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All-ceramic materials in dentistry

Authors:

Abstract

In dentistry, ceramics are often referred to as nonmetallic, inorganic structures primarily containing compounds of oxygen with one or more metallic or semimetallic elements. They are composed of sodium, potassium, calcium, magnesium, aluminum, silicon, phosphorus, zirconium, and titanium. Structurally, dental ceramics contain a crystal phase and a glass phase based on the silica structure, characterized by silica tetrahedra, containing central Si4+ ion with four O− ions. Biocompatibility, esthetics, durability, and easier customization properties have led to the increased usage of ceramics. The specialty of ceramic teeth is its ability to mimic the natural tooth in color and translucency along with its strength. Ceramics have excellent intraoral stability and wear resistance adding to their durability. Basically, the inorganic composition of teeth and bones are ceramics which is hydroxyapatite. Over the past few years, the technological evolution of ceramics for dental applications has been incredible, as new materials and processing techniques are being introduced. The improvement in strength, as well as toughness, has made it possible to expand the range of indications to long-span fixed partial prostheses, implant abutments, and implants. While porcelain-based materials are still a major component in dental science, there have been moves to replace metal ceramics systems with all-ceramic systems. Numerous all-ceramics are being developed which is highly esthetic, biocompatible to tissue, and long-lasting in nature. Advances in computer-aided design/computer-aided manufacturing technologies have led to immense popularity of high-strength ceramic materials. These materials are highly esthetic, biocompatible to tissue, and long-lasting in nature. In this review, we will discuss all-ceramic materials which are used in dentistry.
© 2015 The Saint's International Dental Journal | Published by Wolters Kluwer - Medknow 91
All‑ceramic materials in dentistry
Samarjit Singh Teja, Prerna Hoogan Teja1
Abstract:
In dentistry, ceramics are often referred to as nonmetallic, inorganic structures primarily containing compounds
of oxygen with one or more metallic or semimetallic elements. They are composed of sodium, potassium,
calcium, magnesium, aluminum, silicon, phosphorus, zirconium, and titanium. Structurally, dental ceramics
contain a crystal phase and a glass phase based on the silica structure, characterized by silica tetrahedra,
containing central Si4+ ion with four O− ions. Biocompatibility, esthetics, durability, and easier customization
properties have led to the increased usage of ceramics. The specialty of ceramic teeth is its ability to mimic the
natural tooth in color and translucency along with its strength. Ceramics have excellent intraoral stability and
wear resistance adding to their durability. Basically, the inorganic composition of teeth and bones are ceramics
which is hydroxyapatite. Over the past few years, the technological evolution of ceramics for dental applications
has been incredible, as new materials and processing techniques are being introduced. The improvement in
strength, as well as toughness, has made it possible to expand the range of indications to long-span xed partial
prostheses, implant abutments, and implants. While porcelain-based materials are still a major component in
dental science, there have been moves to replace metal ceramics systems with all-ceramic systems. Numerous
all-ceramics are being developed which is highly esthetic, biocompatible to tissue, and long-lasting in nature.
Advances in computer-aided design/computer-aided manufacturing technologies have led to immense popularity
of high-strength ceramic materials. These materials are highly esthetic, biocompatible to tissue, and long-lasting
in nature. In this review, we will discuss all-ceramic materials which are used in dentistry.
Key words: All-ceramic, alumina, computer-aided design/computer-aided manufacturing, silicate, zirconia
Recent trends show a shift from
metal-ceramics to metal-free restorations
in the dental eld. Several types of all-ceramic
materials have been developed to meet the
increased demands of patients and dentists,
which are highly esthetic, biocompatible, and
long-lasting.[1] Silicate and glass ceramics are
used as a veneer for metal or all-ceramic cores.
High-strength ceramics such as aluminum and
zirconium oxide were developed as a core
material for crowns and xed partial dentures
(FPDs) to extend its range to high load bearing
areas.[2] Due to advances in computer-aided
design (CAD)/computer-aided manufacturing
(CAM) technologies, high-strength ceramic
materials have gained immense popularity.
