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This is the third in a series of pages on dental ceramics. The material
presented on each page is designed to stand alone, but a real understanding of
this material relies on knowledge presented on the two pages that precede it. Many
terms on this page that will be unfamiliar to the casual reader have been
defined there. This series represents a mini course in ceramics
for the beginner, and persons seriously interested in gaining a basic working
knowledge of dental ceramics are advised to take the time to start at the
beginning.
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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 3 continuing
education credits for this course may take the 20 question test at a
cost of $39 and receive their certificate immediately by clicking
here. |
This page covers the history of dental porcelain from its very beginnings in
1903 until the invention of aluminous cores in the 1960's. This page lays
the foundation for understanding the advances in the ceramics technology from
the 1960's through the 1990's which will be the subject of page four.
The pages in this course are as follows:
Traditional Feldspathic Dental Porcelain
Table of
contents (page 3)
The porcelain Jacket crown
In 1903, Charles Land (the grandfather of aviator Charles Lindberg) invented the
first tooth colored full coverage restoration. In retrospect, it would
seem a logical step to make teeth out of porcelain, but he was so far ahead of
his time, that mainstream dentists thought he was a quack. Realistically,
porcelain was white, fairly translucent, and could be made from easily available
and inexpensive porcelain clay. From about 1855 until 1903, the major methods of
repairing teeth was to fill or rebuild them with
amalgam, or with adhesive gold foil. The
gold foil method was the one used by "high class" dentists of the day, but it
was a time consuming and expensive project. Amalgam had a bad name due to
poor formulations until 1895 when G.V. Black standardized the formula.
Amalgam's poor reputation was also due
(undeservedly) to the idea that the mercury in amalgam was poisonous. Neither method resulted in an
especially good looking tooth. Land's idea was to cut the
remaining tooth back and then rebuild the stump using a porcelain covering,
which he called a "jacket".
Land's porcelain jacket was made from feldspathic porcelain clay. It
was fabricated by burnishing a piece of thin platinum foil over a die, and
brushing layers of dry clay over it using a tiny, wet paint brush. The
foil, along with each successive layer was fired in a kiln, and the process was
continued until the porcelain overlying the platinum foil resembled a tooth.
Since platinum is a noble metal the lack of an oxidized layer meant that the
porcelain would not bond to it. After all the firings had been completed, the
platinum was removed and the porcelain "jacket" was luted to the tooth using
zinc phosphate cement.
Gold and silver casting techniques had been used in dentistry for the
fabrication of metal frameworks for partial dentures since the late 1700's, but were not
used to repair decayed teeth until the 1910's, after the invention of the
centrifugal casting machine
in 1907 . The term "gold crown" had been popularized
by the gold foil technique which was used by expensive dentists to rebuild teeth
since about 1855. 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. In
the meantime, porcelain jackets became known as porcelain jacket crowns (PJC's),
probably because it helped dentists who fabricated them to compete with those
who worked only in gold. The issue of dental materials was further
confused in those days by the introduction of
amalgam into the respectable side of dentistry
in 1895. Dentists who worked in gold foil wanted nothing to do with the
newer materials and were constantly engaged in battles with those who used them.
PJC's eventually became a popular restoration in spite of the fact that they
had some serious technical drawbacks. In the first place, the removal of
the platinum foil after the crown was fired meant that there was always a
substantial gap at the margin (the tooth/crown interface) from which the
zinc phosphate cement could leach out and into which leakage of saliva and food
debris could take place.
Second, the porcelain tended to be too opaque to match the surrounding teeth.
Finally, the strength of this old fashioned porcelain crown was not great.
Porcelain jacket crowns could not be used for posterior teeth, and they were
prone to failure even on anterior teeth. However, they were strong enough
to allow Land and his associate, Dr. Edward Bartlett Spaulding to create a shoulder
preparation on a spike, bake a porcelain jacket to fit the spike, cement it
and drive it through a pine board without fracturing the crown.
Dental Feldspathic porcelain
If
you have read the first two pages in this series, you are already familiar
with the diagram above. You can see that domestic porcelain
contains about equal amounts of kaolin and quartz, and also a great deal of
feldspar when compared with other forms of clay. Feldspar is a naturally
occurring glass that contains silica, fluxes and alumina, all neatly bound
together. Feldspar in fact binds all three forms of clay bodies together
once it has been fused at high temperature. The process of turning the
crystalline silica (SiO2), along with associated fluxes and modifiers into glass
is called vitrification.
