Well-extended diagnostic casts are required for every RPD case; it is generally preferred that the casts are fabricated after the extraction phase, particularly when extraction decisions are straightforward. Occasionally it is necessary to generate diagnostic casts before extractions are specified, in order to provide more information for extraction decisions. In these cases students should understand that the pre-extraction models are not sufficient for framework design, nor for custom tray fabrication; post-extraction diagnostic casts must be present at all RPD fabrication appointments.
Diagnostic impressions are made with alginate in stock trays, with rope wax peripheral extensions. It is important to emphasize that soft tissue extension in diagnostic casts for RPD cases is as important as tooth details; all vestibules should be fully visible and accessible in the alginate impression.
In all RPD cases except those involving a denture in the opposing arch, the diagnostic casts must be mounted via facebow and centric record. For distal extension cases lacking a stable tripod of natural tooth stops, a record base must be fabricated on the diagnostic cast, and a second patient appointment is needed for occlusal records.
RPD Survey and Framework Design (Laboratory)
An overview analysis of the case should include the mounted casts and pertinent oral and radiologic findings. The operator should be aware of the Kennedy partially edentulous classification, biomechanical demands for the proposed prosthesis, esthetic requirements, occlusal status, and any circumstances that make proposed abutments sub-optimal or other teeth questionable. Factors that should be considered at this time that cannot be derived from the diagnostic casts include abutment mobility, endodontic and restorative status of abutment teeth, and depth of vestibules, patient cognition, compliance and manual dexterity. Preliminary decisions are then made concerning rest and clasp assembly type and location, major connector design, and any unusual features that may be anticipated.
Diagnostic Cast Survey
Rest locations are 1) marginal ridges adjacent spaces in tooth bounded cases, 2) mesial marginal ridges for distal extension cases, 3) areas least likely to interfere with natural tooth occlusion, 4) at least one additional rest anterior to the fulcrum axis in distal extension cases (to serve as the indirect retainer) an anterior lingual plate also provides indirect retention.
Acrylic Retention Posts:
Occlusal plane and occlusal prematurity correction should be prescribed and attempted on the mounted casts. The most typical problem involves teeth that are unopposed and have extruded. Correction tactics are, in descending order of preference:
Planning for modification of a prospective RPD, because of potential future tooth loss, is based on prognostic uncertainty and framework/acrylic base design to provide components near the suspect tooth or teeth. This should not encourage under-treatment of problematic dentition, nor is it possible routinely to replace accidental tooth loss in RPDs with no nearby components. Welding or soldering new cast components onto an existing RPD involves risky and heroic laboratory technique, and a new RPD is almost always a better service.
Surveyed Abutment Crowns
Faculty members should investigate the future abutment utility of any tooth receiving a crown next to an edentulous space. For example, an RPD may be planned but may be refused by the patient - if future RPD fabrication is even remotely possible, the crown should be designed as an RPD abutment. Before beginning a survey crown, there should be a clear RPD design on a diagnostic cast, showing prosthetic components that will be received by the new crown. Extra tooth preparation is required in areas receiving rests, or on the side of malposition if an axial realignment is desired (for example, if buccal malposition is to be corrected, the buccal aspect of the tooth must be over prepared). In the event a record base is necessary for the centric record, the final crown impression should be extended sufficiently to soft tissue landmarks. As a reminder, the record base should be fabricated on the crown master cast before the die is prepared, or on a second pour solid cast, and the occlusal record always requires a second appointment - in these cases, preparation records or full arch Regisil records are usually completely insufficient for accurate mounting. After mounting, the lab will return a full contour wax pattern to the student, who is then required to recontour the wax to receive RPD components: guide plane, rest, height of contour and undercut for clasp placement. Faculty must approve the wax pattern modification. When the final crown is returned, the student and faculty member must resurvey the crown, and if necessary, correct contours by reshaping with burs or porcelain addition, then reglaze and polish, before cementation. When multiple abutment crowns are fabricated simultaneously on multiple master casts, it is strongly advised that a diagnostic alginate impression is made of the abutment crowns in place I the mouth before they are cemented; a quick setting cast can be fabricated and abutment contours can be checked by surveyor. The RPD final impression can be fabricated immediately after the abutment crowns have been cemented.
Custom Impression Tray
Prior to the tooth modification / final impression appointment, a custom impression tray is fabricated. The tray outline is drawn with pencil on the diagnostic cast. A single sheet of spacer / block out (baseplate wax) is placed over soft tissue, and two sheets are placed over the remaining teeth. Stops are created in 3-4 areas (non-critical tooth occlusal/incisal surfaces are preferred over soft tissue surfaces) by removing all layers of wax. The cast and wax are lubricated, then Triad material is adapted to the cast, carved to the pencil outline, and a vertical handle is adapted. The Triad is light cured for 2 minutes externally and 2 minutes internally, and the wax spacer layers are removed from the tray while it is still warm from the light curing process. The tray is trimmed and smoothed with acrylic burs; large impression material retention holes are placed through the tray plastic (20 for lower trays, 30 for upper trays).