UNIVERSIDADE ESTADUAL PAULISTA

“JÚLIO DE MESQUITA FILHO”

Instituto de Ciência e Tecnologia

Campus de São José dos Campos

ORIGINAL ARTICLE DOI: https://doi.org/10.4322/bds.2024.e4021

Braz Dent Sci 2024 July/Sept;27 (3): e4021

This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in

any medium, provided the original work is properly cited.

Dental zirconia generations: a comparative study of translucency

parameters and light transmittance across varying thicknesses

Gerações de zircônia dentária: um estudo comparativo dos parâmetros de translucidez e transmitância de luz em diferentes

espessuras

Ra’fat I. FARAH1* 

1 - Qassim University, College of Dentistry, Department of Prosthetic Dental Sciences, Al-Mulaydah, Qassim, Saudi Arabia.

How to cite: Farah RI. Dental zirconia generations: a comparative study of translucency parameters and light transmittance across

varying thicknesses. Braz Dent Sci. 2024;27(3):e4021. https://doi.org/10.4322/bds.2024.e4021

ABSTRACT

Objective: This in vitro study evaluated the translucency and polymerization light transmission of zirconia discs,

produced from different generations of dental zirconia materials and with a consideration of varying thicknesses.

Material and Methods: Disc-shaped specimens (0.5mm, 1.0mm, 1.5mm, 2.0mm, and 3.0mm) were produced

from three A1 pre-shaded monochrome zirconia generations: conventional (3Y-TZP (inCoris TZI C); high

translucency 4Y-PSZ (Cercon HT) and super translucent 5Y- PSZ (Cercon XT). A total of 90 discs were prepared.

Translucency measurements were conducted using an intraoral spectrophotometer on each specimen against

white and black backgrounds. Polymerization light transmission was assessed by measuring the transmitted light

through each specimen emitted from conrmed emission spectra light-polymerizing units using a digital optical

power meter. Two-way ANOVA tests (α=0.05) were utilized to evaluate the differences between the inuences

of the zirconia generation and thickness parameters. Results: Different zirconia generations and thicknesses

displayed distinct TP and transmittance values, with 5Y-PSZ exhibiting the highest values, which decreased as

thickness increased. A signicant interaction was found between zirconia generation and thickness on TP and

transmittance (p≤0.001), with more pronounced differences at lower thicknesses and non-signicant differences

at 3mm thickness. Conclusion: The study demonstrates that the generations and thickness of zirconia, and their

interactions, signicantly impact light transmission and TP values. Extra translucent cubic zirconia (5Y-PSZ)

exhibits the highest TP, making it a more preferable choice for achieving predictable aesthetic outcomes that

mimic teeth translucency and high polymerization light transmittance, ensuring adequate photopolymerization

of the underlying resin luting cement.

KEYWORDS

Ceramics; Polymerization; Translucency; Transmittance; Zirconia.

RESUMO

Objetivo: Este estudo avaliou in vitro a translucidez e a transmissão da luz de polimerização através de discos

de zircônia, produzidos a partir de diferentes gerações de materiais de zircônia odontológica com variadas

espessuras. Material e Métodos: Espécimes em forma de disco (0,5 mm, 1,0 mm, 1,5 mm, 2,0 mm e 3,0 mm)

foram produzidos a partir de três gerações de zircônia monocromática pré-sombreada A1: convencional (3Y-TZP

(inCoris TZI C); alta translucidez 4Y-PSZ (Cercon HT) e super translúcida 5Y-PSZ (Cercon XT). Um total de 90

discos foram preparados. As medições de translucidez foram conduzidas usando um espectrofotômetro intraoral

em cada espécime contra fundos branco e preto. A transmissão de luz de polimerização foi avaliada medindo a

luz transmitida através de cada espécime, emitida a partir de unidades de polimerização de luz de espectros de

emissão conrmados usando um medidor de potência óptica digital. Testes ANOVA dois fatores (α = 0,05) foram

utilizados para avaliar as diferenças entre as gerações de zircônia e diferentes parâmetros de espessura. Resultados:

Braz Dent Sci 2024 July/Sept;27 (3): e4021

Farah RI

Dental zirconia generations: a comparative study of translucency parameters and light transmittance across varying thicknesses

Farah RI Dental zirconia generations: a comparative study of

translucency parameters and light transmittance across

varying thicknesses

INTRODUCTION

The field of prosthodontics underwent a

signicant transformation in the late 1990s with

the introduction of zirconia [1]. This material,

known for its remarkable mechanical properties

and pure white color, rapidly became a popular

alternative to metal framework materials in

fixed dental prostheses (FDPs), which were

often veneered with more translucent aesthetic

ceramics [2].