Zirconia, especially yttria-containing tetragonal
zirconia polycrystal (Y-TZP), offering
increasingly greater performance from a
mechanical standpoint, has expanded its range
from single crowns and short-span FPDs to
multiunit and full-arch zirconia frameworks as
well as implant abutments and complex implant
superstructures to support xed and removable
prostheses.[3,4] This overview presents the
current knowledge of all-ceramic materials, their
composition and processing mechanisms, and
possible future trends.
heatpressed CeraMiCs
In the early 1990s, the lost wax press technique
was used as an innovative processing method
for producing all-ceramic restorations. The
dental technicians are usually familiar with
this technique, which is commonly used to
cast dental alloys. In addition, the equipment
needed to heat-press dental ceramics is relatively
inexpensive. The rst generation of heat-pressed
dental ceramics contains leucite as a reinforcing
crystalline phase, whereas the second generation
is lithium-disilicate based.
Leucite glass ceramics
The first generation of heat-pressed dental
ceramics contain leucite as a reinforcing
crystalline in a concentration of 35–45 vol%.[5]
Address for
correspondence:
Dr. Prerna Hoogan Teja,
HNO 771/A, Joginder
Vihar, Phase II,
Mohali - 160 055,
Punjab, India.
E-mail: den_capricorn@
yahoo.co.in
Review Article
Department of
Prosthodontics,
Swami Devi Dyal
Hospital and Dental
College, Panchkula,
Haryana, 1Department
of Orthodontics and
Dentofacial Orthopedics,
Bhojia Dental College
and Hospital, Bhud,
Baddi, Himachal
Pradesh, India This is an open access article distributed under the terms of the
Creative Commons Attribution-NonCommercial-ShareAlike 3.0
License, which allows others to remix, tweak, and build upon the
work non-commercially, as long as the author is credited and the new
creations are licensed under the identical terms.
For reprints contact: reprints@medknow.com
How to cite this article: Teja SS, Teja PH. All-ceramic
materials in dentistry. Saint Int Dent J 2015;1:91-5.
Access this article online
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DOI:
10.4103/2454-3160.177930
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92 The Saint’s International Dental Journal ‑ Vol 1, Issue 2, July‑December 2015
Teja and Teja: All- ceramic materials in dentistry
The molding procedure is conducted at 1080°C in a special,
automatically controlled furnace. The leucite crystals are
formed through a controlled surface crystallization process in
the SiO2-Al2O3-K2O glass system. Because of the difference
in the coefficient of thermal expansion (CTE) between
leucite crystals and glassy matrix, tangential compressive
stresses develop around the crystals on cooling. These
stresses lead to crack deection and improved mechanical
performance.[6] These materials exhibit a exural strength of
120–180 MPa and a CTE of 15–18.5 × 10−6/K m/m.[7] Examples
of leucite-reinforced glass ceramics are VITA VMK 68 (VITA
Zahnfabrik, Bad Sackingen, Germany), Finesse All-Ceramic
(Dentsply, York, PA, USA), Optec OPC (Jeneric, Wallingford,
CT, USA), and IPS Empress (Ivoclar Vivadent, Schaan,
Principality of Liechtenstein). This material is suitable for
fabrication of inlays, onlays, veneers, and crowns. Favorable
clinical long-term data with high survival rates has been
described for IPS Empress inlays, onlays (90% after 8 years),[8]
veneers (94.4% after 12 years),[9] and crowns (95.2% after
11 years)[10] in the dental literature. Leucite glass ceramics
can also be machined with various CAD/CAM systems.
Multicolored blocks have been recently developed to reproduce
color transitions and shading as well as different levels of
translucency to reproduce natural teeth.[11] But with the
introduction of lithium disilicate glass ceramics, which have
signicantly improved mechanical and esthetic properties, the
use of leucite-reinforced glass ceramics has declined.
Lithium disilicate glass ceramics
The second generation heat-pressed ceramics contain about
65 vol% lithium disilicate as the main crystalline phase.[6] A
signicantly higher strength of 350 MPa was achieved with a
glass ceramic of the SiO2-Li2O-K2O-ZnO-P2O5-Al2O3-La2O3
system by precipitating lithium disilicate (Li2Si2O5) crystals.
