Current Oral Health Reports
https://doi.org/10.1007/s40496-023-00344-1
Zirconia: More and More Translucent
Jenni Hjerppe1
· Mutlu Özcan2,3
Accepted: 26 July 2023
© The Author(s) 2023
Abstract
Purpose of Review Yttria-stabilized zirconium dioxide, 3 mol% Y-TZP (zirconia, 3Y-TZP) was introduced as a prosthetic
material to provide metal-free, tooth-colored, and durable material option for the patients. However, its optical properties
are not ideal. This review describes the different strategies to increase translucency of zirconia material and summarizes the
current knowledge of translucent zirconia for fixed prosthodontic applications.
Recent Findings One of the most common ways of increasing the translucency of zirconia is to add the cubic phase by
increasing Y2O3 content. Y2O3 4Y mol% and Y2O3 5Y mol% partially stabilized zirconia materials seem to have better optical properties compared to 3Y-TZP materials but with less favorable mechanical properties.
Summary Despite the attempts to develop a translucent zirconia material, its optical properties are still far from those of
natural tooth structures. Possible solution for achieving more translucent and durable zirconia material could be utilizing
nanocrystalline zirconia. The production of nanocrystalline zirconia is yet very technique-sensitive, and the sintering process
needs to be well controlled. Additional research in this field is needed before recommendation for clinical use. In the future,
the challenge will be in achieving balance between improved translucency without sacrificing from mechanical properties.
This would apply not only for subtractive but also additively manufactured zirconia ceramics.
Keywords Zirconia · Translucency · Monolithic · Prosthodontics · Dental materials
Introduction
Dental materials used for restoring teeth attempt to imitate
translucency and light reflectance of natural tooth structure. Glass ceramic materials suit best for this purpose, as
their optical properties resemble the ones of enamel and
dentine [1]. However, the lower mechanical properties of
glass ceramic-based materials limit their use in load-bearing
areas of the mouth [2, 3]. Yttria-stabilized zirconium dioxide, 3 mol% Y-TZP (zirconia, 3Y-TZP) was introduced as a
prosthetic material to provide metal-free, tooth-colored, and
* Jenni Hjerppe
jenni.hjerppe@zzm.uzh.ch
1
Clinic of Reconstructive Dentistry, Center of Dental
Medicine, University of Zurich, Plattenstrasse 11,
CH-8032 Zurich, Switzerland
2
Division of Dental Biomaterials, Clinic of Reconstructive
Dentistry, Center of Dental Medicine, University of Zurich,
Plattenstrasse 11, CH-8032 Zurich, Switzerland
3
Clinic of Masticatory Disorders and Dental Biomaterials,
Center of Dental Medicine, University of Zurich,
Plattenstrasse 11, CH-8032 Zurich, Switzerland
durable material option for the patients [4]. However, due
to polycrystal material structure, zirconia has some optical
restrictions.
Color and translucency are essential components in esthetic
appearance of a tooth or a dental restoration. Human perception of color is based on the light emission and its transmission, absorption, and reflection on the surface of an object
[5]. Individual interpretation of a color as well as the material
properties and surrounding conditions (illumination and light)
play a role in perception and producing a color in dentistry.
Different Phases of Zirconia
Un-stabilized zirconia exists in three different phases,
depending on the surrounding temperature: monoclinic
(m) <1170 °C, tetragonal (t) 1170–2370 °C, and cubic (c)
>2370 °C [6]. Monoclinic zirconia grains are 3–5% larger
compared to tetragonal ones. When un-stabilized zirconia
material cools down from >1170 °C, tetragonal grains are
transformed into monoclinic where 3–5% volume expansion
occurs. The sudden volume expansion causes cracks and
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Current Oral Health Reports
defects in the material, decreasing mechanical strength of
monoclinic zirconia. Tetragonal zirconia instead has better
mechanical properties, although it does not exist as such at
room temperature. By partially stabilizing the zirconia with
3 mol% of Y2O3 tetragonal zirconia can be maintained also
at room temperature [6, 7].
