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.e4267
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Braz Dent Sci 2024 July/Sept;27 (3): e4267
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.
Effect of repeated pressing on the fracture resistance of
heat-pressed glass ceramic crowns
Efeito da re-prensagem na resistência à fratura de coroas de cerâmica de vidro injetadas
Mohamed Magdy EL SHAZLI1 , Amr EL-ETREBY1,2 , Fatma Adel MOHAMED1
1 - Ain-Shams University, Faculty of Dentistry, Fixed Prosthodontics Department, Cairo, Egypt.
2 - Tallinn Health Care College, Tallinn, Estonia.
How to cite: El Shazli MM, El-Etreby A, Mohamed FA. Effect of repeated pressing on the fracture resistance of heat-pressed glass ceramic
crowns. Braz Dent Sci. 2024;27(3):e4267. https://doi.org/10.4322/bds.2024.e4267
ABSTRACT
Objective: This study examines the impact of re-pressing four different glass ceramic materials on the fracture
resistance (FR) of single crowns. Material and Methods: Fifty-six heat-pressed crowns were fabricated from
four glass ceramic materials. Crowns were divided into 4 groups (n=14): lithium disilicate IPS Emax press LDS1,
lithium disilicate LiSi press LDS2, zirconia reinforced lithium silicate Celtra press ZLS, and zirconia reinforced
lithium disilicate Vita Ambria ZLDS. Two subgroups (n=7) were created for each group. Group (P) crowns were
made from fresh ingots. Group (R) crowns were made from re-pressed buttons. Samples were then subjected
to fracture resistance (FR). Failure load was indicated by an audible crack and veried by a dramatic decline
in the load-deection curve, as recorded using computer software. The load under which crowns fractured was
ultimately recorded in Newtons (N). The properties of the glass ceramic crowns were characterized before and
after re-pressing by scanning electron microscope (SEM), X-ray diffraction (XRD), and energy dispersive X-ray
(EDAX). Results: Numerical data were tested for normality using the Kolmogorov-Smirnov and Shapiro-Wilk
statistical tests. The results indicated that ceramic type had a signicant effect on FR (p-value < 0.001). The
thermal technique used also had a signicant effect on FR (p-value = 0.036). Group LDS1 showed the highest
FR (1765.8N), while Group ZLDS showed the lowest FR (1247N). When comparing (P) to (R) groups, XRD
revealed no variation in the primary crystalline structure. EDAX revealed no difference in chemical makeup
between groups. Conclusion: Re-pressing improves the studied glass ceramics crowns’ resistance to fracture.
KEYWORDS
Flexural strength; Glass ceramics; Lithium disilicate; Recycling; Zirconium oxide.
RESUMO
Objetivo: Este estudo examina o impacto da re-prensagem de quatro diferentes materiais de cerâmica de vidro
na resistência à fratura (RF) de coroas unitárias. Material e Métodos: Cinquenta e seis coroas injetadas foram
fabricadas a partir de quatro materiais de cerâmica de vidro. As coroas foram divididas em 4 grupos (n=14):
dissilicato de lítio IPS e.max Press LDS1, dissilicato de lítio LiSi press LDS2, silicato de lítio reforçado com zircônia
Celtra press ZLS e dissilicato de lítio reforçado com zircônia Vita Ambria ZLDS. Dois subgrupos (n=7) foram
criados para cada grupo. As coroas do grupo (P) foram feitas a partir de lingotes novos. As coroas do grupo
(R) foram feitas a partir de lingotes re-prensados. As amostras foram então submetidas a testes de resistência à
fratura (RF). A carga de falha foi indicada por um estalo audível e vericada por uma queda dramática na curva
de carga-deexão, conforme registrado por software de computador. A carga sob a qual as coroas fraturaram foi
registrada em Newtons (N). As propriedades das coroas de cerâmica de vidro foram caracterizadas antes e depois
da re-prensagem por microscópio eletrônico de varredura (MEV), difração de raios X (DRX) e espectroscopia de
raios X por dispersão de energia (EDAX). Resultados: Os dados numéricos foram testados quanto à normalidade
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Braz Dent Sci 2024 July/Sept;27 (3): e4267
El Shazli MM et al.