Since then, a new generation of zirconia has

emerged, offering pre-shaded options suitable

for monolithic restorations. Although the optical

properties and translucency of zirconia may

not be ideal for anterior teeth, it has secured

its position as the most prescribed material for

fabricating posterior crown restorations [3-6].

Recent advancements have resulted in the

development of highly and extra translucent zir-

conia, widening its applications in aesthetic zones

of FDPs by providing optimal optical properties

and translucency to match the appearance of

adjacent natural teeth, especially when a high

degree of translucency is desired [7-9]. This

progress in zirconia’s evolution is attributable to

several strategies employed by manufacturers,

such as reducing alumina content to 0.05 wt%

or less, rening zirconia grain size, enhancing

processing density, and controlling the manufac-

turing process to minimize air pockets and their

density [10,11].

The most successful strategy has been to

increase the cubic crystalline form content in

zirconia’s microstructure [6,10,11]. The cubic

polymorph of zirconia, which boasts superior

translucency compared to its tetragonal

counterpart, offers a constant refractive index

in all directions due to its optically isotropic

nature. Additionally, the larger grain size of

the cubic polymorph in zirconia reduces light

scattering at grain boundaries, reducing their

visibility and allowing more light to pass through

zirconia restorations [6,10]. However, this

manufacturing strategy results in a trade-off: the

cubic zirconia has lower strength and fracture

toughness than its tetragonal counterpart due

to the absence of transformation toughening

mechanisms present in the metastable tetragonal

phase and the relatively larger particle size of

the cubic phase compared to the tetragonal

phase [4,9,12]. To stabilize and maintain a

polymorphic equilibrium favoring the cubic form

as a major crystalline phase, additional stabilizing

oxides such as yttria (Y2O3) are incorporated.

Conventional zirconia contains about 3mol%

yttria (3Y), which is insufcient for achieving

the desired translucency [6,10,11]. By raising the

yttria content to 4mol% (4Y) and 5mol% (5Y),

the cubic form’s presence increases to about 30%

and 50% respectively. This enhancement in cubic

zirconia content has led to improved translucency

in zirconia-based dental restorations [8,9]. It is

noteworthy that while the increase in cubic phase

content by raising yttria concentration enhances

translucency, it concurrently reduces mechanical

properties. Specically, 4Y-TZP zirconia exhibits

a flexural strength of 600 to 900 MPa and

fracture toughness of 2.5 to 3.5 MPa·m1/2, and

5Y-TZP zirconia shows a flexural strength of

700 to 800 MPa and fracture toughness of

2.2 to 4 MPa· m1/2. This is in contrast to more

opaque zirconia, which has a exural strength

of 1,000 to 1,400 MPa and fracture toughness

of 3.5 to 4.5 MPa·m1/2 [10,13,14].

The increased translucency of zirconia,

coupled with advances in bonding zirconia

to tooth structures using new generations of

10-Methacryloyloxydecyl Dihydrogen Phosphate

(10-MDP)-containing adhesive luting resin

cements, has expanded potential applications

of zirconia in the aesthetic and minimally

Diferentes gerações e espessuras de zircônia exibiram distintos valores de TP e transmitância, com a 5Y-PSZ

exibindo os valores mais altos, que diminuíram conforme a o aumento da espessura. Uma interação signicativa

foi encontrada entre a geração de zircônia e a espessura em TP e transmitância (p≤0,001), com diferenças mais

proeminentes em espessuras menores e diferenças não signicativas em espessuras de 3 mm. Conclusão: O estudo

demonstra que as gerações e a espessura da zircônia, e suas interações, impactam signicativamente a transmissão

de luz e os valores de TP. A zircônia cúbica extra translúcida (5Y-PSZ) exibe o TP mais alto, tornando-a uma

escolha mais preferida para atingir resultados estéticos previsíveis que se assemelham a translucidez dos dentes

e a alta transmitância de luz de polimerização, garantindo a fotopolimerização adequada do cimento resinoso.

PALAVRAS-CHAVE

Cerâmicas; Polimerizações; Translucidez; Transmitância; Zircônia.

Braz Dent Sci 2024 July/Sept;27 (3): e4021

Farah RI

Dental zirconia generations: a comparative study of translucency parameters and light transmittance across varying thicknesses

Farah RI Dental zirconia generations: a comparative study of

translucency parameters and light transmittance across

varying thicknesses

invasive bonded restoration eld within xed

prosthodontics [15-17]. As zirconia usage

continues to grow, it’s crucial to further investigate

its optical properties [18].

