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.e3959
1
Braz Dent Sci 2023 Oct/Dec;26 (4): e3959
Comparison of shear bond strength and color reproduction of two
different high-performance polymers veneered with two different
thicknesses of resin composite: an in-vitro study
Comparação da resistência ao cisalhamento e reprodução de cor de dois diferentes polímeros de alto desempenho
revestidos com duas diferentes espessuras de resina composta: um estudo in vitro
Gehad Abdelgawad AHMED1 , Amr EL-ETREBY1 , Ghada ABD EL-FATTAH1
1 - Ain Shams University, Fixed Prosthodontics Department. Cairo, Egypt.
How to cite: Ahmed GA, El-Etreby A, Abd El-Fattah G. Comparison of shear bond strength and color reproduction of two different high-
performance polymers veneered with two different thicknesses of resin composite: an in-vitro study. Braz Dent Sci. 2023;26(4):e3959.
https://doi.org/10.4322/bds.2023.e3959
ABSTRACT
Objective: to evaluate the effect of material type and veneer thickness on the nal color and shear bond strength
of PEEK bilayered restorations. Material and Methods: sixty-four square-shaped core specimens (7×7×2), were
fabricated by CAD/CAM technology from two high-strength polymers, BioHPP (High-Performance Polymer, group
B n=32), and Pekkton® ivory (PolyEtherKetoneKetone, group P n=32) then were veneered with resin composite
(crea.lign dentin A3). Specimens from each group were divided (n=16) according to the veneer layer thickness
(0.5 & 1 mm), then according to the test applied (n=8), either ΔE or shear bond strength (SBS). Specimen
surfaces were treated according to the manufacturer’s instructions. Specimens were veneered according to the
manufacturer’s instructions.The mode of failure was evaluated under a stereomicroscope at 40 x magnication
after the shear bond strength test. Results: data showed parametric distribution and variance homogeneity and
were analyzed using two-way ANOVA. The signicance level was set at p<0.05 for all tests. Statistical analysis
was performed with the R statistical analysis software version 4.3.1.Results of two-way ANOVA showed that
material types and veneer thicknesses had an individual signicant effect on the color change. For the shear
bond strength, only the sample thickness (1mm) had a signicant effect (p=0.033). The majority of samples
in different groups presented a mixed failure mode with all the differences being not statistically signicant
(p>0.05). Conclusion: the thickness of the resin composite veneer can signicantly affect the nal esthetic
outcome and shear bond strength of a bilayered restoration.
KEYWORDS
Composite resins; Polymers; Polyetheretherketone; Polyetherketoneketone; Shear strength.
RESUMO
Objetivo: avaliar o efeito do tipo de material e da espessura do revestimento na cor nal e na resistência ao
cisalhamento de restaurações em duas camadas de PEEK. Material e Métodos: foram fabricados 64 espécimes
com núcleo de formato quadrado (7x7x2) usando tecnologia CAD/CAM a partir de dois polímeros de alta
resistência, BioHPP (Polímero de Alto Desempenho, grupo B n=32) e Pekkton ivory (Polieterecetona cetona,
grupo P n=32), que foram então revestidos com resina composta (crea.lign dentin A3). Os espécimes de cada
grupo foram divididos (n=16) de acordo com a espessura do revestimento (0,5 e 1 mm), e depois de acordo
com o teste aplicado (n=8), seja ΔE ou resistência ao cisalhamento (SBS). As superfícies dos espécimes foram
tratadas de acordo com as instruções do fabricante. Os espécimes foram revestidos de acordo com as instruções
do fabricante. O modo de falha foi avaliado sob um estereomicroscópio com ampliação de 40x após o teste de
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Braz Dent Sci 2023 Oct/Dec;26 (4): e3959
Ahmed GA et al.
