Journal of the Mechanical Behavior of Biomedical Materials 88 (2018) 170–175

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Journal of the Mechanical Behavior of

Biomedical Materials
journal homepage: www.elsevier.com/locate/jmbbm

Load-bearing capacity of lithium disilicate and ultra-translucent zirconias T

⁎
Jing Yan, Marina R. Kaizer, Yu Zhang
Department of Biomateirals & Biomimetics, New York University College of Dentistry, New York, NY 10010, United States

A R T I C LE I N FO A B S T R A C T

Keywords: Objective: The aim of this study was to evaluate the load-bearing capacity of monolithic lithium disilicate (LiDi -
Load-bearing capacity IPS e.max CAD) and novel ultra-translucent zirconia restorative systems of various compositions: 5Y-PSZ (5 mol
Lithium disilicate % yttria-partially-stabilized zirconia) and 4Y-PSZ (4 mol% yttria-partially-stabilized zirconia); relative to a 3Y-
Ultra-translucent zirconia TZP (3 mol% yttria-stabilized zirconia) control.
Elastic modulus
Materials and methods: Experiments were carried out with 10 disc specimens (Ø12 ×1 mm) per ceramic mate-
Flexural strength
rial. The zirconia intaglio surface (as machined) was sandblasted (50 µm Al2O3 at 2 bar), while LiDi was etched
Layer thickness
with 5% HF for 20 s. The ceramic discs were then adhesively bonded onto a dentin-like substrate (G10, a high-
pressure fiberglass material) using Multilink Automix cement and Monobond Plus primer, producing a ceramic/
cement/dentin-like substrate trilayer structure. The bonded specimens were stored in water for 3 days at 37 °C
prior to a Hertzian indentation flexural radial fracture test. The plate-on-foundation theory was used to validate
the load-bearing capacity of the trilayer systems based on the flexural tensile stress at the ceramic intaglio
(cementation) surface—a cause for bulk fracture of ceramic onlays.
Results: The experiment data showed that, when bonded to and supported by a dentin-like substrate, the load-
bearing capacity of LiDi (872 N) is superior to the 5Y-PSZ (715 N) and can even reach that of 4Y-PSZ (864 N),
while 3Y-TZP still holds the highest load-bearing capacity (1195 N). Theoretical analyses agree with experi-
mental observations. The translucency of 5Y-PSZ approaches that of LiDi, which are superior to both 4Y-PSZ and
3Y-TZP.
Conclusions: When adhesively bonded to and supported by dentin, lithium disilicate exhibits similar load-
bearing properties to 4Y-PSZ but much better than 5Y-PSZ.

1. Introduction zirconia (3Y-TZP, 3 mol% yttria stabilized zirconia polycrystals) is the

strongest (800–1200 MPa) and toughest (3.5–4.5 MPa m1/2) dental
Monolithic dental restorations fabricated with CAD/CAM tech- ceramic, but it is fairly opaque. The translucency of dental zirconia has
nology are gaining popularity among patients and practitioners due to been improved by increasing the yttria content. The novel ultra-trans-
their ease of fabrication and cost-effective characteristics (Li et al., lucent zirconias (4Y-PSZ and 5Y-PSZ, respectively, 4 mol% and 5 mol%
2014; Zhang and Kelly, 2017). The monolithic crowns in the posterior yttria partially stabilized zirconia) have increased volume fractions of
area of the mouth are mainly fabricated with two types of ceramics: the optically isotropic cubic-phase (> 50%), which have effectively
lithia-based glass-ceramics or zirconia (yttria-stabilized polycrystals). increased the materials’ translucency. However, the increase in cubic-
Particular characteristics of these two classes of materials guide their content also results in reduced strength and toughness (Mao et al.,
clinical indications, as well as the clinicians preferences (Belli et al., 2018; Zhang et al., 2016a; Zhang and Lawn, 2018), likely to a level
2017; Tong et al., 2016; Wendler et al., 2017; Zhang, 2014; Zhang and similar to that of LiDi.
Lawn, 2018; Zhang et al., 2013a). The most widely used lithia-based Although the fracture resistance of ultra-translucent zirconia is
glass-ceramic is lithium disilicate (LiDi), which is available in a variety significantly lower than that of traditional dental zirconia, their elastic
of shades and opacities. Also, its glassy matrix allows for excellent modulus is the same (200–210 GPa), which is significantly higher than
bondability, using the traditional etch/silane technique. Nonetheless, that of LiDi (95–105 GPa); both being stiffer than dental hard tissues
these materials have only moderate strength (400–600 MPa) and (enamel ~70 GPa and dentin ~18 GPa) (Kinney et al., 2003; Poolthong
toughness (2–2.5 MPa m1/2). On the other hand, traditional dental et al., 1998; Zhang and Lawn, 2018). It's well known that the elastic