Effect of r epeated pressing on the fractur e resistance of heat-pres sed glass ceramic crowns
El Shazli MM et al. Effect of repeated pressing on the fracture resistance of
heat-pressed glass ceramic crowns
INTRODUCTION
The most prevalent dental ceramics include
glass ceramics, densely sintered alumina, and
zirconia-based ceramics. Heat pressing is a
method for processing glass ceramic restorations
that, unlike other techniques such as sintering,
has a wide range of applications in dental
restoration due to its simplicity and its ability to
produce decreased porosity, reduced shrinkage,
increased exural strength, and better marginal
t and crystalline distribution within the glassy
matrix. During the manufacturing of glass
ceramics, the glassy phase is converted into the
crystalline phase, resulting in a glassy matrix with
numerous crystalline phases [1]. The ultimate
crystalline form is determined by the glass
composition, nucleating agent, and manner of
heating. The form and size of the crystals have a
substantial impact on mechanical properties [2].
Lithium disilicate glass ceramics exhibit
greater exural strength and fracture toughness
than other glass ceramics [3]. They also offer
excellent aesthetic restoration due to their
intrinsic translucency. IPS e.max Press is an
aesthetically pleasing, translucent, heat-pressed
lithium disilicate ceramic with a exural strength
of 400 MPa and a 70% volume of needle-shaped
lithium disilicate crystals, making it suitable for
short-span xed partial dentures [2]. Initial LiSi
Press is another high-strength lithium disilicate
glass-ceramic manufactured with unique High-
Density Micronization (HDM) technology that
equally distributes micro-crystals, rather than
larger crystals that ll the entire glass matrix [4].
Ohashi et al. [5] reported that Lisi and IPS e.max
press have relatively similar strengths.
Heat-pressed ceramics use the lost wax
technique, in which ceramic ingots are pressure-
pressed into a mold in a pneumatic press
furnace [6]. When the lithium disilicate-pressed
restoration is removed, the sprue and button
portions are discarded, leaving a substantial
amount of ceramics unused. Using the same
ingot to press multiple restorations at once can
save money but may not always be practicable.
In some dental labs, it is found to be more
beneficial to use residual materials (leftover
sprues and buttons) to produce new restorations,
rather than squandering the residual sprue and
button components. Recycling ceramic material
is an economical way to reduce the expense of
restoration [7].
A variety of studies have investigated the
effect of re-pressing residual material on the
biaxial exural strength of heat-pressed glass
ceramics. Gorman et al. and AlBakry et al. reported
no significant difference in biaxial flexural
strength or fracture toughness after re-pressing
lithium disilicate ceramic [1,7]. X-ray diffraction
was utilized to characterize the crystalline
phase and scanning electron microscopy was
used to examine the microstructure. Repeated
pressing revealed no difference in crystalline
composition. Chung et al. was determined that
lithium disilicate glass-ceramic may be re-pressed
while retaining good mechanical qualities and not
considerably affecting the crystalline composition
of the material [8].
In contrast to Chung et al., who concluded
that re-pressing produced a statistically signicant
increase in the exural strength of re-pressed
lithium disilicate-reinforced glass-ceramic
material (Empress®2) [8], Tang et al. [6] detected
significant differences in three-point fracture
toughness, exural strength, and hardness after
re-pressing of IPS e.max press. The density of
lithium disilicate ceramics (IPS e.max Press)
reduced and porosity increased after two heat
pressing events. Flexural strength, Vickers
hardness, and fracture toughness all dropped
dramatically.
Recently, different companies have added
different percentages of ZrO2 to develop zirconia-
toughened glass-ceramics, reinforcing ceramic
usando os testes estatísticos de Kolmogorov-Smirnov e Shapiro-Wilk. Os resultados indicaram que o tipo de
cerâmica teve um efeito signicativo na RF (valor p < 0,001). A técnica térmica utilizada também teve um
efeito signicativo na RF (valor p = 0,036). O grupo LDS1 apresentou a maior RF (1765,8N), enquanto o
grupo ZLDS apresentou a menor RF (1247N). Ao comparar os grupos (P) e (R), a DRX não revelou variação na
estrutura cristalina primária. A EDAX não revelou diferença na composição química entre os grupos. Conclusão:
A re-prensagem melhora a resistência à fratura das coroas de cerâmica de vidro estudadas.
