finished denture which, with its defect, may produce an unsatisfactory prosthesis.

b. A properly poured master cast can be ruined by contact with water since artificial stone is soluble in a ratio of one part artificial stone to 600 parts of water. A master cast should not be completely immersed in water to soak for an extended time. Two to 3 minutes in water is enough to wet a master cast for most purposes. If it must be soaked longer, a saturated slurry solution should be used. The level of slurry water should be below the tissue surface of the cast. The wet

Figure 5-14. Proper method of soaking an edentulous cast.

5-11. Modification of the Maxillary Master Cast

a. The tissue surface of the maxillary master cast is modified by the dental officer to improve the retention of the maxillary complete denture in the mouth. This modification, called the posterior palatal seal, can be done any time after the master cast is trimmed and before the denture is ready to be processed.

b. The retention of a maxillary complete denture depends partly on a tight seal between the peripheral borders of the denture and the oral tissues. This seal prevents air from entering between the denture and the oral tissues. The posterior palatal seal of the maxillary complete denture is used to seal this border against the resilient tissues of the soft palate. The seal consists of a ridge or "dam" of denture base material across the posterior border of the maxillary denture.

c. The dental officer is responsible for modifying the master cast to create the posterior palatal seal. He decides on the location, width, depth, and outline for the seal. The most widely used procedure consists of scribing the master cast in the form of a trench or groove which will produce a ridge of denture base material on the tissue surface of the maxillary complete denture. The scribing may be done with a number 8 bur or the blunt end of a carving instrument. The groove should not be sharp, since a sharp edge of acrylic resin in the finished denture will irritate the tissues of the soft palate.

d. The posterior palatal seal is produced by scribing a groove 1 mm. deep and 1 mm. wide from the hamular notch on one side to the hamular notch on the other side, crossing the palate by curving through or just anterior to the fovea

palatina at the midline. Another method is to scribe a groove 3 to 4 mm. wide in the same position as the one previously described. Some dental officers consider the wider design less irritating to the tissues of the soft palate.

5-12. Occlusion Rims

When the master cast is completed, preparations are made for the patient's next appointment. A baseplate, which temporarily represents the base of the final denture, is adapted to the master cast. It may be made from shellac baseplate material, autopolymerizing acrylic resin, heat-polymerizing acrylic resin, or thermoplastic vinyl resin which can be vacuum-formed. The baseplate must be rigid enough to resist ordinary biting pressures. It must also be neat, clean, and smooth enough to be placed in the patient's mouth without causing discomfort. Next, a horseshoe-shaped wax block is attached to the baseplate. The whole unit is called an occlusion rim. Occlusion rims are used by the dental officer to record measurements needed for denture fabrication. Rims also serve as a framework upon which denture teeth are set. Occlusion rims are stabilized so they will fit the patient's mouth accurately and prevent unnecessary movements during the recording of the maxillomandibular relationship.

5-13. Adaptation of Baseplates to Master Cast

a. Shellac Baseplate.

(1) For maxillary cast (fig 5-15).

(a) Undercuts on the master cast are blocked out by being filled with moistened asbestos or modeling clay. Asbestos is the easier to use (A, fig 5-15).

(b) The master cast is soaked in slurry water for 2 to 3 minutes to prevent the baseplate from sticking to it (B, fig 5-15).

(c) The proper shellac baseplate form is selected, centered, heated, and softened on the master cast (C, fig 5-15). Enough heat must be applied with either a Bunsen burner or an alcohol torch to make sure the baseplate will retain the shape to which it is adapted. Shellac baseplates have a tendency, called memory, to return to their original shape if enough heat is not applied. Overheating is avoided. For comfort, the fingers should be moistened with water while manipulating the material.

(d) Using an alcohol torch, the operator softens and adapts small portions of the facial area of the baseplate to the peripheral roll on the cast. Either a rubber wheel with rounded edges, a pencil eraser, or a burnisher can aid greatly in pressing the material to place without punching through it (D, fig 5–15).

(e) Excess material is trimmed with a pair of shears or scissors leaving about 6 mm. of material around the edges of the cast. Again, small portions of the facial edge at a time are warmed and the excess is folded back to form a smooth, well-rounded border. The peripheral roll should approximate the borders desired in the finished denture in both contour and thickness (E and J, fig 5-15).

(f) The posterior border is warmed and folded over onto the palatal portion. If reinforcement is needed in this area, a piece of wire can be adapted to the posterior border and some of the baseplate folded over it (F, fig 5-15).

(g) The edges which have been folded over should be smoothed with a hot spatula (G, fig 5-15). Extra baseplate material can be added where needed by heating small strips in the flame and sealing them in place with the spatula.

(h) The baseplate is trimmed with an arbor band and smoothed with a rubber wheel so that no sharp edges remain (H, I, J, and K, fig 5-15).

(2) For mandibular cast (fig 5-16). The shellac baseplate for the mandibular master cast is adapted the same as for the maxillary master cast (A, B, and C, fig 5-16).

b. Acrylic Resin Baseplate. A well-fitting, stabilized baseplate can be made from either autopolymerizing or heat-polymerizing acrylic resin. A baseplate of this material is better than a shellac baseplate because it does not warp or otherwise distort during the heating and cooling to which it is subjected in subsequent procedures. Autopolymerizing acrylic resin baseplates are fabricated by either the dough or sprinkle-on method described in paragraph 5–5b (2). Heatpolymerizing acrylic resin baseplates may be fabricated as described in paragraph 5–5b (3).