⁎
Correspondence to: Department of Biomaterials and Biomimetics, New York University College of Dentistry, 433 First Avenue, Room 810, New York, NY 10010,
United States.
E-mail address: yz21@nyu.edu (Y. Zhang).

https://doi.org/10.1016/j.jmbbm.2018.08.023
Received 28 June 2018; Received in revised form 10 August 2018; Accepted 19 August 2018
Available online 21 August 2018
1751-6161/ © 2018 Elsevier Ltd. All rights reserved.
J. Yan et al. Journal of the Mechanical Behavior of Biomedical Materials 88 (2018) 170–175

modulus mismatch between ceramic and supporting structure influ- Al2O3 at 2 bar), and LiDi was acid etched (5% HF for 20 s), following
ences the load-bearing capacity of the ceramic (Timoshenko and recommended protocols for their clinical use. After that, all samples
Woinowsky-Krieger, 1959). The same is valid for dental ceramic re- were cleaned in an ultrasonic water bath for 2 min and dried. Adhesive
storations when supported by dental structures (Zhang et al., 2009). A bonding was carried out using Multilink Automix cement and
previous finite element analysis (Ma et al., 2013) showed that although Monobond Plus primer (Ivoclar Vivadent, Amherst, NY). A static load of
the flexural strength of zirconia (3Y-TZP) is 2.5 times higher than that 1 kg was used for 120 s to standardize cement thickness. The samples
of the LiDi glass-ceramic, the difference in load-bearing property is were then light cured for 4 intervals of 30 s at directions 90º apart. The
significantly reduced when these materials are bonded to enamel sup- ceramic/cement/G10 trilayer specimens were stored in water for 3 days
ported by dentin. Therefore, we hypothesize that LiDi should have si- at 37 °C to allow the continuous polymerization and complete hydration
milar or even superior load-bearing capacity to that of novel ultra- of the cement layer prior to the Hertzian load-to-fracture test. The
translucent zirconias, owing to their similar flexural strength but fracture resistance test was performed by loading the ceramic/cement/
smaller elastic mismatch between LiDi and dental hard tissues relative G10 trilayer specimens at the top ceramic surface with a rigid tungsten
to zirconias. This study experimentally evaluated the load-bearing ca- carbide (WC) indenter (r = 3.18 mm) under the loading rate of 1 mm/
pacity of LiDi and zirconia bonded onto a dentin-like substrate, which min. It is true that the elastic modulus of WC is 3 times that of the
was validated by the plate-on-foundation theory. stiffest opposing ceramic prostheses. The theory of contact mechanics
and our previous study (Ma et al., 2013) have shown that although the
2. Materials and methods elastic modulus of the indenter has profound influence on the initiation
of the near-contact cone cracks, it has little effect on the onset of the far-
2.1. Materials and sample preparation field flexural radial cracks. To suppress the formation of cone cracks
and to achieve a uniform force distribution, a thin piece of nitrile foil
Disc-shaped specimens (Ø12 mm × 1 mm thickness, final dimen- was placed between the ceramic surface and the loading ball. The
sions) were prepared from three dental zirconias (the Luxisse series; schematic of the test configuration along with the specimen geometric
Heany Industries, USA) and lithium disilicate (IPS e.max CAD; Ivoclar parameters is shown in Fig. 1. Critical load for the onset of radial
Vivadent, Lichtenstein). The surface of each disc was grinded dry after fracture was registered. All ceramic discs were then carefully peeled off
cutting, using a 320-grit (~35 µm) silicon carbide paper, to simulate the of their G10 substrate for optical microscopy examination. By using a
