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The Composition of Porcelain Alloys

This series represents a mini course in dental alloys for the beginner, and persons seriously interested in gaining a basic working knowledge of dental alloys are advised to take the time to start at the beginning.

If all five pages are read in order,  the reader will gain a good understanding of just what dental alloys really are, their internal crystalline structures, how they differ from each other and how different alloys are utilized in various applications.

Composition of porcelain alloys (Alloys that are formulated to act as substructures for Porcelain Fused to Metal (PFM) restorations)

PFM alloys are classified according to the proportion and types of noble metals they contain.

High noble alloys

High Noble alloys have a minimum of 60% noble metals (any combination of gold, palladium and silver) with a minimum of 40% by weight of gold.  They usually contain a small amount of tin, indium and/or iron which provides for oxide layer formation which in turn provides a chemical bond for the porcelain.  High noble alloys have low rigidity and poor sag resistance.  They may be yellow or white in color.  There are three general types of High noble alloys:

 Gold-Platinum alloy

Developed as a yellow alternative to otherwise white palladium alloys, these can be used for full cast as well as metal-ceramic restorations.  More prone to sagging, they should be limited to short span bridges.  A typical formula is Gold 85%;  Platinum 12%;  Zinc 1%;  silver to adjust the expansion properties (in some brands).

Gold-Palladium alloy

Can also be used for full cast or metal-ceramic restorations.   Palladium has a high melting temperature, and even fairly small amounts of it will impart a white or gray color to the finished alloy.  The palladium content reduces the tendency of the casting to sag during porcelain firing.  These alloys usually contain indium, tin or gallium to promote an oxide layer.  A typical formula is Gold 52%;  Palladium 38%;  indium 8.5%;  Silver to adjust the expansion properties (in some brands).

Gold-copper-silver-palladium alloys

These have a low melting temperature and are not used for metal-ceramic applications.  They contain silver which can cause a green appearance in the porcelain, and copper which tends to cause sagging during porcelain processing.  A typical composition is Gold 72%;  Copper 10%;  Silver 14%;  Palladium 3%.

Noble Alloys

Noble alloys contain at least 25% by weight of noble metal.  This can mean gold, palladium or silver.  Any combination of these metals totaling at least 25% places the alloy in this category. They are the most diverse group of alloys.  They have relatively high strength, durability, hardness and ductility.  They may be yellow or white in color.  Palladium imparts a white color, even in small amounts.  Palladium also imparts a high melting temperature.

Gold-copper-silver-palladium alloys

Note that this classification is also included under the high noble category.  The difference here is that the proportion of gold and palladium is a great deal less than its high noble cousin.  More copper, and silver are in the mix in its place.  These alloys have a fairly low melting temperature, and are more prone to sagging during application of porcelain.  Thus they are used mostly for full cast restorations rather than PFM applications.  A typical formula is: gold 45%;  Copper 15%;  Silver 25%;  Palladium 5%.

Palladium based alloys offer a less expensive alternative to high noble alloys since they can cost between one half and one quarter as much as the high gold alternative.

Palladium-copper-gallium alloys

These are very rigid and make excellent full cast or PFM restorations.  They do contain copper and sometimes are prone to sagging during porcelain firing.  The gallium is added to reduce the melting temperature of the alloy as a whole.  A typical formula is Palladium 79%;  Copper 7%;  Gallium 6%.

Palladium-Silver and Silver-Palladium alloys

As the name(s) imply, the recipes for these alloys vary depending on the relative content of palladium and silver.  These were popular in the early 1970’s as a noble alternative to the base metal alloys with which they were designed to compete.   Higher palladium alloys are popular for PFM frameworks.  Higher silver alloys are more susceptible to corrosion and the silver may lead to greening of the porcelain unless precautions are taken.  On the other hand, they have high resistance to sagging during porcelain firing and are very rigid, so they are good for long spans.  They are also more castable (more fluid in the molten state), easier to solder and easier to work with than the base metal alloys.  Typical recipes include: Palladium 61%;  silver 24%;  Tin (in some formulas).   Another is: Silver 66%;  Palladium 23%;  Gold (In some formulas, a low percentage of gold was included to satisfy insurance requirements regarding the definition of nobility in the alloy.)

