Shape

Sharp angles or pits on the veneering surface of a metal-ceramic restoration should be avoided because they can contribute to internal stress in the final porcelain.⁶ Convex surfaces and rounded contours should be created so that the porcelain is supported without development of stress concentrations (Fig. 24-3). In addition, a smooth surface facilitates wetting of the framework by the porcelain slurry.

Fig. 24-3 Substructure design for an anterior partial fixed dental prosthesis. The metal should be shaped to support an even thickness of porcelain.

The intended metal-ceramic junction should be as definite (90-degree angle) and as smooth as possible to make finishing easier during all fabrication stages (Fig. 24-4). The metal framework must be sufficiently thick to prevent distortion during firing. A minimum of 0.3 mm is advocated for the noble metal alloys; 0.2 mm is sufficient for base metal alloys, which can be finished thinner and still withstand distortion because of their higher fusing ranges, moduli of elasticity, and yield strengths (see Chapter 19).

Fig. 24-4 The metal substructure should have a distinct margin for finishing the veneer.

The mechanical properties of a metal-ceramic restoration depend largely on the design of the substructure that supports the ceramic veneer. The metal-ceramic interface must be at least 1.5 mm from all centric occlusal contacts and must be distinct to facilitate the removal of excess porcelain. The veneering surface must be finished to a smooth texture with rounded internal angles to allow proper wetting by the opaque porcelain.

Investment Removal

After the framework has been cast, all investment should be removed ultrasonically with airborne particle abrasion or with steam (according to the alloy manufacturer’s directions). The phosphate-bonded investments, which must be used with the high-fusing metal-ceramic alloys, are more difficult to remove from the metal surface than are conventional gypsum-bonded investments. Hydrofluoric acid dissolves the refractory silica component of the investment. However, this acid is extremely dangerous and must be handled very cautiously.⁷ A small spill on the skin results in painful acid burns, and slight exposure to the fumes may produce severe corneal damage. Less dangerous substitute solutions (e.g., Stripit ^*) are available.

Careful examination of the internal aspect of the framework may reveal small investment particles. Several cycles of ultrasonic cleaning may be necessary to eliminate all residual investment. If air abrasion is used for investment removal, the margins of the framework must be protected to prevent damage by the abrasive particles.⁸

Oxide Removal

The oxide layer that has been formed on the metal surface during casting must be partially removed with either acid or air abrasion. For optimal metal-porcelain bonding, the alloy manufacturer’s directions should be followed precisely, because achieving a successful bond depends on a controlled thickness of the metal-oxide layer.

Metal Finishing

Care is needed when the veneering surfaces are ground, to avoid dragging the metal over itself, which could entrap air and grinding debris (which later results in bubbling or contamination of the porcelain). Finishing the surface in one direction with light pressure helps avoid trapping debris between folds of the metal, which is a problem when high-gold content alloys with high elongation values are used (Fig. 24-5).

Fig. 24-5 A, Correct way to prepare the veneer area. The metal should be ground in the same direction. B, Incorrect multidirectional grinding can trap debris in the high-noble alloys.

Surface finishing should be performed with ceramic-bound stones, because the organic binders used in conventional rotary instruments are a potential source of contamination. Carbide burs also may be used safely. After the surface has been smoothed, it should be air abraded with aluminum oxide according to the manufacturer’s instructions. This creates a satin finish on the veneering surface that is easily wettable by the porcelain slurry (Fig. 24-6).

Fig. 24-6 Metal preparation. A and B, Castings prepared by grinding. C, Satin finish obtained by air-abrasion. D and E, Scanning electron micrographs of metal after grinding with a stone (D) and with air-abrasion (E).

(D and E, Courtesy of Dr. J. L. Sandrik.)

Thickness

A dial (or metric) caliper is invaluable for verifying that the metal substructure conforms to all specified minimum dimensions (Fig. 24-7). Metal thickness of less than 0.3 mm may lead to distortion during firing. Sometimes the margin is thinned to a knife edge so that a metal line is not visible. There is no evidence that thinning the margin adversely affects the fit of restorations cast in contemporary alloys.^9,10 The all-porcelain labial margin design can provide a better appearance, particularly when a high smile line and thin gingival tissue necessitate good esthetics at the labial margin.

Fig. 24-7 Dimensions should be verified with calipers.

Finishing

Finishing of the metal-ceramic interface is a difficult laboratory procedure and requires attention to detail. The axial surfaces and visible portion of the metal collar should be contoured and finished to a rubber wheel stage before preparation of the veneering surface is attempted (Fig. 24-8A and B). At this time, the margin itself is left untouched. Round stones and carbides can be used to finish the veneering surface at the metal-ceramic interface (Fig. 24-8C), and the desired right-angle configuration is then easily obtained. Any remaining irregularities can be blended easily with barrel-shaped stones (Fig. 24-8D).

Fig. 24-8 Preparing the substructure. A, Nonveneering surface finished to the rubber wheel stage. B and C, Metal-ceramic junction delineated with a tungsten carbide bur. D, Veneering surface dressed with a ceramic-bound stone. (Note: To avoid perforation, check the metal thickness regularly with calipers.) E, Air abrasion of the veneering area. The margins have been protected by soft wax.

When the grinding phase is completed, a satin finish is obtained by air abrading the veneering surface with a fine-grit alumina (Fig. 24-8E).

Oxidizing

To establish the chemical bond between metal and porcelain, a controlled oxide layer must be created on the metal surface (Fig. 24-9). In noble-metal alloys, iron, tin, indium, and gallium are the base elements used for oxide formation (see p. 748).

Fig. 24-9 Metal-ceramic substructure after cleaning and before oxidizing in the porcelain furnace.

The oxide layer is typically obtained by placing the substructure on a firing tray, inserting it into the muffle of a porcelain furnace, and raising the temperature to a specified level that sufficiently exceeds the firing temperature of the porcelain. A vacuum is created in the firing chamber, which, although insufficient to remove adherent gases, reduces the thickness of the oxide layer. The incorrect term degassing often is used interchangeably with oxidizing (see p. 750).

The specific procedure may vary slightly, depending on the alloy used. High-gold content ceramic alloys usually are held at the oxidizing temperature for several minutes. The first porcelain application can be performed as soon as the casting has cooled to room temperature after it is removed from the furnace.

Many of the lower gold content alloys contain more base elements, which can result in a thicker oxide layer. Some of these systems therefore do not require the work to be held at the oxidizing temperature for any length of time. To reduce excessive surface oxides, some manufacturers recommend briefly air abrading the casting with alumina or placing it in hydrofluoric acid after firing.

Because of lower costs, the use of nonnoble or base metal alloys for metal-ceramic restorations is now widespread. These alloy systems undergo continuous oxide formation. Although the techniques for different systems vary, most manufacturers prefer not to oxidize substructures made of base metal alloys. Instead, they recommend performing the first porcelain application immediately after cleaning. Because the extent of oxide formation cannot be easily controlled, there is a potential for failure through the thick and brittle oxide layer with these alloy systems. However, it may be of no significance with other alloy systems.

