Page 19 - CEREC Q4|2012

quarter 2 | 2013 | cerecdoctors.com | 15
desired substructure. This 3-D data set
is used to create either an enlarged die
upon which ceramic powder is packed
(Procera, NobelBiocare, Zurich, Switzer-
land) or to machine an oversized part for
firing by machining blocks of partially
fired ceramic powder (Cercon, Dentsply
Prosthetics.; Lava, 3M-ESPE; Y-Z, Vita
Zahnfabrik). Both of these approaches
rely upon well-characterized ceramic
powders for which firing shrinkages can
be predicted accurately.6,7
Polycrystalline ceramics tend to be
relatively opaque compared to glassy
ceramics; thus, these stronger mate-
rials cannot be used for the whole-wall
thickness in esthetic areas of prostheses.
These higher-strength ceramics serve
as substructure materials upon which
glassy ceramics are veneered to achieve
pleasing esthetics. Laboratory measures
of the relative translucency of commer-
cial substructure ceramics are available,
both for a single-layer of materials and
for those that are veneered.8,9
Green Machining of
Oversized Parts
Machining of tougher structural
ceramics such as alumina and especially
transformation-toughened zirconia
(see following section) was much more
difficult, requiring heavier machinery
and longer milling times, and quite
often involved limited tool life. Further
advances in the manipulation of 3-D data
sets, along with the fruits of a decade
of research into ceramics processing,
provided the underpinning for an inno-
vative solution proposed by Filser and
Gauckler at the University of Zürich
involving machining of an oversized part
froma ceramic block only lightly sintered
to what is termed the “initial sintering”
stage.10-12 With very careful control over
both the ceramic powder particle size
distribution and particle packing density,
it become possible to predict the over-
sized shape needed that would then
shrink to the desired “net shape.” This
technique has been variously termed
“green machining” or “soft machining”
in dental literature. This technique
allowed the individually customized and
high-tolerance-parts dentistry required
to be manufactured from polycrystalline
ceramics such as alumina and zirconia.
As of today, the last major advance in
dental ceramics comes with the intro-
duction of transformation-toughened
zirconia.12-14 This ceramic is arguably the
most complex material ever introduced
for dental use and, as will be discussed
later, its introduction has not been
without a “learning curve” that we are
still going up. Two other major changes
currently underway involve: 1) the estab-
lishment of dedicated industrial-quality
manufacturing centers for fabrication
of prostheses; and 2) the application of
engineering design research into clinical
and laboratory practices to optimize
durability and esthetics.
Transformation-Toughened
zirconium Oxide
Potentially the most interesting polycrys-
talline ceramic nowavailable for dentistry,
transformation toughened zirconia,
needs further explanation since its frac-
ture toughness (and hence strength)
involves an additional mechanism not
found in other polycrystalline ceramics.
While fracture toughness and strength
are outside the scope of this paper, it is
sufficient here to understand toughness
simply asmeaning the difficulty in driving
a crack through a material.
Unlike alumina, zirconium oxide is
transformed fromone crystalline state to
another during firing. At firing tempera-
ture, zirconia is tetragonal and at room
temperature monoclinic, with a unit
cell of monoclinic occupying about 4.4
percent more volume than when tetrag-
onal. Unchecked, this transformation
was a bit unfortunate since it would lead
to crumbling of the material on cooling.
In the late 1980s, ceramic engineers
learned to stabilize the tetragonal form
at room temperature by adding small
amounts (approximately 3 – 8 mass%)
of calcium and later yttrium or cerium.13
Although stabilized at room tempera-
ture, the tetragonal form is really only
“metastable,” meaning that trapped
energy still exists within the material
to drive it back to the monoclinic state.
It turned out that the highly localized
stress ahead of a propagating crack is
sufficient to trigger grains of ceramic to
transform in the vicinity of that crack
tip. In this case, the 4.4 percent volume
increase becomes beneficial, essentially
altering material conditions around
the crack tip and shielding it from the
outside world (more formally stated,
transformation decreases the local
stress intensity).
Although most dental zirconia is
a bit opaque and coping need to be
veneered for high esthetics, these pros-
theses can be quite life-like. Zirconia
is not as opaque as In-Ceram alumina,
and can be internally colored as can
lithium disilicate. With fracture tough-
ness twice or more than that of alumina
ceramics, transformation-toughened
zirconia represents an exciting potential
substructure material. Possible problems
with these zirconia ceramics may involve
long-term instability in the presence of
water, porcelain compatibility issues,
and some limitations in case selection
due to their opacity. However, as of this
writing, three-year clinical data involving
many posterior single-unit and three-
unit prostheses (plus one five-unit) have
revealed no major problems (discussed
more fully below).
Zirconia Porcelain/
Substructures Issues
Two issues are of concern with zirconia;
one quite real and one potential. Of
real concern are reports of significant
percentages of single-unit and multi-unit