PALAVRAS-CHAVE
Resistência à exual; Cerâmicas de vidro; Dissilicato de lítio; Reciclar; Óxido de zircônio.
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El Shazli MM et al.
Effect of r epeated pressing on the fractur e resistance of heat-pres sed glass ceramic crowns
El Shazli MM et al. Effect of repeated pressing on the fracture resistance of
heat-pressed glass ceramic crowns
structures through crack interruption [9,10].
Celtra® Press is a newly released zirconia-
reinforced lithium silicate (ZLS) containing 10%
ZrO2 as a nucleating agent that shows exural
strengths of more than 500 MPa following
power ring at 760°C. A study by Yehia et al.
demonstrated that re-pressing enhanced the
exural strength of Celtra [11].
Ambria is another ZLS with a biaxial strength
of 450 MPa after pressing at 890°C; this increases
after annealing to 600 MPa [12]. Heat tempering
is a widely used technique for strengthening glass
ceramics by enhancing the size of lithium disilicate
crystals [10,13-15]. The crystal’s microstructure
becomes more interlocking and closely packed,
with the result that crack propagation must follow
a more tortuous path [16-18]; this signicantly
increases exural strength [13]. Heat tempering
is a technique whereby a heat-pressed ceramic
crown is heated to a temperature just above the
glass transition region, yet below its softening
point [19]. Abo-Elezz et al. [12] reported that
heat tempering with a temperature 5% below
pressing temperature increased the biaxial
exural strength of IPS e.max Press, initial LiSi
Press, Celtra Press, and VITA Ambria crowns.
According to previous studies, Celtra® Press
and Vita Ambria have mechanical properties
that are comparable to commonly used lithium
disilicate glass ceramics [12]. However, there is a
dearth of information in the literature about the
effect of re-pressing on the microstructure and
mechanical characteristics of recently released
ZLS and ZLDS glass ceramics, as well as their
comparison to other LDS glass ceramics. The null
hypothesis of this study was that re-pressing
has no inuence on the fracture resistance or
microstructure of four studied glass ceramic
materials, and that ceramic type has no effect on
fracture resistance.
The present study aims to assess the
impact of repeated heat-pressing on the fracture
resistance of four glass-ceramics, and describe the
microstructural characteristics of freshly-pressed
and re-pressed materials using X-ray diffraction
(XRD), energy dispersive X-ray analysis (EDAX),
and scanning electron microscopy (SEM).
MATERIALS AND METHODS
This study employs a power analysis based on
a previous work [20] to test the null hypothesis,
which stated that there would be no signicant
difference in fracture resistance for tested
groups. Using G*Power version 3.1.9.7 [21],
we determined an alpha (α) level of 0.05, beta
(β) level of 0.2, and effect size (f) of 0.595.
The anticipated total sample size (n) was 48,
with 12 samples in each group and 6 samples in
each subgroup. In this investigation, 56 samples
were used, with 14 in each group and 7 in each
subgroup.
Specimen preparation
A computerized numerical control lathe-cut
milling machine (CNC premium 4820, imes-
icore, Eiterfeld, Germany) was used to prepare
an acrylic resin lower rst molar prototype for
a lithium disilicate crown (1.5 mm occlusal and
axial reduction, 1 mm shoulder nish line). It was
subsequently duplicated using silicon duplicating
material (Replisil 22N, Dentecon, Germany),
then chemical cure epoxy resin (Chemapoxy
150, MBC) was poured into the silicon mold
and allowed to set for 24 h. Each die was then
magnied using loops to look for aws.
