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.e3706
1
Braz Dent Sci 2023 Jan/Mar;26 (1): e3706
Mechanical behavior of different machinable ceramic crowns using
vertical and horizontal preparations: an in-vitro study
Comportamento mecânico de diferentes coroas cerâmicas usináveis usando preparos verticais e horizontais: um estudo
in vitro
Ingy NOUH1 , Nancy RAFLA1 , Kamal EBEID1
1 - Ain Shams University, Faculty of Dentistry, Fixed Prosthodontics Department, Cairo, Egypt.
How to cite: Nouh I, Raa N, Ebeid K. Mechanical behavior of different machinable ceramic crowns using vertical and horizontal
preparations: an in-vitro study. Braz. Dent. Sci. 2023;26(1):e3706. https://doi.org/10.4322/bds.2023.e3706
ABSTRACT
Objective: the aim of this study was to compare the mechanical behavior of different ceramics when used in thin
vertical preparations versus traditional horizontal preparation. Material and Methods: two stainless-steel dies
were milled to simulate a minimally invasive vertical preparation (VP) and a traditional horizontal preparation
(HP) for an all-ceramic crown of a maxillary rst premolar. The stainless-steel dies were duplicated using epoxy
resin. Eighty monolithic crowns were milled and divided into 2 groups according to preparation design. Each
design group was subdivided into 4 sub-groups according to material (n=10): IPS e.max CAD (lithium disilicate),
Bruxzir shaded zirconia (full contour zirconia), CeraSmart (resin nanoceramic) and CEREC Tessera (advanced
lithium disilicate). The crowns were cemented on their relevant epoxy resin dies using self-adhesive resin cement.
All specimens were subjected to 15,000 thermocycles and then loaded to fracture in a universal testing machine.
Data were analyzed using two-way ANOVA and Tukey pair wise comparison test. Results: the fracture resistance
mean values of the VP (1344 + 118 N) was signicantly lower than the HP design (1646 + 191 N). Ceramic
crowns made of full contour zirconia had higher fracture resistance mean values (2842 + 380 N) than advanced
lithium disilicate (1272 + 125 N) followed by lithium disilicate crowns (983 + 52 N) and resin nanoceramic
(882 + 61 N). Conclusion: both vertical and horizontal preparations, regardless the different ceramic materials,
showed clinically acceptable fracture resistance values.
KEYWORDS
Dental crown; Prosthodontics; Zirconia; Lithium disilicate; Hybrid ceramics.
RESUMO
Objetivo: o objetivo deste estudo foi comparar o comportamento mecânico de diferentes cerâmicas quando
utilizadas em preparos verticais nos ou preparos horizontais tradicionais. Material e Métodos: dois modelos
de aço inoxidável foram fresados para simular um preparo vertical minimamente invasivo (PV) e um preparo
horizontal tradicional (PH) para uma coroa totalmente em cerâmica de um primeiro pré-molar superior. As
matrizes de aço inoxidável foram duplicadas usando resina epóxi. Oitenta coroas monolíticas foram fresadas
e divididas em 2 grupos de acordo com o desenho do preparo. Cada grupo foi subdividido em 4 subgrupos de
acordo com o material (n=10): IPS e.max CAD (dissilicato de lítio), zircônia Bruxzir (zircônia de contorno total),
CeraSmart (resina nanocerâmica) e CEREC Tessera (dissilicato de lítio avançado). As coroas foram cimentadas
em suas respectivas matrizes de resina epóxi usando cimento resinoso autoadesivo. Todos os espécimes foram
submetidos a 15.000 ciclos térmicos e então carregados até a fratura em uma máquina de teste universal. Os
dados foram analisados usando ANOVA com dois fatores e teste de comparação por pares de Tukey. Resultados:
os valores médios de resistência à fratura do PV (1344 + 118 N) foram signicativamente menores do que
PH (1646 + 191 N). As coroas de cerâmica feitas de zircônia de contorno total apresentaram maiores valores
médios de resistência à fratura (2842 + 380 N) do que dissilicato de lítio avançado (1272 + 125 N), seguido por
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Braz Dent Sci 2023 Jan/Mar;26 (1): e3706
Nouh I et al.
Mechanical behavior of different machinable ceramic crowns using vertical and horizontal preparations: an in-vitro study
Nouh I et al. Mechanical behavior of different machinable ceramic crowns
using vertical and horizontal preparations: an in-vitro study
INTRODUCTION
The conservation of tooth structure has
become the primary goal of restorative dentistry.
However, combining maximum conservation
with minimally invasive preparations without
compromising esthetics and strength of the nal
restoration has always been challenging [1,2].
