Alloys for Porcelain-Fused-to-Metal Restorations

Fig 14-1 A classification of alloys for PFM restorations.

Last, in a balanced decision, cost should be considered relative to these criteria. Factors such as single-unit or multiple-unit, presence or absence of metal occlusal surfaces, span length, and porcelain brand often indicate different alloy choices. A practitioner using only one alloy is unlikely to make an optimal choice in every clinical situation.

Table 14-1 Composition (%) of alloys for PFM restorations*

ALLOYS   Au   Pt   Pd   Ag   Sn   In   Ga OTHER METALS

High gold

74–88

0–20

0–16

0–15

0–3

0–4

Zn < 2; Fe < 0.5; Ta <1

Gold-palladium (no silver) 45–68 0–1 22–45 0–5 2–10 0–3 Zn < 4

Gold-palladium-silver

42–62

25–40

5–16

0–4

0–6

0–2

Zn 0–3

Palladium-copper 0–2 0–1 66–81 0–8 0–8 3–9 Cu 4–20; Zn 0–4

Palladium-silver

0–6

0–1

50–75

1–40

0–9

0–8

0–6

Zn 0–4; Mn 0–4

Nickel-chromium               Ni 59–74; Cr 10–22

Nickel-chromium-beryllium

 

 

 

 

 

 

 

Ni 70–80; Cr 12–15; Be 0.6–2

Cobalt-chromium               Co 54–65; Cr 24–32

Titanium

 

 

 

 

 

 

 

CP Grades 2 and 4

Titanium alloys               Ti-6Al-4V; Ti-Nb-Al

*Courtesy of A. Prasad.

Image Classifications

Before discussing alloy characteristics, it is worth reviewing the terms noble, precious, semiprecious, and nonprecious. Noble metals are defined on the basis of their chemical properties; that is, they resist oxidation and are not attacked by acids. Eight metals meet this definition but only four are widely used in dental alloys: gold, palladium, silver, and platinum. (See appendix C for all noble metals, including gold, platinum, palladium iridium, rhodium, ruthenium, osmium, and silver.) These metals give noble metal alloys their inert properties in the mouth.

The term precious refers only to cost, which is controlled by supply and demand. Many elements in the periodic table, including the eight noble metals, are precious by today’s standards.

The term semiprecious was originally coined for noble metal alloys that contained significant amounts of silver, and it subsequently has been applied to a variety of alloys, some of which are mixtures of precious and nonprecious ingredients. It is advisable to drop the term semiprecious from the dental vocabulary, as it is not well defined and leads to much confusion.

Nonprecious alloys are composed of nonprecious ingredients, except for the common inclusion of 1% to 3% beryllium, a precious but ignoble metal. Most non-precious alloys are based on a combination of nickel and chromium, although cobalt-chromium and iron-based alloys also exist. See Table 14-1 for the compositions of alloys commonly used in PFM restorations.

When an alloy is chosen for a particular clinical situation, a number of characteristics have clinical significance and should be considered. Among the most important of these characteristics are physical and chemical properties, casting accuracy, and porcelain-metal compatibility.

Image Physical and Chemical Properties

Color is one of the most obvious physical properties of an alloy. Although the color has no biologic significance, it is equated with quality in the minds of many clinicians. Sometimes this factor seems to matter more to the clinician than to the patient.

When the gold content of an alloy is decreased and metals such as silver and palladium are substituted, yellow color is lost. These less yellow dental alloys are not yet widely accepted. The profession’s desire for gold color is so strong that gold-colored semiprecious and nonprecious alloys are commercially available, even though their other physical and chemical properties fall far short of those of even the cheapest white alloys. In some countries, yellow alloys of copper and nickel are currently popular.

If an alloy is gold colored, it must contain copper, gold, or both. However, an alloy can contain substantial amounts of gold or copper without appearing yellow. Examples of this apparent contradiction are jewelers’ white gold and some popular gold alloys for PFM restorations (eg, Degudent U, Evonik Degussa; SMG-3, Ney Dental). The latter products contain more than 80% gold, yet no yellow color is seen because of the strong whitening effects of palladium and platinum. Color can be a misleading indicator of composition; dentists should consider other physical and chemical properties as more important than color when a casting alloy is selected.

