Dentaltown March 2018 - 64

Introduction
A 2015 survey reported that the mostprescribed ceramic material for posterior
crowns was monolithic zirconia, with lithium
disilicate most often prescribed for anterior crowns.1 This increase in prescribing
full-contour zirconia as the favored posterior
ceramic material is due to several factors:
* An increased demand for
aesthetic restorations.
* The increased cost of metal
(gold $1,340/ounce).
* Zirconia being a strong
aesthetic material.
* The development of
CAD/CAM systems.
Zirconia was introduced as a framework
for dental restorations, which was veneered
with feldspathic porcelain. At the time of its
introduction, few computer-aided design and
computer-aided manufacturing (CAD/CAM)
systems were available to fabricate milled
zirconia restorations.
With the development of newer, more
accurate CAD/CAM systems, milling
zirconia restorations became commonplace.
The introduction of zirconia was a perfect
fit with the improved CAD/CAM systems,
producing relatively inexpensive aesthetic
restorations with improved mechanical
properties.
Zirconia has evolved through several
significant modifications, and clinicians
must be aware of the properties and
differences between zirconia materials

64

MARCH 2018 // dentaltown.com

because these factors affect the success or
failure of the final restoration.

What is dental zirconia?
Zirconium is a soft silver-colored metal
found as a mineral called zircon (ZrSiO4).
It is mined primarily from large deposits
in Australia and Africa and shipped to
processing plants such as Tosoh (Japan), the
largest producer of refined zirconia powder.
Zircon is purified and during the production
process metal oxides are added: yttrium
to stabilize the polycrystals; aluminum to
prevent water corrosion; hafnium oxide to
reduce pore development; and other coloring
components to produce a powder that is
pressed into a disk or block and partially
sintered.
The partially sintered zirconia is milled
into a framework or restoration, then further
sintered, which produces approximately
20 percent shrinkage in the finished restoration. The density of zirconia is recorded
on each block or puck, because density
determines the shrinkage during sintering.
The mill enlarges the final restoration
to compensate for the shrinkage during
sintering.
Zirconia is a polymorphic material
that may exist in three crystallographic
forms, depending on temperature and
pressure: monoclinic (stable at room temperature up to 1,170° C), tetragonal (stable
at 1,170-2,370° C), and cubic (stable from
over 2,370° C to its melting point, 2,716° C).
At room temperature, pure zirconia is present
in the most stable phase, monoclinic. As the
temperature rises to about 1,170° C, the
monoclinic phase transforms into
the tetragonal phase, accompanied by a shrinkage in volume of
approximately 4-5 percent. The
tetragonal phase converts into the
cubic phase at about 2,370° C,
with only minimal changes in
volume.2-3


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Table of Contents for the Digital Edition of Dentaltown March 2018

Dentaltown March 2018 - Intro
Dentaltown March 2018 - Cover1
Dentaltown March 2018 - Cover2
Dentaltown March 2018 - 1
Dentaltown March 2018 - 2
Dentaltown March 2018 - 3
Dentaltown March 2018 - 4
Dentaltown March 2018 - 5
Dentaltown March 2018 - 6
Dentaltown March 2018 - 7
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Dentaltown March 2018 - Cover3
Dentaltown March 2018 - Cover4
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