High-temperature X-ray diffraction studies revealed that both
lithium metasilicate (Li2SiO3) and cristobalite form during the
crystallization process before the growth of lithium disilicate
(Li2Si2O5) crystals.[12] The nal microstructure consists of,
5 µm in length and 0.8 µm in diameter, highly interlocked
lithium disilicate crystals. Tangential compressive stresses
develop around the crystals due to thermal expansion mismatch
between lithium disilicate crystals and glassy matrix, potentially
responsible for crack deflection and strength increase.
Multiple crack deections develop due to crystal alignment
after heat pressing of lithium disilicate glass ceramic. The
lithium disilicate ceramic was introduced as IPS Empress 2
(Ivoclar Vivadent) in 1998 and is moldable as leucite glass
ceramics but at a lower temperature of 920°C. The CTE is
10.5 × 10−6/K m/m.[13] High survival rates were observed for
anterior and posterior IPS Empress 2 crowns (95.5% after
10 years).[14] IPS e.max Press (Ivoclar Vivadent) is the newly
developed pressable lithium disilicate glass ceramic with
improved physical properties (exural strength, 440 MPa)
and translucency through a different ring process in the
SiO2-Li2O-K2O-ZnO-P2O5-Al2O3-ZrO2 system. This can be
used in a monolithic application for inlays, onlays, and posterior
crowns or as a core material for crowns and three-unit FPDs in
the anterior region. Apatite glass ceramics are recommended
for veneering. Clinical data exhibited high survival rates for
IPS e.max Press onlays (100% after 3 years),[15] crowns (96.6%
after 3 years),[16] monolithic inlay-retained FPDs (100% after
4 years),[17] and full crown retained FPDs (93% after 8 years).[18]
Recently, IPS e.max CAD (Ivoclar Vivadent) has been designed
for CAD/CAM processing technology. The milled lithium
disilicate block is exposed to a 2-stage crystallization process.
Approximately, 40 vol% lithium metasilicate crystals are
precipitated during the rst stage with their crystal size ranging
from 0.2 to 1.0 µm. At this precrystallized state, the CAD/CAM
block exhibits a exural strength of 130–150 MPa, which in turn
allows simplied machining and intraoral occlusal adjustment.
The nal crystallization occurs after milling of the restoration at
850°C in vacuum. The lithium disilicate crystallization occurs
after the meta-silicate crystal phase is dissolved completely. The
above process also converts the blue shade of the precrystallized
block to the selected tooth shade which in turn results in a glass
ceramic with a ne grain size of approximately 1.5 µm and
a 70% crystal volume incorporated in a glass matrix.[11] The
exural strength of CAD/CAM – processed lithium disilicate
glass ceramic is 360 MPa. Because of its favorable translucency
and shade assortment, the material can be used for fully
anatomic (monolithic) restorations with subsequent staining
characterization or as a core material with a subsequent coating
with veneering ceramics. Its use is recommended for anterior or
posterior crowns, implant crowns, inlays, onlays, and veneers.
Preliminary clinical results on single crowns revealed high
survival rates (100% after 2 years).[19]
drypressed and sintered CeraMiCs
Densely sintered alumina-based ceramics produced by dry
pressing, followed by sintering have been available since
the early 1990s. The technique involves computer-aided
production of an enlarged die in order to compensate for
sintering shrinkage (12–20%). Dry pressing and sintering of
a high purity alumina-based core ceramic are then performed
done at high-temperature (1550°C). This results in a highly
crystalline ceramic with a mean grain size of 4 µm and
exural strength of 601 73 MPa.[20] The high-strength core is
then veneered with translucent porcelain to obtain adequate
esthetics. Clinical results demonstrate an excellent in vivo
performance at 15 years.[21-23]
slipCast CeraMiCs
In-Ceram Alumina (VITA Zahnfabrik) was the rst all-ceramic
system available for the single-unit restorations and three-unit