surface produced by CAD/CAM milling. Descriptions of the materials combination of reflected and transmitted light illumination, it was
and sintering temperatures are given in Table 1. The microstructure of possible to confirm that the fracture does indeed originate from the
these materials was observed on highly polished (0.5 µm) and thermally cementation radial cracks and not from the near-contact cone cracks.
etched (zirconia) or acid etched (LiDi) samples by field emission
scanning electron microscopy (Zeiss Merlin FE-SEM). To prevent grain 2.3. Plate-on-foundation theory
growth, thermal etching was carried out at a relatively low temperature
(1250 °C for 20 min) and fast heating rate (20 °C/min). For grain size The plate-on-foundation theory was used to predict the theoretical
analysis, at least 300 grains were measured using the linear intercept load-bearing capacity of the trilayer systems (ceramic/cement/dentin-
method (ASTM Standard, E112, 2013). A correction factor of 1.56 for like substrate) according to their elastic gradients (Fig. 1).
tetrakaidecahedral grains was used (Wurst and Nelson, 1972). Critical load (PR) for the onset of radial fracture from the ceramic
intaglio surface was calculated by the plate (ceramic restoration) on
2.2. Fracture resistance experiments foundation (cement/dentin assembly) theory. The mathematical model
is composed of three equations (Kim et al., 2003; Timoshenko and
The free-standing biaxial strength of each material (n = 10) was Woinowsky-Krieger, 1959):
determined by the piston-on-three-balls test, using a loading rate of
1 mm/min, following the ISO 6872 (ISO/FDIS, 6872, 2015). Bσd 2
PR =
To simulate the fracture resistance of ceramic prostheses supported log ( )
CE
E* (1)
by tooth dentin, ceramic specimens of each material (n = 10) were
bonded to a dentin-like substrate (G10, Acculam, USA). G10 is a glass where E is the flexural modulus of the ceramic Table 1, zirconia (=
fiber reinforced epoxy resin, with elastic modulus E = 18.6 GPa similar 210 GPa) and lithium disilicate (= 95 GPa) (Ma et al., 2013); σ is the
to human dentin (Kinney et al., 2003). G10 rods were flattened and flexural strength of the ceramic, which was investigated in this study
finished with 45 µm diamond grinding, then soaked in water for 21 (Table 1); d is the thickness of the ceramic. C (≈ 1) and B (= 1.35) are
days for complete hydration. the dimensionless constants (Miranda et al., 2003); E * is the effective
For adhesive bonding, the G10 surface was acid etched (5% HF for modulus of the cement/dentin (G10) layer which is based on contact
2 min), while the zirconia intaglio surface was sandblasted (50 µm mechanics (Gao et al., 1992; Hu and Lawn, 1998; Kim et al., 2003):

Table 1
Properties of materials used in this study.
Material Manufacturer Sintering condition Composition (%) Modulus, Ea (GPa) Strength, σ (MPa) Thickness, d/h (mm)

Zirconia
Zpex (3Y-TZP) Heany Dental 1530 °C for 2 h t-ZrO2: 71, c-ZrO2: 29 210 904 (57) A 1.0 ± 0.2
Zpex 4 (4Y-PSZ) Heany Dental 1450 °C for 2 h t-ZrO2: 43, c-ZrO2: 57 210 749 (29) B 1.0 ± 0.2
Zpex Smile (5Y-PSZ) Heany Dental 1450 °C for 2 h t-ZrO2: 31, c-ZrO2: 69 210 593 (90) C 1.0 ± 0.2
Glass-ceramic
IPS e.max CAD Ivoclar Vivadent 820 °C for 2 min + 840 °C for 7 min Crystals: 70, Glass: 30 95 488 (28) C 1.0 ± 0.2
Cement
Multilink Automix Ivoclar Vivadent Light cured Glass, DMA, HEMA 7.9 114a 0.04 ± 0.01
Composite
G10 Acculam Lab fabricated Glass fiber, Epoxy 18.6 379 a
15

Different letters indicate statistical difference among materials, within each property.
a
Data from manufacturers.

