In general, a fixed bridge needs a metal support for strength. The veneer coating may be acrylic, composite, or ceramic. Newer ceramic materials, including alumina, leucite, and zirconium, have increased strength that in some cases may eliminate the metal substructure.

In general, the metal-ceramic crown combines certain favorable properties of metal in its substructure and of ceramic in its veneer coating.

Advantages

Disadvantages

The amount of tooth reduction necessary for the metal-ceramic crown depends on the metal and ceramic thickness. The necessary thickness of the metal is 0.5 mm, whereas the minimal ceramic thickness is 1.0 to 1.5 mm. Therefore, the tooth reduction is approximately 1.5 to 2.0 mm. With this porcelain-metal sandwich, a shoulder preparation is generally necessary for adequate tooth reduction.

If the tooth reduction is less than 1.5 mm at the marginal area, only metal can be present in that area. If porcelain is applied on metal that has been reduced in thickness because of lack of space, marginal metal distortion is likely during the firing cycle. If the porcelain thickness is reduced to compensate for the reduced space, the opaque porcelain layer is likely to be exposed or to dominate, leading to an unaesthetic result. If both the porcelain and metal have adequate thickness, then the crown is overcontoured (Fig. 9-3).

There are many techniques with which to construct a porcelain margin with optimal aesthetics, proper fit, and correct contour (emergence profile).

It is possible to cover the metal correctly with porcelain in the marginal area, but most often the aesthetic results fall short of expectation in the most critical area. Incident light that transmits through the porcelain and reflects from the metal often creates a shadowing effect. If porcelain is present only at this marginal area, light transmission and reflection through the porcelain and the tooth create the proper blend between the marginal aspect of the crown and the tooth.

Originally the metal was finished slightly shy of the edge of the shoulder, with porcelain extending to the edge. Another technique finished the metal at the axiocaval line angle of the preparation, creating a porcelain margin that totally covered the horizontal shoulder. With both techniques, however, shadowing was still present. To create proper light transmission and reflection of the porcelain-tooth interface, the metal should be finished to about l to 2 mm above the axiocaval line angle of the shoulder (Fig. 9-4).

Noble alloys in general do not oxidize on casting. This feature is important in a metal substrate so that oxidation at the metal-porcelain interface can be controlled by the addition of trace oxidizing elements. If oxidation cannot be controlled on repeated firings, porcelain color may be contaminated, and the bond strength may be weakened. Noble alloys are gold, platinum, and palladium. A silver alloy that oxidizes is considered semiprecious.

The base metal or nonprecious alloys most often used in the construction of a metal-ceramic crown are nickel and chromium. Because these alloys readily oxidize at elevated temperatures, they create porcelain to metal interface problems. The oxidation must be controlled by a metal-coating treatment, which is somewhat unpredictable. Casting and fitting are also difficult. Authorities agree that a noble alloy is preferable.

Ceramic adheres to metal primarily by chemical bond. A covalent bond is established by sharing O₂ in the elements in the porcelain and the metal alloy. These elements include silicon dioxide (SiO₂) in the porcelain and oxidizing elements such as silicon, indium, and iridium in the metal alloy.

The criteria for selecting a specific porcelain include:

The elements in the opaque layer create the chemical bond of the porcelain to the metal substrate. The opaque layer masks the color of the metal and is the core color in determining the final shade of the crown.

Opacious dentin is an intermediary modifying porcelain that affords better light transmission than the opaque layer, in part because of its optical properties. Opacious dentin is less opaque than the opaque layer but less translucent than the body (dentin) porcelain. It is also used for color shifts or effect properties.

The principal difference between shoulder and body porcelain is the firing temperature. Because the shoulder porcelain is established before the general buildup, its color and dimension must remain stable during subsequent firings. Therefore, the shoulder porcelain matures at a higher temperature than the subsequent body porcelain firings.

Segmental buildup refers to the method of applying the porcelain powders in incremental portions horizontally. Each increment differs from the others in opacity and translucency or hue, value, or chrome. This technique is used to construct a crown that attempts to mimic the optical properties of a natural tooth (Fig. 9-5).

