Non-precious or predominantly base metalswere
first introduced at the end of 1960s. By classification their noble content (gold, platinum, silver, palladium) is less than
25 %. In reality most of the metals in this category do not contain any precious metals at all or their content is negligible.
When first introduced they were heavily used
as a base for PFM ( porcelain fused to metal crowns ) as a low cost alternatives for the gold based crowns. These metals have
some advantages in use for PFMs. 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 and Beryllium, two of the most commonly used constituents used to make base metal alloys, can cause allergic
reactionswhen 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.
studies have shown that very high intake of nickel and beryllium is known to be carcinogenic (cancer causing).
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. The
existence of nickel and beryllium restorations do not cause any carcinogenic action.
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.
In recent years with the price of gold moving really high, and insurance reimbursement rates staying almost the same, more
and more dentist use non-precious metals for use as a base for PFM crowns.
or noble metals contains somewhere between 25% and 60% of any combination of precious metal. This is the most diverse
group of alloys. It is more economical solution compare to high noble metal, also it has advantages to the high noble metal
in casting of long span bridges.Precious
or high noble minimum content of noble metals is 60% (any combination of gold, palladium, platinum and
silver). The gold content is 40% minimum by weight. They usually contain a small amount of other metals
such as tin, indium, zinc, copper 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. Depending on the content hey may be yellow
or white in color. The high noble metals shouldn't be used in very long span bridges. During the baking
of porcelain the long span bridges tend to deform.Titanium alloys are also used as a base for the PFM crowns, but due to the special
requirements of porcelain and high cost found only limited use on the market. With the development of cad-cam technologies
one can expect titanium base PFM crowns to find more use in the dental market. Today cad-cam technology for the PFM crowns allow
to produce wax forms which can be casted in metal of choice or to mill direct titanium castings.
Enter second column content here