A total of 56 heat pressed glass ceramic
crowns were fabricated using the heat press
technique. The crowns were divided based on
the material used into four groups (n=14). These
comprised Group (LDS1): Lithium disilicate
glass ceramic (IPS e.max Press, Ivoclar), Group
(LDS2): High Density Micronization (HDM)
Lithium disilicate glass ceramic (GC initial LiSi
Press, GC), Group (ZLS): Zirconia reinforced
lithium silicate glass ceramic (Celtra Press,
Dentsply Sirona), and Group (ZLDS): Zirconia
reinforced lithium disilicate glass ceramic (VITA
Ambria, VITA Zahnfabrik). Each group was then
subdivided into two subgroups (n=7) based
on the ingot used. These subgroups comprised
Subgroup (P): Samples fabricated from new
ceramic ingots, and Subgroup (R): Samples
fabricated from the button material remaining
from all the pressing group samples.
Exocad computer software version
2017 (Exocad GmbH) was used to design wax
patterns for glass ceramic crowns, which were
then milled using a 5-axis milling machine (VHF
CAM 5-S1, VHF) to standardize the anatomy,
thickness, and contour of the crowns while
eliminating all operator variables involved in the
fabrication process.
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El Shazli MM et al.
Effect of r epeated pressing on the fractur e resistance of heat-pres sed glass ceramic crowns
El Shazli MM et al. Effect of repeated pressing on the fracture resistance of
heat-pressed glass ceramic crowns
The tooth to be restored was chosen at the
beginning of the production process (Figure 1A),
and the suggested design was picked. The die was
scanned, and a 3D image (virtual model) was
produced on a screen (Figure 1B), using a desktop
Identica blue scanner (MEDIT Corp., Seoul,
Korea). The proposed design was subsequently
modied, and a 5-axis milling machine (Dima
Mill Wax, KULZAR, Germany) was used to mill
the crown wax pattern (Figure 1C). Under 2.5x
magnication, the wax patterns were evaluated for
t, accuracy, and marginal adaptation (Figure 1D).
Patterns that were flawed were eliminated.
The IPS Press VEST (Ivoclar Vivadent, Zurich,
Switzerland) was then used to invest the patterns
after they had been sprued. After 30 minutes, the
wax was removed using a wax burn-out furnace.
Ingots of Groups (LDS1), (LDS2), (ZLS) and
(ZLDS) were pressed in a heat press furnace EP
3000 (Ivoclar Vivadent AG, Schaan/Liechtenstein)
(Figure 1E). The thermal cycle used for each
material is shown in (Table I). Crowns for (P)
groups were extracted from the investment
ring through air abrasion with 110 μm alumina
particles (Cobra, Renfert) under 4 and 2 bar
pressure (representing rough and soft divesting,
respectively). The pressed LDS1 crowns were
then separately submerged in 1% hydrouoric
acid Invex liquid (Ivoclar Vivadent AG, Zurich,
Switzerland) and cleaned in an ultrasonic
cleaner for 10 minutes, in accordance with
the manufacturer’s instructions, to remove the
investment’s reaction layer. To fabricate the
subgroup (R) specimens, the remaining buttons
from the pressing process were trimmed, and all
the previously mentioned steps were repeated
using the trimmed leftover buttons to produce
re-pressed crowns (Figure 1F, G). Both groups’
crowns were examined on the die (Figure 1H)
and glazed using IPS Ivoclar Glaze Paste (Ivoclar
AG, Zurich, Switzerland).
Figure 1 - Graphic Diagram Of Specimen Preparation. A: Selection Of Tooth To Be Restored On Exocad. B: Virtual Model. C: Milled Wax Pattern.
D: Wax Pattern Try-in On The Corresponding Dies. E: Ingot Placement Inside Investment. F: Re-pressing Of Finished Buttons. G: Divesting And
Sprue Cutting. H: Restoration Was Checked For Its Adaptation. I: Sample Undergoing Fracture Resistance Testing.
Table I - Pressing parameters of the four ceramic materials
Materials Start
temperature (°C)
Heating
rate (°C/min)
Maximum
temperature (°C)
Holding
time (min)
Press
time (min)
Pressing
pressure (bar)
IPS e.max Press Group (LDS1) 700 60 917 25 3 3
LiSi Press Group (LDS2) 700 50 910 30 3 3
Celtra Press Group (ZLS) 700 40 865 30 3 3
VITA Ambria Group (ZLDS) 700 60 890 25 3 3
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Braz Dent Sci 2024 July/Sept;27 (3): e4267
El Shazli MM et al.
