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A Course in Dental Alloys
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| 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.
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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.
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Dentists and allied dental professionals often
seek CE courses from ADA CERP recognized providers to fulfill their
CE requirements for re-licensure. Most state and
provincial licensing boards will accept CE credits issued by ADA
CERP recognized providers. In the spring of 2003, the FDI World
Dental Federation became the first internationally based CE provider
to be granted ADA CERP recognition.
Please contact your state board directly for their specific rules
and regulations. Most states approve supervised self-study courses
that are ADA CERP accredited.
Those interested in receiving 2 continuing
education credits for this course may take the 20 question test at a
cost of $30 and receive their certificate immediately by clicking
here. |
Castable metal alloys used in dentistry
Their history, the types, their uses and their toxicities
Metal Castings are
made by fabricating a hollow mold, pouring a molten metal into it, allowing the
metal to solidify and separating the now solid metal casting from the mold.
Ultimately, all metallic objects originate from castings. In dentistry, metal castings are used to restore teeth, replace teeth, and as
frameworks for removable partial dentures. Today, metal castings are also used
as metal frameworks to support porcelain crowns or fixed partial dentures in order to
produce strong, and yet very esthetic restorations.
The history of the lost wax technique in dentistry
The lost wax technique was probably invented in ancient China or
Egypt. The technique consists of carving a wax replica of an item
that is to be duplicated in gold. The wax is invested (imbedded) in
plaster or clay and burned out leaving an image (hole) where the wax used to be.
Then the image is filled with molten gold through a small hole in the
investment. This technique works quite
nicely for fairly large castings, but gravity alone is not sufficient to draw
gold into the very fine detail necessary to fabricate a tiny filling for a
tooth.
Prior to 1855, dentistry
consisted mostly of extracting decayed and abscessed teeth and replacing them
with some sort of removable denture. Silver amalgam, made from shaved silver
coins mixed with mercury,
was invented in France in 1819, but was an unreliable filling material due to the haphazard way it was formulated.
While itinerant entrepreneurs traveled the countryside plugging amalgam into
decayed teeth, most reputable
dentists refused to use it. Gold leaf was first used to fill
teeth in about 1483 by Giovanni d'Arcoli, but
the technique was extremely tedious and expensive and only the most wealthy and
determined patients could afford and withstand having their decayed teeth
repaired this way.
The cohesive gold foil technique was perfected and codified in 1855.
It was much less tedious and less expensive than using gold leaf and made
restoration of decayed teeth a real option for a wider swath of consumers. The gold foil
technique consists of a rather laborious
and still fairly expensive process involving hammering tiny pieces of pure
(cohesive) gold foil into an
equally laboriously prepared cavity preparation. Only reasonably affluent people could
afford this sort of dentistry, but it was reliable, and gold foil became
the industry standard for repairing damaged teeth. In 1895, G.V. Black
standardized a reliable and safe formula for
dental amalgam . This made it possible for the average
(non wealthy) person
to save a decayed tooth rather than having to extract it. Unfortunately, not all dentists
were on board with the mercury, so many remained wedded to the gold foil
technique.
In 1907, William H. Taggart invented a centrifugal casting
machine for use with the lost wax technique. With centrifugal force
replacing gravity as a method of filling the casting image inside an investment,
it became possible to cast small, highly detailed objects. He worked up the
procedures for the technique and patented it, however he eventually lost the
patent when it was discovered that a Dr. Philbrook of Denison, Iowa had
published a paper on the subject twenty five years before. Taggart's
procedure involved carving a wax pattern directly inside of a patient's open
mouth. Today, a dentist takes an impression and sends it to a dental
laboratory. The lost wax technique is explained below:
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A dentist laboriously drills out the decay from a
tooth and refines the shape of the preparation, being careful that there are no
undercuts
which might interfere with an unrestricted path of withdrawal.
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The dentist then takes an impression of the prepared tooth.
This impression is then sent to the lab for fabrication of the restoration.