Porcelain Manufacture

Dental porcelain is produced from a blend of quartz (SiO₂), feldspar (potassium aluminum silicate orthoclase, sodium aluminum silicate albite), and other oxides. During manufacture, the materials are heated to high temperature to form a glassy mass and then rapidly cooled by quenching them in water, which causes the glassy mass to fracture into many small fragments. The resulting product is called a frit. This process may be repeated several times, after which the frit is ball-milled until the desired particle size distribution is obtained. Because fritting takes place at temperatures much higher than those used in the fabrication of a dental restoration, most of the chemical reactions between raw materials occur before they are used in the dental laboratory. Typical compositions are provided in Table 24-1, although the actual compositions vary according to the proposed use of the end product. Most formulations designed for metal-ceramic use are similar to that described by Weinstein and colleagues.^11,12 They consist of a mixture of two frits: a low-fusing glass frit and a high-expansion frit consisting of crystalline leucite (KAlSi₂O₆) (Fig. 24-10) with tetragonal symmetry (Fig. 24-11). This combination overcomes the two principal difficulties in veneering metal with ceramic: having a porcelain firing temperature well below the melting range of the metal and having a sufficiently high thermal expansion compatible with the metal. After firing in the laboratory, dental ceramics consist of about 20% volume tetragonal leucite crystals dispersed in a glassy matrix.¹³ The structure of this glassy matrix is a random Si-O network. The silicon atom combines with four oxygen atoms in a tetrahedral configuration (Fig. 24-12). These tetrahedra may be linked into a chain with both covalent and ionic bonds, which leads to a metastable structure. However, such a Si-O network would have a very high melting point. Usually, potassium and sodium are added to the glass composition to help break down the Si-O network and are therefore known as glass modifiers. In dental ceramics, potassium and sodium are initially provided by the feldspars. Two desirable consequences result: (1) The softening temperature of the glass is reduced, and (2) the coefficient of thermal expansion is increased. The manufacturer adjusts the oxide content so that the dental ceramic’s coefficient of thermal expansion is close to the corresponding value for the alloys used to make the substructure. If the composition of the glass is not properly adjusted, extensive breakdown and reorganization of the Si-O network may occur, leading to a crystallization of the glass (also called devitrification). The change in lattice structure from a vitreous to a crystalline form (devitrification) is shown schematically in Figure 24-13. This phenomenon may occur partially in dental ceramics if a ceramic restoration is fired too often, and it is typically associated with an increase in the coefficient of thermal expansion and opacity.

Table 24-1 COMPOSITION OF HIGH-, MEDIUM-, AND LOW-FUSING BODY PORCELAINS (WEIGHT PERCENTAGE)

Fig. 24-10 Scanning electron micrograph of polished and etched leucite-containing dental ceramic, showing a tetragonal leucite crystal.

Fig. 24-11 Crystalline structure of low-temperature (tetragonal) leucite.

Fig. 24-12 A and B, The Si-O tetrahedral configuration.

Fig. 24-13 Change in the Si-O network structure from a glassy (A) to a crystalline (B) form.

(Modified from Kingery WD, et al: Introduction to Ceramics, 2nd ed. New York, Wiley & Sons, 1976.)

Feldspar also contains alumina (Al₂O₃), which acts as an intermediate oxide to increase the viscosity and hardness of the glass. As a result, dental porcelain has a good resistance to slump or pyroplastic flow; this resistance is necessary for obtaining the desired configuration of the restoration.

Porcelain Technique

Dental porcelain is usually received from the manufacturer in powder form, which is mixed with either water or a water-based glycerin-containing liquid to form a paste of workable consistency. This mixture is then used to make a restoration with the required configuration. Several condensation techniques (e.g., vibration and blotting) are used to remove as much excess water as possible. The porcelain particles are drawn together during condensation by capillary action. Proper condensation minimizes steam generation during the drying phase of firing. When the mass is heated, individual porcelain particles conglomerate by sintering. The viscous flow of unfused particles results in wetting and bridging between such particles (Fig. 24-14). Consequently, a loss of interstitial space occurs, accompanied by as much as a 27% to 45% volumetric shrinkage after firing.¹⁴

Fig. 24-14 Vitreous sintering. Partial melting of the unfused particles leads to their bonding. Observe the necking caused by the glass flow.

(Modified from Van Vlack LH: Elements of materials science, ed 2, Reading, Mass, 1964, Addison-Wesley.)

Types of Porcelain

The following porcelain blends are produced for the different roles they play in the fabrication of metal-ceramic restorations: opaque, body, and incisal porcelains.

Opaque porcelain

This is applied as a first ceramic coat and performs two major functions: it masks the color of the alloy, and it is responsible for the metal-ceramic bond.

Opacifying oxides are added to the original porcelain blend. The density of these oxides is greater than that of the glass matrix. Consequently, the oxides of tin, titanium, and zirconium have a higher refractive index than do the components of the glass matrix (feldspar and quartz): 2.01 to 2.61 and 1.52 to 1.54, respectively. When a specific range of oxide particle sizes is used, most of the incident light is scattered and reflected rather than transmitted through the porcelain, effectively masking the color of the alloy substrate.

A scanning electron microscope (SEM) view of the alloy-porcelain interface¹⁵ for a high noble metal alloy is shown in Figure 24-15. When the region around the interface is examined for specific elements (a technique known as elemental mapping), the concentrations of aluminum and titanium can be identified. These appear as dense regions in the figure, which indicates that discrete oxide particles of aluminum and titanium are in the opaque porcelain, probably just beneath the surface. By comparing the elemental map with the photomicrograph of the interface, it is possible to identify the distribution and size of the subsurface opacifying oxide particles. Silicon examined in this way also demonstrates a uniform porcelain distribution, which is to be expected.

Fig. 24-15 Alloy-porcelain interface for a high noble metal alloy system: Degudent U (Degussa) and Vita normal (Vident). A, Scanning electron microscopic view of the interface. B to D, Elemental maps of aluminum (B), silicon (C), and titanium (D).

(From Laub LW, et al: The metal-porcelain interface of gold crowns [abstract no. 874]. J Dent Res 57:A293, 1978.)

Porcelain-Alloy Bonding

William A. Brantley

Leon W. Laub

The formation of a strong bond between the opaque porcelain layer and the cast alloy is essential for the longevity of the metal-ceramic restoration. Extensive research since the 1970s has provided insight into the important factors for achieving metal-ceramic bonding. Early work¹⁸ established the importance of wetting the alloy surface by the porcelain at the firing temperature. Although similar measurements of contact angles have not been reported for current dental porcelains and casting alloys, good wetting is essential for minimizing porosity at the metal-ceramic interface. A detailed relationship between the elevated-temperature contact angle and metal-ceramic bonding has not been established, but the research by O’Brien and Ryge¹⁸ indicates that perfect wetting (a contact angle of 0 degrees) does not occur.

In the model by Borom and Pask,¹⁹ an idealized continuous lattice structure is considered across the metal-ceramic interface for chemical bonding. This would be achieved in principle by incorporating certain oxidizable elements in the porcelain composition that can diffuse into the casting alloy at the elevated temperatures of the porcelain firing cycles and have the same equilibrium chemical potentials in both the metal and ceramic. However, in reality the situation at the dental metal-ceramic interface does not fit this idealized model. Research on gold²⁰ and high-palladium^21,22 alloys used for ceramic veneering has shown that the structure of the oxidized regions is highly complex, and similar results would be anticipated for detailed studies of other types of oxidized dental casting alloys. The existence of multiple phases in the oxidized region of the alloy indicates that the proposed continuity¹⁹ of atomic bonds cannot generally be achieved across the metal-ceramic interface, except possibly at sites where the glass matrix of the porcelain is in contact with the solid solution matrix of the alloy.

