Empress
Ceramics are defined as man-made solid objects formed
by nonmetallic and inorganic raw materials that are
baked at high temperatures. The traditional porcelain
is composed of three naturally occurring minerals: pure
white clay, silica, and feldspar. The basic components
of dental porcelain are silica and feldspar. Additional
components are aluminum oxide as well as pigments and
opacifying agents, depending on the application. Only
denture teeth contain clay as the third basic component
of traditional porcelain. All baked dental porcelains
contain small crystals (leucite and/or alumino-silicate
crystals) that are embedded in a silicate matrix. The
relative amount of crystals and glass depend on the
specific porcelain. Leucite, a reaction product of potassium
feldspar and glass, is a particularly important component
of dental porcelain, because it affects its optical
properties, thermal expansion, strength, and hardness.
Until recently, all-porcelain restorations were used
with caution because of two inherent problems: the risk
of brittle fracture and the abrasive wear of opposing
tooth structure. Brittle fracture is generally attributed
to the rapid, uninterrupted propagation of cracks, usually
beginning at a flaw in the ceramic. This failure is
often initiated at the internal surface. Aluminous oxide
has been added to increase the strength of dental porcelains.
However, aluminous crowns fit very poorly due to the
shrinkage during baking procedures. Therefore, dental
ceramics fused to metal substrates have been the preferred
treatment modality.
Metal bases affect the aesthetics of porcelain
by decreasing the light transmission through the
restoration and by creating metal-ion discoloration.
In addition, some patients may have allergic reactions
or other sensitivities to metals. These drawbacks
have prompted the development of new all-ceramic
systems that do not require metal, yet have the
high strength and precision fit of ceramometal
systems. Among the systems available today are the powder-slurry
ceramics (e.g. Optec HSP, Duceram LFC), castable ceramics
(e.g. Dicor), machinable ceramics (e.g. Cerec Vitablocks,
Dicor MGC), infiltrated ceramics (e.g. In-Ceram),
and pressable ceramics (e.g. IPS Empress and Optec
Pressable Ceramic).
IPS Empress is a heat-pressed glass ceramic that
has superior mechanical properties for several
reasons. The high shrinkage of leucite crystals
creates compressive stress in the vitreous phase,
which prevents the development of surface cracks.
The randomly oriented leucite crystals are tightly
packed in the vitreous phase and stop the propagation
of micro-cracks. The combination of heat pressing,
initial firing, and stain and glaze of the veneers
creates an additional 50% increase in strength.
This higher cohesive strength and fracture toughness
allows for thicker areas of porcelain with a lesser
risk of fracture. Unbonded glazed IPS Empress has
a flexural strength of 215 MPa compared to 71 MPa of
feldspatic porcelain, 114 MPa of Dicor, 167 MPa of
Optec HSP, and 419 MPa of In-Ceram. IPS Empress
is bonded to dentin with a resin cement after conditioning
of the surfaces of both the restoration and the
prepared tooth, as will be explained later in
the description of the procedure. This further
increases its fracture resistance significantly
and reduces microleakage. The occlusal wear of
IPS Empress is in the range of enamel due to its
fine-grain structure. Heat-pressed glass-ceramic
crowns have survival rates of 100% after 33 months in
function and 95% after 24 months in function. In addition,
all-porcelain restorations allow direct light to
penetrate. The amount of scattering versus transmission
of light depends on the chemical composition of
the porcelainÍs glass matrix, the size
and structure of the crystalline phase, and the
processing technique. Light penetration adds a
translucency to the restorations that is comparable
to natural teeth even under compromised light conditions,
since there is no metal substructure that interferes
with the transmission of light. The transillumination
quality also improves the natural pale-pink appearance
of the adjacent marginal gingiva.
top^
|
