62 09.2017 |
ZIRCONIA: EVOLUTION OF A UNIQUE UNIVERSAL RESTORATIVE
The introduction of zirconia-based ceramics
as a restorative dental material has
generated much interest in the dental
profession. The mechanical properties of
zirconia are the highest ever reported for
any ceramic used in prosthetic dentistry.
The strength of zirconia has allowed the
incorporation of high-strength all-ceramics
into its use for posterior FDP.
14 High strength,
coupled with the possible high esthetics
that zirconia offers, allows the material to
become a highly valuable option in our
The basis for the valued strength displayed by zirconia is its unique crystalline
structure and its behavior under loads. Zirconium dioxide (ZrO2), also known as zirconia, is a white crystalline oxide of the metal
element zirconium. Its most naturally occurring form is the rare mineral baddeleyite,
though zirconium metal used for dentistry
is obtained from the zirconium-containing
mineral ore called zircon. After being processed and purified, these powders can be
further processed to produce somewhat
porous bodies that can be CAD/CAM-milled
with great precision. Once densely sintered,
a polycrystalline ceramic material is produced which, unlike most other dental ceramics, contains no glass phase.
Over the last several years, many high-
strength ceramics have been developed for
the construction of metal-free restorations.
Several studies have evaluated different all-
ceramic systems and offered conclusions
on where these ceramic systems may be
used with success in the oral environment.
Luthy et al. measured average load-bearing
capacities for several ceramic systems and
found 518 N for alumina-based restorations,
282 N for lithium disilicate-based restora-
tions, and 755 N for zirconium restorations.
Raigrodski et al. also analyzed several dif-
ferent all-ceramic systems and concluded
that the all-ceramic systems he studied were
only to be used in the anterior, for single-
crown restorations, and possibly three-unit
FPDs. He also concluded that because of the
higher strength of zirconia, this material
offers a wider area of restorative options in
the oral cavity, including posterior single
units and multiunit restorations.
To circumvent the need for veneering
altogether, another option is to develop
fracture-resistant, partially translucent
monolithic ceramics. Monolithic all-ceramic
restorations are becoming more accepted
due to higher strength and by avoiding weak
veneer-core interfaces. All-ceramic restorations—such as IPS e.max (lithium disilicate),
Wieland (zirconia), and 3M Lava Plus (
zirconia)—offer several all-ceramic monolithic,
restorative options that have acceptable
esthetics and eliminate the need for veneering ceramics altogether.
During the early 2000s, zirconia was perceived
to be a material that would cause high wear
to the opposing dentition. Today, this perception is being challenged. Multiple studies have
shown how zirconia can be a material that is
gentle to opposing dentition, compared to
glass ceramics that are layered on PFM restorations.
17 This low-wear property can be
attributed to zirconia’s microstructure, and
its small grain size that allows for a mirror
polished surface to be created that is kind to
opposing enamel surfaces.
An in vitro study by Jeung YS et al. evalu-
WEAR OF ZIRCONIA VERSUS
ated the wear of enamel opposing zirconia
surfaces. The zirconia surfaces appeared to
be less abrasive to enamel than feldspathic
porcelains. The study also found that pol-
ished zirconia without glazing is less abrasive
than zirconia-glazed surfaces.
20 A second
study by Janyavula et al. also evaluated the
wear of enamel by full-contour zirconia pol-
ished and glazed. A wear simulator and pro-
ducing a 4 mm slide at 20 N rate was applied
and samples evaluated using a noncontact
3-D profilometer. Although the wear of glazed
zirconia was more than polished zirconia, it
was still less than commonly used porcelains
for PFM restorations.
These results were further investigated in a
six-month clinical study that evaluated the
wear on opposing dentition for monolithic
full-contour zirconia prostheses. Twenty
monolithic crowns were placed in patients,
and mean vertical loss for specimens, antagonists, and contralateral was recorded.
Both mean and maximum enamel wear were
significantly different between the antagonists of the zirconia crowns and the contralateral antagonists. Under clinical conditions,
monolithic zirconia crowns seemed to be
associated with more wear of opposed enamel than the natural teeth, but the amount of
wear was comparable to, if not less than,
other ceramic systems.
The difficulty in reproducibly achieving good
esthetics with PFM restorations and the desire
for metal-free solutions has led to the increased
use of zirconia. The unique optical properties
of zirconia require a new and different under-
standing of how the materials are managed.
Translucency and color are important
and often inseparable variables for dental
restorations, and translucency may be an
innate optical property of the zirconia mate-
rial related to its crystalline structure.
High strength, coupled with the
possible high esthetics that zirconia
offers, allows the material to become
a highly valuable option in our