Porcelain, then, can be defined as a highly
vitrified ceramic body. The term "ceramic body" includes both the glass,
and a refractory
skeletal structure which retains it shape first through the
sintering
process, and then through the final
fusing process. The key here is that
porcelain is not simply a form of glass. It is glass with a
refractory internal structure. In domestic
porcelain, the refractory structure forms a sort of skeleton around which the
feldspathic glass can flow so that the final ceramic body keeps its shape
without slumping, distorting or simply melting into a puddle on the floor of the
kiln. This was why Dr Land's porcelain crowns were reasonably strong. Since
it was made from domestic porcelain, each crown contained both glass, and a
refractory skeletal structure to strengthen it.
This refractory skeleton was composed of
about equal parts of
kaolin which is an alumino silicate, and fine grains of pure
silica in the form of
quartz. These materials are present in in the form of crystals which
remain unmelted except at their points of contact where they fuse together
during the sintering process. Kaolin however, is opaque due to the fact
that much of it it remains in crystalline form throughout the ceramic body. The
opacity is the result of the internal scattering of light by refractory alumina
in the form of kaolinite
crystals. (Click
here for more on why crystals cause opacity.) This was the reason that Land's porcelain jackets were not
very esthetic, and this opacity remained a recurring problem in early dental
porcelains.
In light of this problem, ceramic technologists began to
formulate feldspathic porcelains with less and less kaolin until by 1938 kaolin was omitted
entirely. The glass itself was strengthened by the addition of various
stabilizers
such as boron and dissolved alumina, but these did not entirely make up for the
loss of the crystalline alumina substructure. Very fine particles of
refractory quartz were left in the formulation in order to give the glass enough
structure to resist sagging and to stop cracks from propagating through the
structure, but unfortunately, as the proportion of aluminous kaolin
decreased, the strength of the glass declined. (Note--Quartz crystals
have a refractive index close to that of the surrounding glass, and therefore do
not have as much of an effect on the
optical qualities of the glass as do alumina
crystals.) Thus feldspathic dental
porcelains slowly began to to be formulated with only a weak refractory skeleton
composed of quartz particles and became more prone to failure as a result.
On the other hand, they were (and still are), highly esthetic materials for
building tooth-like structures.
The Aluminous
Core crown
In 1965, dental ceramic technologists were
driven by the clinical failures of existing porcelain jacket crowns to address
the problem of weakness. They did this by adding large quantities (up to
50%) of finely ground aluminum oxide particles to the raw materials used to make
their feldspathic porcelain glasses. Most of it remained in crystalline form. This
creates an aluminous porcelain, a material that was already in use in industrial
applications. While the added alumina recreated the refractory internal
skeleton that had strengthened Land's original porcelain, it also reintroduced
the problem of opacity. The solution was to cut back the facing of the
aluminous crown and overlay it with a veneer of the more esthetic feldspathic porcelains
that were formulated without the alumina particles. Thus, the aluminous
porcelain was used as a substructure, or core, over which a veneer could
be applied. This substructure became known as an aluminous core.
| Note the historical progression. Dental porcelain started
out essentially as domestic porcelain, which was really a form of
pottery. As its esthetics improved, more and more of its
refractory aluminous substructure was removed until it became just a
complex feldspathic glass fortified with grains of quartz. During this
time, the strength of the resulting porcelain bodies decreased
until, in the 1960's it was decided that the benefits of an
aluminous substructure outweighed the esthetic deficits. Thus,
with the addition of refractory particles of alumina, dental porcelain moved from the realm of glass back into the world
of "pottery". |
 Fracture
in brittle solids is nearly always initiated at a small internal or
surface defect such as a scratch, or a microcrack that develops due to
uneven shrinkage of the glass structure during cooling. When a
tough, crystalline material such as alumina or silicone in particulate
form is added to a glass, the entire mass is strengthened because a
crack cannot penetrate the alumina particles as easily as it can the
glass. When the crack encounters a well fused silica or aluminum
oxide crystal, it cannot continue to propagate unless it encounters
weaknesses in the glass surrounding the crystal. When
crystalline inclusions in a glass structure are used in this capacity,
they are known as crack stoppers.
