Tooth Modification and Final Impression
Preparation of RPD features in natural teeth should be conservative, and routinely local anesthesia is not used. Carbide burs are recommended for enamel/dentin/composite/metallic restorations; diamond burs are recommended for porcelain. Patients who have existing crowns on abutment teeth, requiring modification, should be informed of the consequences of perforating a crown (usually the perforation is patched with direct restorative material) and of fractured porcelain (smooth and re-polish small fractures, new crown required for large fractures). Guide planes are prepared with a cylindrical bur or diamond and are aligned with the path of insertion (conveniently assessed with a periodontal probe). Posterior tooth bounded spaces should have conservative guide planes; anterior guide planes should be full length if the cervical aspect of the abutments constrict, to reduce or eliminate proximal dark triangles between natural and artificial teeth. For distal extension abutments, a conservative guide plane is placed in the occlusal half of the distal proximal surface, to facilitate stress release of the clasp assembly under heavy occlusal loading of the RPD. Axial recontouring often accompanies guide plane preparation, in order to optimize height of contour location. Rests are prepared to create positive apical seat and minimum clearance of 1.5mm. Anterior lingual rests should be positioned apical to areas of occlusal function; posterior occlusal rests are usually located near centric contacts, and reduction of opposing surfaces during tooth modification may be necessary to achieve the full clearance. Anterior pothole rests are created with #4 round bur, finished with #6; posterior occlusal rests are created with #6 round bur, finished with #8; cingulum rests are prepared with a flat-end bur, such as inverted cone or cylinder. Retentive dimples are created with a #4 round bur. All modifications should be polished before final impression.
Prior to the impression, the teeth should be assessed for cleanliness, and if necessary, the teeth should be scaled and polished with pumice. Border molding is recommended for the floor of the mouth and for distal extension areas. Polyvinyl siloxane light body impression material is recommended, with corresponding tray adhesive. Prior to the impression, take care to block out severe undercuts and proximal embrasures with wax or light cured block out material, to avoid locking the impression in the mouth. The mouth is dried and impression material is injected into rests, around the abutment teeth, and over the occlusal surfaces. The loaded tray is seated onto its stops and held immobile until the material sets. The impression is removed from the mouth and inspected. If acceptable, it is disinfected and sealed in a baggie for transport to the lab. Submit a prescription requesting master cast fabrication (die stone).
Master Cast and Chrome Lab Prescription (Laboratory)
Casts intended for RPD framework fabrication should be highly detailed, contain all prosthetic anatomy and landmarks, be made of vacuum-mixed die stone without voids, inclusions or debris, should have a 1" thick and continuous base, trimmed close to the buccal and labial vestibules with 3mm wide land area, smooth flat tongue area, and no sharp projections. The cast must be mounted with an appropriate opposing cast, with indexing between master cast and mounting stone; the cast should be separated from the mounting stone. The master cast is surveyed and the design from the diagnostic cast is transferred onto the master cast. The framework drawing on the master cast should be detailed, sharply defined and accurate. The framework design is also drawn on the laboratory prescription, with a concise written description of the major connector, each clasp assembly, and any non-routine details.
Typical design variations include:
Lack of inter-arch space - Because passive eruption of unopposed natural teeth can result in diminished occlusal space for prosthetic replacement, the following tactics are recommended:
When the design is complete, the cast is reattached to its mounting with external glue or sticky wax, and the case is submitted to the lab with articulator and instructor-approved prescription.