The Classification of Crown and Bridge 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. 

The classification and formulation of crown and bridge gold alloys (used for all-gold restorations)

Prior to the introduction of Porcelain Fused to Metal (PFM) restorations, gold based alloys were virtually the only castable alloys used in dentistry.  There were four types:

Type I

Type I was hard enough to stand up to biting forces, but soft enough to burnish against the margins of a cavity preparation.  It was used mostly for one surface inlays. 

Type II

Type II was less burnishable, but hard enough to stand up in small multiple surface inlays that did not include buccal or lingual surfaces. 

Type IV

Type IV was used for partial denture frameworks, and was not used in fixed prosthetics. 

Type III

The most commonly used type of gold for all-metal crowns and bridges is Type III.  It is still used whenever a patient requests an all gold restoration such as an all gold crown, inlay or onlay.  A typical type III gold alloy has approximately the following formula:

The purpose of each component is as follows:

Gold

Gold is a "noble metal".  In other words, it resists tarnish and corrosion and will participate in very few chemical reactions, which means that it is non toxic and hypoallergenic.  It is also highly ductile and malleable and has a relatively low melting point, which are major factors accounting for its use by people in early historical periods.  Gold's long civilizational lineage and incorruptibility made it a natural first choice for use in dentistry.  It forms the bulk of the composition of the alloy. 

The other noble metals are:, palladium, silver, tantalum, platinum, iridium, osmium, ruthenium, and rhodium.  The classification of noble metals is an ancient one and is rather loosely defined since silver certainly tarnishes, and copper is sometimes included in the list. 

 

Copper

Copper is the principal hardener.  It is necessary for heat treatment and is usually added in concentrations of greater than 10%

Silver

Silver lowers the melting temperature and also modifies the red color produced by the combination of gold and copper.  It also increases ductility and malleability.

Palladium

Palladium, (another noble metal) raises the melting temperature, increases hardness and whitens the gold, even in very small concentrations.  It also prevents tarnish and corrosion and acts to absorb hydrogen gas which may be released during casting causing porosity. 

Zinc

Zinc acts as an oxygen scavenger and prevents the formation of porosity in the finished alloy.  It also increases fluidity and reduces the surface tension in the molten state improving the casting characteristics of the alloy.

For more on nearly all the individual metals used to formulate dental alloys, click here.

 

Porcelain alloys

Until the mid 20th century, gold and amalgam were virtually the only materials available for the restoration and replacement of posterior teeth.  Metal was the only game in town.  Porcelain jacket crowns were available for front teeth, but they did not fit very well, and they were prone to easy fracture.  In 1962 that all changed when Dr. Abraham Weinstein patented the first gold based alloy upon which porcelain could be baked.  The metal substructure reinforced the porcelain and gave it the durability and the strength to resist fracture in the mouth.  It made it possible for the first time to replace missing teeth with natural looking tooth colored fixed bridgework.  In addition, due to the accuracy of the lost wax technique, the appliances could fit the tooth preparations exactly.  Producing a metal framework that was compatible  with a durable porcelain superstructure was not an easy task:

Porcelain will not chemically bond with gold by itself.  There needs to be a mix of trace elements in the composition of the alloy to allow the formation of an oxide layer on its surface, which then bonds the porcelain to the metal.  The three oxide-forming elements are iron, indium and tin.  Porcelain is, itself, made of metal oxides.  Thus it will bind with the oxides on the surface of the gold framework.

The necessity for the formation of metal oxides on the surface of the underlying casting means that ions from the metal casting will mix with the porcelain, potentially affecting the color, reflective properties and translucency of the finished product.  Thus the porcelain must be formulated to overcome these effects.

Porcelain melts at high temperatures (between 850°C and 1350°C depending on the type of porcelain used).  It is applied as wet powder over the metal framework, and baked, or fired in order to fuse the powder particles together.  This means that the metal substructure upon which the porcelain is applied must resist sagging and deformation while being held at this high temperature for several hours while the porcelain is fused over it.  Otherwise, the casting will not fit the teeth in the mouth.

The metal is opaque and generally has a gold or gray color.  Porcelain must be translucent, or it fails the tests of esthetics.  There must be a mechanism to "opaque" the underlying metal framework, or the finished appliance will have a gray cast and not look real.

The index of thermal expansion of the metal must be nearly identical to that of the porcelain.  Otherwise, the porcelain will simply shatter off of the framework as it cools after being fired.  If the metal shrinks less than the porcelain during cooling, the porcelain will "craze" (develop little cracks throughout its structure).  If the metal shrinks too much more than the porcelain during cooling, the porcelain will "shiver" (the opposite of crazing, sort of like "puckering", but having the effect of breaking the porcelain off of the framework.

Ideally, porcelain should be under slight compression in the final restoration.  This objective is accomplished by selecting an alloy/porcelain combination in which the alloy contracts slightly more than the porcelain on cooling to room temperature.  Compression of the porcelain reduces the likelihood that cracks will propagate throughout the structure during service.

All porcelains used to veneer metallic substructures contain leucite crystals.  These crystals serve two functions in the porcelain.  They act to limit the propagation of cracks in the porcelain veneer, and they serve to increase the index of thermal expansion of the porcelain.  By carefully adjusting the proportion of leucite crystals in the glass, it can be made to "fit" the metallic substructure during the sintering and fusing phases of manufacture.

How porcelain is applied to a metal coping

In the image below, a cast metal coping is placed back on the die after the buccal gingival margin is removed.  This is done in order to allow a butt porcelain margin so that no metal will show in the final crown:  (Thanx to Bothell Dental lab for these images.)

Next, a thin layer of opaque porcelain powder (frit) is layered over the metal in order to mask the underlying darkness.  Otherwise, the finished crown would always show a gray caste.

After the opaque layer is fused onto the metal coping, the first layer of overlying porcelain is applied with a wet paintbrush.  Different shades of frit are applied over various parts of the crown in order to make the finished tooth look more natural:

The coping, along with its "green" porcelain is removed from the die and placed in a vacuum kiln and fired at about 1700 degrees F:

The green porcelain shrinks during its firing, so a second layer of porcelain frit is layered over the first bake. 

When the technician has finished rebuilding the correct contours, he or she then replaces the crown in the vacuum kiln for its second and final firing. For a thorough understanding of glass and porcelain, Students and dental professionals should consult  my five page  "Course in Dental Ceramics for the beginner"

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