Effect of r epeated pressing on the fractur e resistance of heat-pres sed glass ceramic crowns
El Shazli MM et al. Effect of repeated pressing on the fracture resistance of
heat-pressed glass ceramic crowns
The interior surface of each crown was etched
for 20 seconds using a 9.5% hydrouoric acid gel
(Porcelain Etchant, BISCO, USA). It was then rinsed
with water and dried with oil-free, moisture-free air.
The internal surface of each restoration was treated
with a silane coupling agent (Prehydrolyzed Silane,
BISCO, USA) for one minute, and each was then
allowed to air dry for ve seconds.
Fifty-six (56) epoxy resin substrates were
acid-etched for 15 seconds using 37% phosphoric
acid. The surfaces were then rinsed with water
and allowed to air dry for 20 seconds. Transparent
dual-cured self-adhesive resin cement (Breeze,
Pentron Clinical) was used to cement the
crowns. Next, the crowns were installed on the
appropriate dies using light finger pressure.
For the 56 samples, an axial load of 5 kg was
applied for 10 minutes after any excess luting
material was removed with a brush. The luting
substance was light-cured for 20 seconds on each
surface. The specimens were left for 24 hours
before the fracture test to ensure complete setting
of dual-cured self-adhesive resin cement.
Fracture resistance
The 56 samples underwent a fracture
resistance test using a universal testing machine
(Instron 3345, Instron, USA) with a 5 kg load
cell. Using a metallic rod, a compressive load
was applied to the occlusal surface’s midpoint
(Figure 1I). Only the inclined planes of the buccal
and lingual cusps were contacted by the rod, at a
crosshead speed of 1 mm/min. A tin foil sheet was
placed between the crown’s occlusal surface and
the applicator tip for stress distribution. Failure
load (detected via an audible cracking sound with
a sharp drop at the load-deection curve) and
fracture values were recorded in Newtons (N).
Mode of failure of fractured samples
Failure modes were inspected and classied
into 3 groups (Table II): Cracking, Chipping or
partial fracture, and Catastrophic fracture [20].
Scanning electron microscopy (SEM)
The tting surface of a representative cracked
or fractured crown from each group was etched
for 90 seconds with 9.8% hydrouoric acid, then
cleaned, steamed, dried, and coated with sputter
gold. A 10000x magnication scanning electron
microscope (Leo Supra 55, Jena, Germany) was
used to investigate the fractured samples.
X-ray diffraction analysis (XRD)
Representative powder samples were
scanned using 20-40 degrees Cu Kα X-ray angle,
Figure 2 - Pie chart representing percentage distributions of failure
modes in all groups.
Table II - Frequency distribution of failure modes
Group Crack Chipping or partial
fracture
Catastrophic or
fragments fracture
P
-value Effect size
(v)
n%n%n%
LDS1P 3 42.9 4 57.1 0 0
0.531 0.389
LDS2P 3 42.9 2 28.6 2 28.6
ZLSP 4 57.1 3 42.9 0 0
ZLDSP 5 71.4 2 28.6 0 0
LDS1R 3 42.9 1 14.3 3 42.9
LDS2R 3 42.9 3 42.9 1 14.3
ZLSR 5 71.4 2 28.6 0 0
ZLDSR 5 71.4 2 28.6 0 0
Total 31 55.4 19 33.9 6 10.7
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El Shazli MM et al.
Effect of r epeated pressing on the fractur e resistance of heat-pres sed glass ceramic crowns
El Shazli MM et al. Effect of repeated pressing on the fracture resistance of
heat-pressed glass ceramic crowns
2θ with a step size of 0.04 degrees and 5 sec-step
intervals (X’pert PRO; PW 3040/60, Almelo,
Netherlands).
Energy dispersive X-ray analysis (EDAX)
EDAX was carried out for one sample
from each group to quantify elements by X-ray
microanalysis (FEI Czech SEM, Brno, Czech
Republic).