Specically, the translucency of zirconia, a

key optical property, plays a critical role in dental

restorations [3,19,20]. Translucency is evaluated

by measuring a material’s translucency parameter

(TP) and light transmittance. The TP represents

the color difference between a material with a

uniform thickness when placed against white

and black backgrounds, which is important in

predicting the aesthetic outcome of a restoration.

This allows for the replication of natural teeth

translucency or the concealment of underlying

colors in cases with dark foundations or tooth

shades [8,21,22].

Light transmittance, dened as the ratio of

light transmitted at a specic wavelength to the

amount of light before transmission, is essential in

indirect zirconia restorations. The process involves

reectance, internal scattering, and absorption of

polymerization light before it reaches the luting

cement, which is responsible for attenuating the

polymerizing light after it passes through and

exits the restoration. Previous research indicates

that light transmission through ceramic materials

impacts the photopolymerization of resin luting

cement, potentially leading to inadequate

polymerization of photopolymerizing or highly

light-dependent dual polymerizing luting resin

cements [22-24].

While the optical properties of zirconia

ceramics are well-documented, research on these

properties—specically, light transmittance and

translucency parameter (TP), which are essential

for the aesthetic outcome of zirconia restorations

and the effective photo-polymerization of

underlying resin cement—remains limited in

the context of the newest highly translucent

cubic zirconia. To the best of my knowledge,

no study has yet investigated the combined

effects of generation type and thickness on

light transmittance and TP. Furthermore, there

is a signicant gap in research using materials

from the same manufacturer and shade,

which is critical for accurately determining the

impact of yttria content and thickness and for

minimizing confounding factors that could affect

transmittance and TP. Therefore, this study aimed

to compare the TP and transmittance of zirconia

discs (of the same shade and from the same

manufacturer) fabricated from new generations

of high and extra-high translucent zirconia

at varying thicknesses with those made from

conventional 3Y-TZP. The null hypothesis that

was tested is that there would be no signicant

difference in the light transmittance and TP

values between highly translucent cubic zirconia

and 3Y-TZP.

MATERIALS AND METHODS

This study explored the optical properties

of three generations of A1 pre-shaded

monochrome dental zirconia materials from

the same manufacturer (Dentsply Sirona Inc.;

Charlotte, NC), outlined in Table I. The materials

tested comprised a conventional 3mol% yttria-

tetragonal zirconia polycrystal (3Y-TZP) (inCoris

TZI C, mono L blocks), a high translucency

4mol% yttria-partially stabilized zirconia (4Y-

PSZ) (Cercon HT, 98.5mm discs), and a super/

extra translucent 5mol% yttria-partially stabilized

zirconia (5Y-PSZ) (Cercon XT, 98.5mm discs).

Circular disc-shaped specimens with

a uniform diameter of 10 mm and various

thicknesses (0.5mm, 1.0mm, 1.5mm, 2.0mm,

and 3.0mm) were designed using a 3D modeling

software program (Meshmixer; Autodesk, Inc.;

San Rafael, CA). The 3D digital images were

exported as STL files and opened in a CAM

software (inLab CAM SW16.1; Dentsply Sirona

Inc.; Charlotte, NC) to nest the les within the

respective zirconia CAD/CAM discs and blocks

(Figure 1). The specimens were then milled

using a 5-axis computer numerical control (CNC)

dental milling machine (inLab MC X5; Dentsply

Sirona Inc.; Charlotte, NC). In total, 90 discs

Figure 1 - Schematic representation of the process of virtually

nesting a specimen’s design ﬁles into a CAD/CAM zirconia disc in

a software program.

Braz Dent Sci 2024 July/Sept;27 (3): e4021

Farah RI

Dental zirconia generations: a comparative study of translucency parameters and light transmittance across varying thicknesses

Farah RI Dental zirconia generations: a comparative study of

translucency parameters and light transmittance across

varying thicknesses

were prepared, with six specimens for each

combination of thickness and zirconia generation

type. Sample size calculations were initially

conducted using G*Power software (version

3.1.9.6, Heinrich Heine University Düsseldorf).

An a priori power analysis, utilizing an effect

size (f) of 0.91 derived from the findings of

Supornpun et al. [8], determined that a total

of 30 specimens would be required to achieve

80% power for detecting signicant differences

at an alpha level of 0.05 in an ANOVA test.

Despite the initial calculations, the decision was

made to fabricate 6 specimens for each material

and thickness combination, consistent with the

number of specimens used in related research,

resulting in a total of 90 specimens to enhance

the robustness of the study.

