corrected impressions for extension base areas (softens at 132 F / 55.55 C).
Red-brown material
Obtainable in stick form for border molding.
Red, gray, green (modelling compound)
- 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.
Impression waxes and natural resins
Used for:
> Corrective impression
> Bite registration
Impression waxes and natural resins
Iowa wax & Korecta wax are examples of this.
impression waxes
- 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- Only ones that can be withdrawn without permanent deformation.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.
Elastic impression materials
Example of reversible hydrocolloid
Agar-agar
- Fluid at higher temperatures, gel on reduction in temperature.
- For fixed restorations
- Acceptable accuracy
Reversible hydrocolloid
- 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
Mercaptan Rubber-base / Polysulfide polymer (thiokol) impression material
- 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.
Polyether impression materials
- 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.
Silicone based materials
Possible causes of an inacurate / weak cast (7)
- Distortion of hydrocolloid material
- Ratio of water to powder is too high = weak
- Improper mixing = weak / chalky
- Trapping of air (due to insufficient vibration)
- Soft or chalky surface from retarding action of the hydrocolloid or absorption of necessary water for crystallization by dehydrating hydrocolloid.
- Premature separation of the cast from the impression.
- Failure to separate the cast from the impression for an extended period.
A stock tray should?
* Be sufficiently _______ to avoid distortion.
* Fit in the mouth about _______ clearance.
* Permits _____ and _____ to fit the mouth.
- Rigid
- 4-5mm clearance
- Trimming & Shaping