Statistical analysis
Statistical analysis was carried out using
IBM SPSS Statistics, Version 23.0. (Armonk,
NY: IBM Corp). The normality of the numerical
data was explored using Kolmogorov-Smirnov
and Shapiro-Wilk tests. Data were presented as
mean ±standard deviation (SD). To study the
effect on fracture resistance of ceramic type,
thermal procedure and their interactions, a two-
way analysis of variance (ANOVA) test was used
followed by Bonferroni’s
post-hoc
test for pair-
wise comparisons. Fisher’s exact test was used
to compare failure modes in different groups.
The signicance level was set at
P
<0.05.
RESULTS
Regardless of ceramic type, pressing showed
significantly lower mean fracture resistance
than re-pressing (
p
-value = 0.036, v = 0.094).
Regardless of heat pressing treatment, whether
with pressing or re-pressing, different ceramic
types showed a signicant difference in mean
fracture resistance. Pair-wise comparisons
of ceramic types revealed that LDS1 showed
statistically higher mean fracture resistance than
LDS2, and ZLS showed statistically lower mean
values. ZLDS showed the lowest mean fracture
resistance with a non-signicant mean fracture
resistance difference from ZLS (Table III).
Results of Fisher’s exact test for comparison
between failure modes in different groups showed
that there was no signicant difference between
failure modes in different groups (
p
-value =
0.531, v = 0.389). Percentage distributions of
failure modes in all groups are presented in
Figure 2.
Scanning electron microscopy (SEM)
The 10000x magnification SEM image
observation revealed multilayered rod-shaped
lithium disilicate crystals in the LDS1P group
(Figure 3A). On the other hand, the LDS1R group’s
lithium disilicate crystals seemed better oriented,
aligned parallel to the direction of pressing with an
increase in both width and length. The crystals this
time arranged themselves in an interconnecting
pattern (Figure 3B). LDS2 specimens differed
signicantly from LDS1 specimens in terms of
microstructure. An interlocking microstructure
created by multilayered platelet-shaped crystals
is depicted in (Figure 3C). After re-pressing, a
more interconnected microstructure is exhibited
(Figure 3D).
Lath-like crystals with randomly oriented
regular and irregular forms were visible in SEM
images of ZLSP specimens (Figure 3E). These
crystals grew longer and wider after being
re-pressed. Additionally, they came to resemble
belts (Figure 3F). In both the pressed (Figure 3G)
and re-pressed (Figure 3H) specimens, needle-
shaped particles were visible in the ZLD SEM images.
The particles’ size increased after re-pressing.
X-ray diffraction analysis
XRD analysis of both pressed and re-pressed
samples revealed crystalline phases, with lithium
disilicate being the primary crystalline phase
for LDS1, ZLDS and LDS2 groups, and lithium
silicate being the primary crystalline phase for
ZLS (Figure 4).
Table III - The mean, standard deviation (SD) values and results of a two-way ANOVA test for comparison between the fracture resistance (N)
values of ceramic types: IPS e.max Press Group (LDS1),LiSi Press Group (LDS2),Celtra Press Group (ZLS) and VITA Ambria Group (ZLDS) with
each thermal procedure.
Thermal
procedure
LDS1 (n = 7) LDS2 (n = 7) ZLS (n = 7) ZLDS (n = 7)
p-
value
Effect size
(Partial eta
squared)*
Mean SD Mean SD Mean SD Mean SD
Pressing 1780.9 A127.4 1370.3 B116.8 1274.5 BC 109.9 1177.8 C125.4 <0.001* 0.547
Re-pressing 1750.7 A172 1520.3 B158.8 1408.7 BC 152.8 1306.4 C255.6 <0.001* 0.396
*Eta squared measures the proportion of the total variance in a dependent variable that is associated with the membership of different
groups defined by an independent variable. Partial eta squared is a similar measure in which the effects of other independent variables and
interactions are partialled out [22].
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El Shazli MM et al.
