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Figure 1. Second, virtually any ceramic
within this spectrum can be consid-
ered as being a “composite,” meaning
a composition of two or more distinct
entities (to be explained below). This
concept is then helpful in comparing and
contrasting resin-based composite CAD/
CAM block materials – essentially one
having a resinmatrix filledwith particles,
and the other a glass matrix filled with
particles. Quite a number of seemingly
different dental ceramics canbe shown to
be very similar or closely related to each
other when reviewed within the frame-
work these two simplifying concepts
provide. Two examples of the utility
of these concepts include these basic
truths: 1) highly esthetic dental ceramics
are predominantly glassy, and higher-
strength substructure ceramics are
generally crystalline, and; 2) the history
of development of substructure ceramics
simply involves an increase in crystalline
content to fully polycrystalline. Figure 2
provides basic composition details and
commercial examples of many esthetic
and substructure dental ceramics orga-
nized based on their matrix and filler.
Predominantly Glassy Ceramics
Dental ceramics that best mimic the
optical properties of enamel and dentin
are predominantly glassy materials.
Glasses are 3-D networks of atoms
having no regular pattern to the spacing
(distance and angle) between nearest or
next-nearest neighbors; thus, their struc-
ture is “amorphous,” or without form.
Glasses in dental ceramics derive prin-
cipally from a group of mined minerals
called feldspar, and are based on silica
Fig. 3: Feldspathic
glasses 3-D network
3
(silicon oxide) and alumina (aluminum
oxide), hence feldspathic porcelains
belong to a family called aluminosili-
cate glasses.2 Glasses based on feldspar
are resistant to crystallization (devitri-
fication) during firing, have long firing
ranges (resist slumping if temperatures
rise above optimal) and are extremely
biocompatible. In feldspathic glasses,
the 3-D network of bridges formed by
silicon-oxygen-silicon bonds is broken
up occasionally by modifying cations
such as sodium and potassium (and yes,
even a smidgen of lead) that provide
charge balance to non-bridging oxygen
atoms (Fig. 3). Modifying cations alter
important properties of the glass; for
example, by lowering firing tempera-
tures or increasing thermal expansion/
contraction behavior.
Lead in Dental Ceramics?
During the summer of 2009, the media
was full of alarming news about an FDP
(fixed denture prosthesis) in Columbus,
Ohio, outsourced from a lab in China that
contained lead and was alleged to have
damaged the woman’s mouth in which
it was inserted. As the dust settled, it
became obvious that dental porcelains
are a really lousy source of dietary lead.
Far better to get your lead by drinking
water or eating bread, fruits and soups.
On a daily basis, these foods will provide
you with thousands of times more lead.4
Further, lead from these natural sources is
more easily digested than that fromdental
porcelain. So, hands down, if you want to
maintain blood lead levels, stick with the
health food supermarket, where lead is
available at far higher levels, ismore easily
absorbed and is far more economical than
it is from the dental office.
All tongue-in-cheek aside, the above
is spot-on accurate. Bottom line is that
lead is found as a trace element in virtu-
ally everything that comes into contact
with earth. Plants grown in soils along
with surface water and ground water all
extract tiny amounts of lead. In 2005, a
comprehensive study was published
analyzing the French diet. Forty-one
different categories of food including
regional, seasonal and national sources
were analyzed.4 Turns out, the average
French person eats 18.4 micrograms of
lead each day. “Ah, maybe this explains
the French!” you say. (And they would
respond, “But who in France is average?)
Some perspective for those finding this
information challenging: one microgram
is one millionth of a gram. With gold at
$960 an ounce, 18.4 micrograms of gold
would fetch 6.25 one-hundredths of a
penny (0.0625¢).
Back to dental restorations. The Ohio-
via-China prosthesis of ill repute referred
to was found to contain about 300 parts-
per-million of lead, actually a bit less, but
let’s stay with 300 parts-per-million to be
conservative. Assuming all the porcelain
fromthis crown is eaten in 10years (which
we know is absurd), the daily plus-up in
lead would be .078 micrograms. That’s a
whopping increase of .000004 percent.
If this crown were eaten over one year,
.00004 percent.
Particle-Filled Materials
Filler particles are added to the glass
or resin matrix composition in order to
improve mechanical properties and to