As a meta-stable material, tetragonal zirconia has ability
to resist crack propagation up to certain extend. The stress
caused by a crack induces t-m transformation at the crack
tip, which leads to local volume expansion of 3–5%, and the
compressive stresses are preventing the crack propagation
[6, 7]. This phenomenon, called transformation toughening,
is the basis of high mechanical properties of tetragonal zirconia. On the other hand, t-m transformation can also lead
to detrimental changes on material surface which occur due
to environmental stresses such as presence of water, body
fluids, or steamed water [8]. This phenomenon, low-temperature degradation or aging, can be caused for example by
steam pressure in autoclave. Due the effect of water steam,
tetragonal grains on material surface turn into monoclinic,
which is leading to sudden volume expansion and water penetration into the material [9, 10].
chipping rate of veneering ceramic including non-anatomical framework design, thickness of the veneering ceramic,
fast cooling phase during porcelain firing process, uneven
heat distribution in zirconia material, and glass transition
temperature and thermal expansion of veneering ceramic as
well as roughness of the veneering ceramic after occlusal
adjustments [23–28]. Minor and major ceramic chippings,
varying from 12 to 32 %, are reported in clinical studies on
teeth over 8- to 10-year follow-up time [21, 29] and up to
50% in implant-supported restorations in 10-year follow-up
time [30]. Size of the restoration affects the reported chipping rate as well. Single crowns have shown lower chipping
rates [21] compared to long-span multiple-unit FDPs, as in
a 10-year randomized controlled clinical trial, 4- to 5-unit
FDPs have shown higher chipping rates compared to 3-unit
FDPs [29]. Figure 1 illustrates a fractured 3Y-TZP multipleunit FDP after 3.7 years of use.
In order to avoid the chipping of the veneering ceramic,
more translucent zirconia materials were introduced [31,
32•, 33]. These materials can be used monolithic, without
veneering ceramic. Figure 2 illustrates a monolithic implant
crown made of 5Y-TZP material.
Mechanical Properties of 3Y‑TZP
Color and Translucency in Dentistry
Zirconia is by far the strongest ceramic material for dental
applications. Flexural strength of 3% mol yttria-stabilized
zirconia (3Y-TZP) is high, varying from 900 to 1400 MPa
[11–13] and fracture toughness from 7.4 to 11.5 MPa m1/2
[14–16]. On the other hand, flexural strength of lithium
disilicate-based glass ceramic materials is varying between
330 and 550 MPa and fracture toughness from 1.39 to 2.04
[17, 18]. However, it has been shown that flexural strength
of lithium disilicate-based glass ceramic materials is higher
after adhesive bonding [19].
The relationship between the colors is defined by Munsell
color space [34]. The Munsell color space consists of three
different parameters: hue, chroma, and value. Hue is defined
as a color shade; value is describing lightness/brightness of
a color shade; and chroma is the intensity of the color shade.
When white light is passing through a prism, each hue of
visible light range has different wave length, varying from
400 nm for violet to 700 nm for red color [35]. When white
light is interacting with an object, some of the wavelengths
are absorbed and some reflected. The reflected wavelengths
produce the visual color. Hence, certain object can appear
in different colors when different light sources are used. In
3Y‑TZP as a Prosthetic Material
Dental zirconia is a polycrystal material, which does not
contain any glass phase [1]. Therefore, it is very opaque and
can be used mainly as framework material for dental crowns
and FDPs. For esthetic purposes, the framework has to be
veneered with feldspathic ceramic.
In clinical studies about 3Y-TZP material, minor amounts
of 1.9–5.9% framework fractures of multiple-unit FDPs have
been seen in 5 to 10 years of follow-up time [20–22]. Typically, the multi-unit FDPs fracture on the connector area,
and the frameworks are therefore often designed bulky in
order to achieve enough stability for the connector areas.
Veneering ceramic seems to be clinically the weakest link
of 3Y-TZP restorations. Several factors can affect to the
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Fig. 1 Clinical failure of 7-unit 3Y-TZP FDP with framework fracture
(black arrow) and veneer chipping (white arrow) after 3.7 years of
clinical use
Current Oral Health Reports
Fig. 2 Monolithic, highly polished 5Y-TZP implant crown
human eye, more precise in the retina, cone and rod cells
register the light and send to the brain signals, which will
be interpreted as colors [36].
Numerical values of color can be calculated by using
CIELAB color space. The International Commission on
Illumination (CIE) has defined a system where values
given by spectrophotometric device can be translated
into color parameters of L*, a*, and b* [37]. L* applies
to lightness of an object, a* to a hue scale of red to
green, and b* to a hue scale of yellow to blue. Using
the CIELAB color space enables comparison of different materials and objects in terms of color and appearance. However, the results of the CIELAB color space
can be difficult to interpret in a clinical environment.