Effect of r epeated pressing on the fractur e resistance of heat-pres sed glass ceramic crowns
El Shazli MM et al. Effect of repeated pressing on the fracture resistance of
heat-pressed glass ceramic crowns
EDAX analysis
EDAX system is attached to a scanning
electron microscopy instrument that allows the
microscope’s imaging capabilities to identify the
specimen of interest. The information produced
by the EDAX analysis consists of spectra with
peaks that represent the constituent elements of
Figure 3 - Representative SEM Of The Sample Surface At X10000 Magnification. A: LDS1P, B: LDS1R, C: LDS2P, D: LDS2R, E: ZLSP, F: ZLSR, G:
ZLDSR,H: ZLDSP.
AB
CD
E
F
G
H
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Braz Dent Sci 2024 July/Sept;27 (3): e4267
El Shazli MM et al.
Effect of r epeated pressing on the fractur e resistance of heat-pres sed glass ceramic crowns
El Shazli MM et al. Effect of repeated pressing on the fracture resistance of
heat-pressed glass ceramic crowns
the sample under study. It also permits image
analysis and elemental mapping of a sample.
It can be quantitative, semi-quantitative, or
qualitative. Through mapping, it also shows
the spatial distribution of the elements. Pressed
and re-pressed samples showed no difference in
composition (Figure 5).
DISCUSSION
The re-pressing of lithium disilicate and
leucite glass-ceramics has been documented in the
literature. However, ndings differ regarding how
re-pressing affects the mechanical and physical
characteristics of these glass-ceramic materials.
Figure 4 - XRD peaks for all groups showing the main crystalline phases.
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El Shazli MM et al.
Effect of r epeated pressing on the fractur e resistance of heat-pres sed glass ceramic crowns
El Shazli MM et al. Effect of repeated pressing on the fracture resistance of
heat-pressed glass ceramic crowns
Furthermore, the existing literature contains little
information regarding how re-pressing affects
the mechanical and physical characteristics of
ZrO2 lithium silicate glass-ceramics.
The results of the present study allow the
null hypothesis to be rejected, as the re-pressing
and different types of glass ceramics was found to
have a signicant impact on fracture resistance.
Different types of ceramic had statistically
signicant differences regardless of the heating
process used, with LDS1 (1765.8 N) exhibiting
the highest mean fracture resistance. ZLDS
(1247N) exhibited the lowest mean fracture
resistance, differing from ZLS in a way that was
not signicant. The microstructural characteristics
(as measured by SEM) of the LDS1 specimens,
which showed pore-free multilayered rod-shaped
lithium disilicate crystals producing a high
interlocking microstructure, may correspond to
this nding. Moreover, LDS1’s higher pressing
temperature may promote greater crystal
development and interlocking.
These ndings are consistent with research
conducted by Wang et al. [23] on the effect of
heat-pressing temperature on the microstructure
and exural strength of lithium disilicate glass-
ceramic, which found that the IPS e.max press
at the highest temperature produced the most
pore-free structure. Furthermore, heat-tempering
lithium disilicate glass ceramics enhances its
exural strength, as reported by Sun et al. [24];
this was explained by a shift in crystal morphology
from spherical to rod-shaped.
Another important factor that signicantly
affects ceramic material strength is the number
of crystal llers in the material. Glass ceramics
are made by melting glass and carefully heating
it with nucleating chemicals until the desired
degree of crystallinity is reached. During these
processes, the glassy phase transforms into the
crystalline phase, and the materials that remain
are composed of a glassy matrix with several
embedded crystalline phases [25]. In glass
ceramics, nucleation is the main controlling
process for crystallization. The prevailing
mechanism is determined by the chemical
composition of the nucleating chemicals and the
parent glass [26]. ZrO2 was used as a nucleating
agent, which aided in volume crystallization.
of glasses while impeding crystal development.
This could explain why ZLS and ZLDS have
lower flexural strengths than LDS1, because
ZrO2 increases the viscosity of the heat-pressed
ceramic and inhibits the growth of lithium
metasilicate and lithium disilicate crystals during
heat tempering [2,27].
Figure 5 - EDAX analysis of pressed and repressed samples.
