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After studying this chapter, the student will be able to do the following:
1. Discuss the factors that affect treatment planning for a fixed indirect restoration.
2. Explain the lost-wax casting process used in dentistry to make metal restorations.
3. Describe the types of alloys used to make all-metal crowns, ceramometal crowns, and partial denture frameworks.
4. Recall the types of porcelain used to simulate the color of teeth.
5. List the advantages and disadvantages of all-metal, ceramometal, and all-ceramic restorations.
There are many types of fixed indirect restorations. Indirect restorations are those restorations that are constructed outside the mouth. Fixed restorations cannot be removed from the oral cavity; they are luted (cemented) in place. Fixed indirect restorations can be classified in two ways: by the amount of tooth structure they restore or by the material from which they are made.
I. Classification by Amount of Tooth Structure Restored
Inlays are intracoronal (inside the crown) restorations that replace small to medium amounts of tooth structure, as shown in Figure 1.3. They are most commonly used to restore pits, fissures, and grooves (class I) as well as proximal surfaces (class II) of posterior teeth. They do not restore the cusps. They are retained by luting cements.
Onlays (or overlays) involve replacing more tooth structure than inlays. In addition to the pits, fissures, and proximal surfaces of an inlay, onlays restore one or more cusps and, at times, the entire occlusal surface of a tooth. Onlays are used when the likelihood of cusp fracture is high. The onlay protects the cusps from occlusal forces. Onlays, like inlays, are retained with intracoronal retention and luting cements. An onlay is shown in Figure 10.1.
FIGURE 10.1. Photograph of several teeth restored with 30-year-old cast gold restorations. Tooth #3 is restored with an inlay. Tooth #4 is restored with an inlay/onlay combination. Tooth #5 is restored with an onlay.
A veneer is a thin layer of material that covers another material (like the shell of a hard-boiled egg). In dentistry, veneers are restorations that are placed on the facial surface of anterior teeth to treat an esthetic problem, such as discolorations, rotations, or spaces (diastemata) (Fig. 10.2). Veneers are often used in conjunction with orthodontic or periodontal treatment. Two types of veneers are used.
FIGURE 10.2. Photographs of teeth A. prepared for indirect veneers and B. the veneers cemented in place. (Courtesy of Ultradent Products, Inc.)
1. Direct veneers use bonded composites, as shown in Figure 5.9. Direct veneers may not involve the removal of significant tooth structure. If tooth structure is not removed, such veneers are considered to be a reversible treatment.
2. Indirect veneers use a ceramic material, such as porcelain. Usually, the facial surface is prepared to provide space for the veneering material. Therefore, indirect veneers are not a reversible procedure. Indirect veneers involve an impression, sometimes a temporary restoration, a second appointment, and a laboratory fee. Therefore, they cost more overall than direct veneers. At one time, composite materials were also used, but discoloration was a problem. Today, porcelain is popular because a single veneer can utilize a variety of porcelain shades and translucencies. Indirect veneers are supported by, and are bonded to, the underlying tooth structure.
Crowns are used to restore teeth that have lost a significant amount of tooth structure. They are used when intracoronal retention is unavailable or when the tooth needs to be surrounded and held together by the restoration. A full gold crown is shown in Figure 1.4. A porcelain crown is shown in Figure 1.10F. Ceramometal crowns are shown in Figure 10.3.
FIGURE 10.3. Photographs of porcelain-fused-to-metal crowns restoring teeth #7 and #10. A. Labial view of both teeth. B. Lingual view of tooth #10.
E. Complex Restorations
Complex restorations combine the features of inlay, onlay, and crown restorations, as shown in tooth #4 of Figure 10.1. A complex restoration is designed by the dentist after caries, and the loss of tooth structure has been assessed.
F. Dental Bridge
A bridge replaces missing teeth. Typically, a bridge has a crown, called a retainer, at each end. The retainers are supported by teeth called abutments. The replacement teeth are called pontics. A bridge may replace one tooth or several teeth, as shown in Figure 1.5.