Manufacturers incorporate in the casting alloy composition small amounts of certain base metals that form oxides^23,24 and contribute chemical bonding to the metal-ceramic adherence. Investigators using the electron microprobe and SEM^15,25-30 have shown that these elements accumulate at the metal-ceramic interface and form an interfacial oxide layer. For noble metal alloys, elements having a major role for porcelain adherence are iron (high-gold alloys), tin and indium (lower gold content, palladium-silver, silver-palladium, and high-palladium alloys), and gallium (high-palladium alloys). For base metal alloys in which the principal elements are nickel and cobalt, chromium oxidation provides chemical bonding for porcelain adherence, whereas titanium oxidation fulfills this role for the titanium casting alloys.

Figure 24-16A is an SEM photomicrograph of the interface for a high-palladium alloy bonded to dental porcelain. This alloy undergoes complex external and internal oxidation during the porcelain firing cycles. The internal oxide particles in the palladium solid solution grains are too small (less than 1 to 2 μm in diameter) for accurate compositional determinations by x-ray energy-dispersive spectroscopic analysis with the SEM. X-ray diffraction²¹ has shown that CuGa₂O₃ and SnO₂ are present in the oxidation region when the alloy surface receives standard airborne particle abrasion with 50 μm of aluminum oxide before oxidation. Figure 24-16B shows line scans obtained with the SEM for major elements in the metal and ceramic near the interface. Variations in the x-ray counts occurred when the line scan crossed a region of internal oxidation.

Fig. 24-16 A, Secondary electron (scanning electron microscopic [SEM]) photomicrograph of the metal-ceramic interface for Liberty (Heraeus Kulzer) palladium-copper-gallium (Pd-Cu-Ga) high-palladium alloy (A) bonded to VITA VMK 68 (Vident) dental porcelain (P). The grain boundaries of the alloy (M) have been widened by the formation of oxide (O) deposits, and there are numerous very small oxide particles within the grains. Scale bar = 10 μm. B, Elemental line scans perpendicular to the metal-ceramic interface obtained by x-ray energy-dispersive spectroscopic analyses for the Liberty alloy. Because the raw SEM data have not undergone matrix corrections, the relative elemental concentrations (x-ray counts) are qualitative, but the indicated trends are appropriate. O, oxygen; Si, silica; Sn, tin.

(From Papazoglou E, et al: New high-palladium casting alloys. Studies of the interface with porcelain. Int J Prosthodont 9:315, 1996.)

Factors Affecting the Bond

Most metal-ceramic systems require that the cast alloy be subjected to an initial oxidation step before the several layers of dental porcelain are fired. (A notable exception is the palladium-copper-gallium [Pd-Cu-Ga] high-palladium alloy Freedom Plus,^* in which the oxidation step does not need to be performed before firing the opaque porcelain layer.) This step has also been called a conditioning bake or degassing. The latter term, which is frequently used in the dental laboratory industry, is inaccurate, because this procedure’s purpose is to oxidize the metal surface for subsequent adherence to the fired porcelain. Historically, some clinicians thought that the heating cycle might result in loss of gases incorporated in the alloy during melting. However, this occurs during solidification because of the much greater solubility of atmospheric gases in the molten alloy than in the solidified alloy; it results in the formation of microscopic porosity³¹ in the casting.

The oxide layer between the metal and ceramic should have an optimum thickness for a strong metal-ceramic interfacial bond. This was demonstrated in the 1970s for selected noble and base metal alloys.³² Research has shown that particular care is required with the base metal casting alloys to avoid excessively thick oxide layers.³³ Beryllium is added to some nickel-chromium (Ni-Cr) alloy compositions to lower the melting range; beryllium also has an effect on the thickness of the oxide layer.³⁴ Some systems require the application of a bonding agent before firing of the opaque porcelain. Certain formulations consist of colloidal gold suspensions that are fired on silver-colored, gold-based ceramic alloys for esthetic purposes. SEM examination of the interfaces has shown that in Ni-Cr alloys, the bonding agents may increase or decrease the width of the interaction zone between the metal and ceramic.²⁹ An analysis of bonding agents for several Ni-Cr alloys indicates that they contain elements found in porcelain (e.g., aluminum, tin, and silicon).³⁴ For certain specific brands of Ni-Cr alloys, the bonding agent appears to increase the adherence between the alloy and the opaque porcelain. The manufacturer indicates whether a bonding agent is necessary or beneficial.

Airborne particle abrasion with aluminum oxide (alumina) is routinely performed on the alloy castings to create surface irregularities and to provide mechanical interlocking with the opaque dental porcelain, which has sufficiently low viscosity in the firing temperature range to flow into these microscopic openings. Early studies found no effect of such surface roughening in the interfacial resistance of gold-platinum-palladium (Au-Pt-Pd),³⁵ gold-palladium-silver (Au-Pd-Ag), and Ni-Cr³⁶ systems to shear loading. More recent research with a Pd-Cu-Ga high-palladium alloy showed that controlled amounts of mechanical surface roughening that yielded greater notch depth for the irregularities increased the metal-ceramic bond strength, with greater improvements from coarse roughening.³⁷

The linear coefficients of thermal expansion for the metal (α_M) and ceramic (α_C) must closely match to achieve a strong interfacial bond. Typically, α_M values range from 13.5 to 14.5 × 10⁻⁶/degree Celsius; α_C values range from 13.0 to 14.0 × 10⁻⁶/degree Celsius.³⁸ The slightly higher coefficient for the metal causes the ceramic to be in a beneficial state of residual compressive stress at room temperature (Fig. 24-17). (The thermal contraction and expansion coefficients are assumed to be equal, and residual stress is developed in the ceramic only below its glass transition temperature, at which time viscous flow is no longer possible.) Porcelain is much stronger in compression than tension, and residual tensile stress in the porcelain must be avoided, to prevent fracture of the restoration.

Fig. 24-17 The ceramic-metal bond at the firing temperature and at room temperature when the thermal coefficient of expansion of the metal is 0.5 × 10⁻⁶/degree Celsius greater than that of the ceramic, which thus places the ceramic in compression at room temperature.

(Adapted from Craig RG, et al: Dental Materials: Properties and Manipulation, 7th ed. St. Louis, Mosby, 2000.)

Adherence between the alloy casting and porcelain is very important in fixed prosthodontics, and investigators have used a variety of test configurations with shear, tensile, flexural, and torsional loading to determine the metal-ceramic bond strength. Ideally, the interfacial bond is strong enough so that fracture of the test specimen occurs entirely within the porcelain (cohesive failure). In one early study,³⁹ no significant difference was found in the diametral tensile strength of commercial dental porcelains for air firing and vacuum firing. Lower tensile strength values of 28 MPa (4100 psi) for opaque porcelain, in comparison with 42 MPa (6100 psi) for gingival porcelain, were attributed to greater porosity and the inclusion of opacifying oxides in the latter. In addition, vacuum firing appeared to have little effect on the porosity of the opaque porcelain. On the basis of this research, the tensile strength of the metal-ceramic bond should exceed 28 MPa to have cohesive failure through the porcelain rather than failure at the interface. Measurement of the shear strength of dental porcelain⁴⁰ allows a similar prediction for the minimum interfacial shear strength required for cohesive shear failure through the ceramic. Results from several studies^34,41,42 in which researchers measured the tensile bond strength of metal-ceramic systems were consistent with these concepts. Cohesive failure within the porcelain occurred at 15 to 39 MPa (2200 to 5700 psi), whereas shear bond strengths ranged from 55 to 103 MPa (8000 to 15,000 psi). For many of the shear bond strength determinations, a mixed mode of failure was observed, in which adhesive failure at the metal-ceramic interface extended into the porcelain, which fractured cohesively.