Alumina particles are far stronger than the glass, and are more
effective than quartz crystals in preventing crack propagation.
The inclusion of tiny alumina crystals into the glass in concentrations
of 40-50% increases the flexural strength of the feldspathic
porcelain on the order of about 2-3 times. Unfortunately,
since aluminum oxide in crystalline form is opaque, it is necessary to veneer a layer of
more esthetic traditional feldspathic porcelain over the visible surface
of the aluminous core.
The esthetic veneers on aluminous cores, as
well as the glass matrix between the aluminum particles are still
feldspathic glass. This is true even
though many brands of porcelain do not actually contain naturally occurring feldspars,
but are mixed up from the raw materials, just like any other form of
manufactured glass. Feldspathic glass does not, in fact, have to be
manufactured from feldspar. It just has to have the correct general
chemical formula.
Dental feldspathic glasses contain the same
proportions of chemical constituents as the original feldspar; one flux molecule
bonded to one aluminum oxide molecule bonded to a larger number of silica
molecules, all fused into an amorphous glass gel. The aluminum oxide in
feldspathic glass acts as a
stabilizer and is part of its molecular structure.
As a stabilizer, it is not
in particulate or crystalline form. Without crystalline structures, this form of
alumina does not cause internal scattering of light and does not adversely affect the
optical qualities of the glass.
When aluminum oxide is used as a stabilizer, the glass is called an
aluminosilicate glass.
Aluminosilicates are much stronger than most other types of clear glasses.
On the other hand, when one speaks of
aluminous porcelain, one is
speaking of a clear glass to which refractory grains of alumina have been
added making it more fracture resistant, but also more opaque.
Frits
The porcelain used by dental technicians
today is not a simple mixture of ingredients. The porcelain ingredients
have already been combined and fired once by the manufacturer. The
manufacturer adds metal oxides to adjust the color, opacity, strength,
thermal expansion and other
characteristics, fires the mixture to the correct temperature and then quenches
the molten glass in cold water. The thermal shock of the hot glass hitting the
water causes it to shatter into very fine particles which are retrieved and
ground even finer into a powder known as a frit.
The frit is mixed with a binder (often starch and
sugar), packaged and delivered to the dental technician to veneer the crowns and
bridges he creates for the dentist. Thus in making a PFM or a porcelain jacket
crown, there are no chemical reactions taking place in the dental lab kilns.
That has already been taken care of at the glass factory. Technicians
build both aluminous cores and esthetic veneers using powdered frits. This is
accomplished with the
powder condensation technique and vacuum firing.
The powder condensation technique
The particles in a porcelain frit average about 25
microns with a wide distribution of particle sizes so that smaller particles can
fit between larger ones. This allows the technician to create a fairly dense mass of
powder while fabricating the porcelain veneer. ("Veneer" as used
here refers to the porcelain placed over a metal, aluminous or ceramic
substructure.)
The powder condensation technique consists of applying
layers of frit to the substructure using a
brush dipped in water. The wet brush is then dipped into the dry powder which
adheres to the brush by capillary action. The wet frit is then "painted"
over the substructure (a refractory die or a crown or bridge core), until a fairly
thick layer of frit, in approximately the correct shape has been built up.
The brushing action serves to compact the frit particles to create a body of
fairly densely packed powder over the die or core. Each stroke of the
brush further compacts the particles bringing water to the surface.
The excess water is blotted off the
greenware body
with tissue paper. Each time it is blotted off, the water is drawn toward
the tissue, and in the process the surface tension of the water draws the
frit particles on the opposite side of the body together. Thus surface
tension is ultimately responsible for condensing the powdered frit.
Vacuum
firing
The body is allowed to dry out and
then fired inside an automated kiln at a temperature appropriate to the
porcelain being applied (Older porcelains were fused at 1350°C, but modern
porcelains fuse between about 800 °C and
1100 °C .)
This sinters and fuses the frit into a dense porcelain which has shrunk
considerably since it was first placed in the kiln.
In order to reduce
porosity within the porcelain body, the air pressure inside the kiln chamber
is reduced to about a tenth of the ambient atmospheric pressure. This
reduction in pressure draws gas out of the unmelted body, leaving less gas
to cause porosity when it melts. After fusion temperature has been
reached, the automated kiln begins to reduce the temperature. At about
55 °C , below the upper firing
temperature, the vacuum inside the kiln is released. As the air
pressure approaches normal atmospheric pressure, any air bubbles inside the
melted porcelain are reduced in volume to one tenth of their original size.