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(a) (b) P
r

E Ceramic d
R

Ec Cement h

Es Dentin

Fig. 1. Schematic of the Hertzian load-to-fracture test configuration used to determine the critical load of radial crack.

(a) (b)

0.5 m 0.5 m

(c) (d)

0.5 m 0.5 m
Fig. 2. The zirconias are polycrystalline ceramics, with (a) 3Y-TZP having average crystal size of 0.54 ± 0.04 µm, (b) 4Y-PSZ 0.95 ± 0.03 µm, and (c) 5Y-PSZ
1.33 ± 0.06 µm. (d) IPS e.max CAD is a glass-ceramic material composed of ~70% lithium disilicate and lithium phosphate crystals of 2–4 µm average size.

L
E by the color difference between the specimen on black (B) and white
E * = Ec ⎛⎜ s ⎞⎟ (W) backgrounds (Kaizer et al., 2017):
⎝ Ec ⎠ (2)
where Es and Ec are the modulus of cement (7.9 GPa scientific document TP = (LB* − LW* )2 + (aB* − aW* )2 + (bB* − bW* )2 (4)
from Ivoclar) and dentin-like substrate (18.6 GPa), and L is an experi- where L* , a* , and b* refer respectively to the lightness, redness to
mentally determined dimensionless function which is described by Eq. greenness, and yellowness to blueness coordinates in the CIE color
(3) (Kim et al., 2003): space (CIE, 2004).
h
γ CR is the ratio of spectral reflectance of the light (Y) of the specimen
L = exp ⎧−⎡α + βlog ⎛ ⎞ ⎤ ⎫ on black (YB) and white (YW) backgrounds (Nogueira and Della Bona,
⎨
⎩ ⎢
⎣ ⎝ d ⎥ ⎬
⎠⎦ ⎭ (3)
2013):
where α = 1.18, β = 0.33, γ = 3.13 (Kim et al., 2003); and h is the
YB
thickness of cement measured in cross-sectioned specimens (~ 40 µm). CR =
YW (5)

2.4. Translucency parameter and contrast ratio L* +16 ⎞ 3

Y=⎛ Yn
⎝ 116 ⎠ (6)
Highly polished (0.5 µm diamond grits) disc specimens (n = 3) were
measured by a calibrated dental colorimeter (SpectroShade Micro; The specified white stimulus (Yn) is normally chosen since it is a
MHT). Color coordinates CIEL*a*b* were measured over standard perfect reflecting diffuser, that is, Yn = 100. CR values range from 0
backgrounds (black L* = 1.8, a* = 1.3, b* = −1.5 and white L* = (for a transparent material) to 1 (for a totally opaque material).
95.7, a* = −1.3, b* = 2.6). To ensure optical continuity, a drop of
glycerol (n = 1.472) was placed between the specimen and background 2.5. Statistical analyses
(Nogueira and Della Bona, 2013).
The translucency parameter (TP) of the material can be determined Four outcomes were investigated in this study: flexural strength,

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J. Yan et al. Journal of the Mechanical Behavior of Biomedical Materials 88 (2018) 170–175

load-bearing capacity, translucency parameter, and contrast ratio. Data 1500

a
for each outcome was separately analyzed for normality and equality of Experiment
variances, and then subjected to 1-way analysis of variance (ANOVA).

Fracture load, P (N)

A Theory
Multiple comparisons were performed using the Tukey test. The sig-
nificance level was set at 5%. 1000 b b
B
B
c C