Subsequently, the focus for evaluating the metal-ceramic interfacial bond was directed toward measurement of porcelain adherence rather than determination of bond strength. Anusavice and associates⁴³ reported a finite element analysis of the tests that had been used to measure metal-ceramic bond strength (e.g., pull-shear, three-point bending, and four-point bending). Two major problems were revealed for all the bond strength tests: The stress varied with position along the metal-ceramic interface (particularly near porcelain termination sites), and there was a lack of the pure shear stress conditions that were considered necessary to simulate the loading generally expected to cause clinical failure. Furthermore, the small mismatch between the thermal contraction coefficients of the metal (α_M) and ceramic (α_C) results in an unknown amount of residual stress at the interface, and an idealized value of metal-ceramic bond strength assumes the presence of a residual stress-free interface.

To avoid these problems, O’Brien proposed a completely different approach,^44,45 focusing on the mode of failure of metal-ceramic specimens or restorations rather than measurement of bond strength. The adhesive and/or cohesive failure can occur at six possible sites or combinations of those sites (Fig. 24-18). Adhesive failure can occur (1) at the porcelain-metal interface if no oxide layer is present, (2) at the metal oxide-metal interface, and (3) at the porcelain-metal oxide interface. Cohesive failure can occur (4) through the porcelain, which is the desirable mode; (5) through the metal oxide layer; and (6) through the metal.^* This approach for evaluating the metal-ceramic bond by determining the area fraction of adherent porcelain on fractured test specimens was adopted in the American National Standards Institute/American Dental Association (ANSI/ADA) specification no. 38 for metal-ceramic systems,⁴⁶ although the method of microscopic measurement was not specified.

Fig. 24-18 Possible modes of failure of alloy-porcelain restorations.

(Modified from O’Brien WJ: Evolution of dental casting. In Valega TM Sr, ed: Alternatives to Gold Alloys in Dentistry [DHEW Publication No. (NIH) 77–1227, p 5]. Washington, DC, U.S. Government Printing Office, 1977.)

A quantitative x-ray spectrometric method was developed by Ringle and coworkers⁴⁷ to measure porcelain adherence. The fracture surfaces of metal-ceramic specimens loaded to failure in biaxial flexure are examined with the SEM, with x-ray energy-dispersive spectroscopic analysis. This method is based on the principle that silicon is a major element in dental porcelain but is largely absent in dental alloys (except as contaminants from investments or polishing abrasives used to prepare specimens). The amount of dental porcelain remaining on the metal surface of the fractured specimen is readily determined by measuring the silicon Kα signal, with necessary calibration measurements on the oxidized alloy surface before porcelain application and on the porcelain surface before testing the specimen. This technique has been used to measure the oxide adherence to a variety of ceramic alloys⁴⁸ and the porcelain adherence to high-palladium alloys^49,50 and to titanium and an alloy of titanium-aluminum-vanadium (Ti-6Al-4V).^51-54 Another philosophical change in the recommended method for evaluating the metal-ceramic bond has occurred with the introduction of International Organization for Standardization (ISO) standard no. 9693 for dental porcelain fused to metal restorations,⁵⁵ which contains a three-point bending test. Lenz and colleagues⁵⁶ performed a finite element analysis for this test, considering the stress concentrations that arise at the porcelain termination sites, although the effect of unknown residual stresses arising from the mismatch between α_M and α_C could not be included.⁵⁷ Investigators using several Pd-Ga high-palladium alloys with identical values of elastic modulus found no correlation between the porcelain adherence measured by the x-ray spectrometric method^49,50 and the force to failure by the three-point bending test in ISO standard no. 9693⁵⁵ and Lenz and colleagues’ analysis.⁵⁶ These experimental results cast doubt on the effectiveness of x-ray spectrometric technique^48,49 in measuring porcelain adherence. In future experimental research in this area, investigators may use a fracture-mechanics approach to evaluate the metal-ceramic interfacial bond.

Other important factors that affect the metal-ceramic bond are the surface treatment of the alloy before the porcelain is fired and the atmosphere of the porcelain furnace during firing. As previously mentioned, air abrasion of the cast alloy is typically performed before the oxidation step to help remove surface contaminants that remain from devesting and to help clean the casting and provide microscopic surface irregularities for mechanical retention of the ceramic. The oxidation step for the alloy can be performed in air or by using the reduced atmospheric pressure (approximately 0.1 atm) available in dental porcelain furnaces. A much thinner oxide layer is formed if the alloy is oxidized at this reduced atmospheric pressure, in comparison with the thickness for oxidation in air. One manufacturer (Heraeus Kulzer, Armonk, New York) recommends performing the oxidation step in air and then carefully reducing the thickness of the oxide layer before firing the opaque porcelain. Manufacturer’s recommendations for the oxidation of the alloy and the porcelain firing cycles must be followed. An early study⁵⁸ showed that the shear bond strength of porcelain-gold alloy specimens was 60% greater when air firing was employed; another study at that time⁵⁹ revealed that the tensile bond strength varied according to the furnace atmosphere used. The shear bond strength of porcelain-nickel–based alloy specimens was greater when firing was performed in oxidizing atmospheres than in non-oxidizing or reducing atmospheres.⁶⁰ More recently, Wagner and associates³⁷ found that the use of a reducing atmosphere severely reduced the bond strength of a Pd-Cu-Ga high-palladium alloy, which confirmed the role of alloy oxidation during the standard porcelain firing cycles. Areas of current research activity are surface modification of cast titanium with chemical solutions,⁵² control of interfacial variables (different porcelain firing environments and sputtered gold film),⁵³ and use of special gold⁵⁴ and silicon nitride⁶¹ coatings to improve the porcelain adherence.

…

Opaque Porcelain

For a proper mechanical bond and chemical interaction at the interface, opaque porcelain must wet the surface easily. It becomes the primary source of color of the restoration and must mask the color of the metal, even in thin layers. Opaque thickness generally should not exceed 0.1 mm; otherwise, achieving an esthetic result without overcontouring the restoration becomes impossible, although a greater thickness may be needed to mask the darker oxide of some alloys.⁶² Small amounts of zirconium oxide and titanium oxide, in conjunction with alumina, act as the opacifying agents to block the darker color of the oxidized metal. Some of these oxides are also present in the body porcelain. Manufacturers supply opaque porcelains in paste and powder form (Fig. 24-19).

Fig. 24-19 Metal-ceramic porcelain. Opaque porcelains are available as powders or pastes.

(Courtesy of Ivoclar Vivadent, Amherst, New York.)

