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Zirconia
With
the exception of zirconium oxide, existing ceramics systems lack reliable
potential for the various indications for bridges without size limitations.
Zirconium oxide with its high strength and comparatively higher fracture
toughness seems to buck this trend. With a three-point bending strength
exceeding nine hundred mega-Pascals, zirconium oxide can be used in
virtually every full ceramic prosthetic solution, including bridges, implant
supra structures and root dowel pins. The fact that zirconium oxide has been
used in the industrial production of root dowel pins since nineteen ninety
reaffirms the belief that its high strength yields clinical durability. The
high strength and toughness are the result of a material-specific crystal
transformation, specifically from a tetragonal to a monocline crystal
structure, which stops cracking at the source. The increase in volume
resulting from this transformation inhibits cracking and increases strength
by an order of magnitude. In a nineteen ninety-two publication, Garvy
compared zirconium oxide with hardened steel. The similarities in material
properties proved astounding. The bending strength, modulus of elasticity,
thermal expansion coefficient and specific gravity of both materials are
comparable. The fact that both materials can attribute their strength to the
same martensitic transformation of crystal structure accompanied by a nearly
identical volume increase makes the comparison even more striking. In
addition, both materials are opaque to X-ray.
Due to its specific material properties, zirconium oxide ceramic,
referred to by the abbreviation Y-TZP, has been used for quite some time in
orthopaedics as part of hip joint implants. Previous attempts to extend its
application to dentistry were thwarted by the fact that this material could
not be processed using traditional methods used in dentistry. The arrival of
computerized dentistry enables the economically prudent use of zirconium
oxide in such elements as base structures such as copings and bridges and
implant supra structures. Special requirements apply to dental materials
implanted for longer than a period of thirty days. Several technical
requirements include high strength, corrosion resistance and defect-free
producability at a reasonable price. The primary requirement, however, is
biocompatibility, which means that there should be no rejection response,
infection or any other problem related to the introduction of material in
tissue. In vitro testing of zirconium oxide has thus far elicited no
unfavourable responses when combined with cells or tissues. Moreover, short
and long-term in vivo testing indicate excellent biocompatibility. This is
further substantiated by the results of various clinical trials extending
over a period of more than eight years, each of which reveals no
unfavourable tissue responses.
Ever more
stringent requirements are being placed on the aesthetics of teeth. Metals
and porcelain are currently the materials of choice for crowns and bridges.
The demand for full ceramic solutions, however, continues to grow.
Consequently, industry and science are increasingly compelled to develop
full ceramic systems. In introducing full ceramic restorations, such as base
structures made of sintered ceramics, computerized dentistry plays a key
role. When discussing aesthetics, we must not focus solely on natural
colored porcelain inlays, onlays and veneers milled with the aid of a
computer, but also the application of various layers of dental glass ceramic
on base structures. To increase the aesthetics of zirconium oxide, a glass
ceramic layer can be applied onto the structure’s surface. Research
focuses primarily on the strength of the bond between the zirconium oxide
and the glass ceramic and the strength of the entire structure in terms of
the difference in thermal expansion coefficient. The application of glass on
zirconium oxide is infrequently discussed in the scientific literature. The
first aim is to gain a good overview of the functional properties of the
glass ceramic-zirconium oxide system in terms of the thermal compatibility
of the components, interface integrity, form stability of glass ceramic when
used in ceramic shoulders, integrity of the glass ceramic in terms of
porosity and in vitro testing of fatigue sensitivity and strength of
restorations. These results can be used to assess the risks associated with
the clinical application of this type of restoration.
Due in part to
the growing demand for aesthetic and biocompatible, metal-free restorations,
this research is important to predict an evidence-based estimate of clinical
reliability. This will make it possible to evaluate the values found using a
mathematical analysis of the designed restoration model based on the finite
elements method. A number of international evaluation standards find their
use in testing the functional product requirements and validating the
production method. These standards, however, do not always respond to
questions which are key to long-term in situ performance. At the same time,
one of the study aims is to generate more definitive conclusions regarding
the guidelines for use in dental indications for this type of restoration.
The study will also address how the restoration is produced to determine
which parameters are essential for the resulting quality of the restoration.
The final aim is to improve the quality of the crown or prosthetic elements.
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