PATIENT VISIT 2– FINAL IMPRESSIONS
Laboratory Procedures

B. Pouring And Trimming The Master Casts

1. Vacuum mix 42 cc of water and 150 g. of buff stone for 20 seconds.  Slowly vibrate the stone into the impression.  Additional stone is added in the same area until the impression is filled without entrapment of air.  Pour the other impression in the same manner.
2. Allow the stone to remain undisturbed for a minimum of 45 minutes.

Separating the Impression from the Master Cast

1. Remove the boxing and beading  wax or Play-Doh.
2. Soak the impression and cast in warm water (140 degree F) until the border molding compound is soft, about 4-5 minutes.  Overheating will cause the compound to stick to the cast. 
3. Carefully remove the impression from the cast using a lab knife.  The compound should be soft and allow you to remove the cast with a minimum of force.
4. Flatten the base of the casts using the model trimmer.  Make the base of the cast parallel to the average height of the ridge (mean foundation plane).  The base of the cast is no thinner than 11-13mm.

5. If necessary, the sides of the cast should also be trimmed.  Also reduce the height of the land area if it exceeds 2 mm above the depth of the vestibule.

The protective land should be 3-4 mm wide.  It should be 4-6 mm wide on the posterior border of the mandibular cast.  Bevel the land area approximately 30-45 degree with a lab knife or a rotating instrument.  The tongue area should be flat and smooth and no more than 3 mm above the depth of the vestibule

Caution: Do not trim away any part of the impression surface of the cast in the pterygomaxillary notch areas until the posterior limit is established at the next patient visit.

6. Both the maxillary and mandibular casts are indexed by placing grooves or notches in the base of the cast.  Using a stone on the lathe and suction, make three V-shaped index grooves in the base of the cast, two grooves in the posterior of the cast, and one in the midline in the anterior of the cast.  Indexing permits removal and accurate replacement of the casts on the articulator mounting.  Alternatively, two grooves extending the length and width of the cast base and intersecting at right angles can be placed using a stone in the lathe for purposes of indexing.  All grooves should be 3-5 mm deep and approximately 10 mm wide.

Criteria for Grading Complete Dentures

Master casts and record bases and occlusion rims for complete dentures must be graded through one of the faculty prior to bringing the patient to the clinic for visit 3.  Although the master cast and record bases and occlusion rims have separate criteria for grading, we recommend you show them both to one of your faculty at the same time to use your time most efficiently.  Step 4, Posterior palatal seal is evaluated after visit 3.
These are the grading criteria for the master casts.

3. Master Casts
     - Grade A -Base of cast parallel to ridges (mean foundation plane).  Cast is 11-13 mm thick in the thinnest areas.  Land areas clearly present, are contoured to follow depth of vestibules, beveled at a 30-45 degree angle, and are no more than 2 mm above the depth of the vestibules.  Land areas are 3-4 mm wide.  Land is 4-6 mm wide on the posterior border of the mandibular cast.  Vestibules present and 1-3 mm deep.  All areas of denture support present in cast with no obvious over or under extensions.  Repositioning indexes present and approximately 10 mm wide and 3-5 mm deep.  Tongue area flat and smooth and no more than 3 mm above depth of vestibules.  Casts neat and clean with no voids or blebs and non-tissue surfaces smoothed.
    - Grade B  - Minor discrepancies from Grade A which will not compromise the quality of the denture.
    - Grade C  - Master cast is adequate to make a clinically acceptable complete denture, but marginally so.  Cast thickness beyond criteria above, less than 11 mm thick.  Land areas not smooth, does not follow vestibule, and/or has insufficient width.  Vestibule present, but insufficient depth causes a future loss of contour to denture flange.  Repositioning indices present, but too deep or shallow.  Cast not neat, smooth, and clean.  Wax remains in undercuts on cast. 
    - Grade E - Master cast prevents clinically acceptable complete denture treatment.  Master cast is broken and/or glued in place