In light of minimally invasive dentistry, the advent of a new generation of particle-filled and high strength ceramics, hybrid composites, and techno-polymers in the recent decade has provided an extended palette of dental materials, broadening the clinical indications in fixed prosthodontics. In the last two decades, there is an increase in the demand for non-metallic restorations. Zirconia is one of the recent advances in dental materials that comes under the ceramic oxide group. This article gives an overview of zirconia in the field of prosthodontics and crown & bridge.
Key words: zirconia, ceramics, advances in dentistry
Advancements in technology leave their footprint
in the progress of the dental and medical fields
for Achieving supreme precision, better esthetics
to mimic, and ally with the biological tissues.
Structural ceramics have been improved and have
become increasingly popular in dentistry, intending
to replace the infrastructure of metallic dental
prostheses. Because of its physical, mechanical,
and optical qualities, zirconia has become a
versatile and promising material, expediting its
CAD/CAM technology used for various prosthetic
treatments.
The metal zirconium (Zr) has an atomic number
of 40. Martin Heinrich Klaproth, a chemist, was
the first to discover it in 1789. It was first used
as a biomaterial in the 1970s. Though its usage
was reported in the late ’90s, its usage as a hip
replacement material and fixed prosthodontics
were since 2004.1
Zr does not exist in its purest form in nature. It can
be found as a free oxide (Zirconia, ZrO2) with the
mineral name Baddeleyite or in combination with
silicate oxide with the mineral name Zirconate
(ZrO2x SiO2). After the purification process, the
material produced can be used as a biomaterial
ceramic.
Biocompatibility of zirconia (cellular) or systemic adverse responses to this
material. Collagen fiber orientation and bone
levels in implants around zirconia and titanium
implant necks were identical. The fibers in both
materials run parallel-oblique and parallel to
the implant surface; plaque accumulation was
identical at zirconia and titanium abutments.2
There are 3 phases:3
There are 3 phases:3
ZrO2 is a polymorphic substance used in three
different shapes: monoclinic, tetragonal, or
cubic. The monoclinic phase is stable at room
temperatures up to 1170°C(1167), the tetragonal
phase at temperatures between 1170 and 2370°C,
and the cubic phase at temperatures above 2370°C
(2367).(Figure:1) However, these alterations are
linked with considerable volume changes: When
zirconium oxide is heated during the monoclinic
to tetragonal transformation, it loses 5% of its
volume; yet, when it is cooled, it gains 3% to 4%
of its volume.
The tetragonal to monoclinic phase transition
produces cracks in bulk zirconia samples and
reduces strength and toughness. The high-temperature tetragonal phase can be stabilized
at room temperature by changing the composition
using Mg, Ca, Sc, Y, or Nd doping. Single-phase
tetragonal zirconia stability is enhanced by highly
soluble trivalent stabilizers such as yttria, which
induce vacancies, or tetravalent stabilizers such
as ceria, oversized or undersized concerning
zirconium. The most common stabilizer for dental
applications is yttria (Y2O3). The addition of 3 to 5
mol% of Y2O3 results in a stabilized core ceramic
referred to as yttria-stabilized zirconia or yttria-stabilized tetragonal zirconia polycrystals (Y-TZP).
Mg-PSZ core ceramics have also been made from
magnesia (MgO). Ceria (Ce2O3) is used as a
stabilizer in a Ce-TZP/Al2O3 core ceramic. Another
possibility for stabilizing the tetragonal phase at
room temperature is to reduce the crystal size to
less than 10nm.
The structural stabilization of zirconia by yttria
results in a significant proportion of metastable
tetragonal phase. This metastable tetragonal
phase strengthens and toughens the structure
by a localized transformation to the monoclinic
phase when tensile stresses develop at crack
tips (Figure:2). The resulting volume expansion
adjacent to the crack tips produces a high local compressive stress around the crack tips, which
increases the localized fracture toughness and
inhibits the potential for crack propagation. Many
types of crack shielding processes are possible,
including micro cracking, ductile zone formation,
and transformation zone formation. Because of
this strengthening and toughening mechanism,
the yttria-stabilized zirconia ceramic is sometimes
referred to as “ceramic steel.”
One property of zirconium oxide that has not been
well studied is the phenomenon of low-temperature
degradation or “aging.” Water and non-aqueous
solvents are involved in the formation of zirconia
hydroxides along a crack. This process accelerates
the expansion of the fracture and can result in
reduced strength, toughness, and density, leading
to failure of the restoration.
Metal-ceramic restorations with opaque cores do
not meet the need for aesthetic restorations and
other ceramic materials. The translucency of the
most robust zirconia-based ceramic crowns, on the
other hand, is said to be less than that of lithium
disilicate glass ceramics, which have been shown to have excellent aesthetic performance. Among
non-zirconia core materials, an optimal esthetic
result has been reported with Procera AllCeram, a
99.9% aluminum oxide densely sintered ceramic,
and IPS Empress lithium disilicate glass-ceramic.
In 2005, it was renamed IPS e.max Press (Ivoclar
Vivadent AG), which had better translucency and
mechanical properties.
Alumina and glass-ceramic have, respectively,
fair to high relative translucency; nevertheless,
their mechanical properties are lower than
ZrO2 ceramics. The structure and thickness of the
zirconia matrix and the physical characteristics
and degree of glazing of the veneering porcelain
affect light transmission through Y-TZP. Cekic-Nagas I et al. 2012 concluded that measuring the
degree of conversion of different resin luting agents
beneath zirconia ceramic materials may produce
better clinical outcomes—the greater thickness
of zirconia results in lower light transmittance.4
Zirconia ceramics are commonly used in dentistry
as framework materials and are usually milled
from a zirconia block using a CAD/CAM machine
device. Blocks can be milled at three different
stages: orange, pre-sintered, and fully sintered.