anterior bridges with a high-strength ceramic core fabricated
with a slip-casting technique. A slurry of densely packed
(70–80% wt) Al2O3 is applied and sintered to a refractory die
at 1120°C for 10 h which produces a porous skeleton alumina
particles. This is inltrated with lanthanum glass in a second
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Teja and Teja: All- ceramic materials in dentistry
ring at 1100°C for 4 h to eliminate porosity and to increase
strength. The core is veneered with a feldspathic porcelain.[24]
In-Ceram Zirconia (VITA Zahnfabrik) is a modied form of
original In-Ceram Alumina systems, with the addition of 35%
partially stabilized zirconia oxide to the slip composition to
strengthen the ceramic. Traditional slip casting technique can
be used and the material can be copy-milled from prefabricated,
partially sintered blanks and then veneered with feldspathic
porcelain. As the core is opaque and lacks translucency, the use
of this material for the anterior region becomes problematic.[25]
ZirConia CeraMiCs
In the early 1990s, zirconia was introduced to dentistry[26] and
in recent years, a large number of publications have appeared
in the literature. Zirconia is a polymorphic material that occurs
in three temperature-dependant phases which are monoclinic
(room temperature to 1170°C), tetragonal (1170–2370°C),
and cubic (2370°C to melting point).[26] Yttrium-oxide
(Y2O3 3% mol) is added to pure zirconia to stabilize the
tetragonal phase at room temperature, enabling a phenomenon
called transformation toughening to occur. The partially
stabilized crystalline tetragonal zirconia transforms to the more
stable monoclinic phase with an associated 3–5% localized
expansion.[27] This increase in volume counteracts further
crack propagation by compression at the tip of the crack.[28]
High exural strength (900–1200 MPa)[29,30] and high fracture
toughness (9–10 MPa.m1/2) for zirconia have been reported.[29]
The most commonly used zirconia are glass-inltrated zirconia
toughened alumina ceramics (In-Ceram, VITA Zahnfabrik)
and 3 mol% Y-TZP. Y-TZP has been used for root canal posts,
frameworks for posterior teeth, implant-supported crowns,
multiunit FPDs, resin-bonded FPDs, implant abutments, and
dental implants.
Zirconia and computer‑aided design/computer‑aided
manufacturing
An assay of CAD/ CAM systems has evolved since F. Duret
introduced the concept in 1971.[31,32] Most of the available CAD/
CAM systems shape blocks of partially sintered zirconia.[26]
Milling from partially sintered blocks involves machining
enlarged frameworks in a so-called green state. These blocks
are then sintered to their full strength, which is accompanied
by shrinkage of the milled framework by approximately 25%
to the desired dimensions. Examples of these systems are
CERCON (Dentsply Friadent, Mannheim, Germany), LAVA
(3M ESPE, Seefeld, Germany), Procera (Nobel Biocare,
Gothenburg, Sweden), Ekton (Straumann, Basel, Switzerland),
and Cerec (Sirona, Bensheim, Germany). The advantage
of industrialized block fabrication and reproducible and
consistent CAM resulted in increased process reliability, and
cost-effectiveness of CAD/CAM-fabricated restorations.[33]
Survival
One of the major drawbacks of zirconia as compared
with metal-ceramics is low-temperature degradation and
was first described by Kobayashi et al.[34] in 1981. At
relatively low temperatures (150–400°C), slow tetragonal to
monoclinic transformation occurs, initiating at the surface
of polycrystalline zirconia and subsequently progressing
into bulk of the material.[26,35] Transformation of one grain is
accompanied by an increase in volume, which causes stress
on surrounding grains and microcracking. Water penetration
into these cracks then exacerbates the process of surface
degradation, and the transformation progresses. The growth
of transformation zone results in surface microcracking, grain
pullout, and nally surface roughening, which ultimately leads
to strength degradation.