The images presented here show how a gold crown is fabricated in the
laboratory.
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Then, a sprue (simply a small wax rod) is attached to the wax
replica. In the image below, the sprue is the green extension from the
crown down toward the casting ring cap at the bottom. The bulb in the
sprue serves as a reservoir for the gold to help equalize the pressure of
the liquid gold so it flows evenly into each wax pattern
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The casting ring cap is then fitted over the casting ring.
The casting ring serves as a container to hold the plaster (actually in
improved investing plaster) which is then flowed around the wax patterns.
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Once
the investment has set, the casting ring cap is removed leaving the sprus
sticking up out of the now hard investment. The cylinder, with its invested wax,
is placed in a very hot oven.
When the wax burns
away, the plaster in the ring then contains a space in the shape of the original
wax filling (actually a hollow three dimensional image of the filling with
attached sprue).
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The image is then filled with molten gold using a centrifugal casting machine.
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By immersing the still hot plaster with its gold innards in
water, the plaster would shatter away leaving behind the casting which includes
the gold filling and the attached sprue.
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After removing the sprue,
the gold casting was polished up and cemented into the original cavity preparation in the
tooth.
This technique works equally well for fillings in teeth as well
as full gold crowns. When a casting soes not replace the cusp of a tooth,
it is called an inlay:
When a casting replaces one or more cusps on a tooth it is
called an onlay:
 Taggart's centrifugal casting machine made it possible to apply
enough "gravity" to force the molten gold into the tiny invested image
of a filling.
The picture on the left shows a modern version of Taggart's invention. The
white piece with the hole in it is a small crucible used to melt the gold
alloy with a gas and forced air torch. The burned out image
(originally invested in a metal cylinder, called a casting ring), is
placed behind the hole in the crucible. The orange stand contains a spring
which has been wound several times in preparation for the casting operation.
Once the image is in place and the alloy has been melted, the technician allows
the locking pin that sticks up on the left side of the base to drop. This
releases the armature, and when the technician lets the armature loose, the
armature, along with the crucible and its attached casting ring spins at
considerable speed. The crucible apparatus swings out so that it is
facing the counterweights on the opposite side of the armature.
Centrifugal force forces the melted alloy through the hole in the crucible and
molten metal proceeds to fill the image in the casting ring behind it.
Unfortunately, Taggart's technique did not produce
the accuracy that many dentists demanded for these small restorations, so most
dentists still resisted the introduction of cast metal restorations in favor of
gold foil or the newly improved silver amalgam, both of which always produced the
tightest restorations possible. Still, even in the 1910's, wealthy people wanted high class dentistry and were
willing to pay more for the privilege of not having to sit around suffering while the
dentist hammered gold into the cavity preparation. Thus, cast gold
restorations began to compete successfully with gold foil almost immediately, in
spite of the fact that the castings did not fit the preparation perfectly.
Since gold was the metal used to make the crowns worn by kings, it suited the
mentality of the day to think of being able to afford the services of an
expensive dentist as something that brought a royal distinction to the patient.
Thus the term "gold crown" was something like an advertising slogan. The
term "crown" was used to denote any gold restoration applied to a single
tooth, including gold foil restorations, inlays and onlays. Today, the
term "crown" is reserved for any full coverage restoration, whether gold or
porcelain, and the terms "inlay", "onlay" and "filling" are used to denote
restorations that cover only a part of a tooth's clinical crown.
In
1929, Coleman and Weinstein invented cristobalite investment to replace
the plaster of Paris, eliminating most of the shrinkage and distortion problems
which had plagued the production of gold castings up to that point.
(Cristobalite is one of the three crystalline configurations of silica. It
has unique thermal expansion qualities which makes it especially suitable as an
investment material for metal casting.) Even
cristobalite investment did not produce perfect castings, and it was not until
the 1940's that cristobalite investment materials were formulated that
compensated for all of the distortions encountered in the original lost wax technique.
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