After the first
firing, more layers of powder are added to fill out the ceramic body to the
correct shape and size. Porcelain powders of several different shades are
used, and it takes considerable time for a technician to develop the art and the
skill necessary to create a ceramic structure that looks like a natural tooth.
The composition of feldspathic dental porcelain
A typical dental feldspathic glass contains approximately
the following proportion of constituents. (The porcelain contains
refractory crystalline elements as well.)
Silica
Silica is contained in dental porcelain in two separate forms.
The first type is in the form of
feldspathic glass in which it is
combined with aluminum oxide and a flux and does not have a crystalline
structure. In this capacity silica is the major
glass
former in the porcelain. The concentrations expressed
in the table above reflect this type of silica. The second type of silica is
in the form of refractory crystalline quartz particles which are
dispersed through the glassy phase to act as
crack stoppers. Quartz
crystalline inclusions have the unique property of remaining nearly
invisible within the glassy matrix due to the similarity of their
respective
refractive indexes.
Aluminum oxide
Aluminum oxide (Al2O3), also
exists in dental porcelain in two forms. It, like silica, is a component of feldspathic glass, and in this form
aluminum oxide is used as a
stabilizer. As a stabilizer, it
is combined on a molecular level with the amorphous silica matrix.
In this form, it toughens and waterproofs the glass without affecting
its optical properties. However, aluminum oxide also exists in the
form of tiny crystals dispersed throughout the glass
matrix. When it is in crystalline form, it is known as
alumina. In this form it strengthens the glass
by acting as crack
stoppers, but it also diffuses light and causes
opacity.
Boric Oxide
Like silica, boric oxide is another glass former. Used alone,
the glass it creates is useless since it is soluble in water, but when
combined with silica in concentrations over 5%, it toughens the glass
and makes it able to withstand mechanical and thermal shock much better
than ordinary silica glass without the boron. This is especially
useful in dental porcelains which are subjected to chewing and
bruxing forces as well as to extremes of hot and cold (hot coffee followed by ice cream etc.).
Potash (K2O) and Soda (Na2O)
Both of these alkaline oxides are used as
fluxes
to lower the melting temperature of the glass, and both are natural
constituents of
feldspar. Both
sodium and potassium are chemically quite similar, but there are major
differences between them when it comes to fluxing glass. The soda
does a better job of lowering the melting temperature than potash, but
the potash is often used in greater concentration because it increases
the viscosity of the molten glass, thus reducing slumping and running in
the kiln.
Other metal oxides
Certain metal oxides are added to the glass in order to produce
different colors and opacities thereby causing the porcelain to mimic
the correct tooth shade and translucency. For example,
- oxides of iron impart a brown color
- copper oxide produces a green color
- small amounts of titanium oxide produce a yellowish brown color
- cobalt oxide imparts a blue color
- Manganese oxide produces a lavender color
- Zirconium, cerium, titanium and tin oxides, when used as
refractory crystals produce opacity
The inherent
weakness of feldspathic glass structures
 If
you calculate the compressive and tensile strength of glass from
the bond strengths between the atoms in the glass matrix, theoretically
they should be about 100 times stronger than they actually are.
The reason that the reality is so different from the theory is that
glass and porcelain are brittle materials. In other words, their
coefficient of elasticity is extremely low, and tiny surface defects can
concentrate stresses at a single point, multiplying the strain in that
area to the breaking point. The maximum strain a glass can
withstand is 0.1%. (Stress is the force applied to
an object, while strain is the actual mechanical movement,
or deformation in the object produced by the stress.) The stress
in the diagram to the right is applied at the red arrow. All of
the microcracks on the internal surface are preexisting due to tensile
stresses during cooling. The stress on the crown in this image concentrates the
strain at these pre-existing internal microcracks causing one of them to
fracture.
When a concave/convex porcelain object like a crown cools in the
kiln, the outside cools more rapidly than the interior because
porcelain is a good insulator. The outside surface contracts and
hardens before the inside surface. Thus the inside surface is
prevented from contracting as it cools by the harder outer parts of the
structure. This places the interior surface under tensile stress.