Core/framework fractures
Fractures within the zirconia core ceramic are reported at
7% for single crowns after 2 years at 1–8% for FPDs after
2–5 years. Occlusal overloading caused by bruxism (crown
fracture after 1 month[36]) or trauma (connector fracture in
ve-unit FPD after 38 months[37]) and insufcient framework
thickness of 0.3 mm (crown-abutment fractures in three-unit
FPD[38,39]) were mentioned as main reasons for zirconia core
bulk fractures. Fractographic analyses of clinically failed
zirconia crowns showed that radial fractures propagating
upward from cementation surface site resulted in bulk
fractures.[40] Microscopic examinations of failed zirconia-based
FPDs revealed that core bulk fractures were most commonly
located in the connector area and initiated from the gingival
surface, where tensile stresses were the greatest because of
occlusal loading.[41,42]
Veneering ceramic cohesive fractures
Cohesive fractures within the veneering ceramic (chipping) are
the most frequent reason for failures, irrespective of the applied
zirconia veneer system. Veneer fracture rates are reported at
2–9% for single crowns after 2–3 years[43,44] and at 3–36%
for FPDs after 1–5 years.[45,46] Implant-supported zirconia
restorations revealed even higher rates at 8% for single crowns
after 6 months[47] and 53% for FPDs after 1 year.[48] Impaired
proprioception and rigidity of osseointegrated implants
correlated with higher functional impact forces might further
exacerbate cohesive veneer fractures. Fractographic analyses
of clinically failed veneered zirconia restorations revealed
cohesive veneer failures, with cracks originating from the
occlusal surface and propagating to core-veneer interface,
leaving an intact core.[41]
The veneering ceramic material (exural strength 90–120 MPa)
is weak compared with the high-strength core material
(900–1200 MPa).[29,30] As a result, it is prone to failure at low
loads during the masticatory function. However, attempts
to improve the microstructure and mechanical properties of
veneering ceramics with the development of glass-ceramic
ingots for pressing veneering ceramics onto zirconia
frameworks did not result in an increased reliability of the
veneering ceramic.[49,50] Residual stresses in bilayer crowns and
FPDs are associated with the possibility of thermal gradients
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94 The Saint’s International Dental Journal ‑ Vol 1, Issue 2, July‑December 2015
Teja and Teja: All- ceramic materials in dentistry
being developed in these structures during cooling. For zirconia
veneer all-ceramic systems, the low thermal conductivity of
the zirconia (approximately 3 Wm/K)[51] results in the highest
temperature difference and therefore, very high residual
stresses. In addition, thick layers of veneering ceramics on
zirconia cores are highly susceptible to generating high tensile
subsurface residual stresses resulting in unstable cracking or
chipping.[52]
Framework design
The lack of uniform support of veneering ceramic because of
improper framework design has been discussed as a possible
reason for chipping fractures. With the introduction of
CAD/CAM technologies in dentistry, excessive veneer layer
thickness (>2.5 mm) was created because of the uniform layer
thickness of the copings for crowns and bar-shaped connectors
for FPDs. Improved customized zirconia coping design derived
from the conventional porcelain fused to metal technique
has been recommended to provide adequate support for the
veneering ceramic.[53] A dual-scan procedure of die and full
contour wax pattern has been merged to fabricate the desired
framework. Preliminary in vitro studies showed that cohesive
fractures within the veneering ceramic could not be avoided
with the improved support, but the size of the fractures was
signicantly decreased[54,55] and failure initiation was shifted
toward higher loads.[56]
Minimizing core failures
Laboratory technicians and clinicians should follow the precise
sequence steps in manufacturing zirconia-based restorations,
with the knowledge that zirconia as a framework material is
potentially damaged by surface modications and improper
laboratory and clinical handling techniques.[57] Grinding or
sandblasting of surfaces with high or mild/low-pressure ranges
is implicated as a factor in inducing the formation of surface
microcracking that could be detrimental to the long-term
performance of the restorations and lead to unexpected
failures.[58] With respect to the highly deleterious effect on
zirconia reliability,[59] postsintering surface modications of
zirconia frameworks at the dental laboratory or under clinical
circumstances should be avoided.
newer ConCepts
Manufacturers are now shifting toward the development of
monolithic all-ceramic materials instead of bilayer all-ceramic
systems to remove the most common failing layer of the system
and to avoid inherent residual thermal stresses. Monolithic
zirconia ceramic restorations are being researched in high-load
bearing areas.[60] Separate core and veneer layers than can be
joined with nonthermal methods with CAD/CAM technology
are evolving at rapid pace. Subtractive CAD/CAM approaches
are being now complemented with additive CAD/CAM
approach.[61]
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conicts of interest.