Body and Incisal Porcelains

As with opaque porcelain, the selection of body and incisal porcelains is based largely on their esthetic properties. However, the amount of shrinkage that occurs when these powders are fired must also be considered. The volume of body and incisal porcelains usually shrinks as much as 27% to 45% during a first firing¹⁴; opaque porcelain, on the other hand, may exhibit some cracking during an initial bake, but it remains relatively stable dimensionally. Lower-fusing metal-ceramic porcelains^* have become popular.⁶³ These materials should be considered when opposing enamel wear is likely to be a problem, because in vitro they tend to exhibit lower abrasiveness than do conventional formulations.⁶⁴

Porcelain Application

Armamentarium (Fig. 24-20)

• Porcelain modeling liquid

• Paper napkin

• Glass slab or palette

• Tissues or gauze squares

• Two cups of distilled water

• Glass spatula

• Serrated instrument

• Porcelain tweezers or hemostat

• Ceramist’s sable brushes (Nos. 2, 4, and 6) and whipping brush

• Razor blade or modeling knife

• Cyanoacrylate resin

• Colored pencil or felt-tip marker

• Articulating tape

• Ceramic-bound stones

• Flexible thin diamond disk (about 20 mm in diameter)

Fig. 24-20 A, Armamentarium for porcelain application. B, Whip Mix Pro Press 100 (left) and Pro 100 (right) porcelain furnaces. C, DENTSPLY Centurion porcelain furnace.

(B courtesy Whip Mix Corporation, Louisville, Kentucky; C courtesy DENTSPLY International, York, Pennsylvania.)

Step-by-step procedure

After the metal substructure has been oxidized, it must be inspected carefully. An uninterrupted oxide layer should cover the entire surface to be veneered.

Opaque porcelain (Fig. 24-21)

1. After selecting the opaque bottle, shake it to mix the powder thoroughly. Then place it on the bench to allow the smaller pigment particles to settle. Over time, all porcelain powders segregate into layers of different particle size if left undisturbed.

2. Dispense a small amount of powder on a glass slab or palette. Add some modeling liquid and mix it with the spatula. Metal instruments should not be used in mixing, because metal particles could rub off and act as contaminants. The proper opaque consistency should “hold an edge” for a few seconds.

3. Moisten the substructure with some of the liquid, and pick up a small bead of opaque with the tip of the brush or spatula. Apply it to the coping, which should be held with the porcelain tweezers.

4. Use light vibration to spread the material thinly and evenly. Moving the serrated instrument back and forth over the handle of the tweezers creates the necessary disturbance. Excess moisture that comes to the surface can be blotted off with a clean tissue. Vibration may not be as necessary with the so-called paint opaques.

5. Apply a second bead on top of the first, and spread it in a similar manner. To minimize the entrapment of air when the two masses meet, do not apply opaque porcelain adjacent to the initial mass. If the moisture content is properly controlled, condensing poses little difficulty. A mix that is too wet will slump and produce too thick a layer on the substructure, especially in the concave areas near the porcelain-metal junction.

6. Once the veneering surface is covered, add more material to a dry base. Wetting the initial application before adding more porcelain may be necessary. If not, the moisture will be absorbed immediately by the dry base layer before a new material can be properly condensed and distributed, which results in a porous and weakened application (similar to constructing a sand castle with wet sand on a dry beach). The addition of more liquid and further vibration will resolve the problem.

7. When the entire veneering surface has been covered, remove any excess material from other surfaces with the side of a slightly moistened brush. If the metal immediately adjacent to the veneer has been properly prepared and smoothed, it is not difficult to remove the excess porcelain. However, this crucial task is often overlooked, and its omission can make metal polishing much more difficult.

8. After removing any excess porcelain, carefully inspect the inside of the restoration for porcelain particles. A stiff, dry, short-bristle brush can be used to remove the particles.

9. Before firing, inspect the opaque application to see that it satisfies the following criteria:

• The entire veneering surface is evenly covered with a smooth layer that masks the color of the metal.

• There is no excess anywhere on the veneering surface.

• There is no opaque on any external surface adjacent to the veneer.

• There is no opaque on the internal aspect of the substructure.

If these criteria have been met, the coping is transferred to a sagger tray and placed near the open muffle of the porcelain furnace for several minutes. This allows moisture to evaporate. When drying is completed (this varies according to specific manufacturer’s recommendations), reinspect the work for any residual excess opaque powder. Material that was previously overlooked is clearly visible, because the chalky white porcelain contrasts with the darker oxidized metal. A stiff, short-bristle brush can again be used to remove any remaining porcelain. The opaque is then fired according to manufacturer’s recommendations.

10. After the first firing, remove the work from the muffle and set it aside to cool to room temperature.

11. At this time, inspect the opaque veneer for cracks, thin spots, and general adequacy of coverage. When the veneer is removed from the furnace, it appears yellow; however, when it has cooled, the more representative matte-white color is apparent. Fired opaque should have an eggshell appearance. If necessary, a second application of opaque can be made. Small cracks and fissures are common after the first firing. This problem can be resolved by applying moisture, followed by a thin mix of opaque carefully condensed into the fissures. When correcting a thin area where the color of the metal has not been masked completely, moisten the surface before applying a second coat (to facilitate wetting).

12. After firing, check that the opaque application (Fig. 24-22) meets the following criteria:

• Relatively smooth, even layer masking the color of the framework.

• Eggshell appearance.

• No excess on any external or internal surface of the restoration (which would prevent it from seating fully on the die).

Fig. 24-21 Opaque porcelain application. A, Substructure is oxidized. B, Porcelain is applied. Vibration can be used to help spread the opaque into an even, thin film (C). D, Application of additional opaque. E, After drying in front of the furnace, the opaque powder should have a uniform matte-white appearance. Excess must be removed before firing.

Fig. 24-22 Evaluation of the opaque porcelain.

Body and incisal porcelains

When a satisfactory opaque layer has been fired, the body and incisal porcelains (Fig. 24-23) can be applied. The use of several porcelains in one restoration is common. Body porcelains with increased opacity^* may be used where less translucency is required (e.g., the gingival area of the pontic, incisal mamelons) to mimic existing anatomic features of adjacent natural teeth. Special neck powders can be applied on the cervical third, and incisal powders on the incisal edge, to simulate natural enamel. In general, the restoration is built to anatomic contour; when it is acceptable, a cut-back similar to that made during the waxing stage allows for a veneer of the more translucent incisal porcelain.

1. Dispense the neck, body, incisal, and other powders on a glass slab or palette. If the same slab was used for the opaque porcelain, any opaque residue must be removed.

2. Mix the powders with the recommended liquid or distilled water. The moisture content for these powders should be the same as for opaque porcelain. Specially formulated liquids that allow longer manipulation than is possible with conventional glycerin-containing liquids are available.

3. Wet the previously fired opaque layer with a small amount of the liquid, and place a bead of neck powder on the cervical portion of the veneering surface. Gentle patting with a brush and light tapping on the cast produces adequate vibration during the preliminary stage of condensation. A tissue is held close for removal of excess surface moisture. During the entire buildup procedure, the facial surface should not be blotted with tissue, because the smaller pigment particles might be removed. Blotting consistently from the lingual aspect is recommended and results in superior esthetics.

4. After placing the neck powder and sculpting it, build the veneer to anatomic contour with body porcelain. Use the adjacent and opposing teeth as a guide. Where contact is anticipated between the wet buildup and stone cast, the cast can be coated with a small amount of cyanoacrylate resin, immediately blown into a thin layer. This seals the surface and prevents the absorption of moisture from the buildup.

