Steps 13-15


Figure 5: Cast is sectioned at the distogingival line angle of the distal abutments. Tapered grooves are made before assembling the RPD altered cast impression on the sectioned cast. Irreversible hydrocolloid is used for the boxing technique.

Step 13: The edentulous area(s) of the master is/are removed using a disc and an acrylic bur on a lathe.  The master cast should be trimmed so that there is no interference with the peripheral extension of the impression tray(s) and up to the abutment teeth.  Retention grooves tapered toward the occlusal surface are placed in the master cast adjacent to the edentulous area(s). (Fig. 5)

Step 14: Place the framework back on the master cast so that the rests and the other components are completely seated.  It is important that one or two millimeters of the major connector past the internal finish line be exposed so that it will be resting on the altered portion of the master cast.  This will serve as a reference point to ensure that the framework is correctly related to the edentulous area(s) during tooth arrangement and processing of the acrylic denture base.  The framework should be secured to the master cast with wither sticky wax or household glue on the major connector.

Step 15: An irreversible hydrocolloid technique is recommended because boxing wax may not readily adhere to elastomeric impression materials. (Fig. 5)  The technique is as follows.

(1) Select a denture cup or other plastic container that is approximately 10 mm wider than the master cast and the impression tray(s).
(2) Using blockout compound (Buffalo Dental Mfg. Co., Brooklyn, NY) or Play-Dough, position the master cast in the boxing form with the tissue surface up.
(3) Vacuum spatulate a watery mix of irreversible hydrocolloid (two scoops with six measures of water per arch) and pour it into the box, leaving three millimeters of lingual flange exposed on a mandibular impression and three millimeters of buccal flange exposed on a maxillary impression.
(4) Remove any irreversible hydrocolloid that may have gotten on the tissue surface of the impression. Using gentle vibration flow, vacuum spatulated improved stone into the boxed impression. This should be done immediately after the irreversible hydrocolloid has set to avoid any dehydration and shrinkage of the material.
(5) When the stone has set, remove the container and irreversible hydrocolloid boxing material.
(6) Trim the master cast with the impression in place to protect the tissue surface from abrasion. The master cast should be trimmed so that four to six millimeters of land area remain.