CIEDE2000 color difference formula was therefore introduced for better correlation with visual inspection [38,
39]. In CIEDE2000 system, the total color difference
(△E00) is calculated from lightness (L), chroma (C), and
hue (H). The system has also thresholds for visual perceptibility (△E 00=0.8) and acceptability (△E 00=1.8),
which is functioning as a quality control tool for clinical
assessments.
A recent in vitro study was comparing the color of anterior monolithic (ML) and veneered zirconia crowns [40].
The veneered crowns were fabricated with cutback technique
and layered with either enamel layering material (bilayer,
BL) or enamel and dentine layering materials (tri-layer, TL).
The CIE L*a*b* values of the crowns (n=5) were measured
with spectrophotometer, and the color differences were calculated with CIEDE2000 formula. The lowest color differences were seen on BL and TL crowns compared to ML in
cervical and middle areas of the crowns. In all crown types,
the cervical and middle areas were mostly under the visual
perceptibility level (△E00≤0.8).
Light refraction, dispersion, transmission, and absorption
affect to the optical behavior and color appearance of each
object. Translucency describes the extent to which light is
transmitted through the object instead of being absorbed or
reflected [35]. Translucency can be evaluated by contrast
ratio (CR), translucency parameter (TP), and refractory
index (RI). CR is measured by evaluating the ratio of reflectance of a specimen placed on a black background compared to a white background [41], whereas TP represents a
color difference of a specimen placed on a black background
compared to a white background [42]. Higher the TP value
means higher translucency value. Materials with TP values
≤ 2 are considered opaque, while they are blocking black
background [43].
Thickness of the material affects directly the translucency parameter (TP) of the zirconia materials. TP values
of different generation zirconia materials in an in vitro study
varied between 16.59–20.40 for 0.5-mm-thick specimens to
5.10–9.17 for 2.0-mm-thick specimens, showing that the
thinner the material, the more translucent it is [44]. For
1-mm-thick specimens, TP values were varying between
11.16 and 15.82, whereas the previously reported TP value
is 18.7 1-mm-thick enamel and 16.4 for 1 mm dentin [45].
The contrast ratio is known to decrease by increasing
translucency [44]. The contrast ratio value 0 is considered
translucent and a value of 1 as fully opaque [46]. The effect
of sintering temperature to contrast ratio (CR) was investigated in an in vitro study [47]. CR values were measured
on specimens sintered in 9 different temperatures from
1300 to 1700 °C. CR was varying from 0.85 for 1300 °C to
0.68 for 1700 °C differences being statistically significant.
In other words, CR was decreasing by increasing sintering
temperature.
Refractive index (RI) defines how much the path of light
is refracted when entering a material. It depends on different wavelengths of light and interaction of light in different
interfaces (grain/grain vs grain/pore) [48, 49]. 3Y-TZP material has higher RI than other ceramic materials, meaning
that the material has high surface reflection and low light
transmission [49].
Strategies to Increase Translucency
in Zirconia Material
There are several methods to increase the translucency of
zirconia material. It is possible to modify the material structure by changing the sintering protocol and amount and type
of dopants, leading to less light refraction and scattering
from the material structure.
During the development of translucent zirconia, modifying 3Y-TZP microstructure was considered as one of the
first actions. Zirconia was made more translucent trying by
increasing material density by eliminating the pores [48, 50].
The large difference in refractive index between the pores
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Current Oral Health Reports
and zirconia material is causing the light scattering by the
porosity and generating opaque optical appearance.
For the first-generation zirconia materials, alumina was
used as sintering aid (3mol% of Y2O3 and 0.25 wt% Al2O3),
which additionally stabilized the material and made it more
resistant to low-temperature degradation [48]. However,
alumina dopant was making the material look opaquer. By
decreasing the alumina content as well as adding other
dopants like lanthanum oxide, the material translucency
could be improved [51]. The optimal level of Al2O3 dopant
used in more translucent generations of 3Y-TZP is <0.25
wt%, providing higher density and better translucency [51].
Other suggested dopants are lanthanum oxide La2O3, magnesium oxide MgO, and neodymium oxide Nd2O3 [52].
