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acid etch and rinse
a special heat treatment,
causing the precipitation
and growth of crystallites
within the glass. Since
these fillers are derived
chemically from atoms of
the glass itself, it stands to
reason that the composi-
tion of the remaining glass is
alteredaswellduringthisprocess,termed
“ceraming.” Such particle-filled compos-
ites are called glass-ceramics. The main
commercial example today is the glass-
ceramic containing 70 vol% crystalline
lithium disilicate filler (Empress 2, now
e.maxPress and e.maxCAD, Ivoclar-
Vivadent). Visiting the factory in Schaan,
Liechtenstein, you will see CAD/CAM
blocks of a nearly clear (slightly amber)
glass – following an additional heat treat-
ment crystallizing the lithium silicate
these blocks turn blue.
Polycrystalline Ceramics
Polycrystalline ceramics have no glassy
components; all of the atoms are densely
packed into regular arrays that are much
more difficult to drive a crack through
than atoms in the less dense and irregular
network found in glasses (Fig. 5). Hence,
polycrystalline ceramics are generally
much tougher and stronger than glassy
ceramics. Polycrystalline ceramics are
more difficult to process into complex
shapes (e.g., a prosthesis) than are glassy
ceramics. Well-fitting prostheses made
from polycrystalline ceramics were
not practical prior to the availability
of computer-aided manufacturing. In
general, these computer-aided systems
use a 3-D data set representing either
the prepared tooth or a wax model of the
control optical effects such as opales-
cence, color and opacity. These fillers
are usually crystalline, but can also be
particles of a higher-melting glass. Such
compositions based on two or more
distinct entities (phases) are formally
known as “composites,” a term often
reserved in dentistry to mean resin-
based composites. Thinking about
dental ceramics as being composites is
a helpful and valid simplifying concept.
Much confusion is cleared up in orga-
nizing ceramics by 1) what the matrix
is, 2) what filler particles they contain
(and how much), and 3) in the case of
ceramics how they got into the glass.
Moderate-strength increases can
also be achieved with appropriate
fillers added and uniformly dispersed
throughout the matrix, a phenomenon
termed “dispersion strengthening.” The
first successful strengthened substruc-
ture ceramic was made of feldspathic
glass filled with particles of aluminum
oxide (app. 55 mass%).5 Leucite is also
used for dispersion strengthening, at
concentrations of around40 to 55mass%,
in products such as Empress CAD for
CEREC (Ivoclar). Commercial ceramics
incorporating leucite fillers for strength-
ening also include a group that are
pressed into molds at high temperature
(OPC, Pentron; Empress Esthetic, Ivoclar
Vivadent; Finesse All-Ceramic, Dentsply
Prosthetics), and a group provided as a
powder for traditional porcelain build-
up (OPC Plus, Pentron; Fortress, Mirage
Dental Systems). Volume%, instead of
mass% (or weight%) is a more useful
measure for comparisons among resin-
based composites, since the density
of the fillers is much higher than the
matrix. Filler loading is generally no
higher than 60 volume% to 65 volume%,
even for composites claiming 80 mass%
to 90 mass% filler content.
Beyond thermal expansion/contrac-
tion behavior, there are two major bene-
fits to leucite as a filler choice for dental
ceramics; the first intended, and the
second probably serendipitous. First,
leucite was chosen because its index of
refraction is very close to that of feld-
spathic glasses — an important match
for maintaining some translucency.
Second, leucite etches at a much slower
rate than the base glass, and it is this
“selective etching” that creates a myriad
of tiny features for resin cements to
enter, creating a good micromechanical
bond. Etching of resin-based compos-
ites can theoretically involve the selec-
tive removal of the surface filler phase,
but etch depth and microporosities are
limited to the size of the filler particles
(e.g. nanometers). This difference is
illustrated in Figure 4. Silane treatment
of the etched ceramic provides further
chemical bonding not available with
resin-based composites.
Glass-Ceramics (special sub-set
of particle-filled glasses)
Crystalline filler particles can be added
mechanically to the glass; for example,
by simply mixing together crystalline
and glass powders prior to firing. In a
more recent approach, the filler parti-
cles are grown inside the glass object
(prosthesis or pellet for pressing into a
mold) after the object has been formed.
After forming, the glass object is given
Glass-matrix ceramic resin-matrix composite
acid etch and rinse ceramics 4
5
Fig. 4: Removal of
glass or filler phase
for bonding
Fig. 5: Simple cubic
crystalline lattice
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