The coefficient of thermal expansion is the exponential expansion of a material as it is subjected to heat. The coefficient is extremely important during joint firing of two dissimilar materials. For example, the coefficient of thermal expansion should be slightly higher (rather than the same) for the metal substrate than for the porcelain coating. This slight difference results in compression of the fired porcelain coating, which gives it greater strength.

The purpose of the metal coping is to ensure the fit of the crown and maximize the strength of the porcelain veneer. The metal must have the proper thickness so as not to distort during the firing. The coping should be reinforced in load-bearing areas, such as the interproximal space and can be strengthened in areas in which metal exists alone, such as the lingual collar. To maximize the strength potential of the porcelain, uniform thickness should be attempted in the final restoration. This thickness can be obtained by designing the wax-up of the framework to accommodate the porcelain layer.

The marginal tooth preparation determines the marginal configuration of the metal-ceramic crown. The three options are:

The design of the metal framework must incorporate four basic interrelationships—strength, aesthetics, contour, and occlusion. In fixed bridgework, however, strength of the substrate plays the dominant role. Therefore, greater attention must be paid to reinforcement of the framework than of a single unit.

Metamerism is the optical property whereby two objects with the same color but different spectral reflectance curves do not match. This property is important in matching the shade of the metal-ceramic restoration to the natural tooth. Even if the colors are the same, different reflectance curves can create a barely noticeable difference.

Fluorescence is the optical property whereby a material reflects ultraviolet radiation; it reflects different hues. Natural teeth can fluoresce yellow-white to blue-white hues. Fluorescence in porcelain is important to minimize metamerism of porcelain to natural teeth in varying light conditions.

Color consists of three properties:

The properties have a practical use in ordering color.

Opalescence is the optical property seen in an opal during light transmission and light reflection. During transmission, the opal takes on an orange-white hue, whereas during reflection it takes on a bluish-white hue. This phenomenon also occurs in the natural tooth as a result of light scattering through the crystalline structure of the opal. The structure size is in the submicron range (0.2–0.5 μm). A porcelain restoration can demonstrate the opal effect by incorporating submicron particles of porcelain into the enamel (incisal) layer.

There is no truly scientific method to analyze the shade of a natural tooth and apply this information to the selection of porcelain and fabrication of the crown. Attempts to establish such a technique have met with limited success. At present, shade determination is designed to match natural teeth with a synthetic replication (shade guide) that results in a range of acceptability rather than an absolute match. The most widely used guide is the VITA Classical Shade Guide by Vident.

The 3D guide was developed over 10 years ago based on the three dimensions of hue, chroma, and value. The VITA 3D-Master Shade Guide (Vident) improves accuracy in selection based on a value numbering system of 0 to 5, with 0 being the brightest and 5 the darkest shade. Chroma is also categorized from 1 to 3, with 1 being the lowest and 3 being the highest. Hue is categorized as left, middle, or right (orange to red).

Digital photography for shade analysis has also been used. Digital photography with Adobe Photoshop has been used in shade selection. Images of the compared tooth and the shade guide are taken in the same vertical plane in a RAW file format. The images are then opened in Photoshop and the background area is turned to black. A duplicate image is made and turned to gray scale. The “HSB” color model is selected in the “INFO” palette, and when the cursor is placed on the shade guide or the compared tooth, the hue will be given by an “H” number, chroma by the “S” number, and the value by the “B” number. The difference in these parameters can be measured between the tooth and the guide. The ceramist can use this information to choose the most appropriate porcelains to match the tooth structure.

Computerized digital shade technology and software are used in conjunction with visual shade guides and digital photography. Instruments store data for various shade guides and use fiberoptics along with spectrometers to aid dentists in determining the tooth shade and color zones. Light waves are sent through the tooth and refracted or reflected back to the receiver in the spectrometer. The digital spectrophotometers determine the shade, hue, chroma, and value. VITA Easyshade Compact by Vident and Crystaleye by Olympus America are two product examples.

