If the onlay, inlay, crown, or bridge is to be made in porcelain or porcelain fused to metal (PFM), matching the shade of the natural teeth is very important and is completed at the first appointment, before the procedure is started. A shade guide, which contains samples of all available shades, is used to match the natural tooth color (Figure 23-1).

Because teeth remain slightly moist from saliva continuously, the color sample from the shade guide should be moistened as well. The shade guide is held close to the teeth to be restored and is compared in a natural light setting. To ensure an exact match, it is best to use natural light. The shade selected is identified by number from the shade guide and is noted on the patient’s record and on the laboratory prescription. The shade guide is a semicritical item that cannot withstand the heat of sterilization. After use, it must be disinfected.

During tooth preparation, the height and contour of the natural tooth or teeth are reduced to remove any diseased tooth structure and to prepare the tooth for the casting. The prepared tooth is shaped so that the cast restoration can fit down over the preparation to resemble its natural tooth (Figure 23-2).

If the coronal portion of the tooth is extensively decayed, fractured, or worn, it may be necessary to provide additional support for the crown.

Core Buildups

A core buildup is the use of amalgam or core material, which is placed on the prepared tooth to provide enough structure to hold a fixed restoration (Figure 23-3). If an amalgam restoration is already in place, this may be shaped and prepared for use as the core.

Post and Core

If the tooth has been treated endodontically, a post is placed into the pulp canal. The core is then built up around the post. This provides additional strength and stability for the crown (Figure 23-4).

For the laboratory technician to create an accurate fitting cast, a final impression is taken to prepare the working cast from stone. As stated in Chapter 22, the final impression includes detail of the preparation that extends slightly below the finish line of the preparation. The best way to achieve this detail and not harm the tissue is with the use of gingival retraction cord.

Gingival retraction cord temporarily pushes the gingival tissue away from the tooth and widens the sulcus. This allows the impression material to flow around all parts of the preparation (Figure 23-5).

Retraction cords are available as untwisted, twisted, or braided. For easier application, the untwisted cord is moistened in water or a vasoconstrictor, and then is twisted just before placement. Once placed, the cord will expand and open the sulcus, ready to receive the impression material.

To control hemorrhage from the preparation procedure, the retraction cord can be impregnated (saturated) with an astringent and a vasoconstrictor. A vasoconstrictor controls bleeding and short-term constriction of the tissue. Special caution must be taken if the patient has cardiovascular disease; the patient should not receive any retraction cord with a vasoconstrictor.

Following the time period of the gingival retraction cord within the sulcus, the retraction cord is removed, the sulcus is rinsed and dried, and the final impression is taken of the prepared tooth and surrounding tissue. Refer to Chapter 22 on the procedure for taking final impressions.

See Procedure 23-1: Placing and Removing Gingival Retraction Cord (Expanded Function).

Provisional coverage, also known as temporary coverage, is a protective covering that is temporarily cemented to the prepared tooth for a single crown or to the abutment teeth for a bridge (Box 23-1).

Provisional coverage is worn until the cast restoration or bridge is returned from the dental lab and readied for permanent cementation. This may take from several days to a few weeks. (The cements used for temporary placement of provisional coverage are discussed in Chapter 20.)

Types of Provisional Coverage

Several types of provisional coverage are available. The dentist will determine the type of temporary based on the needs of the patient. Table 23-2 provides the types most commonly used and their description. In most states, construction and cementation of temporary coverage can be delegated to the certified dental assistant.

TABLE 23-2

Types of Provisional Materials

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Types	Description
Intermediate restorative material (IRM)	Used as provisional for an inlay or onlay preparation. This material is adapted to the prepared area using condensers and carvers to bring about normal anatomy, and then allowed to harden.
Pre-formed polymer crown	These shell-like crowns are available for posterior single crowns and for bridgework. The prostheses are supplied with a hybrid composite resin that bonds with the pre-formed crown.
Pre-formed polycarbonate crown	Made from a strong acrylic resin, the polycarbonate crown is manufactured in a variety of sizes for anterior teeth where appearance is important. The crown is adapted to fit the prepared tooth using an acrylic lab bur and a Burlew wheel, and then is cemented.
Custom acrylic temporary	Made from a tooth-colored acrylic resin, the material is loaded in a vacuum-formed custom tray or alginate impression, seated onto the prepared area, allowed to set, removed from the tray, finished using acrylic burs and finishing stones, and then cemented to provide temporary coverage of a single crown or bridge. See Procedure 23-3.

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See Procedure 23-3: Fabricating and Cementing a Custom Acrylic Provisional Crown (Expanded Function).

At the completion of the first appointment, a laboratory prescription is completed and sent to the lab along with the final impression and bite registration. The laboratory technician can fabricate a single crown or a bridge on the basis of a written prescription from the dentist (Box 23-2). Also known as a work order or requisition, one copy of the prescription is included with the case, and another copy is retained with the patient records (Figure 23-6).

The basic components of a removable partial denture are described in Table 23-3 and shown in Figure 23-9.

Because this must be an exact impression, a custom tray is made and an elastomeric impression material is used. The dentist chooses the type of tray, the material, and the impression technique to be used (see Chapter 22).

Even though accuracy is extremely important, the use of gingival retraction is not required because for a removable prosthesis, the teeth are not prepared below the gingival margin.

In addition to the final impression, it is necessary to take an impression of the opposing arch and to obtain an occlusal registration. Most commonly, this is an occlusal registration, and the technique is described in Chapter 22.

After the final impression, the shade (color) and mold (shape) of the teeth are determined. The manufacturer of the artificial teeth provides a shade guide. To identify the teeth, the mold and shade numbers are imprinted by the manufacturer on the back of each tooth in the shade guide (Figure 23-10).

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