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Questions and Answers

What is the purpose of mixing stone and water in a vacuum mixing machine?

To prevent air bubbles in the mixture (correct)

To speed up the setting time of the stone

To increase the temperature of the stone mixture

To enhance the flavor of the stone

When adding plaster to the impression, where should it be poured?

Evenly across the entire impression

Only in the center of the impression

In one corner of the impression (correct)

Around the edges of the impression

What is the recommended action after the initial set of the stone?

Leave it uncovered until fully dry

Immediately remove the impression from the stone

Trim the excess stone from the impression

Place the impression in slurry water for 3 to 5 minutes (correct)

What is advised to do with the cast after soaking it in slurry water?

Trim it on a cast trimmer Signup and view all the answers

How should the impression be prepared before pouring a base with the same type of stone?

Check the height of the stone after removing it from slurry water Signup and view all the answers

What should be done with the excess stone in the tongue area of the mandibular ridge?

Remove it carefully Signup and view all the answers

What is a common problem area that may occur during the impression process?

Failure to identify a faulty impression Signup and view all the answers

What is modeling plastic impressions mainly composed of?

Gum dammar, prepared chalk, and other materials Signup and view all the answers

What is the primary purpose of a preliminary impression?

For diagnosis or construction of a tray Signup and view all the answers

Which requirement for a preliminary cast ensures accuracy and reliability?

It should have surfaces free of voids and nodules Signup and view all the answers

What is the minimum thickness required for the base of a cast?

15 to 16 mm Signup and view all the answers

Which material is commonly used for complete denture preliminary impressions?

Alginate Signup and view all the answers

What should be done if a preliminary impression shows voids in critical areas?

Reject the impression and remake it Signup and view all the answers

Why is it critical to pour alginate impressions as soon as possible?

To prevent distortion due to dehydration Signup and view all the answers

What should be avoided when trimming the tongue space on a mandibular cast?

Creating undercuts Signup and view all the answers

What is a common characteristic of alginate as an impression material?

It transforms from sol to gel and is irreversible Signup and view all the answers

Study Notes

Preliminary Impressions and Casts

Preliminary impression: An impression made for diagnosing or constructing a tray.
Preliminary cast: A positive copy of the upper or lower jaw tissues, made in an impression, used to create trays.

Requirements for a Proper Cast

All surfaces of the cast to be contacted by the tray or denture should be accurate and free of voids or nodules.
Surfaces of the cast should be hard, dense, and free of any grinding sludge left from trimming.
Casts should extend sufficiently to include all areas for denture support (e.g., mandibular cast extends 3-4 mm beyond retromolar pads).
The peripheral roll of the cast should be complete and no deeper than 3-4 mm; the cast edge extending from this roll should be approximately 3-4 mm wide.
Side walls should be vertical or slightly tapered outward, but not undercut.
The base of the cast should be at least 15-16 mm at its thinnest point.
The tongue space on a mandibular cast should be flat and smooth when trimmed, but the lingual peripheral roll must remain intact.
The cast should not show signs of being wet or washed with tap water.

Materials for Impressions

Alginate (irreversible hydrocolloid): Used for diagnostic casts, removable partial dentures, and preliminary impressions for complete dentures. It transforms from a sol (soluble salt of alginic acid) to a gel (insoluble salt). Pour the impression as soon as possible to avoid distortion due to dehydration.
Modeling plastic (impression compound): Used specifically for complete denture impressions. It is composed of gum dammar, prepared chalk, and other materials.

Pouring Procedure

Examine the impression for voids and pull-away of alginate in critical areas. Discard any faulty impressions.
Wash the impression with a gentle stream of cold water.
Sprinkle stone on the impression to reveal any remaining saliva and then scrub it with a camel hair brush, rinsing with running water.
Remove moisture with a gentle stream of air.
Remove alginate extending beyond the tray back to prevent contact with the workbench.
Weight stone and water according to manufacturer instructions. Store stone in a dry, sealed container.
Mix stone and water in a vacuum mixing machine to eliminate air bubbles.
Carefully pour stone into the impression, avoiding contact with the vibrator.
Add stone to the impression's undercut areas with droplets.
Place impression on a bench or tray holder to set.
Soak the set stone impression in slurry water for 3-5 minutes for better grip on the base material.
Remove impression from slurry water, check the stone height, and pour a base with the same stone type.
Mix the base stone material in the same ratio to the glass or plastic surface and form it into a base.
Add a small amount of stone to the undercut areas of the first pour.
Invert the impression onto a patty and draw stone up onto the sides of the first pour.
Remove excess stone, especially in the tongue area of the mandibular ridge.
Separate cast from impression (approximately 45 minutes to 1 hour).
Inspect the cast for nodules and remove them.
Soak the cast in slurry water to prevent sludge from sticking.
Allow the cast to dry.
Label and identify the cast.