5. To compensate for the firing shrinkage that results when the particles fuse, slightly overbuild the porcelain. A typical metal-ceramic anterior crown shrinks 0.6 mm at the incisal edge and 0.5 mm midfacially⁶⁵ (Fig. 24-24).

6. When the body buildup is completed, assess it for proper mesiodistal, faciolingual, and incisogingival contour.

7. Depending on the desired appearance, make a cut-back for the more translucent incisal powder. Some manufacturers recommend carrying the incisal veneer all the way to the cervical portion of the restoration; others suggest limiting it to the incisal third. An almost infinite variety of possibilities exists, and only with experience can the dentist predict the finished product’s appearance. Whether the cut-back is made with a razor blade, scalpel, or modeling instrument, condensing the body buildup well before cutting back is necessary. This minimizes the risk of fracture during the process. Furthermore, to minimize the chance of damaging the unsupported incisal portion of the buildup, the cut-back should be made from incisal to cervical aspects. Adequate space must exist for the incisal veneer in the interproximal area.

8. Apply the incisal powder in the same manner, and overbuild the restoration as described for body porcelain. The remaining body powder must be wet before application of the incisal powder, and, again, intermittent light vibration helps achieve an acceptable level of condensation. Prolonged condensation should be avoided. It does not reduce porosity⁶⁶ or increase fracture toughness⁶⁷ and may lead to unwanted redistribution of the pigmented particles.

9. Mark the opposing teeth on the stone cast with a red or green felt-tip marker. These markings are not absorbed if the cast first has been coated with cyanoacrylate resin. The articulator can then be closed to allow the antagonists to contact the wet porcelain. If this is done carefully, the markings are transferred onto the buildup without fracturing, and the buildup can be modified to the necessary occlusal scheme. Only red or green dyes, which burn off without leaving a residue, should be used for these markings. Blue or black pigments usually contain metal oxides or carbon, which after firing can discolor the porcelain.

10. Moisten the proximal contact areas immediately before removing the completed buildup from the cast. This reduces the risk of fracturing that portion of the buildup.

11. After the restoration has been removed from the cast, fill in the proximal contact areas. At this time, the work should be reinspected for any excess material beyond the veneering area (which, as before, must be removed before firing). The internal aspect of the coping should be reinspected even more carefully, because enamel powders are quite transparent in thin layers and are not easily detected after firing.

12. Place the restoration on a sagger tray close to the open muffle at the drying temperature recommended by the manufacturer. A drying time of 6 to 10 minutes is usually sufficient. If a restoration is fired prematurely, the residual moisture in the buildup may generate steam, and the accompanying vapor pressure causes the buildup to explode. After the drying process, once it has been determined that no undesired excess material remains, proceed with firing. When the firing is completed, the work should cool to room temperature before further handling. Follow the manufacturer’s recommendations concerning the cooling rate after firing. Incorrect cooling rates may lead to residual stresses that eventually result in porcelain fracture during function. Thermal expansion increases after slow cooling, because additional (high-expansion) leucite crystallizes.⁶⁸ In general, alloys with high thermal expansion coefficients require more rapid cooling than do alloys with low coefficients.⁶⁹

13. Be especially critical when evaluating the first (or low bisque) bake. If the surface is fissured, grind the porcelain before adding any more (Fig. 24-25). The shape of the restoration should conform to the standards set by dental anatomy and the predetermined occlusal scheme for the patient.

14. Remove all excess material with ceramic-bound stones. A flexible diamond disk is imperative for proper shaping of the embrasure spaces. To extend its usefulness, the disk should be kept moist.

15. When the restoration has been contoured and all the necessary areas reduced, certain portions probably require a second application of porcelain.

16. Before a second corrective bake (also referred to as a patch bake), clean the restoration ultrasonically to remove any grinding debris.

17. Place the second body and incisal layers directly on the slightly moistened low bisque bake. Evaluate the color at this time, keeping the restoration moist. Sometimes another bake is needed, particularly for an extensive prosthesis. However, multiple firings lead to devitrification of the porcelain, with a loss of translucency and a decrease in the restoration’s fracture resistance.⁷⁰

Fig. 24-23 Body and incisal porcelain application. A to E, Development of incisal mamelons with opacious dentin. The incisal plaster or silicone putty index is made from the anatomic contour wax patterns (see Chapter 18) and serves as a guide to developing proper incisal edge position. F and G, Cervical and body powders are added to the contour. H, Alternatively, the incisal index can be used to establish incisal edge position. This is especially helpful for extensive restorations. I, Restorations are slightly overbuilt. J, The buildup is smoothed with a whipping brush. K, Restorations are separated with a razor blade before firing. Additional porcelain is added to the proximal contacts. L and M, Restorations after firing of the first body bake. N, Porcelain is added in areas of deficient contour. O, After firing, the proximal contacts are carefully adjusted, and the restorations are seated on the cast. P to S, Restorations are contoured by grinding. Careful attention is paid to the shape and position of line angles and incisal edges. When completed, the restorations are ready for clinical evaluation and final contouring intraorally (see Chapter 30).

Fig. 24-24 Mean shrinkage values from firing of a typical maxillary central incisor metal-ceramic crown.

(From Rosenstiel SF: Linear firing shrinkage of metal-ceramic restorations. Br Dent J 162:390, 1987.)

Fig. 24-25 A, This restoration had a defect (arrow) at the gingival margin after the first firing. Access for repair is made by grinding out such cracks. B, Additional porcelain is applied after the area is moistened. C, The porcelain has been added, and the restoration is ready for a second firing.

Internal Characterization

Internal or intrinsic characterization or staining may be accomplished by incorporating colored pigments in the opaque, body, or incisal powder. These pigments are ceramic in nature and have physical properties similar to those of the porcelain powders.

Most commercially available porcelains have colored opaque modifiers that can be selectively mixed with the opaque to increase the saturation of the desired pigment. A variation of this approach is to use opacified dentin powders that produce a finished restoration with a slightly higher chroma than one prepared with the more translucent dentin powders. Similarly, a translucent powder can be used to enhance incisal translucency (Fig. 24-26). Highly colored glazes, commonly used as surface stains, may be layered within the buildup powders to create special effects (Fig. 24-27).

Fig. 24-26 A and B, Natural incisal appearance has been achieved through subtle layering of porcelains of different translucencies.

Fig. 24-27 Porcelain stain was applied intrinsically to create the effect of discolored dentin in these mandibular incisors seen before firing.

The use of internal stains presents little technical difficulty for operators familiar with metal-ceramic procedures. However, because the pigment is built into the material, if the desired effect is not obtained through internal staining, the porcelain must be stripped from the substructure.

Another technique for internal characterization is to fire the body powders initially, carve them into the desired mamelon shape, and then apply the subsequent enamel powders. A disadvantage of this approach is that an additional firing is needed.

Contouring

The appearance of the finished restoration depends on its color, shape, and surface texture, which can be altered by shaping and characterizing dental porcelain to mimic the appearance of natural teeth (see Fig. 24-23P to S).

The appearance of restorations can be influenced considerably through the selected use of optical illusion (see Chapter 20). The human eye is capable of discerning differences in height and width, but its depth perception is far less developed. Even trained observers experience difficulty when attempting to recognize subtle differences in the third dimension.