The specimens underwent conventional

sintering in a high-temperature furnace

(Sirona Inre HTC speed; Dentsply Sirona Inc.,

Charlotte, NC) following the manufacturer’s

recommendations, Post-sintering, they were

cleansed in an ultrasonic cleaner and their

thicknesses were confirmed using a digital

caliper (Mitutoyo 500-180-30 Caliper, Mitutoyo

Corp., IL, USA) with an accuracy of ±0.02 mm.

Subsequently, each ceramic disc was then

nished and polished in a standardized manner

to achieve a smooth surface using diamond-

impregnated, bullet-shaped zirconia polishing

stones/burs (Zirconia Polishing Kit CA, SHOFU

Dental Corporation, CA, USA).

The specimens’ translucency was assessed

using a calibrated clinical spectrophotometer

(Vita Easyshade Advance, VITA Zahnfabrik;

Bad Säckingen, Germany). This contact-type

spectrophotometer features D65 illumination and

a geometric 2°/0° observer angle and has been

proven to be highly reliable for measuring the

optical properties of dental materials in various

studies [25,26]. With a small aperture size of

3mm within the measurement area and a probe

tip size of 5mm, the device corresponds to the

central area of the ceramic discs [27]. A custom-

designed 3D-printed alignment jig, depicted in

Figure 2, was employed to ensure consistent

alignment of the probe tip with the same central

Table I - Overview of the characteristics of the investigated zirconia generations

Brand Type Composition (wt%) Manufacturer Lot # Shade Flexural

strength*

InCoris TZI C

Pre-shaded

zirconium Oxide

(3Y-TZP)

ZrO2+HfO2+Y2O3 ≥ 99.0%

Dentsply Sirona

Inc.; Charlotte,

2014373410 A1 > 900MPa

Σ Y2O3 + Er2O3 5.6%

Al2O3 ≤ 0.35%

Other oxides (except Er2O3) ≤ 0.2%

3Y-TZP (<15%

)

Cercon® HT

Pre-shaded

zirconium Oxide

4-PSZ (>30%

)

ZrO2

Dentsply Sirona

Inc.; Charlotte,

18047811 A1 1200 MPa

Y2O3: 5%

HfO2: <3%

Al2O3, Other oxides including

Silicon oxide: <1%

Cercon® XT

Pre-shaded

zirconium Oxide

5Y-PSZ (>50%

)

ZrO2

Dentsply Sirona

Inc.; Charlotte,

18044893 A1 750 MPa

Yttrium oxide (Y2O3): 9%

HfO2: <3%

Al2O3, Other oxides including

Silicon oxide: <1%

* According to manufacture data (three-point ﬂexural testing). PSZ, yttria-partially stabilized zirconia; TZP, yttria-tetragonal zirconia polycrystal;

Cubic.

Figure 2 - A specimen is shown being tested against a black

background using a spectrophotometer. A 3D-printed alignment

jig was utilized to precisely position the spectrophotometer at the

center of the specimen.

Braz Dent Sci 2024 July/Sept;27 (3): e4021

Farah RI

Dental zirconia generations: a comparative study of translucency parameters and light transmittance across varying thicknesses

Farah RI Dental zirconia generations: a comparative study of

translucency parameters and light transmittance across

varying thicknesses

area of the specimens. This jig facilitated the

positioning of the probe at a precise 90-degree

angle to the specimen surface, thereby promoting

consistent measurement conditions [28,29].

Furthermore, the specimens were prepared with

at surfaces and were measured against opaque

backgrounds to mitigate any potential edge-loss

effects. Additionally, the selection of the specimen

diameter was carefully considered in relation to

the device’s aperture to minimize the potential

for edge-loss effects, ensuring the reliability of

the translucency measurements [30,31].

The CIE Lab* values of each specimen were

first obtained on a ColorChecker background

(X-rite ColorChecker Passport, X-rite Inc.; MI,

USA) on a black patch background (L* = 20.46,

a* = -0.07, and b* = -0.97) and then on a white

patch background (L* = 96.53, a* = -0.43, and

b* = 1.19). The TP was calculated for each

specimen based on the color differences against

these backgrounds using the following equation:

()

( )

()

W WB WB

TP E76 L LB a a b b

∗∗



=∆ = ∗− + ∗− ∗ + ∗−





(1)

where L* refers to the brightness, a* to the

redness-to-greenness, and b* to yellowness-

to-blueness. The subscript B refers to the color

coordination on the black background and W to

those on the white background [19,21,32,33].