3. Results
500
The SEM images of thermally etched zirconias and acid etched LiDi
are shown in Fig. 2. The zirconia average grain size was
1.33 ± 0.06 µm for 5Y-PSZ, 0.95 ± 0.03 µm for 4Y-PSZ, and
0.54 ± 0.04 µm for 3Y-TZP. The glass-ceramic consisted of ~70 vol% 0
elongated LiDi crystals. The average grain size of LiDi crystals was 3Y-TZP 4Y-PSZ 5Y-PSZ LiDi
1–3 µm in length and 0.1–0.4 µm in width. Fig. 4. Theoretical and experimental load-bearing capacity of zirconia and li-
Table 1 also summarizes the composition, mechanical properties thium disilicate for the trilayer systems—ceramic/cement/dentin-like sub-
and layer thicknesses of various ceramic materials, luting cement, and a strate. Different upper and lower-case letters indicate a statistical difference
dentin analogue material used in the present study. The piston-on-3- among materials for theory and experiment, respectively.
ball biaxial flexural strength of the ceramic prosthetic materials, and
the elastic modulus and thickness of material of each layer (Fig. 1) were
absence of the formation of near-contact induced cone cracks in all
used to predict the critical fracture load of the ceramic/cement/G10
cases [Fig. 3(a) and (c)]. Note: arrows in (a) and (c) indicate the contact
trilayer systems by the plate-on-foundation theory (Eqs. 1–3). It is im-
area. However, stereo optical microscopy examination of the same
portant to note that the biaxial strength of LiDi was similar to that of
specimens using transmitted light illumination revealed the far-field
5Y-PSZ but lower than 4Y-PSZ, while 3Y-TZP remained the strongest
flexural induced radial cracks [Fig. 3(b) and (d)], suggesting that
among all materials. In addition, the elastic modulus of LiDi (95 GPa)
fracture is indeed initiated from the cementation surface of the ceramic
was only half of that of the zirconias (210 GPa), making it much more
discs.
compatible with the dentin-like substrate (18.6 GPa).
Fig. 4 shows that, when bonded to and supported by dentin, the
Rietveld refinement on XRD spectra using MAUD revealed that 5Y-
load-bearing capacity of LiDi can reach that of its 4Y-PSZ counterpart,
PSZ is comprised of approximately 69% c-ZrO2 and 31% t-ZrO2,
and is much higher than 5Y-PSZ. Nonetheless, 3Y-TZP holds the highest
whereas 4Y-PSZ and 3Y-TZP consist of 57% c-ZrO2 and 43% t-ZrO2 and
load-bearing capacity (1195 ± 195 N) among the materials tested.
29% c-ZrO2 and 71% t-ZrO2, respectively. On the other hand, LiDi
Theoretical predictions for critical fracture load were found to agree
consists of a dominant lithium disilicate phase (Li2Si2PO5) couple with
very well with experimental results.
a subordinate lithium phosphate phase (Li3PO4) embedded in a glass
Fig. 5 depicts the TP and CR results for the materials tested of a
matrix.
common thickness 1 mm. 5Y-PSZ has the highest translucency among
Fracture modes in ceramic layers subjected to the Hertzian load-to-
the zirconias, and approaches that of LiDi. These findings are further
fracture test are shown in Fig. 3. Stereo optical microscopy examination
supported by a comparative digital photography.
of the specimen surface using reflected light illumination revealed an

Fig. 3. Representative stereo optical micro-

(a) (b) scopy images show surface features of (a) 3Y-
TZP and (c) 5Y-PSZ discs fractured at 1010 N
and 774 N respectively. Images were obtained
using reflected light illumination. Arrows in-
dicate the contact area, demonstrating the ab-
sence of near-contact induced top surface cone
cracks. (b) and (d) are images of the same
specimens, but acquired using transmitted
light illumination to reveal the far-field flex-
ural induced radial cracks initiated at the ce-
mentation surface of the ceramic discs. Images
were taken on ceramic discs that have been
“peeled off” of their dentin analogue composite
substrate after the Hertzian load-to-fracture
(c) (d) test.

1 mm

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J. Yan et al. Journal of the Mechanical Behavior of Biomedical Materials 88 (2018) 170–175

3Y-TZP 4Y-PSZ 5Y-PSZ LiDi

TP = 24.0 (0.1)C TP = 24.2 (0.6)C TP = 29.7 (0.4)B TP = 34.3 (0.9)A

CR = 0.48 (0.00)A CR = 0.47 (0.01)A CR = 0.37 (0.00)B CR = 0.37 (0.01)B

Fig. 5. Digital photograph illustrating the comparative translucency of the four materials at 1 mm thickness. Average (standard deviation) of translucency parameter
(TP) and contrast ratio (CR). Different letters indicate statistical difference among materials, within each property.