Through selective contouring, the apparent shape of a restoration can be made to look quite different from its actual configuration. The perceived size of a tooth depends on the reflection of its line angles and the relative position and spacing of these reflections. Even though an edentulous area on one side may be slightly larger than a space occupied by the corresponding tooth on the contralateral side, a restoration can be made to appear similar (or even identical) through careful mimicking of the line angle distribution and contours immediately adjacent to the line angles. The clinician can create an illusion that the restoration is narrower than it really is (Fig. 24-28). In addition, by simulating the normal distance between line angles superimposed on a pontic in an edentulous area that is otherwise too narrow, it is possible to create the illusion that teeth are of normal size but merely crowded. Careful application of these principles may trick the casual observer into concluding that the teeth overlap and that a portion of the tooth (or restoration) is behind an adjacent tooth when, in reality, the overlap does not exist.

Fig. 24-28 A and B, The esthetics of an abnormally sized restoration can be improved by matching the location of the line angles and adjusting the interproximal areas. C, The pattern of light reflection depends on the surface texture of the restoration.

(A and B redrawn from Blancheri RL: Optical illusions and cosmetic grindings. Rev Asoc Dent Mex 8:103, 1950.)

The surface texture of a metal-ceramic restoration should resemble that of the adjacent teeth, including selected characterizing irregularities that exist on those teeth. Several rules of light reflection must be remembered when the clinician attempts to accomplish this:

Thus, a smooth restoration can appear larger than one of identical size that has been characterized or textured. Careful study of adjacent teeth and an understanding of how their reflective patterns should be simulated before characterization are essential. Care must also be taken not to “overcharacterize,” which would draw attention to the restoration and reveal that it is artificial.

Glazing and Surface Characterization

Metal-ceramic restorations are glazed to create a shiny surface similar to that of natural teeth (Fig. 24-29). ^* The glazing cycle can be performed concurrently with any necessary surface characterization (see Chapter 30).

Fig. 24-29 A and B, Glazed and polished restorations.

In autoglazing, the contoured bisque bake is raised to its fusion temperature and maintained for a time before cooling. A pyroplastic surface flow occurs, and a vitreous layer or surface glaze is formed. Sharp angles and edges are rounded slightly during this process. Consequently, occlusal contact in porcelain is altered slightly during glazing.

In contrast, in overglazing, a separate mix of powder and liquid is applied to the surface of a shaped restoration, and the restoration is subsequently fired. The firing procedure is similar to that for autoglazing, although there are variations among brands. Because most metal-ceramic restorations include low-fusing porcelain, overglazing is not currently in widespread use.

External Characterization

Surface stains are highly pigmented glazes, which can be mixed with glycerin and water (supplied with most commercially available staining kits).

By moistening the bisque firing, mimicking the appearance of the glazed restoration is made possible. After the desired effect has been obtained by placing selected stains on the surface, the restoration is held outside the open muffle of the glazing furnace, and the stain is allowed to dry.

When it turns white and chalky, any excess that may have been accidentally applied to the metal surface is removed, and the restoration is fired. During this staining and glazing bake, a pyroplastic surface flow occurs, and a glassy layer (or autoglaze) forms on the surface in which the stains are incorporated.

Advantages and Disadvantages

The collarless crown’s most obvious advantage is the esthetic improvement it offers over the conventional metal-ceramic restoration. Plaque removal also is easier when gingival tissues are in contact with vacuum-fired glazed porcelain than when they are contacting highly polished gold. Therefore, porcelain appears to be the material of choice for restorations that will be in contact with gingival tissues.

The difficulties encountered during fabrication, however, limit its application. Although a technically comparable result is feasible,^71,72 the marginal adaptation of these restorations (as currently produced by most commercial laboratories) is slightly inferior to that of cast metal. Because of careless handling, fracture of the unsupported margin is sometimes a problem during evaluation or cementation. Fracture during function is rarely a problem, because the labial margin is not subjected to high tensile stresses.⁷³ In addition, the collarless metal-ceramic restoration is more time consuming and therefore more costly to make.

Indications and Contraindications

A porcelain labial margin is indicated when a conventional metal-ceramic restoration will not create the desired esthetic result. It is contraindicated when an extremely smooth, 1-mm-wide shoulder cannot be prepared in the area of the ceramic veneer. (In this regard, the conventional metal-ceramic restoration is somewhat more “forgiving.”) Although multiple porcelain margins may be used in one fixed prosthesis without sacrificing marginal adaptation, the limitations of the operator and the technical auxiliaries should be carefully and objectively assessed before the dentist and patient commit themselves to a fixed prosthesis consisting of multiple collarless retainers.

Framework Design for Labial Margin

Various framework designs have been proposed with different facial framework reductions⁷⁴ (Fig. 24-32). In general, the more metal reduction, the better the esthetic result. However, the technical procedures become more demanding. Removal of up to 2 mm of the labial framework has been shown not to decrease the fracture resistance of the restoration.^75,76

Fig. 24-32 Labial margin designs for metal-ceramic restorations. A, A thin metal band provides excellent adaptation but is very unesthetic unless it can be hidden subgingivally. For esthetic reasons, this design is rarely used for anterior teeth. B, “Disappearing” margin, sometimes called a conventional margin, is commonly used and is esthetically acceptable in some patients. However, the metal often causes unacceptable grayness of the gingival tooth surface. C to E, Various cut-back designs for labial porcelain margin restorations. Reducing the metal provides better esthetics but makes the laboratory phase more demanding and may result in margin chipping. F, A 360-degree porcelain margin provides excellent light transmission in the gingival area and optimal esthetics; however, the laboratory fabrication is very demanding. This design requires a preparation design that is similar to that for an all-ceramic crown (see Chapter 11) with a circumferential rounded shoulder. Close cooperation between dentist and technician is essential in determining the best labial margin design.

Methods of Fabrication

Several techniques have been proposed to fabricate porcelain labial margins in metal-ceramic restorations: platinum foil matrix, direct lift or cyanoacrylate resin, and porcelain wax.

Platinum foil matrix technique

The platinum foil technique⁴ involves use of a platinum matrix that is spot-welded to the metal substructure. Its primary purpose is to support the porcelain during firing.

Step-by-step procedure (Fig. 24-33)

1. Wax the metal substructure and cast it in the conventional manner. Some technicians prefer to cast a conventional restoration and trim the collar off the casting; others wax the substructure exactly as desired and cast it.

2. To prevent the foil from becoming distorted on removal, block out undercuts apical to the margin. Modeling compound is suitable for this step. Any excess that may have covered the shoulder of the tooth preparation must be carefully cleaned off. Some technicians prefer to use epoxy or electroplated dies so that more pressure can be applied during burnishing without the risk of chipping.

3. Burnish a small piece of platinum foil onto the facial portion of the die where the porcelain margin is to be placed, and extend it a few millimeters onto the axial wall of the preparation.

4. After burnishing, trim it so there is a 2- to 3-mm “skirt” lying cervical to the margin.

5. Carefully place the coping over the foil and, if necessary, scrape the die in the cervical area (on only the facial aspect) to allow the casting to seat. To help stabilize the foil, a drop of sticky wax may be applied.

6. Remove the casting from the die, together with the foil, and position the assembly between the electrodes of an orthodontic spot welder. The foil can now be welded to the framework, which should be done as close as possible to the edge of the metal. Four or five welds are usually adequate for attaching the foil to the substructure. The restoration is then fabricated in a conventional manner, although the facial margin is similar to that for the porcelain jacket crown. After oxidizing,^* the metal is opaqued. The foil itself is not covered with opaque at this time, however. The restoration is built to contour, and the marginal portion is ditched as for the porcelain jacket crown. Some technicians prefer to paint a thin coat of separating liquid on the foil instead of ditching.