II. Classification by Material
A variety of metals are used to restore teeth, as shown in Figures 10.1 and 10.4. Most metallic indirect restorations are made by a casting procedure. Casting involves melting the metal and then pouring or forcing the liquid metal into a mold. The casting process allows custom, complex shapes to be easily produced. Metals are very tough and work well in high-stress situations, but their esthetics are poor.
FIGURE 10.4. Two examples of the need for an indirect restoration. A. A fractured lingual cusp. B. A tooth #30 that has had a multitude of dental treatment, including root canal therapy, root amputation, and a crown. This tooth was deemed to be hopeless and extracted because of pain, mobility, and a periapical radiolucency.
B. Ceramic Materials
Ceramic materials are used when esthetics are important, as shown in Figure 1.10. Ceramic materials can simulate the natural colors and translucency of teeth. Porcelain is the ceramic material most commonly used in dentistry to obtain a variety of colors or shades for a single crown. A variety of materials and processing techniques are employed. Most ceramic materials lack the toughness and fracture resistance required by bridges, but their esthetics can be excellent.
A metal–ceramic combination was developed for dentistry in the 1950s. This material is made by using the same process to bake enamel (porcelain) on metal as that was used to make old-fashioned pots and pans or sinks and bathtubs. The tough, strong metal supports the weak but esthetic ceramic material. The restorations are called ceramometal restorations (or porcelain-bonded-to-metal or porcelain-fused-to-metal crowns) and are considered to be an important “workhorse” in modern restorative dentistry. Ceramometal restorations are shown in Figure 10.3.
D. Composite Materials—Optional
Particle-reinforced composite materials (very similar to those discussed in Chapter 5, Direct Polymeric Restorative Materials) have been adapted for use as fixed indirect restorations. These materials are polymers reinforced by irregularly shaped particles, as shown in Figure 5.8C. They are cured at elevated temperatures, pressures, or both in the dental laboratory. Particle-reinforced composites have seen only limited success because their strength and toughness do not meet those required by crown and bridge materials.
Recently, fiber-reinforced composites have been developed for dentistry. These composites are similar to materials that have been developed for many other industries in which glass fibers are used to reinforce polymeric materials. Imagine a bowl of frozen, leftover spaghetti with sauce. The spaghetti represents the glass fibers; the frozen sauce represents the polymer matrix. The long fibers transfer stresses over a larger area than particles do. Automotive, aerospace, and sports equipment companies use fiber-reinforced composites to manufacture strong, lightweight products. Fiber-reinforced composites are much tougher and stronger than particle-reinforced composites. Clinical results are promising, but these materials are not commonly used.
III. Procedures for Constructing an Indirect Restoration
A. Diagnosis, Treatment Planning, and Designing the Restoration
Lost tooth structure and missing teeth result from a variety of causes. Caries and periodontal disease are the most common. An extreme case involving caries and periodontal disease is shown in Figure 11.10. Trauma or fracture (Fig. 10.4A), esthetic concerns (Fig. 1.10), and congenital deficiencies (Fig. 10.2A) are other reasons to restore or replace teeth. Often, teeth are lost as the result of restoration and re-restoration and multiple dental treatments (Fig. 10.4B).
2. Treatment Planning
A variety of factors affect the success of a restoration. These factors must be assessed during treatment planning.
a. The periodontal status of the patient is critical. Without a stable foundation, the long-term success of any restoration is unlikely.
b. The pulpal (endodontic) status of the tooth must be considered. A simple cold test for vitality is commonly used. A periapical radiograph is important to assess the periodontal and periapical condition of the tooth.
c. Caries risk assessment and testing is becoming more common. If a crown or bridge will likely fail in several years because of recurrent caries, efforts to reduce the risk of caries should precede extensive and expensive restorations.
d. Enough tooth structure must remain to retain the restoration. Badly broken-down teeth may require buildups (also called cores) to provide retention of the final restoration, as shown in Figures 6.11 and 8.1A.
3. Restoration Design
Many factors influence the design of a restoration.
a. The most common factor is the patient’s desire for an esthetic restoration. Esthetic requirements include the shade and translucency of the restoration and influence the selection of the restorative material to be used.
b. The attrition (wear) rate of the restoration should be considered.
c. Biocompatibility of the material must be considered.
d. Location of the margins of a restoration may be placed above the gingiva or in the gingival sulcus. Margin location affects esthetics, retention, periodontal response to the restoration, and likelihood of recurrent caries.
