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After performing the laboratory/clinical exercises in this chapter, the student will be able to do the following:
1. List the differences between the acrylic resin and visible light–cured (VLC) resin tray fabrication techniques.
2. Briefly describe the characteristics of an exothermic reaction.
3. Name the three purposes of an impression tray.
4. Recall the four reasons why a dentist may choose to fabricate a custom tray for a patient rather than using a stock tray.
5. Explain the purpose of the occlusal stops that are designed in a custom impression tray.
6. Discuss two methods of trimming a custom tray.
visible light–cured (VLC) resins
Custom impression trays can be constructed of several types of resins. Autopolymerizing acrylic resins and visible light–cured resins are two types of useful materials for the production of custom impression trays, orthodontic appliances, temporary restorations, and denture fabrication. These materials are discussed in detail in Chapter 5, Direct Polymeric Restorative Materials.
Autopolymerizing resins (Fig. 29.1) may be formed by adding measured amounts of the monomer liquid to the polymer powder and then mixing and molding the resin into the desired shape before the initial set. When the material cools from the exothermic phase (material gives off heat) of the setting reaction, it may be removed from the die, cast, or model and then trimmed and/or polished.
FIGURE 29.1. Tray resin powder and liquid supplied in bulk form and in single-use packaging.
The visible light–cured (VLC) resins are supplied in lightproof plastic packages. After the package is opened, the material is molded to the desired shape and then made rigid by exposure to high-intensity visible light in a light-curing unit.
The purpose of an impression tray can be remembered by using “the 3 Cs”—to carry, control, and confine the impression material. Custom-made impression trays made from either acrylic or VLC resins are used in final impressions for crown and bridge restorations and for denture construction.
Custom rather than stock trays are used for the following reasons:
- The operator can better control the thickness of the material.
- Custom trays can accommodate any anatomic anomaly, such as large lingual tori.
- Custom trays are more stable than stock trays.
- Custom trays can be used again for the same patient.
These are accomplished best with custom impression trays. Use of custom trays is described in Chapter 8, Impression Materials. A custom tray requires a preliminary impression to produce a preliminary cast.
Tips for the Clinician
- If adding a handle to an autopolymerizing resin tray, wet the tray in the area of intended handle placement with monomer. This will enhance the bond.
- Make sure the resin is a doughy consistency before manipulating it. If the resin is a viscous consistency, it will be difficult to adapt into the shape of a tray.
- Construct the handle in the proper shape, size, and position to be functional for taking the impression.
II. Tray-Spacing Procedure (for a Dentulous Custom Tray)
For this discussion, we will assume that fabrication of a custom tray will be a laboratory exercise, with the impression being taken on a Dentoform rather than on an actual patient. For this reason, fabrication of a quadrant tray will be presented so that the subsequent impression can be easily removed from the Dentoform. In clinical practice, however, custom impression trays are usually made as a full maxillary or mandibular arch tray rather than as a quadrant tray for greater stability and ease of articulation. The VLC tray is illustrated with this technique.
Impression materials need a uniform thickness to produce the most accurate casts. The construction of a custom tray uses a blockout procedure and “occlusal stops” to accomplish this uniform thickness. These procedures are the same for both autopolymerizing and VLC resin trays. The blockout procedure adds wax to eliminate or “block out” any cast undercuts that would lock the set custom tray to the cast. The same wax provides an even space for impression material. Stops are holes in the blockout wax that result in a precise seating of the custom tray. The exact location of stops, thickness of wax, and extent of the wax and impression tray depend on the patient, impression material, and clinician’s personal preferences. The following is a general technique useful for most patients. The items necessary to prepare a cast for custom tray construction are listed in Table 29.1.
TABLE 29.1. Armamentarium for Custom Tray Blockout
A. Coat the preliminary cast with Foilcote (Whip Mix Corp., Louisville, KY) or a similar separator material (see Fig. 29.2).
B. Using a Bunsen burner or hot water, soften a piece of baseplate wax that is approximately 2 × 3 inch in size for the quadrant tray. Be careful not to overheat the wax, as this could result in unwanted thin areas.
C. Loosely adapt the warm wax over the tooth to be restored and adjacent teeth (see Figure 29.3). Extend the wax onto the buccal and lingual tissues 3 to 4 mm beyond the gingival margin of the teeth. Adapt a second layer of wax on top of the first. This will give a space of about 2 to 3 mm for the impression material. Trim the excess wax with a warm dental lab knife.
D. Provide occlusal stops by cutting three 1- to 2-mm openings in the wax on the occlusal portion of two teeth, not teeth to be prepared for restoration. When preparing a full-arch custom tray, three occlusal stops should be cut in a tripodal configuration: two posterior stops and one anterior stop. The cut openings in the wax for occlusal stops are shown in Figure 29.4. They allow the tray material to flow into these openings and create a “bump” or a protuberance on the tissue side of the tray. When the tray is made, the tray will seat until these occlusal stops or bumps fit onto their mating surfaces of the teeth in a stable and predictable manner. Precise seating of the tray will provide space for a uniform layer of impression material. Figure 29.5 illustrates this result in cross section. Occlusal stops are pictured on the inside of a tray in Figure 29.6.
E. Place a layer of aluminum foil, tin foil, or foil substitute separating medium over the wax to ensure the tray material separates from the blockout after fabrication. The inside surface of the tray should not be contaminated with wax (Fig. 29.7). Your instructor (or future employer) will select the separating material to be used.