10
Braz Dent Sci 2024 July/Sept;27 (3): e4267
El Shazli MM et al.
Effect of r epeated pressing on the fractur e resistance of heat-pres sed glass ceramic crowns
El Shazli MM et al. Effect of repeated pressing on the fracture resistance of
heat-pressed glass ceramic crowns
Hallmann et al. [2] found that after heat
tempering at 860 °C, the Celtra press exhibited
the lowest biaxial exural strength values when
compared to the IPS Emax press and the Initial
LiSi press. This result is consistent with that of
Radwan et al. [28], who assessed the biaxial
exure strength of several pressable lithium silicate
ceramics and concluded that Celtra press had the
lowest strength and IPS e.max press the greatest.
In contrast, zirconia-reinforced lithium silicate
crowns have a greater mean fracture resistance
value than lithium disilicate crowns, according
to a study by Hamza et al. [29]. This nding may
have been due to the composition of the material,
as adding 10% zirconia may have boosted its
strength. In the present study, examination of
broken samples indicated that ZLDS had the
most frequent failure mode, which was cracking.
Microstructural analysis (SEM) of ZLDS revealed
nanoclusters that were well aggregated and fused
together to form bigger clusters, which may be
responsible for this nding.
The mean fracture resistance of pressed
samples was found to be much lower than that
of re-pressed samples, regardless of the type
of ceramic. SEM examination showed a pore-
free microstructure with an increase in grain
size in LDS1, LDS2, ZLS, and ZLDS following
repressing, which may be accountable for this result.
The increase in grain size signies the continued
existence of the crystallization. process during the
re-pressing process, resulting in the precipitation of
more crystals of lithium silicates. This behavior is
called Ostwald ripening [30], and is common for all
precipitated materials. The microstructure coarsens,
releasing excess surface energy due to small particle
solubility, causing larger grains to grow at the
expense of smaller particles [31]. This nding is in
line with Albakry et al. [1] and El-Etreby et al. [3,20]
who evaluated the impact of re-pressing on glass
ceramics and discovered that re-pressing led to
substantial growth of the crystals, but it is at odds
with Tang et al.’s [6] investigation into the impact
of repressing on the mechanical characteristics and
microstructure of lithium disilicate ceramics, which
led to the conclusion that re-pressing changed the
microstructure by noticeably increasing porosity.
Along with a signicant decline in hardness, fracture
toughness, exural strength, and density. According
to Gorman et al. [7], the mechanical qualities of
lithium disilicate ceramics after repressing remained
constant even after multiple pressings, with the rst
pressing offering the best results.
CONCLUSION
Within the limitations of this
in vitro
study,
the following can be concluded:
1. Re-pressing improves the studied glass
ceramics crowns’ resistance to fracture.
2. Recycling the investigated glass ceramics may
reduce failure and extend their service life.
Limitations
Neither the impact of intra-oral stresses nor the
effect of several pressing cycles were studied in this
research. Further research on additional mechanical
and physical qualities is needed. The ndings of this
study may lend support to the cost-effective reuse of
pressed glass ceramics; however, additional clinical
research is necessary to validate these results.
Author’s Contributions
MMES, AEE, FAM: Conceptualization.
MMES, AEE: Methodlogy. MMES: Writing –
Original Draft Preparation. AEE: Formal Analysis,
Validation. AEE, FAM: Supervision. FAM: Writing
– Review & Editing, Visulalization.
Conict of Interest
The authors have no conicts of interest to
declare.
Funding
This research did not receive any specic
grant from funding agencies in the public,
commercial, or not-for-prot sectors.
Regulatory Statement
This study was conducted in accordance with
all the provisions of the local human subjects
oversight committee guidelines and policies of:
Faculty of Dentistry, Ain Shams University, Cairo,
Egypt (FDASU-REC).
The approval code for this study is FDASU-
Rec EM022192.
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Mohamed Magdy El Shazli
(Corresponding address)
Fixed Prosthodontics Department, Faculty of Dentistry, Ain-Shams University,
Cairo, Egypt
Email: magdygeen@gmail.com
Date submitted: 2024 Feb 13
Accept submission: 2024 July 25