Tensile stresses are stretching forces. Glass has very high
compressive strength, but very low tensile strength. Tiny stress
cracks develop on the interior of the porcelain shell to relieve the
strain. Thus early PJC's tended to fail due to the
microcracking that occurred on the internal surface. The answer to
this problem first appeared in the 1950's with the invention of the
Porcelain Fused to Metal crown (PFM). Since the internal surface of the
porcelain is bonded to a metal coping in PFM restorations, the
metal-porcelain bond prevents the stress cracks from developing.
(The key to the metal/porcelain bond is the formation of metallic oxides
on the surface of the metal. This is covered well in my
Course on dental alloys.)
In the 1980's it became possible to
bond porcelain crowns directly to tooth
structure, effectively turning the tooth itself into an unbreakable core.
This brought about a huge improvement the strength of the porcelain
without the opaque metal framework. In the 1990's, more modern substructures made out of very strong opaque ceramics
were invented. These ceramic substructures could be translucent
and colored the same as natural dentin and are an improvement over metallic
frameworks. They will be discussed in detail on the
last page in
this series.
The
use of unbreakable cores to support weak feldspathic porcelain
The most serious drawback with Feldspathic porcelain when used without a core is its
inherent lack of
strength and toughness. In order to
further strengthen crowns made from esthetic feldspathic porcelain so that it
can stand up to the stresses encountered in the mouth, it is necessary to
reinforce it by layering it over an
unbreakable core. There are three ways in which this can be done:
- Metal cores--Porcelain fused to metal (PFM)
The technique of bonding feldspathic porcelain to a metal framework was
invented in the late1950's by Dr Abraham Weinstein. The metal alloy could
be precisely formed to fit the tooth via the lost wax technique. This
effectively solved the dilemma of poor marginal fit, which had always been a
problem with traditionally built porcelain jacket crowns Since the
alloys could form naturally integrated oxide coatings on their surfaces, the
feldspathic porcelains formulated to veneer these frameworks could bond
intimately with their surfaces. This solved the problem of internal
microcracking which
had plagued porcelain jacket crowns as they had been fabricated up until
that time. The subject of metal alloy frameworks and the dilemma of
forcing porcelain to adhere to them is discussed at
length on my pages on
Dental Alloys. PFM crowns and bridges are still the most
popular and durable tooth colored restorations in the world. Their
major drawback is the opacity and color of the metal substructure, a deficit
that drove the search for more esthetic forms of porcelain restorations.
- Reinforced ceramic cores
The idea of replacing the metal substructure of a PFM restoration with an
opaque white porcelain substructure came about in the 1960's with the
invention of the aluminous
core. At the time these were invented, they were simply a
stronger version of the older feldspathic porcelain jacket crowns.
They were cemented to the tooth using zinc phosphate cement, but the cement
could form no bond to the porcelain crown. Aluminous cores were an
improvement, but they were still weak due to the lack of an underlying
supporting structure which could suppress the natural tendency of concave
vitreous bodies to form microcracks on their internal surfaces . It
wasn't until the 1990's and the 2000's that truly strong aluminous and zirconia
core materials were developed which could approach the strength of a metal
framework. These core materials are opaque, but they can be fabricated
to be very thin and therefore somewhat translucent. They can also be
pigmented with the same colors as the overlying feldspathic veneer.
These materials will be discussed on the fifth and last page of this series.
- Resin bonded ceramics
It was not until the 1970's that the
concept of "bonding" became accepted by the dental profession. At that time, it became possible to
etch tooth enamel and bond resin based restoratives to it. The idea of
actually bonding a porcelain jacket crown directly to tooth structure did
not become practical until the 1980's when it became possible to bond both
porcelain and dentin with an intervening resin cement. This
breakthrough made it possible to strengthen the already existing
aluminous core crown
by using the tooth structure itself as the unbreakable "framework" to
strengthen otherwise weak feldspathic porcelain structures. Even
though bonding strengthened the resulting restoration tremendously, and also
improved the esthetics, aluminous core crowns were still not strong enough
to bond to posterior teeth, or to fabricate fixed bridges. In the 1990's,
new forms of ceramic crowns that could also be bonded directly to tooth
structure and which were stronger and better looking than aluminous core
crowns were invented. These new glass ceramics are the subject
of the next page in this series.
<<--Ceramics 2--Glass
and glazes
Ceramics 4--Glass ceramics-->>
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