To measure the transmittance of the

specimens, a digital optical power meter (Model

1830-C; Newport, CA, USA) was connected

to an optical detector (818-SL/DB; Newport,

CA, USA), capable of detecting wavelengths

in the range of 400–1100 nm, along with an

OD3 attenuation lter. The emission spectrum

of the light-polymerizing units was veried using

an integrating sphere connected to a ber-optic

spectrometer (USB 4000, Ocean Optics; Dunedin,

FL, USA). This ensured compatibility between

the spectral emission of the polymerization LED

light unit (450 nm - 550 nm) and the range of

the optical detector used. Ceramic disc specimens

were carefully centered on the detector’s

attenuation filter lens, and the light guide of

the veried light-polymerizing unit was aligned

over the specimen using a silicone alignment

jig (Figure 3). The intensity of transmitted and

incident light was recorded after activating the

light-polymerizing unit for 10 seconds, both

with and without the interposition of zirconia

disc specimens. Transmittance (T) was then

calculated using the formula:

( )

T I_t / I_0 100%= ×

(2)

where I_t is the intensity of the transmitted

light and I_0 is the intensity of the incident

light [8,12,24].

A two-way analysis of variance (ANOVA)

was conducted to determine the effects of

zirconia generation and thickness on translucency

and transmittance. Data were presented as

mean ± standard deviation. Normality and

homogeneity of variances were assessed using the

Shapiro-Wilk test and Levene’s test for equality

of variances (p>0.05). Statistical analysis was

performed using the IBM SPSS statistical software

program (IBM SPSS Statistics, v20.0; IBM Corp)

(α = 0.05).

RESULTS

Distinct TP and transmittance values were

exhibited by the various zirconia generations and

thicknesses, as shown in Figure 4. The 5Y-PSZ

demonstrated the highest overall TP and

transmittance values, followed by the 4Y-PSZ,

and lastly, the 3Y-TZP. Furthermore, the TP and

transmittance values for all zirconia generations

decreased as the thickness of the specimens

increased.

A significant interaction was observed

between zirconia generation type and thickness

on both TP and transmittance, with p ≤ .001 for

both. As a result, an analysis of simple main

Figure 3 - Photograph of the alignment process of a light polymerizing

unit over the zirconia specimen. The process involves the use of a

silicone jig over the optical detector, which is connected to a digital

optical power meter for the measurement of transmittance.

Braz Dent Sci 2024 July/Sept;27 (3): e4021

Farah RI

Dental zirconia generations: a comparative study of translucency parameters and light transmittance across varying thicknesses

Farah RI Dental zirconia generations: a comparative study of

translucency parameters and light transmittance across

varying thicknesses

effects was conducted, with statistical signicance

receiving a Bonferroni adjustment and being

accepted at the p < .025 level. Statistically

signicant differences in the mean of both TP

and transmittance values were identied among

the three zirconia generations for all thicknesses,

except for the 3 mm thickness (Table II).

Pairwise comparisons were carried out for

each simple main effect, with reported 95%

condence intervals and p-values Bonferroni-

adjusted within each simple main effect.

Differences in both TP and transmittance values

among the three zirconia generations were more

pronounced in lower thicknesses and diminished

as thickness increased. The difference between

inCoris TZI C and Cercon HT in both TP and

transmittance values became insignicant for

thicknesses greater than 1 mm, but the difference

was signicant for 0.5 and 1 mm thicknesses.

Furthermore, there were no signicant differences

among the three zirconia generations in TP

and transmittance values at a 3 mm thickness

(Table III). The effect of varying thickness on both

TP and transmittance values was more noticeable

in the extra translucent 5Y-PSZ zirconia than in

the other two zirconia generations (Figure 4).

DISCUSSION

This study’s ndings highlight that both the

type and thickness of zirconia, along with their

interactions, signicantly affect light transmission

and TP values, leading to the rejection of the

null hypothesis. Specifically, the TP values

of extra translucent cubic zirconia (5Y-PSZ)

were noticeably higher than those of 4Y-PSZ

and conventional 3Y-TZP. Furthermore, 5Y

zirconia demonstrated the highest translucency

nearing the human enamel translucency (TP

of 18) at a 1 mm thickness [34]. This superior

optical resemblance to natural teeth can be

attributed to successful strategies employed

in zirconia manufacturing technology [35].

On the other hand, 4Y zirconia exhibited a

TP of approximately 12.4 at 1 mm thickness,

signicantly less than human dentin TP at similar

thicknesses [34]. The lowest translucency was

observed in conventional 3Y zirconia with a TP of

9 at 1 mm thickness, underscoring the inuence

of the zirconia generation and the effects of

reducing Al2O3 content and increased cubic

polymorph content via elevated Y2O3 stabilizing

content on zirconia translucency [9,10].