7. When the coronal portion has been shaped to a satisfactory contour, burnish the foil and fill in the ditched portion with cervical porcelain.

8. When the desired contour has been obtained after firing, trim the platinum skirt.

9. Leave the platinum that covers the shoulder portion of the preparation, and seat the crown on the original die for final cervical contouring. During subsequent staining and glazing, the platinum foil will remain in place to support the porcelain and minimize rounding of the margin during firing.

10. When characterization and glazing are satisfactory, remove the foil and cement the restoration after verifying the fit one more time.

Fig. 24-33 The platinum foil matrix technique. A, The definitive cast with modified removable dies. B, Castings seated over the platinum foil. C, Spot welding the foil to the metal substructure. D, The trimmed matrix.

Direct lift (cyanoacrylate resin) technique

Because this technique is less time consuming⁵ and easier to perform than the platinum foil technique, it is more widely used. The substructure is fabricated in the same manner, but the die is coated with a layer of cyanoacrylate resin, and the porcelain is condensed directly onto it (because the die no longer absorbs moisture from the wet ceramic buildup). Separation is achieved with a porcelain release agent. As with most techniques, a second bake is usually necessary for satisfactory marginal adaptation.

The principal difficulty associated with the cyanoacrylate resin technique occurs during the characterization and glazing firing. Because the porcelain is not supported as in the platinum foil technique, the margin tends to round off slightly; therefore, special shoulder powders are needed.

Step-by-step procedure (Fig. 24-34)

1. Apply cyanoacrylate resin to the labial margin area of the die. This acts as a sealant of the porous stone. Compressed air should be used to minimize the thickness of the film.

2. Apply porcelain release agent to the shoulder of the prepared die.

3. Seat the opaqued casting on the die.

4. Mix shoulder porcelain, and apply it directly to the die and the opaque porcelain. Light tapping assists in condensation and should be done before the dry buildup is separated from the die.

5. After the first firing of the shoulder porcelain, reseat the crown on the die. At this time, the restoration should be examined for margin discrepancies. A second shoulder firing is usually necessary.

6. Relubricate the die, reseat the crown, and apply a thinner mix of shoulder powder to the margin. Vibration helps the porcelain fill the defect completely. After blotting, the restoration can be separated from the die.

7. When the firing is completed, use a water-soluble marking agent to detect premature contacts. The marking agent is applied to the shoulder, and the restoration is then gently tried on the die. The markings will be visible on the porcelain and the inner aspect of the casting.

8. Adjust any areas of contact of the restoration, and proceed with the conventional buildup of body and incisal porcelains, followed by glazing of the final restoration.

Fig. 24-34 The direct lift (cyanoacrylate resin) technique for a porcelain labial margin. A, Armamentarium. B, Cyanoacrylate resin serves as a sealant of the porous stone die. C, The resin is applied to the die where the porcelain will be in direct contact with the die. Compressed air is used to minimize film thickness. D, Recommended separating medium. E, The separating liquid is applied to the shoulder of the prepared die. F, The opaqued casting seated on the prepared die. G, Mixing the shoulder porcelain. H, Shoulder porcelain is applied in direct contact with the die and opaque porcelain. I, Light tapping is used to assist in condensation. J, Separating the dry buildup from the die. K, The buildup before firing. L, First firing of the shoulder porcelain completed. M, The fixed restoration is reseated on the die. Note the minor marginal discrepancy. N, Before additional porcelain application, the die is relubricated. O, Second application of shoulder porcelain. P, Vibration. Q, Separating after the second shoulder application. R, Water-soluble marking agent for detecting premature contact. S, The marking agent is applied to the shoulder. T, The fired restoration is gently tried on the die. U, Markings are visible on the porcelain and on the internal aspect of the casting. V, Excess porcelain is removed. W, The seated restoration. X, Internal view of the completed shoulder. Y, Conventional buildup with body and incisal powders.

Porcelain wax technique

A mixture of body porcelain and wax (6:1 by weight)⁷⁷ is applied to the die for final adaptation of the porcelain labial margin of the metal-ceramic restoration.

Step-by-step procedure (Fig. 24-35)

1. After coating the substructure with opaque porcelain, lubricate the die with a porcelain release agent.

2. Apply the porcelain-wax mixture to the cervical shoulder. Use an electric waxing instrument to flow it into the proper areas.

3. With a conventional wax-carving instrument, shape the material and blend it into the opaque. It separates easily from the die and can be fired in the conventional manner.

4. A second application is needed. Using the electric waxing instrument, keep the mixture liquid long enough so that capillary action can draw it into the marginal discrepancies. The restoration is completed in the conventional manner. If rounding of the margins occurs during glazing, using a shoulder porcelain rather than a body porcelain in the wax mixture is recommended.

Fig. 24-35 The porcelain wax technique. Special porcelain-wax mixtures (A) and electric waxing instruments (B) are available for this technique. C, The mixture is flowed to place and shaped with conventional carving instruments before firing.

Advantages and disadvantages of each technique are summarized in Table 24-2.

Table 24-2 ADVANTAGES AND DISADVANTAGES OF FABRICATION TECHNIQUES

Method	Advantages	Disadvantages
Platinum foil	No shoulder porcelain (best esthetics)78	Time consuming
	Good marginal adaptation	Technically difficult
	Smooth surface78
	Low plaque accumulation78
Wax suspension	Separates easily	Shoulder porcelain needed
		Less accurate fit79
Direct lift	Least time consuming	Shoulder porcelain needed
		Rougher margins78

Cracks

Surface cracks and fractures in the opaque porcelain are usually of little concern. They can be patched before the body firing begins. Fractures during the bisque bake, however, often are the result of improper condensation, overly rapid drying, or haphazard moisture control. Poor substructure design, resulting in areas of unsupported porcelain, also can lead to porcelain failure (see Chapter 19). After cementation, pinpointing the cause of failure may be difficult. If the substructure is properly designed and the porcelain-metal interface is kept away from direct occlusal contact, cracks and fractures should not develop during normal function.

Bubbles

Even the most experienced ceramist sometimes traps air between the metal and the opaque. Usually this is of little concern. However, if a restoration is fired too many times, the trapped air may appear as blisters that rise to the surface. If this occurs, the porcelain must be stripped, and the procedure is started over.

If bubbles appear after only a few firings, improper casting technique, insufficient metal preparation, and haphazard moisture control can usually be isolated as the cause (Fig. 24-36).

Fig. 24-36 A, Bubbles (arrows) have made this metal-ceramic restoration unacceptable. B, Devitrified porcelain on a metal-ceramic restoration, the result of an excessive number of firings. C, Contamination of the porcelain surface has made this prosthesis unacceptable.

Unsatisfactory Appearance

Poor esthetic results often result from poor communication between the operator and the dental technician (see Chapter 16). An opaque application that is too thick can result in opacity of the veneer. Inadequate tooth reduction, especially in the cervical third and the interproximal areas, is one of the more common causes of a poor esthetic result. Careful communication, based on a thorough understanding and knowledge of relevant laboratory procedures and color science, is essential.