From different dopants, the Al3+, La3+, Mg2+, and Nd3+
cations are segregated to the grain boundaries providing
hydrothermal stability. Dopants seem to have effect to
grain size, since 3Y-TZP materials doped with different
concentrations of Al2O3 only showed larger grain size than
3Y-TZP materials doped with Al2O3 and La2O3 [51]. When
yttria content was increased to 5 mol% (5Y-TZP with 0.05
wt% Al2O3), the grain size was significantly larger. However, in the market, mainly Al2O3 doped zirconia materials
have been available.
Other option for increasing translucency of zirconia is
to increase the grain size. The larger the grains, the less
light reflection and scattering there will be from grain
boundaries, leading to higher translucency [48, 53, 54].
Larger grain size can be achieved by increasing sintering time and temperature [11, 47]. Decreasing the grain
size would also provide more translucent zirconia material. Also, in nanocrystalline structure, the translucency
is dependent on grain size and thickness. Zhang was estimating in his study that for optimal translucency, 1.3 mm
thick specimen would need a grain size of 82 nm, and 2
mm thick specimen would need a grain size of 70 nm [48].
Nanocrystalline structure could be a perfect solution for
achieving a zirconia material with high translucency and
high mechanical properties. However, the production process is still inaccurate resulting to defects and porosities.
Adding the amount of cubic phase by increasing yttria
content is commonly used way of making zirconia more
translucent [55, 56]. Cubic zirconia is optical isotropic,
and therefore, light scattering at the grain boundaries is
not seen [51, 52]. On the contrary, tetragonal zirconia is
anisotropic in crystallographic directions (i.e., in optical
properties tetragonal zirconia grains are birefringent) [55].
When the adjoining tetragonal grains do not have same
crystallographic orientation, refractory index shows discontinuity at the grain boundaries leading to reductions in
light transmittance and more opaque appearance.
There are several translucent zirconia materials with varying yttria content, most commonly 4 mol % and 5 mol%
13
partially stabilized Y-TZP and 8 mol% fully yttria-stabilized
zirconia (YSZ) [48, 57, 58].
Graded zirconia was introduced more than a decade ago
for making the optical and adhesive properties of 3Y-TZP
better [31]. Glass powder composition was infiltrated on
3Y-TZP surface, creating a structure where outer surface
of an object had more esthetic layer, dense 3Y-TZP layer
in the middle and inner glass layer for adhesive bonding.
Specimens (thickness 1.4mm) with graded structure showed
promising results in terms of critical load for radial cracking (1990 N) compared to monolithic specimens (1388 N).
However, the method might be technique-sensitive and timeconsuming, and the scientific evidence is still scarce.
Finally, one of the more modern options for achieving
better optical and esthetic outcomes is to combine layers of different Y2O3 mol% zirconia material (i.e., different translucency) as a multi-layered zirconia block/puck.
The layers have different contents and pigment types,
which leads to natural shade [59]. When fabricating a
restoration with computer-aided design/computer-aided
manufacturing (CAD/CAM) methods, the restoration is
milled in such a way that the more translucent layer of
zirconia is situated on the incisal edge of the restoration.
A recent in vitro study was showing that there was no difference in translucency between the multi-layered 5 Y2O3
mol% and 4 Y2O3 mol% zirconia blocks [59]. However,
another in vitro study was showing that fracture resistance
of crown-shaped specimens was lower for multi-layered
materials compared to conventional 3Y- and 4Y-TZP
materials [60].
Effect of Dopants to 3Y‑TZP Material
Properties
Zhang and co-workers were testing 3Y-TZP materials with
different microstructure, and it could be seen that adding
0.25 wt% Al2O3 and 0.2 wt% La2O3 as a co-dopant resulted
in higher amount of c-ZrO2 compared to 3Y-TZP material
doped with 0.25 wt% Al2O3 only [51]. Similar significant
difference could also be seen in TP of these materials,
3Y-TZP 0.25 wt% Al2O3, and 0.2 wt% La2O3 being more
translucent and having lower CR. By manipulating the
grain boundaries with co-dopants, good long-term hydrothermal stability and higher mechanical properties could
be achieved [52].