The tooth or teeth are prepared with specific restorative materials in mind. Different materials require different amounts of tooth reduction and shapes of the margins. Amalgam and inlay preparations are shown in Figure 1.3. Crown preparations are shown in Figures 1.10B and 8.1A.
After the preparation is complete, an impression of the preparation, adjacent teeth, and opposing teeth is taken. Full-arch impressions result in more accurate articulation of models and less adjustment of a complex restoration before cementation. Several impressions were shown in Figure 1.8A and in Chapter 8.
D. Fabrication of a Temporary Restoration
The patient will not like the appearance and feel of the prepared tooth. In addition, exposed dentin can be very sensitive to heat, cold, and air. A temporary restoration (also called a provisional restoration) is constructed and cemented at the time of preparation, as shown in Figure 1.10C. The temporary restoration is left in place until the final restoration is received from the lab and permanently cemented. Chapter 35 presents techniques for the fabrication of a temporary crown.
E. Laboratory Procedures
The description and design of the restoration is written on a laboratory prescription (Fig. 10.5). This prescription becomes a part of the patient record. Impressions are disinfected and then poured in the dental office or a commercial laboratory. The laboratory constructs the final restoration according to the dentist’s instructions. A variety of restorations and prostheses are shown in Chapters 1, 10, and 11.
FIGURE 10.5. A laboratory prescription. (Courtesy of Christopher DiConcilis, Smithfield, PA.)
F. Cementation of the Restoration
At a second appointment, the temporary crown is removed, and the temporary cement is cleaned from the prepared tooth. Scaling instruments or pumice and a prophy cup are used. “Try-in” of the restoration is next. The restoration is seated on the preparation and carefully evaluated. The proximal contacts are checked with floss. The margins are examined with an explorer. The occlusion is evaluated with articulation paper, and the esthetics are examined with the patient’s input. If all is acceptable, the restoration is cemented (see Fig. 7.1). Completed crowns are shown in Figures 1.4 and 1.10E.
G. Completion of the Treatment Plan
After the restoration is cemented, the remaining treatment plan is completed. The final step in the treatment plan should be to determine the recall status of the patient based on periodontal concerns, caries risk, chronic problems, and oral hygiene home care practices.
IV. Casting Process—Optional
Dentistry has used the “lost-wax casting technique” for a century to produce metal restorations. This process first constructs the restoration in wax. The wax is then replaced with metal in the same shape and size as the original wax pattern. The lost-wax casting technique is shown in Figure 10.6.
FIGURE 10.6. Photographs showing the fabrication of a full gold crown (the same crown as shown in Figs. 1.4 and 8.1). A. Wax pattern with the sprue attached. B. Wax pattern attached to the sprue base (also called crucible former) and the casting. Note the reproduction of shape and size. C. Wax pattern and casting ring with a paper liner. The casting ring slips over the wax pattern and fits into the sprue base. D. Sprued and invested wax pattern inside a sectioned casting ring. E. Cross section through a casting ring after burnout and a completed casting. F. Completed casting on the die.
A. Wax and Waxing
Wax is easy to mold and shape. All that is required is a heat source and a few simple instruments. Creating the shape of the restoration in wax (wax pattern) requires skill and involves both the art and the science of dentistry, as shown in Figure 10.6A. The art is creating a reproduction of the shape of the lost tooth structure. The science is the requirement for the shape to function as the original tooth structure did (or should).
2. Dental Waxes
Dentistry uses a variety of waxes with different melting, working, and handling properties. Several are shown in Figure 10.7. The composition of a wax product is usually proprietary and is typically a combination of natural and synthetic waxes. Wax is very soft compared to other dental materials. Distortion of wax is not typically visible to the unaided eye, but wax is unstable. A wax pattern can distort over time to a degree sufficient to affect the fit of restorations.
FIGURE 10.7. Photograph of several dental waxes: rope wax (top), baseplate wax (middle left), sticky wax (middle center), inlay wax (middle right), and boxing wax (bottom).