Figure 4 - Bar chart illustrating the variation in translucency parameter and transmittance across diﬀerent specimen thicknesses for each of

the three generations of zirconia.

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Farah RI

Dental zirconia generations: a comparative study of translucency parameters and light transmittance across varying thicknesses

Farah RI Dental zirconia generations: a comparative study of

translucency parameters and light transmittance across

varying thicknesses

Table II - Table demonstrating the distinct main eﬀects of zirconia specimen thickness on both Translucency Parameter and Transmittance for the three zirconia generations

Specimen Thickness

Translucency Parameter Transmittance

Sum of

Squares df Mean

Square F

p-Value

Partial Eta

Squared

Sum of

Squares df Mean

Square F

p-Value

Partial Eta

Squared

0.5 mm Contrast 300.629 2 150.314 71.372 < 0.001* .656 375.231 2 187.616 106.037 < 0.001* .739

Error 157.956 75 2.106 132.700 75 1.769

1 mm Contrast 182.191 2 91.096 43.254 < 0.001* .536 264.250 2 132.125 74.675 < 0.001* .666

Error 157.956 75 2.106 132.700 75 1.769

1.5 mm Contrast 108.974 2 54.487 25.871 < 0.001* .408 155.254 2 77.627 43.874 < 0.001* .539

Error 157.956 75 2.106 132.700 75 1.769

2 mm Contrast 43.934 2 21.967 10.430 < 0.001* .218 103.721 2 51.861 29.311 < 0.001* .439

Error 157.956 75 2.106 132.700 75 1.769

3 mm Contrast 8.876 2 4.438 2.107 .129 .053 9.031 2 4.516 2.552 .085 .064

Error 157.956 75 2.106 132.700 75 1.769

df: degrees of freedom.

Braz Dent Sci 2024 July/Sept;27 (3): e4021

Farah RI

Dental zirconia generations: a comparative study of translucency parameters and light transmittance across varying thicknesses

Farah RI Dental zirconia generations: a comparative study of

translucency parameters and light transmittance across

varying thicknesses

Table III - A comprehensive pairwise comparison of the mean changes in both Translucency Parameter and Transmittance among the three zirconia generations, considering diﬀerent specimen thicknesses

Specimen

Thickness

(I) Zirconia

Generation

(J) Zirconia

Generation

Translucency Parameter Transmittance

Mean (I-J) Std. Error

p-Value

95% CI

Mean (I-J) Std. Err

p-Value

95% CI

Lower

Bound

Upper

Bound

Lower

Bound

Lower

Bound

0.5mm

InCoris Cercon HT -4.258* .838 < 0.001 -6.310 -2.207 -4.367* .768 < 0.001 -6.247 -2.486

Cercon XT -9.975* .838 < 0.001 -12.02 -7.923 -11.100* .768 < 0.001 -12.981 -9.219

Cercon HT InCoris 4.258* .838 < 0.001 2.207 6.310 4.367* .768 < 0.001 2.486 6.247

Cercon XT -5.717* .838 < 0.001 -7.768 -3.665 -6.733* .768 < 0.001 -8.614 -4.853

Cercon XT InCoris 9.975* .838 < 0.001 7.923 12.027 11.100* .768 < 0.001 9.219 12.981

Cercon HT 5.717* .838 < 0.001 3.665 7.768 6.733* .768 < 0.001 4.853 8.614

1mm

InCoris Cercon HT -3.033* .838 .002 -5.085 -.982 -3.250* .768 < 0.001 -5.131 -1.369

Cercon XT -7.733* .838 < 0.001 -9.785 -5.682 -9.250* .768 < 0.001 -11.131 -7.369

Cercon HT InCoris 3.033* .838 .002 .982 5.085 3.250* .768 < 0.001 1.369 5.131

Cercon XT -4.700* .838 < 0.001 -6.752 -2.648 -6.000* .768 < 0.001 -7.881 -4.119

Cercon XT InCoris 7.733* .838 < 0.001 5.682 9.785 9.250* .768 < 0.001 7.369 11.131

Cercon HT 4.700* .838 < 0.001 2.648 6.752 6.000* .768 < 0.001 4.119 7.881

1.5mm

InCoris Cercon HT -2.050 .838 .050 -4.102 .002 -1.867 .768 .052 -3.747 .014

Cercon XT -5.933* .838 < 0.001 -7.985 -3.882 -6.950* .768 < 0.001 -8.831 -5.069

Cercon HT InCoris 2.050 .838 .050 -.002 4.102 1.867 .768 .052 -.014 3.747

Cercon XT -3.883* .838 < 0.001 -5.935 -1.832 -5.083* .768 < 0.001 -6.964 -3.203

Cercon XT InCoris 5.933* .838 < 0.001 3.882 7.985 6.950* .768 < 0.001 5.069 8.831