4. Discussion and low modulus ceramic material, minimize the luting cement thick-
ness, and wherever it is possible, preserve the enamel structure. The
Due to the superior mechanical properties and ease of fabrication in higher elastic modulus of enamel coupled with its better resin bonding
CAD/CAM systems, zirconia and lithium disilicate have already become ability relative to dentin can further increase the load-bearing property
the materials of choice for posterior all ceramic restorations (Rekow and longevity of ceramics (Layton and Clarke, 2013; Layton et al.,
et al., 2011; Zhang, 2012). From the biaxial flexural test, we found that 2012; Rojpaibool and Leevailoj, 2017).
lithium disilicate has similar flexural strength (488 MPa) to 5Y-PSZ We acknowledge that this study has several limitations. All ceramic
(593 MPa), whereas 4Y-PSZ and 3Y-TZP are around 1.5 and 2 times, restorations are susceptible to premature fracture under cyclic fatigue
respectively, stronger than lithium disilicate. Our findings suggest that loading (Wendler et al., 2018; Zhang et al., 2013b). In the case of radial
zirconias, particularly 3Y-TZP and 4Y-PSZ, are much more suitable for fracture due to flexural stresses at the ceramic intaglio surface, loss of
freestanding stress-bearing applications, such as a multi-unit or long load-bearing capacity is predominantly due to moisture assisted slow
span fixed dental prosthesis (Pjetursson et al., 2015; Zhang and Ma, crack growth (SCG) (Bhowmick et al., 2005; Ramos Nde et al., 2016).
2009). Although all oxide ceramics are susceptible to SCG, the crack velocity
When these ceramic restorative materials are bonded to and sup- exponent for LiDi (20 ± 3) is smaller than that of zirconia (25 ± 2),
ported by dentin, the rank of load-bearing capacity can change, due indicating that LiDi would exhibit faster degradation of load-bearing
primarily to the difference in key mechanical property E in conjunction property relative to zirconia (Zhang and Lawn, 2004). It is important to
with layer thicknesses d (ceramic restoration) and h (luting cement). note that the velocity exponent values cited here was determined from
From the perspective of ceramic restorative materials, a high elastic ceramic plates on compliant substrates, which had already considered
modulus ceramic plate provides protection for the underline tooth foundation (cement/dentin-like substrate) creep induced loading rate
structure by bearing the (occlusal) contact stress (stress shielding) effects on the onset of radial cracks (Huang et al., 2007).
(Dejak et al., 2012; Zhang et al., 2016b). This, however, makes the Dental restorations often have significant curvatures at the ce-
ceramic restoration more prone to flexural damage, especially for low mentation intaglio surface (Rekow et al., 2009). Studies from our labs
strength materials with small thicknesses. For this reason, high modulus (Kim et al., 2009) and elsewhere (Qasim et al., 2005; Rudas et al., 2005)
and low strength ceramics, such as alumina (which has a modulus twice have shown that flexural radial cracks require a higher load to initiate
as high as zirconia but a strength only half of that of zirconia), are much in the curved structures relative to their flat counterparts. However,
more prone to flexural fracture relative to zirconia (Christensen and these radial cracks can propagate rapidly to the margins of the curved
Ploeger, 2010; Selz et al., 2014). specimens. In addition, curved structures are also more prone to contact
From the perspective of the cement/dentin foundation, a higher induced cone cracks, which can potentially result in restoration chip-
elastic modulus foundation offers better support for the ceramic plate ping (Kim et al., 2009).
by preventing ceramic flexure upon (occlusal) contact loading (Ma
et al., 2013; Rojpaibool and Leevailoj, 2017). It is important to note that
5. Conclusions
the elastic modulus of the foundation is an effective (or combined)
modulus of the cement layer and dentin analogue substrate. Since the
When adhesively bonded to a dentin-like support, lithium disilicate
modulus of most commercial luting cements (Ec ~ 2 – 10 GPa) is much
restorations exhibit similar load-bearing properties to that of new ultra-
lower than the modulus of dentin and enamel, it is essential to keep the
translucent zirconias (i.e. 4Y- and 5Y-PSZ), suggesting that lithium
thickness of the cement layer small in order to achieve a high founda-
disilicate glass-ceramics have great potential for durable and esthetic
tion modulus. A thin cement layer is beneficial not only to improve the
minimally invasive restorations. In the case of the 3 zirconia materials,
load-bearing capacity, but also to ensure an adequately fitted restora-
there exists a tradeoff between the load bearing property and translu-
tion.
cency. 5Y-PSZ is most translucent (being only slightly below that of
Now consider a ceramic/cement/dentin trilayer system, the load-
lithium disilicate), whereas 3Y-TZP possesses the best load-bearing
bearing property of the ceramic restoration is governed not only by its
capacity. 4Y-PSZ occupies the middle ground.
strength and thickness but also the elastic modulus mismatch between
the ceramic plate and cement/dentin foundation. Although the de-
pendence of the fracture load on the modulus mismatch is the logarithm Acknowledgements
to base 10 of the plate to foundation modulus ratio, in the case of
ceramic modulus approaches to that of the tooth support, the effect of The authors would like to thank Prof. Do Kyung Kim and Dr. Minglei
modulus mismatch on the load-bearing property can be significant (Eq. Zhao for XRD analyses, and Dr. Jingxiang Yang for SEM characteriza-
(1)). Therefore, the key for achieving long-term stability of ceramic tions (Figs. 2a and c). This work was supported by National Institutes of
restorations supported by tooth structures is to select a high strength Health, United States (grant numbers. R01DE026772 and
R01DE026279).

