Untitled Quiz

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.

Problem Areas

• Failure to identify a faulty impression.
• Delay in pouring the impression.
• Poor pouring technique.
• Incorrect trimming on the cast trimmer