Cosmetic Dentistry Materials and Methods
This page will give you some deeper information about
the materials that are used in your mouth. Some data
may be very technical or may remind you of chemistry
class. However, many of these materials are in your
mouth for many, many years. So you decide whether you
want to know about them.
Bonding Agents
Cements
Empress Porcelain
Porcelain Fused to Metal
What are Porcelains?
Bonding Agents
The seepage of
fluid to the tooth surface is caused be positive pressure
of the pulp of the tooth. Acetone-based materials are
less influenced by dentinal fluid, which lowers the
bond strength of alcohol or water-based bonding agents.
There are many
other bonding agents available and new ones are regularly
released by dental-product developing companies. However,
it is most important that the dentist understands the
structural biology of the tooth surface and the chemistry
of his favorite bonding agent. This will ensure that
the agent is applied by proper techniques considering
its specific interaction with the tooth surface. Research
has shown that problems arising from improper handling
of the bonding agents pose greater problems than the
lack of bond strength.
Cements
The cements used to bond the restorative materials
to the conditioned tooth surface are filled
resin cements (mostly so-called hybrids). We
use a dual-cure resin cement, which means that
it sets both by its own catalyst and through
activation with monochromatic light. It is available
in six shades. It is filled to 68% with pre-polymerized
micro-particles of different sizes (hybrid),
which minimizes the polymerization shrinkage that is
common with resins.
We also use a phosphate-ester adhesive resin cement that doesn't need
to be cured with light.
The great variety of resin cements makes it difficult for the clinician
to clearly make a material selection based on scientific evidence. It is
therefore important to become familiar with the chemistry and technical requirements
of only a few materials and to gain experience.
Empress
Empress glass ceramics are used for VENEERS and CROWNS.
Their physico-mechanical properties such as modulus
of elasticity, hardness, and coefficient of
thermal expansion are very close to natural
enamel. Glass may be defined as a rapidly undercooled
liquid with an amorphous, noncrystalline structure.
When glass melt is cooled slowly, crystals develop.
Glass ceramics are produced from raw glass by
controlled crystallization. One or more types
of crystals embedded in one or more vitreous phases
result from this "ceramming" process. Empress consists of crystals and a
vitreous phase. The glass matrix merely forms
a putty between abundant leucite crystals.
Different factors account for the improved mechanical
properties of these materials. One factor is
the stress which is set up in the vitreous phase
as a result of the high shrinkage of the leucite
crystals. Secondly, the leucite crystals with
a mean diameter of 3 micrometer are believed to
stop the propagation of micro-cracks within the
glass matrix. The wear of Empress glass ceramics
is in range of enamel. As a result of its fine-grain
structure, the glass ceramic abrades the antagonist
(opposing dentition) enamel not more than natural enamel.
For more information about porcelains, see also
the page explaining porcelains in more detail.
Porcelain-fused-to-metal Crowns
and Bridges
These restorations have an invisible metal core,
which is covered by a layer of feldspatic porcelain.
Usually these restorations are not aesthetic
as all-porcelain (glass ceramic) restoration,
but certain conditions, such as root-canal-treated
teeth and long-span bridges, require the additional
strength derived from the metal core.
What are Ceramics and Porcelains?
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, 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. 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.
All-porcelain restorations allow direct light
to penetrate. The amount of scattering versus
transmission of light depends on the chemical
composition of the porcelain's 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.
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