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J. Yan et al. Journal of the Mechanical Behavior of Biomedical Materials 88 (2018) 170–175

Author agreement/declaration Mao, L., Kaizer, M.R., Zhao, M., Guo, B., Song, Y.F., Zhang, Y., 2018. Graded Ultra-
Translucent Zirconia (5Y-PSZ) for Strength and Functionalities. J. Dent. Res e-
Print. 1–7.
All authors have seen and approved the final version of the manu- Miranda, P., Pajares, A., Guiberteau, F., Deng, Y., Lawn, B.R., 2003. Designing damage-
script being submitted. They warrant that the article is the authors' resistant brittle-coating structures: i. Bilayers. Acta Mater. 51, 4347–4356.
original work, hasn't received prior publication and isn't under con- Nogueira, A.D., Della Bona, A., 2013. The effect of a coupling medium on color and
translucency of CAD-CAM ceramics. J. Dent. 41 (Suppl 3), e18–e23.
sideration for publication elsewhere. Pjetursson, B.E., Sailer, I., Makarov, N.A., Zwahlen, M., Thoma, D.S., 2015. All-ceramic or
metal-ceramic tooth-supported fixed dental prostheses (FDPs)? A systematic review
Conflict of interest of the survival and complication rates. Part II: multiple-unit FDPs. Dent. Mater. 31,
624–639.
Poolthong, S., Swain, M.V., Mori, T., 1998. Ultra micro-indentation of tooth using
All authors state that there is no financial/personal interest or belief spherical and triangular indenters. J. Dent. Res 77 (1129-1129).
that could affect our objectivity. Qasim, T., Bush, M.B., Hu, X., Lawn, B.R., 2005. Contact damage in brittle coating layers:
influence of surface curvature. J. Biomed. Mater. Res B Appl. Biomater. 73, 179–185.
Ramos Nde, C., Campos, T.M., Paz, I.S., Machado, J.P., Bottino, M.A., Cesar, P.F., Melo,
Declarations of interest R.M., 2016. Microstructure characterization and SCG of newly engineered dental
ceramics. Dent. Mater. 32, 870–878.
None. Rekow, E.D., Silva, N.R., Coelho, P.G., Zhang, Y., Guess, P., Thompson, V.P., 2011.
Performance of dental ceramics: challenges for improvements. J. Dent. Res 90,
937–952.
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