Cercon HT 3.883* .838 < 0.001 1.832 5.935 5.083* .768 < 0.001 3.203 6.964

2mm

InCoris Cercon HT -1.667 .838 .151 -3.718 .385 -1.667 .768 .099 -3.547 .214

Cercon XT -3.817* .838 < 0.001 -5.868 -1.765 -5.717* .768 < 0.001 -7.597 -3.836

Cercon HT InCoris 1.667 .838 .151 -.385 3.718 1.667 .768 .099 -.214 3.547

Cercon XT -2.150* .838 .037 -4.202 -.098 -4.050* .768 < 0.001 -5.931 -2.169

Cercon XT InCoris 3.817* .838 < 0.001 1.765 5.868 5.717* .768 < 0.001 3.836 7.597

Cercon HT 2.150* .838 .037 .098 4.202 4.050* .768 < 0.001 2.169 5.931

3mm

InCoris Cercon HT -.850 .838 .941 -2.902 1.202 -.933 .768 .684 -2.814 .947

Cercon XT -1.720 .838 .131 -3.772 .332 -1.733 .768 .081 -3.614 .147

Cercon HT InCoris .850 .838 .941 -1.202 2.902 .933 .768 .684 -.947 2.814

Cercon XT -.870 .838 .907 -2.922 1.182 -.800 .768 .903 -2.681 1.081

Cercon XT InCoris 1.720 .838 .131 -.332 3.772 1.733 .768 .081 -.147 3.614

Cercon HT .870 .838 .907 -1.182 2.922 .800 .768 .903 -1.081 2.681

*Based on estimated marginal means. Std. Err, Standard Error; CI, Conﬁdence Interval.; Mean diﬀerence signiﬁcant at .05 level.; Adjustment for multiple comparisons: Bonferroni.

Braz Dent Sci 2024 July/Sept;27 (3): e4021

Farah RI

Dental zirconia generations: a comparative study of translucency parameters and light transmittance across varying thicknesses

Farah RI Dental zirconia generations: a comparative study of

translucency parameters and light transmittance across

varying thicknesses

TP values of zirconia were assessed across

a broad range of thicknesses, specifically

0.5mm to 3.0mm. An exponential relationship

was discerned between thickness and TP,

displaying signicant increases in translucency

as thickness decreased, echoing findings

from previous studies [8,21,36]. Notably,

variations in thickness had a more pronounced

impact on the TP of 5Y translucent zirconia,

whereas the TP of the least translucent

zirconia was less influenced by changes in

thickness. Even at a thickness of 3.0mm, all

tested zirconia demonstrated some degree of

translucency, presenting TP values surpassing

the 2.0 threshold [21,37]. Below this threshold,

a material is considered sufciently opaque

to obscure a black background. As a result,

the pre-shaded conventional 3Y zirconia

at A1 shade examined in this study may be

unsuitable for cases requiring the masking

of a very dark underlying tooth structure or

foundation restoration, suggesting the need for

a less translucent zirconia material or the use

of masking luting cement [19].

Transmittance and TP, while closely related,

describe distinct aspects of light interaction with

materials. The TP measurements in this study

accounted for the full visible light spectrum of

Illuminant D65 emitted by the spectrophotometer.

However, the polymerization of resin cement

in dental applications typically employs blue

light, which matches the absorption spectrum

of the camphorquinone photo-initiator [38].

The polycrystalline structure of zirconia, comprised

of varying grain sizes, orientations, and optical

properties, inuences its interaction with light,

resulting in light attenuation coefficients that

change as a function of wavelength. Additionally,

the presence of air pockets/pores, anisotropic

nature, large birefringence, and high refractive

index in tetragonal zirconia signicantly impact

translucency, particularly at the short wavelengths

used for polymerization [5,7.10]. Thus, separate

measurements of polymerization light transmittance

were conducted to account for these factors.

The study results demonstrated that the

transmittance of incident polymerizing light

varied depending on the thickness and type of

zirconia used, with the highest transmittance

reported for 5Y and lower thicknesses. For 4Y

zirconia, a 10% transmittance was observed

at a 1 mm thickness and 14% at a 0.5 mm

thickness. These values align with those reported

by Liebermann et al. [24], who found 8%

and 15% transmittance at 1 mm and 0.4 mm

thicknesses, respectively. The transmittance

value for 5Y zirconia was slightly lower than the

values reported by Harada et al. [3] for an ultra-

translucent zirconia brand containing Y2O3 at

9.32 wt%. This is similar to the yttria content

for the 5Y zirconia used in this study. However,

the transmittance values reported in Harada’s

study were for the full spectrum of light, with a

wavelength range of 380 to 780 nm.