Comparison of shear bond strength and color reproduction of two different high-performance polymers veneered with two different thicknesses of resin composite: an in-vitro study
Ahmed GA et al. Comparison of shear bond strength and color reproduction of
two different high-performance polymers veneered with two
different thicknesses of resin composite: an in-vitro study
INTRODUCTION
PEEK is (-C6 H4 -OC6 H4 -O-C6 H4 -CO-) [1]
a polycyclic, aromatic, thermoplastic polymer that
is semi-crystalline with a linear structure [2]. It’s
obtained as a result of the binding of ketone and
ether functional groups between aryl rings [3]. It’s
highly resistant to high temperatures (over 300c°),
because of its chemical structure [4]. PEEK has
excellent chemical resistance, thermal insulation,
poor electrical and thermal conductivity, low
friction, low density (1.32g/cm3) [3] and tensile
strength of 90-100 MPa similar to that of enamel
and dentin, which makes it a suitable material
for xed prosthodontics restorations [5]. It has a
Young’s modulus of 3.6GPa, which is close to that
of bone [6], and is an opaque material with low
plaque afnity and a sterilizing capability. PEEK
becomes an alternative material for titanium
alloy in implant dental restorations (fixture,
abutments, crowns, xed & removable denture
frameworks) because it solves most problems
associated with titanium [3]. The off-white color
of PEEK requires to be veneered either with resin
composite or ceramics to mimic the natural tooth
color with better light reectivity and increased
esthetics [7].
BioHPP is a high-tech thermoplastic polymer
based on PEEK. It consists of an aromatic
backbone molecular chain interconnected by
ketone & ether functional groups with a density of
1.3-1.5 g/cm3 [8]. It contains ceramic micro llers
which occupy about 20% of its volume [5]. These
ceramic fillers increase the material strength
optimizing its mechanical properties, allowing
occlusal forces to be transmitted from the
weak organic matrix to the stronger inorganic
llers, decreasing the probability of incidence
of cracks, fracture, and plastic deformation or
even fracture [9]. It has a modulus of elasticity
(~ 4 GPa), very close to bone which makes
BioHPP a successful treatment modality in
implantology [9]. It permits the chewing forces to
be easily transmitted between the BioHPP implant
and surrounding bone (shock-absorbing action),
decreasing the risk of fracture [8]. BioHPP could
be processed by modern CAD/CAM technology
and the conventional lost wax method [10].
PEKK is a new material produced
by Cendres+Metaux, a top product among
thermoplastics. It was launched for xed dental
prosthesis due to the double ketone bond
in its chemical structure which increases its
compressive strength up to 80% higher than
PEEK [11]. PEKK is characterized by a crystalline
and amorphous structure which improves its
mechanical and chemical properties [12]. BioHPP
& PEKK polymers are of great interest for dental
applications. Despite their superior properties,
their opaque color is considered a limitation to
be used as monolithic restorations [13].These
polymers must be veneered either with ceramic
(indirectly) or with resin composite (directly or
indirectly), to enhance the nal esthetic outcome
of dental restorations.
In the present study, direct composite
veneers were applied on a core of BioHPP
&PEKK with two different thicknesses. Resin
composite material has many benefits when
used as a veneering material, such as improving
the esthetics of restorations made of opaque
white polymers to a degree that could mimic the
neighboring tooth structure, being less abrasive
to natural teeth, being bio-compatible with
surrounding tissues and could be easily repaired
in clinics or labs [14].
According to previous studies, only limited
information is available on the effect of varying
the thickness of resin composite veneer in
resistência ao cisalhamento. Resultados: os dados apresentaram distribuição paramétrica e homogeneidade
de variância e foram analisados por ANOVA de duas vias. O nível de signicância foi estabelecido em p<0,05
para todos os testes. A análise estatística foi realizada com o software de análise estatística R, versão 4.3.1. Os
resultados da ANOVA de duas vias mostraram que os tipos de materiais e as espessuras do revestimento tiveram
um efeito signicativo individual na mudança de cor. Para a resistência ao cisalhamento, apenas a espessura da
amostra (1mm) teve um efeito signicativo (p=0,033). A maioria das amostras em diferentes grupos apresentou
um modo de falha misto, com todas as diferenças não sendo estatisticamente signicativas (p>0,05). Conclusão:
a espessura do revestimento de resina composta pode afetar signicativamente o resultado estético nal e a
resistência ao cisalhamento de uma restauração em duas camadas.
PALAVRAS-CHAVE
Resinas compostas; Polímeros; Polieteretercetona; Polietercetonacetona; Resistência ao cisalhamento.
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Braz Dent Sci 2023 Oct/Dec;26 (4): e3959
Ahmed GA et al.