Frameworks made from green and pre-sintered zirconia are milled in an enlarged form to
compensate for the shrinkage during sintering,
usually 20%-25% for a partially-sintered framework.
The color of the zirconia can be individualized
with the addition of oxides to the green-stage
framework. Milling completely sintered zirconia
blocks is a time-consuming operation that wears
the diamond burs out faster and costs more.
The longevity of an indirect restoration is closely
related to the integrity of the cement at the margin.
One technique commonly used to condition
the ceramic surface is that of air abrasion.
Air abrasion with aluminum oxide particles is
often used to eliminate pollutants and improve
micromechanical retention between the resin
cement and the restoration; these particles may
be silica-coated or not. With zirconia ceramics,
hydrofluoric acid etching and common silane
agents are ineffective. Several coating agents were
used in other experiments to improve the formation
of chemical bonding with zirconia. However, only
those containing a phosphate monomer agent
effectively formed a stable bond with zirconia
materials.
Zirconia in its different nanoforms like nanotubes,
nanofibers, powder, whiskers can be used
to reinforce the complete denture prosthesis.
Nishiyama H et al., with their study, concluded that
complete maxillary dentures with nano-zirconia
frameworks might be an alternative prosthetic treatment option. And zirconia can be used to
fabricate frameworks as an alternative to metal
frameworks. But it has the disadvantages of
brittleness when fabricated in thin sections as
a clasp.
Kakehashi et al. experimented with zirconia
ceramic post clinically and reported that the
zirconia post showed a high success rate. In
another study, 79 zirconia dowels with direct resin
core building were retrospectively evaluated. All
dowels were cemented adhesively, and no failures
were observed.7
There are two types of zirconia
Two types include: (Figure:4)
Monolithic zirconia:
These are pure zirconia
crowns made of a single block of zirconia, making
it more durable without any fear of cracks or
chipping off but less esthetic when compared to
layered zirconia. It is indicated in posterior crowns
and bridges.
Layered zirconia:
These are not pure zirconia, as it comes with a
zirconia core layered with ceramic on the top, improving the esthetics. It has chances of a
ceramic chip off, so indicated in anterior crowns
and bridges.
The tooth preparation needed to accommodate a
zirconia restoration is essentially a porcelain-fused
to- metal crown with a few modifications. The 3M ESPE recommendations for its Lava zirconia 1.5
to 2.0 mm of incisal/occlusal reduction 1.5 to 2.0
mm of axial reduction. The range of reduction is
related to aesthetic needs. More the tooth reduction,
more space is available for the lab technician to
appropriately layer various porcelains to achieve
better aesthetics. The axial taper should be greater
than or equal to 4 degrees. The horizontal angle
of the margin should be greater than or equal to 5
degrees. Due to the limitations of the die-scanning
process and the subsequent machine milling,
sharp angles in the preparation must be avoided.
A circumferential deep chamfer or rounded
shoulder at the gingival margin is recommended.
Ninety-degree shoulders, troughs at the margins,
feather-edge margins, undercuts, or sharp line
angles are unacceptable. Furthermore, the
technician needs to consider the final shade and
select an appropriately colored zirconia that allows
layering of various translucencies of porcelain to
develop a restoration that demonstrates “color
from within.”
Placement of zirconia restorations can be via
standard cementation or by bonding.
Due to zirconia’s inherent strength, conventional
cement-like zinc phosphate or polycarboxylate can
be used; however, these types of cement may not
be the first choice due to their physical properties
as well as their opaque nature. Opaque cement
may show through the zirconia and affect the final
appearance of the restoration. Glass ionomer,
resin-modified glass ionomer, and self-etching
resin types of cement have all been used with
success. These have the potential to enhance
aesthetics. In the case of short or extremely tapered
preparations, a bonded resin cement may be best.
Zirconia does not etch with hydrofluoric acid due
to the lack of a glass matrix, nor does it contain
silica to allow silane coupling to occur.
Ceramics from aluminum, titanium, and zirconium
oxides have been used for root form, endosteal
plate-form, and pin-type dental implants. Kohal
and Klaus presented 8the first clinical report of
zirconia dental implants in the literature. The
compressive, tensile, and bending strengths
exceed the strength of compact bone by 3 to 5
times. The aluminum, titanium, and zirconium
oxide ceramics have a clear, white, cream, or light
grey color, which is beneficial for applications such
as anterior root form devices. When compared
to other types of synthetic biomaterials, minimal
thermal and electrical conductivity, minimal
biodegradation, and minimal reactions with
bone, soft tissue, and the oral environment are
also recognised as advantages. Although initial
testing revealed that these polycrystalline alumina
materials had adequate mechanical strengths,
long-term clinical results revealed a functional
design and material limitation. The established
chemical biocompatibilities, improved strength
and roughness capabilities of sapphire and
zirconia, and the basic property characteristics
of high ceramics continue to make them excellent
candidates for dental implants. Monolithic
zirconia offers enhanced mechanical properties
for implant restorations, but development is
needed to optimize esthetics. L.D. Friedlander
conducted a retrospective study that suggests that
ZrO endosseous implants can achieve a survival
rate similar to titanium implants with healthy and
stable soft and hard tissues. Levartovsky et al. in
2019 in their clinical research, conclude that the
survival and success rate of monolithic zirconia
restorations installed in patients with bruxism
was excellent.9
The use of zirconia-based fixed dental prostheses
was gaining importance from the last two decades.
There are some disadvantages like opacity,
wearing opposite natural tooth, decreased strength
in thinner sections on par with the advantages. To
date, the research results are promising. However,
significantly more clinical research is needed
regarding this concept. Overall, the potential for
zirconia-based all-ceramic restorations appears
to be very good.