Some important physical and chemical properties to consider when choosing a cast alloy are:

1. Noble metal content: the weight (or better, the atomic) percentage of the eight noble metals contained in an alloy

2. Hardness: the Vickers hardness number (VHN), a measure of resistance to indentation

3. Yield strength: a measure of the stress required to cause permanent deformation under tension

4. Elongation: the amount of permanent deformation a metal undergoes when loaded to its fracture point

5. Fusion temperature: the approximate temperature at which an alloy separates under its own weight from partial melting

All of these characteristics have clinical significance. The noble metal content determines, to a large extent, the corrosion resistance and inert properties of the alloy. Hardness is important in relation to occlusal wear resistance and finishing and affects polishing properties. Yield strength is necessary in determining load-bearing ability, especially in fixed partial dentures. Elongation relates to margin-finishing properties, especially important in partial veneer crowns and abutments. However, the elongation value for an alloy may be clinically irrelevant if the yield strength is high. To use the potential elongation, stresses exceeding the yield strength must be applied to move the metal. Within each group of alloys, yield strength generally increases with increasing hardness. Fusion temperature is important in relation to solder melting ranges and correlates with sag resistance.

Image Porcelain-Metal Compatibility

Thermal expansion, bond strength, and composition are also properties to consider when choosing among alloys for PFM restorations, as these characteristics determine porcelain-metal compatibility. Thermal expansion is important because a state of zero residual stress is desirable for porcelain in the final restoration. Such a state is achieved when the total expansions and contractions of the porcelain and metal are matched between the porcelain firing temperature and room temperature.

Porcelain-to-metal bond strength ensures retention of porcelain both in the oral environment and during thermal processing, when the induced thermal stresses can be high.

Composition is a key factor in porcelain-metal compatibility because some components of an alloy can affect the color of the porcelain, perhaps compromising the esthetics of a restoration. Among the alternative alloys, those containing silver are often associated with porcelain color changes and can cause “greening” of some brands of porcelain. The mechanism behind this discoloration is an exchange between silver from the alloy and sodium from the porcelain. The exchange process requires an oxidizing atmosphere, but a subsequent reducing atmosphere is required to produce the colloidal precipitate responsible for color changes in the porcelain.

Image Other Properties

Because the cross-sectional area of metal used in PFM restorations is usually smaller than that used in all-metal restorations, physical properties such as yield strength of the alloy are crucial in design. Stress in turn controls the minimum allowable dimensions of critical areas like connectors. The elastic modulus is equally important because it determines the flexibility of the metal framework. Flexibility is inversely proportional to elastic modulus; an alloy with a high elastic modulus will flex less under load than an alloy of low elastic modulus.

Chemical properties are important because they affect tarnish resistance, corrosion resistance, and thermal stability. Thermal properties are critical in alloys for PFM restorations because the alloy must have a sufficiently elevated melting temperature range to provide dimensional stability during the porcelain firing cycle. Thermal creep results in distortions such as sag in fixed partial denture frameworks and margin opening during the porcelain firing cycles.

Table 14-2 Typical properties of alloys for PFM restorations

GROUP VHN ELASTIC MODULUS
PSI × 1066 (GPa)
YIELD STRENGTH
PSI (MPa)
SPECIFIC GRAVITY

High gold

182

13 (90)

65,000 (448)

18.3

Gold-palladium (no silver) 220 18 (124) 83,000 (572) 13.5

Gold-palladium-silver

218

16 (110)

63,600 (439)

13.8

Palladium-copper 425 14 (96) 166,000 (1,145) 10.6

Palladium-silver

242

20 (138)

77,000 (531)

11.1

Nickel-chromium 257 29 (207) 58,000 (400) 8.7

Nickel-chromium-beryllium

357

31 (213)

116,000 (800)

7.8

Casting accuracy must, of course, be sufficient to provide clinically acceptable castings. In addition to dimensional accuracy (a strong function of technique), the mold-filling ability also contributes to casting accuracy.

Biocompatibility involves a number of factors, among them cytotoxicity and tissue irritation. Potential biologic hazards from the base metal

Only gold members can continue reading. Log In or Register to continue

Related Posts

May 28, 2016 | Posted by in Dental Materials | Comments Off on Alloys for Porcelain-Fused-to-Metal Restorations
Premium Wordpress Themes by UFO Themes