Comparison of shear bond strength and color reproduction of two different high-performance polymers veneered with two different thicknesses of resin composite: an in-vitro study
Ahmed GA et al. Comparison of shear bond strength and color reproduction of
two different high-performance polymers veneered with two
different thicknesses of resin composite: an in-vitro study
bi-layered restorations regarding shear bond
strength. The mechanical performance of
bi-layered restorations depends not only on the
nature of the veneering material used but also on
the thickness of the material. It was shown that
a thicker veneer yields a lower value of exural
strength [15].
According to the polymer pyramid,
high-performance polymers (HPP) and PEKK
(PolyEtherEtherKetone), belong to the highest-
performance plastics based on their chemical
structure which is made up of repeating units of
macro-molecules that give rise to a long polymeric
chain structure in 3 dimensions, the process that
provides them with superior mechanical and
biological properties. Because of their inferior
coloration, they can’t be used to restore anterior
teeth for the sake of esthetics unless they are
veneered with highly esthetic material such as
resin composite [16]. Thus, it was a goal to
study the effect of varying thicknesses of resin
composite on the ΔE color factor.
Recently, the utilization of PEKK in the medical
and dental elds has increased due to its promising
high biomechanical properties such as compressive,
tensile, and exural strength. The reason is the
addition of ketone groups within its molecular
structure which makes the material more versatile
in surface modication, bonding, and improved
temperature compared to PEEK [16].
High-strength polymers present low surface
energy and resistance to any surface modication
by different chemical treatments, so it becomes
difcult to achieve adequate bond strength with
resin composite veneers without being classically
treated rst [17]. Different methods of surface
treatments were discussed by previous studies
to show the effects of mechanical and chemical
treatments on PEEK surfaces. A scoping review
by Machado et al. [18] discussed the available
surface treatments and adhesives for PEEK to
increase its bond strength with resin-based
materials. It was concluded that sulphuric
acid etching and alumina particle air abrasion
followed by applications of bonding agents
containing MMA, PETIA, and dimethacrylates are
the most effective choices to increase resin-based
materials’ adhesion to PEEK.
Spectrophotometers are the most widely
used devices in measuring color for research work
because these devices enable the dentist to perform
an accurate and reliable objective analysis [19].
The spectrophotometer functions on the principle
of measuring light energy reected from an object
in the visible spectrum [20]. It contains an optical
system for measuring, a detector, and a means of
converting the light obtained into a signal that
could be analyzed [21].
Since spectrophotometers can detect small
color differences at a level that is not possible
with the human eye, an important issue of color
science in dentistry is to establish a reference
value for the evaluations of results detected by the
color device in terms of ΔE. Thus, it’s important to
understand whether this color difference can be
considered clinically relevant or not. If the ΔE is
greater than 1 and less than 3.3, it’s considered to
be detectable by a skilled operator, but clinically
accepted. But if the ΔE is greater than 3.3, it’s
regarded to be unacceptable because it could be
detected by untrained observers [22].
A study by Shiraishi and Watanabe [23]
mentioned that the average transmittance of light,
translucency, and opalescence parameters of the
ceramics are signicantly affected by the type
of ceramic and its thickness. When light passes
through a material, it gradually loses intensity
and interacts in 2 ways, either scattering and/
or absorption. Thus, the larger the thickness of
the material, the greater the degree of scattering
and/or absorption of light.
Therefore, the purpose of the present in vitro
study was to evaluate the color reproduced from
veneering two different thermoplastic polymeric
materials (BioHPP & PEKKTON), with two
different thicknesses of Crea.lign resin composite
material (0.5 & 1mm), followed by an evaluation
of the shear bond strength. The hypothesis
is null since there was no difference in shear
bond strength and color reproduction between
different tested groups concerning the material
and thickness.
MATERIALS AND METHODS
Preparation of specimens
Sixty-four Specimens of BioHPP® (Bredent
GmbH & Co.KG, Germany), in addition to PEKK
(Cendres+Métaux), were cut from BreCAM blank
and Pekkton® ivory blank, respectively (Table I).
A design of rectangular-shaped blocks with
dimensions of 7×7×16 mm (5 blocks for each
material), was designed by a software program
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Braz Dent Sci 2023 Oct/Dec;26 (4): e3959
Ahmed GA et al.
Comparison of shear bond strength and color reproduction of two different high-performance polymers veneered with two different thicknesses of resin composite: an in-vitro study
Ahmed GA et al. Comparison of shear bond strength and color reproduction of
two different high-performance polymers veneered with two
different thicknesses of resin composite: an in-vitro study
(Exocad GmbH, Germany). The STL les were
exported to the milling machine to fabricate the
blocks of both materials. Blocks were embedded
into ready-made plastic mold tubes (Ø13mm
20 mm). Cold cure acrylic resin material was
injected into the tubes & left to bench cured
for 2 hours (This step was done to facilitate
the sectioning step of the peek blocks). Blocks
were sectioned with a water-cooled slow-speed
precision saw (IsoMet 4000; Buehler). The sample
size was determined using the R statistical
analysis software version 4.1.3 for Windows (R
Foundation for Statistical Computing, Vienna,
Austria. URL https://www.R-project.org/.) Thirty
two specimens from each material were ready to
form the core parts. The PEEK core disc thickness
was veried using an electronic digital caliper
(INSIZE, Jiangsu, China)
Air borne-particle abrasion with 110- μm
aluminum oxide grit at a distance of (10-
15 mm), with a pressure of 5 to 6 bars in a
sandblasting device (Oxyker TRIO, Manfredi,
Italy) was performed for 20 seconds for each
sample. Subsequently, the discs were cleaned
ultrasonically in a bath with deionized water
(L&R, Kearny, NY, USA)
Veneering with resin composite
Resin composite (crea.lign, Bredent, Senden,
Germany), and dentin shade A3 were selected.
Custom-made split Teflon metal molds were
designed with 2 designs according to the applied
test. The first design (for color reproduction
testing), is square in shape with dimensions of
7×7×2.5 mm and 7×7×3 mm to apply 0.5 &
1 mm composite layers respectively. The second
design (for SBS testing), is circular in shape
Ø3×0.5 mm and Ø3×1 mm to standardize
0.5 & 1 mm resin composite layers. Thicknesses
were veried using an electronic digital caliper
(INSIZE, Jiangsu, China).
Air-abraded surfaces of all discs ware
then conditioned with a thin layer of Visio.link
(Bond-lign; bredent GmbH & Co KG), and light
polymerized (bre-Lux Power Unit, bredent GmbH
& Co KG), for 90 seconds (wavelength range
370 - 400 nm), according to the manufacturer’s
Recommendations.
For the color reproduction test, square-
shaped discs, placed into the Teon metal mold,
were additionally coated with a thin layer of
Crea.lign Opaker (Bredent GmbH, Germany), and
light-cured for 360 seconds (Brelux Power Unit).
Crea.lign paste (Bredent GmbH, Germany), of
A3 shade was then injected to ll the mold where
the nal thickness of each bi-layered specimen
was determined by the mold thickness. The nal
polymerization was performed for 360 seconds
according to the manufacturer’s instructions.
For the shear bond strength test, veneering
resin composite (Crea.lign; bredent GmbH & Co
KG), was injected into circular shaped molds
located at the center of the PEEK surface. To place
a uniform thickness, a smooth and bubble-free
layer of composite, a clean glass slab was added
over the composite layer to be initially cured for
15 seconds using hand-held lamp light curing
followed by final polymerization in a Bre-lux
power2 unit box for 360 seconds, according to
manufacturer’s instructions. .
Finally, the color specimens were polished
according to the manufacturer’s instructions
(Toolkit, Bredent GmbH, Germany).
All specimens were stored in distilled
water for 24 hours at 37c°according to ISO
recommendation (International Organization for
Standardization), thermo cycled for 5000 cycles
between 5c° and 55c° with a 30-second dwell
time in a thermocycling machine (Thermocycler
THE-1100; SD Mechatronics).
Spectrophotometric analysis
Color (∆E) measurement was carried out
against a white background in an Agilent Cary
5000 UV-Vis-NIR spectrophotometer (Agilent
Technologies, USA). It is a double-beam direct
ratio recording system based on the emission
of a light beam from a tungsten halogen lamp.
The light beam passes through the double
Table I - Sample grouping
Acc. For material Biohpp (Gp B) n=32 Pekk (Gp P) n=32
Acc. For composite thickness Gp I n=16 (0.5 mm) Gp II n=16 (1mm) Gp III n=16 (0.5 mm) Gp IV n=16 (1 mm)
Acc. For testing SBS Sub-Gp I n=8 Sub-Gp II n=8 Sub-Gp III n=8 Sub- Gp IV n=8
ΔE Sub-Gp I n=8 Sub-Gp II n=8 Sub-Gp III n=8 Sub- Gp IV n=8
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Braz Dent Sci 2023 Oct/Dec;26 (4): e3959
Ahmed GA et al.
Comparison of shear bond strength and color reproduction of two different high-performance polymers veneered with two different thicknesses of resin composite: an in-vitro study
Ahmed GA et al. Comparison of shear bond strength and color reproduction of
two different high-performance polymers veneered with two
different thicknesses of resin composite: an in-vitro study
monochromator, and then it is chopped by
a chopper mirror into the sample beam and
reference beam. The light beam is then detected
by a photo-multiplier which is sensitive to the
visible/ultraviolet region where the wavelength
scan in our measurements was carried out from
380 nm to 780 nm.
CIELab color parameters for each specimen
were then calculated from the diffuse reectance
data by using the color software application and
the ∆E was calculated via the equation:
( ) ( ) ( )
*1 * 2 ² *1 * 2 ² *1 * 2 ² ½EL L a a bb∆−
= +− +
−
(1)
where L* is a measure of the lightness of an
object, ranging from 0 (black), to 100(white).
The a*coordinate is a measure of redness (a
measure of HUE along the red-green axis), where
positive a* is related to the amount of redness,
while negative a* is related to the amount of
greenness. The b* coordinate is a measure of
chroma along the yellow-blue axis where positive
b* is related to the amount of yellowness, and
negative b* is related to the amount of blueness
of the specimen.
Shear bond strength evaluation (SBS)
The shear bond strength was measured with
a universal testing machine (Model 2719-113;
Instron Corp), at a 1-mm/min crosshead speed
and converted to MPa by using the equation
s=F/S, where s is the shear bond strength, F the
load (N) at failure, and S the surface area of the
PEEK core/veneering composite resin interface
(mm2) N/mm2 =MP.
Readymade plastic mold tubes of 13 mm
and 10 mm thicknesses were used to fix the
specimens. Cold cured acrylic resin was poured
into the molds and left to bench cure. Before it
reached to the nal curing stage, the specimen
core was xed into its top part until complete
curing. The samples were positioned in the
machine’s lower jaw so that it was parallel to the
direction of the shear force. The testing device’s
upper moveable compartment was attached
to a stainless-steel rod with a mono-beveled
chisel conguration, and this rod was precisely
positioned on the interface. The universal testing
machine displayed the shear force at fracture in
Newton (N) (the force level at which the specimen
debonds), using a 2.5 kN load cell connected to
a computer. By dividing the fracture load (F) in
Newton by the bonded surface area (A) in mm2,
the SBS in megapascals (MPa) was computed.
Failure mode analysis
The tested specimens were collected and
visually inspected under a stereomicroscope (Zeiss
Discovery V20; Zeiss) at × 40 magnications to
determine the failure mode. An adhesive failure
mode at the PEEK/ veneering composite resin
interfaces in addition to a mixed failure mode
within the PEEK and the veneering composite
resin was observed. Cohesive failures were not
observed (Figure 1).
Statistical analysis
Categorical data were presented as frequency
and percentage values and were analyzed
using Fisher’s exact test. Numerical data were
presented as mean and standard deviation (SD)
values. Normality and homogeneity of variances
assumptions were tested using the Shapiro-
Wilk’s and Levene’s tests, respectively. Data
showed parametric distribution and variance
homogeneity and were analyzed using two-
way ANOVA. The signicance level was set at
p<0.05 for all tests.
Figure 1 - Representative stereomicroscopic images representing failure between the veneering composite resin and PEEK specimen (Adhesive
& Mixed).
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Braz Dent Sci 2023 Oct/Dec;26 (4): e3959
Ahmed GA et al.
Comparison of shear bond strength and color reproduction of two different high-performance polymers veneered with two different thicknesses of resin composite: an in-vitro study
Ahmed GA et al. Comparison of shear bond strength and color reproduction of
two different high-performance polymers veneered with two
different thicknesses of resin composite: an in-vitro study
RESULTS
Descriptive statistics for color change and
shear bond strength values are presented in
Table II and in Figures 2-4. Results of two-way
ANOVA presented in Table III showed that
both the material type and sample thickness
had an individual signicant effect on the color
change (i.e., with group (P) and 0.5 mm thick
samples having signicantly higher color change)
(p<0.001). However, the interaction effect was
not statistically signicant (p=0.261). For the
shear bond strength, only the sample thickness
had a significant effect, with the 1 mm thick
samples having significantly higher strength
values (p=0.033). Summary statistics and results
of intergroup comparisons for failure mode
distribution are presented in Table IV. Results
showed the majority of samples in different
groups to have a mixed failure mode with all
the differences being not statistically signicant
(p>0.05).
DISCUSSION
Many studies have pointed out the potential
of PAEK (Polyaryletherketone) materials in dental
applications since their mechanical properties
are close to those of human hard tissue and
bone, making them a good substrate for dental
restorations and teeth [24].
Currently, dental appearance and esthetics
constitute a signicant concern for the practitioner
and are a major requirement for the patient.
This has encouraged many researchers to study
different esthetic parameters and factors that
affect optical properties such as translucency,
the color of the core, and thickness of the
Table II - Descriptive statistics
Measurement Material Thickness Mean
95% CI
SD Min. Max.
Lower Upper
Color change
(ΔE)
(B) 0.5 mm
2.75 2.54 2.96 0.26 2.42 3.11
1 mm
2.14 1.97 2.31 0.21 1.87 2.34
(P) 0.5 mm
3.26 3.08 3.44 0.23 2.98 3.55
1 mm
2.46 2.35 2.57 0.14 2.32 2.69
Shear bond
strength (MPa)
(B) 0.5 mm
11.99 10.94 13.04 1.31 10.06 13.88
1 mm
14.43 12.39 16.47 2.55 10.31 16.93
(P) 0.5 mm
12.74 12.13 13.36 0.77 11.43 13.60
1 mm
14.23 11.86 16.60 2.96 10.48 18.69
95%CI = 95% confidence interval for the mean; SD = standard deviation; Min. = minimum; Max. = Maximum.
Figure 2 - Box plot showing color change (ΔE) values.
Figure 3 - Box plot showing shear bond strength (MPa) values.
Figure 4 - Stacked bar chart showing failure modes’ distribution.
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Braz Dent Sci 2023 Oct/Dec;26 (4): e3959
Ahmed GA et al.
Comparison of shear bond strength and color reproduction of two different high-performance polymers veneered with two different thicknesses of resin composite: an in-vitro study
Ahmed GA et al. Comparison of shear bond strength and color reproduction of
two different high-performance polymers veneered with two
different thicknesses of resin composite: an in-vitro study
restorative material [25]. It was clear in other
studies that the comparison between two high-
strength polymeric materials such as BioHPP
and PEKK lacks information in terms of their
difference in color parameters and their effect
as core materials on the nal esthetic outcomes.
Through the results of the present study, the
null hypothesis was rejected since both the type
of core material and thickness of the veneering
material had a signicant impact on the color
reproduction (p<0.001), while only the resin
composite thickness had a signicant effect on
the shear bond strength (p=0.033).
The ΔE values in the present study showed
that the CIELab parameters for the Pekkton
veneered group, have statistically higher values
than the BioHPP veneered group (p value<0.001).
However, these values are not perceptible to
the naked eye because ΔE values are > 3.3.
Alsadon et al. [26] confirmed that ΔE values
ranging from 1→3 are perceptible to the naked eye
& ΔE < 3.3 are critical values and therefore clinically
unacceptable. It was the rst study to evaluate the
optical properties of a composite veneered PEKK
indirect restoration. Hussain et al. [27] approved
that detectable color differences are normally
nondiscernible below ΔE values of 1, which
convert to unacceptable color at ΔE< 3.3.
Hsu et al. [28] showed that using PAEK (PEEK
& PEKK), as a substrate material was deduced to
have the best color accuracy (ΔE < 2.9), among
all the specimens tested in the present study.
The results of various color attributes show that
PAEK materials have a better color balance,
higher color saturation, and lower hue compared
to those of the other groups.
In the current study, samples veneered
with 0.5 mm resin composite had statistically
higher values than 1mm thick samples. Such a
result was supported by the El-Sawaf et al. [22]
study that showed that increasing the thickness
of composite veneer will reduce the ΔE values
significantly. It was then concluded that a
0.5 mm thickness of a composite veneer layer
is considered unacceptable because it allows
for a signicant increase in light transmission
Table III - Two-way ANOVA test results
Measurement Variable Sum of squares df Mean square f-value p-value
Color change
(ΔE)
Material
1.03 1 1.03 22.59 <0.001*
Thickness
2.98 1 2.98 65.21 <0.001*
Material *
Thickness
0.06 1 0.06 1.34 0.261
Error
0.91 20 0.05
Shear bond
strength (MPa)
Material
0.47 1 0.47 0.11 0.748
Thickness
23.19 1 23.19 5.27 0.033*
Material *
Thickness
1.36 1 1.36 0.31 0.584
Error
88.08 20 4.40
*Significant (p<0.05); df = degree of freedom.
Table IV - Summary statistics and intergroup comparisons of failure modes
Thickness Failure mode
n (%)
χ2p-value
(B) (P)
0.5 mm
Adhesive
3 (37.5%) 0 (0.0%)
3.69 0.200
Cohesive
0 (0.0%) 0 (0.0%)
Mixed
5 (62.5%) 8 (100.0%)
1 mm
Adhesive
0 (0.0%) 2 (25.0%)
2.29 0.467
Cohesive
0 (0.0%) 0 (0.0%)
Mixed
8 (100.0%) 6 (75.0%)
χ23.69 2.29
p-value 0.200 0.467
8
Braz Dent Sci 2023 Oct/Dec;26 (4): e3959
Ahmed GA et al.
Comparison of shear bond strength and color reproduction of two different high-performance polymers veneered with two different thicknesses of resin composite: an in-vitro study
Ahmed GA et al. Comparison of shear bond strength and color reproduction of
two different high-performance polymers veneered with two
different thicknesses of resin composite: an in-vitro study
with a signicant decrease of masking ability.
The same findings were in accordance with
Ellakany et al. [29]. The outcome of the present
study demonstrated that these two variables (the
type of core material and thickness for composite
veneer), are considered essential criteria for
optical properties and variations in color [25].
Therefore, they have a signicant effect on the
color change (ΔE).
The results of the present study also showed
that the type of core material had no signicant
effect on the shear bond strength values, while
veneer thickness was significantly effective.
According to ISO 10477.23 standardization [30],
the minimum acceptable threshold for the shear
bond strength values is 5MPa, and the optimal
clinical service limit for SBS is 10MPa. In the
present study, all the tested groups reached a high
threshold ranging from (11.99:14.33MPa), so the
results were accepted for clinical requirements.
Graupner et al. [31] stated that a highly
brittle material shows higher toughness with
larger sample thickness, a behavior that seems
to correlate with the elongation at fracture.
Gouda et al. [15] agreed with the present results,
showing that different veneering materials
behaved differently when considering different
thickness ratios between the BioHPP core and the
veneering material. Other studies concluded that
the thicker the restoration, the higher the tensile
stress concentration in the restoration. Thicker
occlusal veneers present superior mechanical
performance than thinner restorations. Direct
conventional and ow resin composite occlusal
veneers present a promising mechanical behavior
when bonded on enamel or dentin. However,
caution is advised when preparing 0.5-mm
minimal thickness restorations [32,33]. Other
studies showed that multiple factors are known to
affect the entire strength of bi-layered restorations
other than the thickness of the veneering material
such as residual stress, interfacial bonding
strength, the direction of loading, as well as the
modulus of elasticity and fracture resistance of
each layer [34].
Considering the effect of the material’s
modulus of elasticity, and the fact that materials
having a more compatible modulus of elasticity
are more likely to bend under load and distribute
stresses more evenly [34], Crea.lign resin
composite was used as a veneering material in the
present study because it has an elastic modulus of
4.4 GPa which showed signicantly high exural
strength values [35]. Additionally, the presence
of 50% Nano ceramic llers in the Crea.lign resin
composite matrix played a role in improving its
mechanical properties and contributing to the high
exural strength values [34]. All the specimens’
surfaces are pretreated by air abrasion with 110 μm
AL2O3 particles followed by the application of
Visio.link adhesive primer. Küçükekenci et al. [13]
revealed in their study, the importance of surface
pretreatment of PEAK and its signicance on the
shear bond strength with resin composite veneers.
Hata et al. [36] concluded that the combination of
MMA containing adhesive primer (Visiolink and
Signum bond), and resin cement after air abrasion
with alumina particles could signicantly improve
the bond strength. Turkkal et al. [37] concluded
that surface pretreatment had a signicant role
in the adhesive failure of bi-layered restorations
regardless of the veneering material strength.
They agreed that bonding between the PEEK
surface and adhesive is only and solely of a
mechanical nature therefore, the use of air
abrasion is recommended to enhance the surface
micro-roughness of PEEK, thus permitting better
inltration of the adhesive material. On the other
hand, it was stated that the use of Visio.link as
an adhesive primer is capable of modifying the
PEEK surface, and thereby create a chemical
bond between the veneering material and the
adhesive [18].
All specimens in the present study were
exposed to artificial aging in an attempt to
simulate the oral conditions in agreement with
previous studies which claim that thermal cycling
did not affect the exural strength or load-bearing
capacity of PEEK [38]. However, other studies
in the literature showed a significantly lower
fracture resistance for the Crea.lign veneered
PEEK cores after aging [34].
Regarding the mechanical behavior of both
PEEK and PEKK, previous studies revealed that
both materials had some differences in their
stress responses. Results by Villefort et al. [39]
showed that PEKK has lower stress concentration
values as a prosthetic screw & base in full arch
prosthesis under the” all-on-four” concept while
lower stress concentration was observed on PEEK
frameworks. A study by El Hussieny et al. [40]
was in contrast to ndings by Alsadon et al. [11]
as the rst reported that the PEEK group showed
lesser strain values in comparison to PEKK. while
Alsadon showed that PEKK had better mechanical
9
Braz Dent Sci 2023 Oct/Dec;26 (4): e3959
Ahmed GA et al.
Comparison of shear bond strength and color reproduction of two different high-performance polymers veneered with two different thicknesses of resin composite: an in-vitro study
Ahmed GA et al. Comparison of shear bond strength and color reproduction of
two different high-performance polymers veneered with two
different thicknesses of resin composite: an in-vitro study
values when compared to PEEK in terms of
tensile, exural, and compressive strengths with
better stress distribution.
The limitations of the study included only
one-way surface pretreatment using air abrasion
with Al2O3 particles. Only one material was used
for veneering. Further investigations are needed
for a better understanding of the effectiveness
of different methods of surface treatment on
bonding to PEEK and PEKK. More clinical studies
are required concerning different veneering
materials.
CONCLUSIONS
In light of the limitations inherent to this
study, it can be deduced that a resin composite
veneer with a 0.5 mm thickness plays a pivotal
role in bilayer PEAK restorations, signicantly
influencing color reproduction and shear
bond strength, thereby highlighting its critical
importance in achieving desired clinical outcomes.
Author’s Contributions
GAA: Original draft preparation, Investigations,
Methodology, Resources, Conceptualization,
Writing - Original Draft Preparation, and Writing
- Review & Editing. AEE: Supervision, Validation,
and Writing - Review & Editing. GAF: Project
Administration, Methodology, Writing - Review
& Editing, and Formal Analysis. All authors have
read and agreed to the manuscript.
Conict of Interest
The authors have no proprietary, nancial,
or other personal interest of any nature or kind
in any product, service, and/or company that is
presented in the present article.
Funding
The present research did not receive any
specic grant from funding agencies in the public,
commercial or not-for-prot sectors.
Regulatory Statement
The present research was approved by the
committee of the Faculty of Dentistry Ain Shams
University Research Ethics Committee (FDASU-
REC).
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Gehad Abdelgawad Ahmed
(Corresponding address)
Ain Shams University, Fixed Prosthodontics Department, Cairo, Egypt.
Email: gehadabdelgawad82@gmail.com
Date submitted: 2023 Jul 13
Accept submission: 2023 Sept 18