Mechanically, the bending strength was higher for
3Y-0.25Al specimens (997 MPa, SD 202) compared to
3Y-0.25Al-0.2La specimens (651 Mpa, SD 77). However,
there were no differences between 3Y-0.25Al-0.2La specimens and other specimens doped with different concentrations of La2O3 and Al2O3 [51].
Current Oral Health Reports
Consequences of Making Zirconia
Translucent
Changing the yttria content of zirconia material can have
effect to several material properties. Additionally, when
more c-phase is present, yttria is concentrated on tetragonal grains. Cubic zirconia is more brittle, and tetragonal
zirconia has lower ability to transformation toughening,
leading to lower mechanical properties [61].
Various in vitro studies show differences in mechanical properties between different Y2O3 mol% zirconia materials with tendency to lower mechanical strength when increasing Y2O3 mol%
and translucency [32•, 57, 62–65]. Flexural strength, fracture
toughness, TP and CR values for different generations of zirconia are summarized in Table 1. The possible explanation for
decrease in mechanical properties lies in the phase content of
zirconia material, since c-phase does not seem to have as high
mechanical properties as t-phase.
Same trend has also been seen in crown-shaped specimens.
An in vitro study about implant crowns made of different generations of zirconia materials was showing that the fully stabilized (Y2O3 8 mol%) zirconia crowns connected directly to the
implant exhibited significantly lower fracture load values (140
N) compared to partially stabilized (Y2O3 3 mol%) translucent
zirconia crowns connected directly to implant (259 N) and partially stabilized (Y2O3 3 mol%) translucent zirconia crowns connected with titanium base to implant (453 N) [75].
Previous in vitro studies show that the airborne particle abrasion increases the flexural strength of 3Y-TZP
material [76, 77]. This is based on the t-m phase
Table 1 Flexural strength, fracture toughness, translucency parameter and contrast ratio of different zirconia generations
Zirconia type Flexural
strength
(MPa)
Fracture
toughness
(m1/2)
Translucency Contrast ratio
parameter**
3Y-TZP
Opaque
4.3–11.5
12–14
3Y-TZP
985–1008
Translucent
4.3–7.0
12–18
4Y-TZP
507–965
3.7–4.4
12.3–12.5
5Y-TZP
377–644
2.4–4.8
9.1–12.4
900–1400
Clinical Indications
Authors
Aboushelib et al. (2008) [14]
Guazzato et al. (2004) [15]
Hjerppe et al. (2009) [11]
Jerman et al. (2021) [66]
Pittayachawan et al. (2007) [12]
Sulaiman et al. (2017) [57]
Tinschert et al. (2007) [16]
Tong et al. (2016) [33]
Yener et al. (2011) [13]
Stawarczyk et al. (2013) [47]
Sulaiman et al. (2015) [44]
Abdulmajeed et al. 2022) [62]
0.84–0.89*
Full-contour anterior and posJerman et al. (2021) [66]
terior crowns and long-span
Sulaiman et al. (2017) [57]
FDPs
Tong et al. (2016) [33]
Zhang et al. (2016) [51]
Sulaiman et al. (2015) [44]
Abdulmajeed et al. (2022) [62]
0.79**–0.87*** Full-contour anterior and
Arcila et al. (2021) [67]
posterior single crowns and
Jeong et al. (2022) [68]
3-unit FDPs
Jerman et al. (2021) [66]
Pereira et al. (2018) [64]
Grambow et al. (2021) [69]
Baldissara et al. (2018) [70]
Abdulmajeed et al. (2022) [62]
0.61–0.75***
Full-contour anterior and
Camposilvan et al. (2018) [32•]
posterior single crowns and
anterior/premolar 3-unit FDPs Harada et al. (2020) [65]
Kongkiatkamon et al. (2022) [71]
Zhang et al. (2016) [51]
Salah et al. (2023) [72]
Hajhamid et al (2023) [73]
De Araújo-Júnior et al. (2022)
[74]
0.85–0.88*
Frameworks for anterior and
posterior crowns to long-span
FDPs
*0.7-mm-thick specimens
**1-mm-thick specimens
***1.2-thick specimens
13
Current Oral Health Reports
transformation-induced volume expansion on material
surface, creating compressive stresses [77]. Another
in vitro study comparing different generations of zirconia
was showing that airborne particle abrasion was increasing the flexural strength of partially stabilized Y 2O 3 3
mol% opaque and Y 2 O 3 3 mol% translucent zirconia
but decreasing the flexural strength of fully stabilized
Y 2O 3 8 mol% zirconia [57]. The authors attributed the
lower mechanical properties of Y 2O 3 8 mol% zirconia
are related to the high c-phase content and lower t-phase
content of the material. When less t-grains are present,
also less transformation toughening occurs. On the other
hand, a study by Chevalier and co-workers demonstrated
that the presence of cubic grains tend to reduce the material resistance to low-temperature degradation [78].