External stains or colorants are frequently used to minimize the differences between natural and ceramic teeth. They should be used rationally rather than empirically. An understanding of the color phenomenon is necessary in all aspects of shade control and is essential if extrinsic colorants are to be used correctly. Extrinsic colorants follow the physical laws of subtractive color.

The understanding of hue, value, and chroma and their effect on external staining of a crown are essential. The major guidelines are as follows:

Hue: Drastic change of the shade of the ceramic restoration by use of external colorants is often impossible. Slight changes in shade may be accomplished (e.g., orange to orange-brown).

Value: External colorants can be used to lower the value of the ceramic. The complementary color of the shade to be altered may have a darkening effect. It is almost impossible to increase the value or shade of the ceramic.

Chroma: Chroma can be successfully increased by external colorants, usually in the gingival or interproximal areas.

External colorants are metallic oxides that fuse to the ceramic unit during a predetermined firing cycle. Although stable in an air environment, they are susceptible to corrosion when subjected to certain oral environments. Depending on the stain and pH of the oral fluids, external colorants may be lost from the ceramic unit over a long period of time.

The most important factor in the strength of a ceramic material is control of small flaws or microcracks, which often are present at the surface and internally. In most cases, the strength of the ceramic depends on surface flaws rather than porosity in the normal range.

In general, the surface hardness of dental porcelains is greater than that of tooth structure, metal alloys, and all other restorative materials. This may lead to excessive wear of the opposing dentition if certain occlusal guidelines are not followed. In the best scenario, the opposing material is porcelain, but results are good if the occlusal loads have good force distribution. Porcelain is contraindicated in patients who engage in bruxism or parafunctional activities in which occlusal overloading may occur.

It is now possible to bond composite or ceramic materials to a fractured restoration. The bond, which may occur on porcelain or on the metal substrate, is sufficiently strong to be resistant in a non– or low stress-bearing area. However, if the fracture occurs in a stress-bearing area, the probability of a successful repair is low.

All-ceramic crowns have been frequently used. As with their predecessor, the porcelain jacket crown, which was introduced at the turn of the century, the main reason for their use is superior aesthetics. Unlike the metal-ceramic crown, which is hindered by the metal substrate, the all-ceramic crown can mimic the optical properties of the natural tooth. However, all other factors—including strength, fit, ease of fabrication, and tooth selection and preparation—may inhibit its use.

The same amount of overall tooth reduction is needed for a metal-ceramic restoration as for an all-ceramic crown (1.0-1.5 mm labially, lingually, and interproximally). However, unlike the metal-ceramic restoration, which will accept any marginal design, marginal tooth preparation for the all-ceramic crown must be a shoulder or deep chamfer (minimum of 1.0 mm tooth reduction) (Fig. 9-6).

BruxZir Solid Zirconia (Glidewell Laboratories) is a monolithic zirconia restorative material used for crowns, bridges, screw-retained implant crowns, inlays, or onlays with no porcelain overlay. The material was originally designed to provide a more durable and aesthetic alternative to posterior metal occlusal PFMs (porcelain fused to metal crowns) and cast gold restorations for demanding situations such as bruxers or restorations with limited occlusal space. These restorations are milled through CAD-CAM (computer-aided design–computer-aided manufacturing) technology and are meant to be fracture- and chip-resistant. Marginal preparation can be feather-edged and a shoulder is not necessary. A conservative preparation is indicated, similar to a full-cast gold crown, so any preparation with at least 0.5 mm of occlusal reduction is acceptable. Occlusal reduction of 1.0 mm is ideal. Labial, lingual, and interproximal reduction is 1.0 mm. Tooth preparation for a BruxZir Solid Zirconia crown is shown in Figure 9-7.

Some manufacturers claim that the newer ceramic materials with high theoretical strength values can be used in place of metal-ceramic restorations for any tooth and for small-unit, anterior fixed bridges. However, the guidelines for usage, such as tooth preparation, are more critical and in general more complicated than for metal-ceramic restorations. It is advisable, therefore, to use the all-ceramic crown in the anterior segment, in which aesthetics is the dominant factor (Fig. 9-8).

All-ceramic crowns may be categorized by composition and method of fabrication: