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.2023.e3614

Braz Dent Sci 2023 Apr/Jun;26 (2): e3614

Color stability and degree of conversion of amine-free dual cured

resin cement used with two diﬀerent translucencies of lithium

disilicate ceramics

Estabilidade de cor e grau de conversão de cimento resinoso dual sem amina usado com duas translucidezes diferentes de

cerâmica de dissilicato de lítio

Nora Maher MELEKA

, Ahmed EL-BANNA

, Dalia I. EL-KORASHY



1 - Ain Shams University, Faculty of Dentistry, Biomaterials Department. Cairo, Egypt.

How to cite: Meleka NM, Elbanna A, El-Korashy DI. Color stability and degree of conversion of amine-free dual cured resin cement used

with two different translucencies of lithium disilicate ceramics. Braz Dent Sci. 2023;26(2):e3614. https://doi.org/10.4322/bds.2023.e3614

ABSTRACT

Objective: The present study aims to evaluate the color stability and degree of conversion of amine-free dual

cured resin cement compared to light cured and amine-containing dual cured resin cements used with two

different translucencies of thin esthetic restorations. Material and Methods: A total of 120 specimens were

prepared for color stability testing (n=60). The specimens were divided into three main groups according to

the resin cement type. Group 1: amine-free dual cured, Group 2: light cured, Group 3: amine-containing dual

cured. Each group was further subdivided according to the ceramic translucency into two subgroups: high and

low translucency. Color stability was assessed by a spectrophotometer before and after thermal aging. For the

degree of conversion assessment (n=60), Fourier transform infrared spectroscopy was used at three different

time intervals. Statistical analysis was performed using multi-factorial ANOVA, followed by one-way ANOVA

with Bonferroni correction. Results: Amine-containing resin cement showed signicantly higher ΔE

and ΔE

in both translucencies (4.5±0.3, 3.5±0.3 respectively for high translucency ceramic and 3.8±0.4, 3.0±0.3

respectively for low translucency) than the other tested cements (p<0.001). The highest degree of conversion

(DC) was shown after 2 weeks by the amine-free dual cured resin cement (86.27±0.74). Conclusion: Amine-

free dual cured resin cement can be an alternative to light cured one for cementation of thin veneers since it

showed comparable color stability and high degree of conversion.

KEYWORDS

Resin cement; Color; Polymerization; Light curing of dental resins; Lithium disilicate.

RESUMO

Objetivo: O presente estudo tem como objetivo avaliar a estabilidade de cor e o grau de conversão do cimento

resinoso dual sem amina em comparação com cimentos resinosos fotopolimerizáveis contendo amina usados

com duas translucidezes diferentes em restaurações estéticas denitivas. Material e Métodos: Um total de

120 espécimes foram preparados para teste de estabilidade de cor (n=60). Os espécimes foram divididos em três

grupos principais de acordo com o tipo de cimento resinoso. Grupo 1: polimerização dupla sem amina, Grupo 2:

fotopolimerização, Grupo 3: polimerização dupla contendo amina. Cada grupo foi ainda subdividido de acordo

com a translucidez da cerâmica em dois subgrupos: alta e baixa translucidez. A estabilidade da cor foi avaliada

por um espectrofotômetro antes e após o envelhecimento térmico. Para a avaliação do grau de conversão (n=60),

a espectroscopia de infravermelho com transformada de Fourier foi usada em três intervalos de tempo diferentes.

A análise estatística foi realizada usando ANOVA multifatorial, seguida de ANOVA um faot com correção de

Bonferroni. Resultados: O cimento resinoso contendo amina apresentou ΔEab e ΔE00 signicativamente maiores

Braz Dent Sci 2023 Apr/Jun;26 (2): e3614

Meleka NM et al.

Color stability and degree of conversion of amine-free dual cured resin cement used with two different translucencies of lithium disilicate ceramics

Meleka NM et al.

Color stability and degree of conversion of amine-free dual

cured resin cement used with two diﬀerent translucencies of

lithium disilicate ceramics

INTRODUCTION

Recently, there is an increasing demand for

esthetic restorations due to an increased esthetic

conscious society [1]. Lithium disilicate is one of

the most commonly used glass ceramics for the

fabrication of laminate veneers, attributed to its

excellent esthetic properties, adequate strength

and bonding capacities which enable its use in

thin esthetic veneers [2,3].

Laminate veneers are commonly delivered

in 0.3-0.7-mm thicknesses and they represent

a more conservative approach compared to

full coverage all-ceramic restorations allowing

superior translucency and esthetics [4,5]. Hence,

they have been highly favored as an esthetic

restoration for anterior teeth [6,7]. Nevertheless,

the nal esthetic outcome can be inuenced by

the color interaction with the underlying tooth

substrate and luting resin cement [8-10].

Resin cements are the materials of choice

for bonding ceramics to dental substrates and

they are considered an essential part for the

success and quality of esthetic treatments [11].

Resin cements are classied according to their

method of activation into chemically cured, light-

cured and dual-cured. In chemically-cured resin

cements, color instability and lack of control over

the working time make clinicians prefer to use

either light-cured or dual-cured resin cements in

luting of restorations [12].

Light-cured resin cements are preferred

for luting ceramic veneers due to their good

color stability, less possibility for air bubble

incorporation and absence of tertiary amine

chemical initiator. However, the possible

attenuation of the light intensity by the thickness,

variable degrees of translucency and type of

ceramic may be a shortcoming [13].

Dual cured resin cements have the

advantages of both light and chemically cured

resin cements. They exhibit extended working

time due to light-controlled polymerization in

addition to the chemical activators that ensure

a high degree of polymerization in deeper areas

where light cannot penetrate [14]. Moreover, it

has been reported that they have a higher degree

of conversion in comparison to the light cured

cements [15].

Color stability of resin cements is a common

problem, particularly when used with thin

translucent restorations such as laminate

veneers [16-18]. There are intrinsic and extrinsic

factors that could affect the color stability of resin

cements. The extrinsic factors such as smoking,

beverages and food components may have the

potential to stain restorative materials [19].

The intrinsic factors are material-related

such as the composition of the resin matrix

and llers, ller particle size distribution, type

of photo-initiator system, as well as degree of

conversion percentage [20]. In addition, thermal

change, UV irradiation, and humidity may

cause intrinsic discoloration by physicochemical

reaction. However, the evaluation of the color

stability of resin cements after thermocycling

is limited [21]. Thus, resin cement should have

long-term color stability to guarantee acceptable

esthetic results [22].

For color assessment, a spectrophotometer is

used since it is among the most accurate tools [23].

In color science, a color difference formula (ΔE) is

designed to give a quantitative representation, and

there are two color difference formulas: CIELAB

and CIEDE2000. For interpretation of these data

into real-life scenarios, it should be compared to

the acceptability threshold (AT), which is the limit

above which the magnitude of color difference

will be considered as clinically unacceptable for

dental esthetics [24-27]. In addition, values above

the AT were categorized as mismatch type [a] or

moderately unacceptable (≤AT×2), mismatch

type [b] or clearly unacceptable (≤AT×3), and

mismatch type [c] or extremely unacceptable

(>AT×3) [26].

em ambas as translucidezes (4,5±0,3, 3,5±0,3 respectivamente para cerâmica de alta translucidez e 3,8±0,4,

3,0±0,3 respectivamente para baixa translucidez) do que os outros cimentos testados (p< 0,001). O maior grau

de conversão (DC) foi mostrado após 2 semanas pelo cimento resinoso dual sem amina (86,27±0,74). Conclusão:

O cimento resinoso dual sem amina pode ser uma alternativa ao cimento polimerizável na restauração de facetas

nas, uma vez que apresentou estabilidade de cor comparável e alto grau de conversão.

PALAVRAS-CHAVE

Cimento resinoso; Cor; Polimerização; Fotoativação de cimentos odontológicos; Dissilicato de lítio.

Braz Dent Sci 2023 Apr/Jun;26 (2): e3614

Meleka NM et al.

Color stability and degree of conversion of amine-free dual cured resin cement used with two different translucencies of lithium disilicate ceramics

Meleka NM et al.

Color stability and degree of conversion of amine-free dual

cured resin cement used with two diﬀerent translucencies of

lithium disilicate ceramics

To solve the discoloration problem, new dual

cured resin cements have been manufactured

without a benzoyl peroxide/amine redox initiator

system to be more color stable [28,29]. Variolink

Esthetic is one of the amine free dual cured resin

cements available in the market that was claimed

by the manufacturer to be the first entirely

amine-free with the new patented photo-initiator

Ivocerin [30-32].

The most commonly used photo-initiator

system in resin-based materials is camphorquinone

(CQ), which needs an amine co-initiator such

as ethyl 4-dimethylaminobenzoate (DMAB) to

react with and create free radicles needed for

the polymerization reaction [33]. These amine

molecules go through oxidation reactions and

result in color change. In dual cured resin cements,

more color change occurs due to the presence of

benzoyl peroxide and the tertiary amine initiator

system of the chemical part, in addition to the

oxidation of unreacted co-initiators from the light

cured part [34].

Ivocerin, the new germanium-based photo-

initiator is an alternative substitute for the

CQ-amine photo-initiator system. It does not

require amine molecules to initiate the reaction

in dental resin-based materials [35,36]. It has

been claimed to be more color stable and show

enhanced curing depths due to its high photo-

reactivity [34].

Adequate polymerization of resin cements

is crucial since low degree of conversion could

compromise their optical, physical and mechanical

properties [37]. Fourier-transform infrared

spectroscopy (FTIR) is used to measure DC%

since it is one of the most precise methods with

fast scanning capacity, good resolution, stability

and accuracy [38,39].

The thickness and opacity of lithium-disilicate

ceramic is known to compromise polymerization

of the resin-based luting composite. However,

studies regarding the effect of the ceramic

interposition have focused mainly on traditional

resin cements [40,41]. Currently, the debate

is whether the amine free dual-cured resin

cement would be as efcient as the light cured

one considering color stability and degree of

conversion when used with thin translucent

esthetic restorations [29,42].

Accordingly, the research hypotheses of the

current study are:

1. There is no difference in the color stability

and degree of conversion of resin cements

with different photo-initiator systems used

in cementation of thin esthetic veneers.

2. The different translucencies of lithium

disilicate ceramics have no effect on the nal

color stability of thin esthetic veneers.

MATERIAL AND METHODS

The materials used in the present study, their

composition, manufacturers and lot numbers are

presented in Table I.

Specimens’ grouping

A total of 120 specimens were prepared for

the present study: 60 bovine-bonded ceramic

specimens were prepared for color stability

assessment and 60 resin cement specimens were

prepared and light cured through the ceramic

specimens for the degree of conversion assessment.

For each test, specimens were divided into three

main groups (n=20) according to the resin cement

type. Group 1 (G1): Variolink Esthetic amine-free

dual cured resin cement, Group 2 (G2): Variolink

Esthetic light cured resin cement and Group

3 (G3): Variolink N amine-containing dual cured

resin cement. Each group was further subdivided

into two subgroups (n=10), according to ceramic

translucency: high translucent ceramic (HT) and

low translucent ceramic (LT) (Figure 1).

Color stability specimens’ preparation

Bovine teeth preparation

Sixty bovine incisors were collected and

selected with intact buccal enamel and average

crown widths of 12 to 15 mm. The teeth were

cleansed of soft tissue remnants and debris with

a sharp scalpel, stored in water in order to remain

hydrated, simulating the oral clinical situation of

human teeth [16].

The roots were then removed 1 mm below

the cemento-enamel junction (CEJ), using a high-

speed handpiece. After separating the roots from

their respective crowns, the enamel of the buccal

surface of each tooth was attened and polished

with 320, 600 and 1200 grit SiC abrasive papers

(3M of Brazil, Sumare, Brazil), under running

water for 1 min [16,18].

Braz Dent Sci 2023 Apr/Jun;26 (2): e3614

Meleka NM et al.

Color stability and degree of conversion of amine-free dual cured resin cement used with two different translucencies of lithium disilicate ceramics

Meleka NM et al.

Color stability and degree of conversion of amine-free dual

cured resin cement used with two diﬀerent translucencies of

lithium disilicate ceramics

Each prepared tooth was embedded in a

polyvinyl mold lled with chemically cured acrylic

resin (Acrostone, Egypt), with the exposed at

buccal surface ushing with the upper border of

the mold to maintain the veneer in a horizontal

level for cementation and color assessment [6,16].

Ceramic specimens’ preparation

To simulate the clinical use of laminate

veneers, thin ceramic specimens were fabricated

from lithium disilicate ceramic CAD/CAM blocks

(IPS e-max CAD), shade A2 with two different

translucencies: high translucent (HT) and low

translucent (LT) [6].

IPS e-max CAD blocks were milled into

10 mm diameter cylinders, then each cylinder

was sectioned using a diamond-coated low speed

precision saw (Isomet 1000, Buehler, Germany),

under copious water coolant, producing disc shaped

specimens of 10 mm diameters and 0.3 ± 0.05 mm

Table I - Materials used in the present study, their composition, manufacturer and lot numbers

Material Description Composition Manufacturer Lot number

Group 1:(G1)

Variolink Esthetic DC

Amine-free dual cured

resin cement

Matrix: UDMA, methacrylate monomers

Inorganic ﬁllers: ytterbium triﬂuoride and

spheroid mixed oxide

Ivocerin photo initiator and hydroperoxide/

thiocarbamide self-cure initiator system

Additional ingredients: stabilizers, pigments

Ivoclar/

Vivadent,

Switzerland

W95564

Shade: Neutral

Group 2:(G2)

Variolink Esthetic LC

Light cured resin

cement

Matrix: UDMA, methacrylate monomers

Inorganic ﬁllers: ytterbium triﬂuoride and

spheroid mixed oxide

Ivocerin photo initiator

Additional ingredients: stabilizers, pigments

Ivoclar/

Vivadent,

Switzerland

Y42601

Shade: Neutral

Group 3:(G3)

Variolink N DC

Amine containing dual

cured resin cement

Base: Barium glass ﬁller and mixed oxide,

ytterbium triﬂuoride, dimethacrylates (BisGMA,

UDMA, and TEGDMA) CQ-amine photoinitiator,

stabilizers and pigments.

Ivoclar/

Vivadent,

Switzerland

Base X51772

Shade: Transparent

Catalyst: Barium glass ﬁller and mixed oxide,

ytterbium triﬂuoride, dimethacrylates, benzoyl

peroxide initiator, stabilizers and pigments.

Catalyst

X49378

IPS e.max CAD/CAM

Blocks:

High translucent (HT)

Low translucent (LT)

Lithium disilicate glass-

ceramic blocks

Shade: A2

SiO2: 57–80%; Li2O: 11–19%; in addition to K2O;

P2O5; ZrO2; ZnO; Al2O3; MgO; coloring oxides.

Ivoclar/

Vivadent,

Switzerland

(HT) Y52153

(LT) Z0047Y

Figure 1 - Specimens’ grouping ﬂowchart

Braz Dent Sci 2023 Apr/Jun;26 (2): e3614

Meleka NM et al.

Color stability and degree of conversion of amine-free dual cured resin cement used with two different translucencies of lithium disilicate ceramics

Meleka NM et al.

Color stability and degree of conversion of amine-free dual

cured resin cement used with two diﬀerent translucencies of

lithium disilicate ceramics

thickness which mimics the lowest thickness

that can be used for ceramic veneers. The nal

thickness of each sectioned slice was veried using

a digital micrometer (Digimatic Micrometer Series

293 MDC-MX Lite, Mitutoyo, Japan).

Ceramic surfaces were then polished using

320, 600 and 1200 grit SiC abrasive papers (3M do

Brazil, Sumare, Brazil). All the ceramic specimens

were sintered in a ceramic furnace (Programat CS,

Ivoclar Vivadent), at 850 °C for 10 min following

the manufacturer’s recommendations [9].

Cementation

All cementation procedures were performed

according to the manufacturer’s instructions.

The enamel surface of each bovine tooth was

etched with 37% phosphoric acid (CharmEtch,

Korea) for 30 s, then washed for 30 s under running

water and air-dried. A layer of adhesive (All bond

universal adhesive, Bisco, USA), was applied and

light cured for 20 s using a LED light curing unit

(3M™ ESPE Elipar™ S10, USA), with an output

intensity of 1200 mW/cm

. The ceramic specimens

were etched with 9.5% hydrouoric acid (Porcelain

etchant, Bisco, USA) for 90 s, then washed under

running water for 30 s and air-dried for 30 s. Silane

coupling agent (Monobond plus, Ivoclar Vivadent),

was applied in a thin coat for 60 s. The luting

material was applied on the treated surface of

each ceramic specimen and carefully seated on

the prepared enamel surface, covered with a clean

glass slab with a 1kg weight on top for 20 s to form

a standardized thickness of cement layer [19].

Next, nal curing was performed for 20 s.

Color stability measurement

Color measurements were performed

for each specimen before and after aging

using a spectrophotometer (Agilent Cary

5000 UV-Vis-NIR, USA). Aging was performed

in the present study using a thermocycling

machine (Thermo-Fisher Scientic, USA), for

5000 cycles at temperatures varying between 5 °C

and 55 °C, with a dwell time of 30 s and transfer

time of 5 s [21]. For each specimen, the average

of three measurements were recorded and used

in the data analysis. The CIE-lab coordinates

were used to calculate the color difference (∆E)

between the baseline color measurement (before

thermocycling) and the nal color measurement

(after thermocycling), by applying the following

two equations [16,24]:

( ) ( ) ( )

* * *



∆







= ∆ +∆ +∆

222

ELab

(1)

where L* corresponds to the degree of lightness

and darkness, whereas a* and b* coordinates

correspond to +a*=red, -a*=green and

+b*=yellow, -b*=blue, respectively.

( ) ( ) ( ) ( )

/ / / * / *∆ = ∆ +∆ +∆









∆∆



22 2

E L KL SL C KC SC H KH SH RT C H SC SH

(2)

where: ΔL, ΔC and ΔH are the differences in

lightness, chroma and hue for a pair of samples

in CIEDE2000, and RT is a rotation function that

accounts for the interaction between chroma and

hue differences in the blue region. The SL, SC,

and SH weighting functions adjust the total color

difference for variation in the location of the color

difference pair at the L, a, and b coordinates, and

the kL, kC, and kH parametric factors are correction

terms for experimental conditions. In the present

study, the kL, kC, and kH were set to 1 [24].

The resultant ∆E values were compared

to the 50:50% perceptibility threshold (PT),

and 50:50% acceptability threshold (AT).

The corresponding CIELAB visual thresholds were

1.2 and 2.7 and for CIEDE2000, were 0.8 and

1.8, respectively.

Degree of conversion specimens’ preparation

Sixty resin cement specimens were prepared

in a split Teon mold (1 mm thickness x 2 mm

inner diameter), supported by a metal ring.

The G1 and G2 resin cements were packed

directly into the mold, while for G3, an equal

amount of base and catalyst pastes were mixed

according to the manufacturer’s instructions and

packed into the mold. A Mylar strip (Quimidrol;

Joinville, SC, Brazil), was then placed on the

top surface of the mold to ensure smoothness of

the specimens, allowing the ceramic disc to be

easily placed over the resin cement and to avoid

inhibition of polymerization by oxygen [37].

The ceramic specimen was then placed

over the mylar strip. All curing procedures were

performed through the thin ceramic specimen for

40 s to simulate the clinical condition. All cured

resin specimens were dry stored in light-proof

containers to avoid additional exposure to light.

Degree of conversion measurement

For degree of conversion (DC) testing,

attenuated-total-reectance/Fourier Transform

Infrared spectroscopy (ATR/FTIR VERTEX

Braz Dent Sci 2023 Apr/Jun;26 (2): e3614

Meleka NM et al.

Color stability and degree of conversion of amine-free dual cured resin cement used with two different translucencies of lithium disilicate ceramics

Meleka NM et al.

Color stability and degree of conversion of amine-free dual

cured resin cement used with two diﬀerent translucencies of

lithium disilicate ceramics

70, Bruker; Ettlingen, Baden, Wurttemberg,

Germany), was used at different time intervals;

immediately after curing, after 24 hours and

after 2 weeks [14]. The absorbance spectrum

was acquired by scanning the specimens over a

1638-1608 cm

-1

range. The DC was calculated

using the following equation [39]:

( )

( ) ( )

% / / /





=−×



aliphatic aromatic aliphatic aromatic

DC 1 C C U U 100

(3)

aliphatic

is the absorption peak at 1638 cm

-1

, C

aromatic

is the absorption peak at 1608 cm

-1

of the cured

specimen. U

aliphatic

is the absorption peak at 1638,

aromatic

is the absorption peak at 1608 cm

-1

of the

uncured specimens.

Statistical analysis and sample size

calculation

Numerical data were explored for normality

and variance homogeneity using the Shapiro-

Wilk and Leven’s tests, respectively. Data showed

parametric distribution and homogeneity of

variances across the groups, so they were

represented as mean and standard deviation

(SD) values and were analyzed using the multi-

factorial ANOVA, followed by one-way ANOVA

for analysis of simple effects with Bonferroni

correction. The significance level was set at

p≤0.05 within all tests. Statistical analysis was

performed with the R statistical analysis software

version 4.0.3 for Windows.

Based on the results of a previous study [34].

Sample size calculation was performed using

G*Power version 3.1.9.7 [43]. By adopting

an alpha level of (0.05) a beta of (0.2) i.e.,

power = 80% and an effect size (f) of (0.486).

The predicted sample size was a total of (60)

samples for each test, and (n=10) per each

experimental subgroup and this was statistically

approved.

RESULTS

Color stability

Two-way ANOVA for ΔE

revealed a

signicant effect for the cement type and ceramic

translucency on color stability, however, the

interaction between the two variables was

insignicant with a p value= 0.26. Mean and

standard deviation (SD) values of color change

(ΔE

) for different cement types and ceramic

translucencies are presented in Table II. Pairwise

comparisons for the effect of cement type

showed that for both the HT and LT ceramic

specimens, G3 showed signicantly higher ΔE

values compared to the other tested groups

(p<0.001)

, where G1 and G2 revealed no

statistically significant difference. Regarding

ceramic translucency, the HT ceramic specimens

had signicantly higher mean ΔE values than the

LT ceramic specimens in all tested groups.

When interpreting the results of the present

study with 50:50% acceptability thresholds

(AT) for the CIELab system, it was revealed that

G1 and G2, only when used with low translucent

ceramics, had ΔE

values below 2.7 and therefore

were considered clinically acceptable.

While for ΔE

values presented in Table III,

it was revealed that for G1 and G3 (amine free

and amine containing dual cured resin cements),

high translucency samples had significantly

higher values than low translucency samples

(p<0.05).

For high translucency samples,

there were significant differences between

values of different cements

(p<0.001)

. For low

translucency samples, there was also a signicant

difference with amine containing DC, having a

signicantly higher value than the other cements

(p<0.001)

. Interpreting these results with the

CIEDE2000 50:50% acceptability thresholds

(AT), all groups had ΔE

values above 1.8 and

Table II - Mean ± standard deviation (SD) of color change (ΔE

) values for diﬀerent types of cements and ceramics

Cement type

Color change (ΔEab) (Mean±SD)

p-value

High translucency Low translucency

G1: Amine free DC 3.2±0.5

2.3±0.2

<0.001*

G2: LC 2.7±0.1

2.2±0.1

0.011*

G3: Amine containing DC 4.5±0.3

3.8±0.4

0.001*

p-value <0.001* <0.001*

Means with diﬀerent superscript letters within the same vertical column are statistically signiﬁcantly diﬀerent. *Signiﬁcant (p<0.05).

Braz Dent Sci 2023 Apr/Jun;26 (2): e3614

Meleka NM et al.

Color stability and degree of conversion of amine-free dual cured resin cement used with two different translucencies of lithium disilicate ceramics

Meleka NM et al.

Color stability and degree of conversion of amine-free dual

cured resin cement used with two diﬀerent translucencies of

lithium disilicate ceramics

were considered clinically unacceptable for both

ceramic translucencies.

Results of the correlation between different

color measurements presented in Table IV

showed that there was a strong positive

correlation between both measurements, which

was statistically signicant (r=0.980,

p<0.001

Degree of conversion (DC)

Multiple repeated measures ANOVA showed a

signicant effect for time and cement type for the

mean DC values; however, the effect of ceramic

translucency and the interaction between the three

variables was insignicant. Statistical analysis of

the DC results recorded the highest mean DC% at

2 weeks’ time interval followed by 24 hours, while

those recorded immediately after curing showed

the lowest mean values. For both G1 and G3 dual

cured resin cements groups, there was a remarkable

increase in DC in the rst 24 hours compared to

the G2 light cured group. After 2 weeks, all groups

revealed comparable increase in DC values.

Mean and standard deviation (SD) values

for degree of conversion % for different cements

and ceramic translucencies are shown in Table V.

Immediate records for both the HT and LT ceramic

specimens presented signicant differences between

the different cement types

(p<0.001)

. G2 showed

the highest mean value followed by G1, while

G3 showed the lowest mean value. Records after

24 hours and after 2 weeks showed no signicant

difference between the G1 and G2 cements, while

the G3 cement showed the lowest mean value for

both ceramic translucencies

(p<0.001).

There was

no statistically signicant difference between the

HT and LT ceramic specimens regarding DC mean

values for the different tested groups.

DISCUSSION

Color is one of the integral parts of esthetic

dentistry that affects patients’ satisfaction and

self-esteem. Color stability of resin cements is

one of the most challenging issues that clinicians

encounter especially with thin translucent

ceramic restorations. Since the color of resin

cements influence the final color of ceramic

restorations, neutral shades of resin cements

were used in the present study to dismiss their

inuence on the nal color. The thermocycling

process was chosen as an aging method to

simulate the thermal uctuation in the intraoral

conditions using 5000 cycles which corresponds

to 6 months of clinical service [29].

There are two major thresholds for assessment

of color difference: perceptibility threshold

(PT) and acceptability threshold (AT). These

thresholds can serve as a guide for evaluating the

clinical performance of dental materials. The PT

represents the magnitude of color difference that

can be visually detected by a professional dental

staff, while AT is the threshold above which the

magnitude of color difference constitutes an

unacceptable limit to dental esthetics [24,26].

Despite considerable efforts, the

identification of ΔE values for the “clinically

acceptable threshold” is a difcult task and a

widely accepted limit remains controversial since

most of these thresholds were documented on the

basis of

in vitro

studies [7]. In order to satisfy the

increase of esthetic demands in recent years, a

new CIEDE2000 color change formula had been

developed to be more sensitive and comparable to

visual observation [24]. The 50:50% perceptibility

(PT) and the 50:50% acceptability (AT) thresholds

for the CIEDE2000 and CIELAB were set 0.8 and

1.2 and 1.8 and 2.7, respectively [24,26,27].

Table III - Mean ± standard deviation (SD) of color change (ΔE

) values for diﬀerent types of cements and ceramics

Cement type

Color change (ΔE

) (Mean±SD)

p-value

High translucency Low translucency

G1: Amine free DC 2.8±0.3

2.1±0.2

<0.001*

G2: LC 2.3±0.1

2.1±0.1

0.170

G3: Amine containing DC 3.5±0.3

3.0±0.3

0.001*

p-value <0.001* <0.001*

Means with diﬀerent superscript letters within the same vertical column are statistically signiﬁcantly diﬀerent. *Signiﬁcant (p<0.05).

Table IV - Correlation between the two-color change measurements

95%CI

Statistic p-value

Lower Upper

0.980 0.97 0.99 35.2 <0.001*

r = Pearson correlation coeﬃcient. *Signiﬁcant (p<0.05).

Braz Dent Sci 2023 Apr/Jun;26 (2): e3614

Meleka NM et al.

Color stability and degree of conversion of amine-free dual cured resin cement used with two different translucencies of lithium disilicate ceramics

Meleka NM et al.

Color stability and degree of conversion of amine-free dual

cured resin cement used with two diﬀerent translucencies of

lithium disilicate ceramics

The results of the present study invalidated

both research hypotheses. The results showed

that G3 amine-containing resin cement has

significantly lower color stability than the

other tested groups with ΔE

= 4.5±0.3 and

3.8±0.4 and ΔE

= 3.5±0.3 and 3±0.3 for

the HT and LT ceramics, respectively, which is

considered clinically unacceptable. This could

be attributed to its chemical composition that

includes amines in the benzoyl peroxide/amine

redox initiator system and CQ-amine photo-

initiator system. These amines are easily oxidized

causing discoloration. Moreover, the oxidation

of the residual monomers in addition to the

presence of Bis-GMA monomer could affect

color stability since they are highly susceptible

to hydrolysis [34].

These results are consistent with a previous

study in which color stability of Variolink N

cement bonded to IPS e.max Press HT ceramic was

evaluated after 384 h in an accelerated weathering

machine, which correspond to one year of

clinical service and recorded ΔE= 4.7±0.7 [32].

On the contrary, a study concluded that Variolink

N showed clinically acceptable values of

ΔE= 2.37 after 10,000 thermal cycles. This could

be attributed to the difference in the specimen

preparation protocol since they used the cement

base paste only as a light cured cement which is

free of benzoyl peroxide initiator [21].

On the other hand, there was no signicant

difference in ΔE

values between the other

two groups (G1 and G2). These groups showed

higher color stability compared to G3. This

could be explained by the presence of dibenzoyl

germanium derivative photo-initiator (Ivocerin),

which is an amine-free redox initiator/activator

system which improves material properties

including color matching and stability as a result

of low pigmentation due to shorter wavelength

range absorption. It also does not require

co-initiators and its color is not as yellow as a QC

photo-initiator [31,33].

Previous studies showed that resin cements

lacking benzoyl peroxide and an amine-redox

initiator system (amine-free), have enhanced

color stability therefore, Ivocerin is considered

to be amine-free since it imparts the bleaching

properties and enhanced color stability of

composite resin [21,35]. The color stability

results of the present study agreed with other

previous studies that evaluated the amine-free

resin cements containing Ivocerin as a photo-

initiator. This enhancement was justied by the

absence of amine and the positive effect of the

Ivocerin photo-initiator [17,28,29].

The translucency of the ceramic specimens

had a signicant effect on the nal color of the

restorations, especially in very thin ceramic

thickness of 0.3 mm as used in the present

study [7]. Results of the current study revealed

that HT ceramic specimens had significantly

higher mean values of ΔE than that of LT ceramic

specimens in all tested groups. For the HT

ceramic, both ΔE

and ΔE

values were above

the acceptability threshold in all cement types,

that’s why for the very thin veneers, HT is not

Table V - Mean ± standard deviation (SD) of degree of conversion values for diﬀerent types of cements and ceramics

Time Cement

DC% (mean±SD)

p-value

HT LT

Immediately after

curing

G1 amine-free 61.3±0.9

60.9±1.6

0.581ns

G2 light cure 74.7±0.8

75.0±0.5

0.487ns

G3 amine-containing 42.7±0.5

42.0±0.8

0.102ns

p-value <0.001* <0.001*

After 24 hours

G1 amine-free 78.7±0.4

78.7±0.8

0.913ns

G2 light cure 78.6±0.3

78.3±0.5

0.221ns

G3 amine-containing 52.3±0.9

53.4±1.0

0.051ns

p-value <0.001* <0.001*

After 2 weeks

G1 amine-free 86.2±0.4

85.7±0.8

0.152ns

G2 light cure 85.9±0.9

85.2±0.6

0.117ns

G3 amine-containing 66.3±0.8

65.7±0.6

0.154ns

p-value <0.001* <0.001*

Means with diﬀerent superscript letters within the same vertical columns are signiﬁcantly diﬀerent*; signiﬁcant (p ≤ 0.05) ns; non-signiﬁcant (p>0.05).

Braz Dent Sci 2023 Apr/Jun;26 (2): e3614

Meleka NM et al.

Color stability and degree of conversion of amine-free dual cured resin cement used with two different translucencies of lithium disilicate ceramics

Meleka NM et al.

Color stability and degree of conversion of amine-free dual

cured resin cement used with two diﬀerent translucencies of

lithium disilicate ceramics

recommended since it could easily reect the

color change of the underneath resin cement [8].

Whereas the ΔE

values of the LT ceramic

were below 2.7 and therefore, considered clinically

acceptable. However, the ΔE

values were still

above 1.8 and according to the classication of

mismatch, it was considered mismatch type (a),

which is moderately unacceptable [26]. It has

been reported that ceramic veneers with lower

translucency promote higher masking ability of

the darkened background than high translucency

veneers. In addition, it was recommended that

specied color matching standards for ceramic

veneers are needed when the thickness of veneers

is less than 0.7 mm [4].

For resin cements to reach their optimal

physical and mechanical properties, the highest

degree of conversion (DC) should be achieved.

Inadequate polymerization of a resin- based

luting agent is associated with problems such

as postoperative sensitivity, recurrent caries,

degradation, discoloration and decrease in

mechanical properties. The unreacted monomer

may leach from the polymerized material and

irritate the soft tissue. Accordingly, DC was

one of the essential parameters evaluated in

the present study [38]. In the current study,

degree of conversion was assessed using the

ATR/FTIR since this technique provides rapid

scanning, good resolution and accuracy [34].

To simulate the clinical situation, resin cements

were polymerized through ceramic discs.

Time is an important factor for evaluation

of the degree of conversion, especially for dual-

cured resin cements to indicate the maximum DC

achieved. In the current study, DC was assessed

at three different time intervals: immediately after

light curing, after 24 hours and after 2 weeks since

auto-polymerization is reported to continue up to

2 weeks [14]. Results of the current study revealed

that DC % signicantly increased with time for all

tested groups. Mean values recorded after 2 weeks

were the highest followed by the 24 hours, while

the immediate mean DC values showed the least

values, as shown in Table V. These results were in

agreement with a previous study [14].

Different cement types showed a signicant

effect on DC% with time (p<0.001). Immediately

after curing, results showed that both G1 and

G3 have significantly lower DC% compared

to G2. This may be due to the difference in

curing modes where both G1 and G3 are dual

cured cements that have a chemical activation

part which need time to reach the maximum

polymerization compared to G2, which is a light

cured cement [34].

Results of the 24 hours and 2 weeks revealed

no signicant differences between G1 and G2,

however, both groups have signicantly higher

DC% than G3. This could be explained by the

difference in chemical composition of the tested

resin cements. Groups G1 & G2 contain a novel

dibenzoyl germanium derivative photo-initiator

(Ivocerin), thus they exhibited a signicantly

higher DC % than that of the conventional

CQ- amine photo-initiator in G3. These results

are consistent with previous studies [15,34].

Ivocerin has low energy bonds, which after

cleavage, yields more active radicals and allow

photo-polymerization by high energy, short

wavelength of light curing units and do not

require amine co-initiators which can improve

material properties in dental resins [33].

The incorporation of this photo-initiator into bulk

ll composite resin has resulted in increased DC,

improved reactivity to curing light and greater

depth of cure [35].

A study evaluated the effect of different

photo-initiator systems on the efficiency of

polymerization of resin cements through ceramic

veneers and concluded that Ivocerin alone or

associated with TPO was shown to be an effective

alternative photo-initiator to substitute CQ. This

is consistent with our results and with other

previous studies [31,34].

In the literature, DC% has been reported to be

inuenced by the translucency and thicknesses of

ceramic restorations. However, in the current study,

ceramic translucency had no signicant effect on

the DC% at different time intervals and for different

types of resin cements. This could be attributed to

the thin ceramic thickness (0.3 mm) used in the

present study which could not result in signicant

light attenuation to jeopardize the polymerization

reaction [41]. The results of the current study agree

with previous studies where they concluded that

in up to or less than 1 mm thick ceramic, the DC

showed similar behavior between dual-cured and

light-cured resin cements [38,40].

Based on the results obtained in the

present study, it could be highlighted that the

nal esthetic outcome of thin ceramic veneers

could be affected by the type of resin cement

and ceramic translucency. Additionally, the

Braz Dent Sci 2023 Apr/Jun;26 (2): e3614

Meleka NM et al.

Color stability and degree of conversion of amine-free dual cured resin cement used with two different translucencies of lithium disilicate ceramics

Meleka NM et al.

Color stability and degree of conversion of amine-free dual

cured resin cement used with two diﬀerent translucencies of

lithium disilicate ceramics

polymerization quality of different resin cements

is inuenced by the modications in the photo-

initiator formulation and curing mechanisms.

Nevertheless, further investigations are required

using different resin cement/ceramic materials

combinations with different thicknesses.

CONCLUSIONS

Within the limitations of the present study,

it was concluded that:

1- The rst hypothesis was rejected since both

amine free dual cured and light cured resin

cements with the Ivocerin photo-initiator

system exhibited better color stability and

showed higher degree of conversion than

the conventional photo-initiator.

2- The second hypothesis was also rejected since

the ceramic translucency had a signicant

effect on color stability were HT ceramics

revealed less color stability than LT ceramics.

Author’s Contributions

NMM, AEB, DIEK: Conceptualization.

NMM, AEB: Methodology and Validation. NMM:

Investigation and Resources. NMM, AEB, DIEK:

Data Curation. NMM: Writing- Original Draft

Preparation. AEB, DIEK: Writing- Review &

Editing. NMM, AEB, DIEK: Visualization. AEB,

DIEK: Supervision. NMM: Project Administrator.

Conict of Interest

The authors declare that they have no

proprietary, nancial, or other personal interest

of any nature or kind in any product, service,

and/or company that is presented in this article.

Funding

The authors declare that this research did

not receive any specific grant from funding

agencies in the public, commercial, or not-for-

prot sectors.

Regulatory Statement

This study was conducted and approved by

the ethical committee of Faculty of Dentistry Ain

Shams University with ethical approval number

FDASU-RecEM 011934.

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Date submitted: 2022 Aug 14

Accept submission: 2022 Nov 09

Nora Maher Meleka

(Corresponding address)

Ain Shams University, Faculty of Dentistry, Dental Biomaterials Department, Cairo,

Egypt.

Email: NoraMeleka@dent.asu.edu.eg