This implies that if a 1mm-thick 5Y zirconia

restoration is cemented using a second-generation

LED light curing unit with an average high-

intensity irradiance of 1500 mW/cm2, the

irradiance reaching the composite luting cement

will be approximately 300 mW/cm2. The resulting

calculated radiant energy will be 6 J/cm2 following

a 20-second polymerization duration. In contrast,

for 1mm-thick 3Y and 4Y zirconia restorations,

only 150 to 195 mW/cm2 of irradiance will reach

the cement, respectively, and the energy will fall

short of the 6 J/cm2 required within the 20-second

timeframe [39]. This data, along with the impact

of both zirconia type and thickness on light

transmittance, can assist clinicians in selecting

the appropriate cement type and polymerization

protocols [15,22]. Furthermore, the choice of a

polymerization light unit with sufcient irradiance

and/or an extended polymerization duration

should be considered in line with the reciprocity

principle when applicable [40,41].

This study presents several limitations due

to the inherent properties of zirconia as a poly-

crystalline material [5]. The translucency of zir-

conia is signicantly inuenced by manufacturing

factors such as grain size, air pockets and their

density, which contribute to its optical proper-

ties, including TP and transmittance [10,42,43].

Although the grain size is a crucial factor in

determining the optical properties of zirconia

ceramics, SEM (Scanning Electron Microscopy)

analysis was not conducted in this research,

representing a limitation that warrants consid-

eration. Furthermore, elements such as zirconia

shade, chromaticity, as well as surface treatments

and characterizations, significantly influence

these properties [8,14,44,45]. In this research,

only one brand of zirconia and a single shade

were examined, without the application of glaze

or surface characterization. These limitations

underscore the need for further investigations

to better comprehend the effects of different

Braz Dent Sci 2024 July/Sept;27 (3): e4021

Farah RI

Dental zirconia generations: a comparative study of translucency parameters and light transmittance across varying thicknesses

Farah RI Dental zirconia generations: a comparative study of

translucency parameters and light transmittance across

varying thicknesses

brands, shades, and surface characterizations on

the TP and transmittance of zirconia-based dental

restorations. Expanded research in this area will

provide a more clinically relevant understanding

of expected aesthetic outcomes and the impact of

light transmittance during polymerization. Ulti-

mately, this knowledge will support the selection

of the most appropriate luting cement materials

and protocols, ensuring long-term success in

dental restorations and more predictable optimal

aesthetic outcomes.

CONCLUSIONS

1. The study reveals that both the type and

thickness of zirconia, as well as their

interactions, significantly influence light

transmission and TP values. Among the

tested zirconia types, 5Y exhibited the

highest values.

2. An exponential relationship was observed

between zirconia thickness and both TP

and light transmission values, with these

values increasing as thickness decreased.

The impact of thickness variations was more

pronounced in the case of 5Y zirconia.

3. To predict material translucency and achieve

optimal aesthetic outcomes, clinicians should

take into account both the type and thickness

of zirconia. Furthermore, clinicians should

consider these factors when selecting the

appropriate cement type and polymerization

protocols.

Author’s Contributions

Farah RI: Conceptualization; Methodology;

Investigation; Data Curation; Formal Analysis;

Software; Writing – Original Draft Preparation;

Writing – Review & Editing; Validation;

Visualization; Supervision; Project Administration.

Conict of Interest

The author declares no conict of interest.

Funding

This research did not receive any specic

grant from funding agencies in the public,

commercial, or not-for-prot sectors.

Regulatory Statement

Given that this study did not involve any

human or animal subjects, it did not necessitate

formal ethical approval. However, it has been

granted an exemption from requiring such

approval by the Ethical Committee of Qassim

University.

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Braz Dent Sci 2024 July/Sept;27 (3): e4021

Farah RI

Dental zirconia generations: a comparative study of translucency parameters and light transmittance across varying thicknesses

Farah RI Dental zirconia generations: a comparative study of

translucency parameters and light transmittance across

varying thicknesses

Ra’fat I. Farah

(Corresponding address)

Qassim University, College of Dentistry, Department of Prosthetic Dental Sciences,

Al-Mulaydah, Qassim, Saudi Arabia

Email: ri.farah@qu.edu.sa

Date submitted: 2023 Aug 30

Accept submission: 2024 July 20