Yttrium stabilizer ions seem to enrich the cubic grains,
while the adjacent tetragonal grains are less stable, and
therefore, more t-m phase transformation takes place.
This does not apply to longer sintering times or higher
sintering temperatures but to aging conditions like moist
environment.
The microstructure and phase content of zirconia
material seem to have an impact on bond strength as
well. Microtensile bond strength of airborne particleabraded conventional zirconia (3 mol% Y2O3) was shown
to be significantly higher (20.86 Mpa, SD 5.12) than
that of airborne particle-abraded translucent zirconia (5
mol% Y 2O 3) (16.39 Mpa, SD 4.36) [79]. This could be
also related to the higher number of cubic grains in the
material.
Micro‑layering, Staining, and Glazing
Despite attempts to develop a translucent zirconia material, its optical properties are still far from those of
natural tooth structures. Possible solution for overcoming the translucency mismatch between zirconia material and natural dental structures and to avoid extensive
veneering ceramic chippings could be micro-layering
of the esthetic (buccal) surfaces of dental restorations
[80]. In micro-layering workflow, zirconia framework
is fabricated from translucent material block with no or
very minimal cutback and veneered with <0.5-mm-thin
ceramic layer which consist of fine-ground ceramic particles and a viscose liquid. Although in thin layer, the
felspathic ceramic forms a three-dimensional network
with natural looking depth effect as well as translucent,
fluorescent, and opalescent optical properties. However,
scientific evidence of micro-layering method is still lacking, and the potential risk of veneer chipping cannot be
excluded at this point.
13
More natural looking appearance and depth effect
can also be achieved by staining and glazing the zirconia framework. Staining of green-stage 3Y-TZP opaque
zirconia might weaken the material’s mechanical properties [81]. In an in vitro study, staining was performed by
dipping the specimens into the coloring liquid, and the
increasing staining time was negatively affecting to flexural strength [81]. Similar effect was seen with 3Y-TZP
translucent zirconia specimens stained by painting the surface with a brush [82], while increased flexural strength
and decreased translucency parameter values were seen in
specimens fabricated from 8 mol% fully yttria-stabilized
zirconia. The differences between different zirconia materials might be due to the material content and the amount
of cubic zirconia present. Additionally, it has been shown
that tension forces might cause delamination of the glazing layer on disc-shaped specimens [83]. However, clinically, this has not been reported so far, and it might not be
a relevant problem.
Conclusions and Future Perspectives
When developing monolithic zirconia materials, more
translucent nanocrystalline structures seem to be promising option [48]. They would provide substantial improvements in translucency while preserving strength properties.
However, it seems to be difficult to produce well-dispersed
homogenous starting powder containing controlled concentrations of stabilizing additives [84 •• ]. Additional
challenge is to avoid excessive grain growth and porosities during sintering process. The problems with producing
methods remain to be solved by scientists in the future.
Other possible options for developing more translucent and
durable zirconia materials are further optimizing of the sintering process, grain size, and addition of different dopants.
The development of translucent zirconia materials continues, and new challenge will be in achieving the balance
between improved translucency without sacrificing from
mechanical properties. This would apply not only for subtractive but also additively manufactured zirconia ceramics
[85–88].
Acknowledgements The authors express their gratitude to Dental
Technician Andrea Patrizi for providing Fig. 2.
Funding Open access funding provided by University of Zurich
Declarations
The Section Editors for the topical collection Dental Restorative Materials are Mutlu Özcan and Paulo Francisco Cesar. Please note that
Dr. Özcan was not involved in the editorial process of this article as
she is a co-author.
Conflict of Interest The authors declare no competing interests.
Current Oral Health Reports
Human and Animal Rights and Informed Consent This review article
does not contain any new data with human or animal subjects collected
by any of the authors. Patient in Fig. 1 signed informed consent for
using the photo in this publication.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
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need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
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