Base Metal alloys

Base metal alloys have been around since the 1970’s. They contain less than 25% noble metal, but in actuality, most contain no noble metal at all.  They can be used for full cast or PFM restorations, as well as for partial denture frameworks.  As a group, they are much harder, stronger and have twice the elasticity of the high-noble and noble metal alloys.  Thus castings can be made thinner and still retain the rigidity to support porcelain.  They have excellent sag resistance and are great for long span porcelain bridges.  They appear at first glance to be the ideal metal for  cast dental restorations, and for a while, they were heavily used for PFM frameworks due to their low cost and high strength characteristics.

Nickel/Beryllium alloys

Unfortunately, Nickel and Beryllium, two of the most commonly used constituents used to make base metal alloys, can cause allergic reactions when in intimate contact with the gingiva.  Since many women (and now men) have been sensitized to these metals by wearing inexpensive skin piercing jewelry, crowns and bridges made from these alloys have been known to cause gingival discoloration, swelling and redness in susceptible individuals.  Note that the allergic reaction is limited to contact gingivitis and effects the gingiva (gums) alone.  There are no known systemic (whole body) allergic reactions reported as a result of exposure to oral appliances made from base metal  alloys.  Allergic reactions appear to be limited to fixed appliances (crowns and bridges).   Nickel containing metals rarely cause allergic dermatitis when used for removable partial denture frameworks.

Very high intake of nickel  and beryllium is known to be carcinogenic (cancer causing).  For the most part, however, alloys containing these metals are ubiquitous in jewelry and even in dental restorations in countries outside of the US, Canada and Europe, and are not associated with any form of cancer when used in contact with skin or mucosa.  The sorts of exposures required for evidence of carcinogenicity to appear are uniquely associated with occupational exposures during the smelting and refining of nickel or beryllium.  In dentistry, the only people known to be at risk of cancer from exposure to these metals are dental technicians who melt nickel and beryllium alloys and are exposed to the fumes.

Base metal alloys also have other disadvantages for the lab technicians and dentists that work with them.  They have a very high melting temperature which makes them more difficult to cast.  They exhibit a high casting shrinkage (about 2.3%) which must be compensated for.  Their hardness makes them difficult to burnish and polish and their high melting temperature makes them difficult to solder.  They are also more prone to corrosion under acidic conditions.

Today relatively few American, Canadian or European dentists order fixed restorations (crowns and bridges) made from base metal alloys.  The companies that sell dental alloys still carry a line of these alloys specifically for making crowns and bridges, mostly for sale outside the US, Canada and Europe.   Most American and European doctors stick with palladium or gold based alloys to avoid the (very rare) possibility of legal problems if a patient turns out to be allergic to the nickel or beryllium they contain.   Nickel-containing alloys and compounds have not been associated with increased cancer risk by oral or dermal routes of exposure (link).  Base metal alloys are often used today in the manufacture of removable partial denture frameworks.  There are two subcategories of base metal alloy:

Nickel-chromium alloys

These contain at least 60% nickel, and may contain a small amount of carbon (about 0.1%) as a hardener.  They also can contain either >20% chromium or <20% chromium with or without beryllium.  These are used now mostly for removable partial denture frameworks.

Cobalt-chromium alloys

These are a nickel free alternative to the nickel-chromium alloys.  They seem to have become the most commonly ordered type of base metal for removable partial denture frameworks.  They can also be used for PFM framework fabrication as well.  The major problem with this formulation is that it is more difficult to work with than the nickel-chromium alloy due primarily to its high melting temperature.  This necessitates the use of specialized casting equipment.  This alloy’s high hardness and low ductility also make it difficult to finish and polish.

NOTE:  Prior to the advent of base metal alloys, the most common alloy used for removable partial denture frameworks was type IV  (extra hard) gold alloy.  It is rarely used in this capacity any more since chrome-cobalt frameworks are lighter, stronger and much cheaper than type IV gold alloy.

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