PATIENT VISIT I – DIAGNOSIS AND PRELIMINARY IMPRESSIONS
Laboratory Procedures

PRELIMINARY (DIAGNOSTIC) CASTS

Definition
A positive likeness of the part (s) of the oral cavity for the purpose of study, treatment planning, and/or fabrication of final impression trays.

Equipment and materials
a. Plaster spatula, Vacuum mixer bowl.
b. Buffalo knife.
c. Yellow stone
d. Wax spatulas
e. Vibrator
f. Plastic Vacuform sheet on which to pour stone bases for the casts
g. Model trimmer

Procedure

Pouring the Preliminary Impressions

1. Rinse off disinfectant and dry the impression thoroughly. block out the tongue space of the mandibular preliminary impression with a piece of baseplate wax or Pay-Doh and seal in place with a hot wax spatula. Position the wax very close to the crest of the lingual border roll. Positioning the tongue spacer more than 2 mm away from the lingual border will result in a large excess of stone in the tongue area of the cast.

2. Measure 42 cc. of room temperature (approximately 72 degrees F) water and pour it into a clean, dry mixing bowl.

3. Sift 150 g. of yellow stone into the water until all the stone is incorporated.

4. Spatulate with a clean spatula for approximately 10 seconds to thoroughly wet the stone.

5. Vacuum mix for 20 seconds.

6. Hold the mandibular impression tray by the handle with the base of the handle resting on the vibrator. Place a small amount of stone into the retromolar pad area of the impression. Add small amounts of stone slowly into this area while inclining the impression, forcing air ahead of the flowing stone, guiding the flow of stone until the entire ridge area is filled. The stone should completely fill the impression and cover all the border areas. Form irregular undercut droplets of stone on the top of the poured impression.

7. Now set the impression aside with the stone facing up. The impression can be placed in the slot of an impression holder located on the laboratory counter. The impression is suspended by the tray handle to avoid distorting the distal areas of the impression. The height of stone from the border should not be more than 10-15 mm. Let this stone attain its initial set, about 10 minutes

8. Reduce the height of the mounds using the cast trimmer and invert the impression to verify the cast will be level and not excessively thick. Trim additional stone from the stone mounds until the inverted tray is level and not excessively thick or thin. A base of 10 mm is sufficient.

9. Add a base by using a new mix of stone. Using a spatula, force the stone around the undercut stone mounds previously formed. Make a patty of stone on a smooth glass slab approximately 1 inch high. now, invert the filled impression into this pile. move the spatula around the impression and draw this stone up the sides. Be sure to have enough stone around all sides to allow for trimming when the stone has set, but do not embed the borders for the metal tray in stone.

10. The same procedure is used when pouring the maxillary impression.

11. Permit the stone to set for at least 30 minutes after the addition of the base (45 minutes total).

Separating the Casts

1. Use a Buffalo Knife to cut away any stone which may have covered the edges of the tray. Carefully cut away any impression material which has overlapped the edges of the tray.

2. Tap the handle of the tray to separate it from the impression material.

3. When using Impression compound, place the impression and cast in a heated water-bath at 140 degrees F for 3 minutes. This will soften the compound and allow you to separate the cast. Do not overheat the compound or it will stick to the cast. Discard the compound.

4. Excess impression compound can be removed from your trays by chilling the tray and chipping off large pieces of the brittle compound. Small amounts of compound can be removed by coating the tray with petroleum jelly and heating with a torch. The compound will then wipe out of the tray easily. Be careful to not burn yourself on the hot metal.

Trimming the Casts

1. Bevel the sharp edge around both casts with a lab knife held at a 45 degree angle, or a bur in the lathe. Remember to use suction when grinding on plaster. It will be difficult for you to make final impression trays if the casts are not trimmed correctly. You can easily break your diagnostic cast while making trays if there is excess stone above the base of the sulcus.

2. The base of the cast should be trimmed approximately parallel to the ridge. The cast should be sufficiently thick to avoid accidental breakage. At least 10-15mm thickness of the base from the deepest extension of the impression surface.

3. Place the patients' name on the back of the casts for identification.

CRITERIA FOR GRADING COMPLETE DENTURES

Preliminary casts and final impression trays for complete dentures must be evaluated prior to bringing the patient to the clinic for visit 2. Although the preliminary cast and final impression trays are separate steps, we recommended you to show the preliminary cast and final impression tray to one of the faculty at the same time to use your time most efficiently. These are the grading criteria for preliminary casts.

Preliminary casts
Grade A	Casts are neat, clean, free of voids and/or defects, have sharp detail. Tongue area of mandibular cast trimmed flat and smooth. Casts reflect entire basal support area. The cast base is approximately 10-15mm thick in thinnest area and is trimmed approximately parallel to the ridge. No impingement of impression tray evident. Cast is labeled with patient name and date. Cast is smoothly trimmed with at least 2-3mm of vestibule present lateral to the deepest part of the vestibule labially and buccally. Cast edges are beveled and smooth. Land area of the cast is trimmed so no undercut is present. The sides of the cast and the edentulous ridges should be parallel and perpendicular to the base.
Grade B	Minor discrepancies from the above which do not compromise the quality of the clinical treatment. Casts reflect correct extension, but minor, non-critical defects in areas of stone not covered by tray are present. Casts slightly thinner or thicker than ideal. Cast is not smoothly trimmed. Cast is not labeled.
Grade C	Cast extension adequate to make a clinically acceptable tray but marginally so. Full vestibular depth missing in isolated areas. Cheek slightly captured in distobuccal areas of mandibular cast. tissue surface of cast somewhat unclear and slightly scarred. Land area slightly interferes with correct tray construction. Blockout wax remains on cast. Stone debris left on cast. Cast is not clean and neatly trimmed.
Grade E	Cast prevents construction of clinically acceptable impression tray. Border extension inadequate for correct tray construction. land areas poorly defined, missing, or so high they prevent proper tray construction. Tongue space not trimmed neatly and/or prevents access to lingual borders. Cast base excessively thick or thin (less than 5mm or greater than 20mm).

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