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... Some reports state a wide range of values of fracture strength of dental ceramic materials, most of which range well above 1000 N, with some up to 5000 N (Leevailoj et al. 1998;Anusavice 2013;Johansson et al. 2014;Baladhandayutham et al. 2015;Zhang et al. 2016). The highest flexural strength of all dental ceramics falls in the range of 900-1200 MPa, exhibited by Zirconia (Anusavice 2013;Teja and Teja 2015). These values exceed commonly encountered average maximum bite force loads of up to 600 N, suggesting that they should not fracture during normal service (Bakke 2006;Hattori et al. 2009;Varga et al. 2011). ...
... However, an aim of this study was to evaluate stresses on restorations and relating it to the commonly available flexural strength data such as those routinely published by manufacturers and used by clinicians. Flexural strength of all-ceramics fall into a wide range with Leucite glass, 120-180 MPa; Lithium Disilicate, 350 MPa; Aluminium Oxide, 600 MPa and the strongest, Zirconia at 900-1200 MPa (Anusavice 2013;Teja and Teja 2015). Unfortunately, as the strength of the materials increase, the translucency decreases, which has a negative effect on the aesthetic performance of the material in the mouth. ...
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Bonfante EA, Rafferty B, Zavanelli RA, Silva NRFA, Rekow ED, Thompson VP, Coelho PG. Thermal/mechanical simulation and laboratory fatigue testing of an alternative yttria tetragonal zirconia polycrystalcore-veneer all-ceramic layered crown design. Eur J Oral Sci 2010; 118: 202–209. © 2010 The Authors. Journal compilation © 2010 Eur J Oral Sci This study evaluated the stress levels at the core layer and the veneer layer of zirconia crowns (comprising an alternative core design vs. a standard core design) under mechanical/thermal simulation, and subjected simulated models to laboratory mouth-motion fatigue. The dimensions of a mandibular first molar were imported into computer-aided design (CAD) software and a tooth preparation was modeled. A crown was designed using the space between the original tooth and the prepared tooth. The alternative core presented an additional lingual shoulder that lowered the veneer bulk of the cusps. Finite element analyses evaluated the residual maximum principal stresses fields at the core and veneer of both designs under loading and when cooled from 900°C to 25°C. Crowns were fabricated and mouth-motion fatigued, generating master Weibull curves and reliability data. Thermal modeling showed low residual stress fields throughout the bulk of the cusps for both groups. Mechanical simulation depicted a shift in stress levels to the core of the alternative design compared with the standard design. Significantly higher reliability was found for the alternative core. Regardless of the alternative configuration, thermal and mechanical computer simulations showed stress in the alternative core design comparable and higher to that of the standard configuration, respectively. Such a mechanical scenario probably led to the higher reliability of the alternative design under fatigue.
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To evaluate the effect of framework design on the fatigue life and failure modes of metal ceramic (MC, Ni-Cr alloy core, VMK 95 porcelain veneer), glass-infiltrated alumina (ICA, In-Ceram Alumina/VM7), and veneered yttria-stabilized tetragonal zirconia polycrystals (Y-TZP, IPSe.max ZirCAD/IPS e.max,) crowns. Sixty composite resin tooth replicas of a prepared maxillary first molar were produced to receive crowns systems of a standard (MCs, ICAs, and Y-TZPs, n=10 each) or a modified framework design (MCm, ICAm, and Y-TZPm, n=10 each). Fatigue loading was delivered with a spherical steel indenter (3.18mm radius) on the center of the occlusal surface using r-ratio fatigue (30-300N) until completion of 10(6) cycles or failure. Fatigue was interrupted every 125,000 cycles for damage evaluation. Weibull distribution fits and contour plots were used for examining differences between groups. Failure mode was evaluated by light polarized and SEM microscopy. Weibull analysis showed the highest fatigue life for MC crowns regardless of framework design. No significant difference (confidence bound overlaps) was observed between ICA and Y-TZP with or without framework design modification. Y-TZPm crowns presented fatigue life in the range of MC crowns. No porcelain veneer fracture was observed in the MC groups, whereas ICAs presented bulk fracture and ICAm failed mainly through the veneer. Y-TZP crowns failed through chipping within the veneer, without core fractures. Framework design modification did not improve the fatigue life of the crown systems investigated. Y-TZPm crowns showed comparable fatigue life to MC groups. Failure mode varied according to crown system.