Preserving the tooth structure through
adequate preparation technique reduces the
crack propagation, redistributes stresses and
subsequently increases the teeth durability [3].
The clinician usually prepares the abutment
to receive the xed partial denture ((FPD) by
establishing a nish line on which the restoration
rests. Finish lines are classied into vertical and
horizontal, where the latter could be supragingival
or subgingival which has been shown to cause
more gingival inammation. Horizontal nish
lines consist of knife or feather-edge margins [4].
The Biologically Oriented Preparation
Technique (BOPT) has been recently suggested
as a minimally invasive type of preparation that
uses a vertical nish line and can be adopted in
many clinical situations. It is commonly used
for periodontally affected teeth with gingival
recession where conservation is a priority
for long-term success of the abutment. This
technique prepares the teeth to receive the
restoration with no nish line; hence removing
the original Cement Enamel Junction (CEJ) and
allowing a newly created Prosthetic Cement
Enamel Junction (PCEJ) [5,6].
The tooth then receives an interim restoration
allowing gingival adaptation and thickening
which a crucial phase in the BOPT, then the
final restoration is manufactured, where the
marginal gap between the FPD and the tooth in
conventional methods are eliminated [7].
The BOPT has been under study to determine
the long-term success of the restorations and the
effect of lack of nish line on the periodontal
health. Studies have recently shown signicant
success of anterior and posterior FPDs with
abutment teeth having good marginal and
gingival [5,8,9].
For the success of a nal restoration, it is
necessary to meet three main criteria; marginal
stability, fracture strength and esthetics [10,11].
With the BOPT is crucial that the choose restorative
material can be used in thin thicknesses yet
without compromising its mechanical properties.
Lithium Disilicates provide adequate strength
o f 300-400 MPa combined with adequate
esthetics and high bonding properties, while
monolithic Zirconia has higher exural strength
of 900-1200 MPa promoting its use in very thin
preparations. These two materials have been
evaluated with feather edge finish lines and
showed acceptable results; however, they have
high wear on the opposing dentition [9,12].
Resin Nano ceramics provide acceptable
esthetic, has higher modulus of resilience and higher
exural strength in comparison to conventional
feldspathic CAD/CAM ceramics. Moreover, these
Resin Ceramics are easily polished with no need
for extra time for crystallization [13,14]. Advanced
Lithium Disilicates are novel ceramics introduced
to the market using different methods of fabrication
where the addition of Virgilite crystals increases its
exural strength >700 MPa. How their properties
are compared to conventional lithium disilicates
is still unclear. They can be used to restore single
crown, partial restorations and FPDs replacing a
missing tooth up-to the second premolar with no
need for further crystallization [15].
MATERIAL AND METHODS
Two standardized stainless-steel dies were
prepared by a milling machine (Gerossa, Cairo,
Egypt) to simulate different crown preparation of a
maxillary rst premolar: vertical preparation with
0.1 mm feather-edge margin (VP) or horizontal
preparation with 1 mm radial shoulder (HP) as
shown in Figure 1. The stainless-steel dies were
machined with a height of 5.5 mm, a diameter of
coroas de dissilicato de lítio (983 + 52 N) e resina nanocerâmica (882 + 61 N). Conclusão: preparos verticais
e horizontais, independentemente dos diferentes materiais cerâmicos, apresentaram valores de resistência à
fratura clinicamente aceitáveis.
PALAVRAS-CHAVE
Coroa dental; Prótese dentária; Zircônia; Dissilicato de lítio; Cerâmica híbrida.
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Braz Dent Sci 2023 Jan/Mar;26 (1): e3706
Nouh I et al.
Mechanical behavior of different machinable ceramic crowns using vertical and horizontal preparations: an in-vitro study
Nouh I et al. Mechanical behavior of different machinable ceramic crowns
using vertical and horizontal preparations: an in-vitro study
7.5 mm, a 12° total occlusal convergence angle
and a at occlusal surface [16]. A 45° occlusal
bevel was prepared at the occluso-axial line angle
on the buccal side for exact repositioning of the
crowns during seating and cementation and to
act as an anti-rotational feature.
Each stainless-steel die was duplicated into
an epoxy resin die using a non-shrink epoxy
resin material (Kempoxy 150 transparent,
CMB, Cairo, Egypt). 40 epoxy resin dies were
fabricated from each design. Each epoxy resin
die was scanned using CEREC Omnicam intraoral
scanner (Dentsply Sirona, Bensheim, Germany).
Eighty full contour monolithic ceramic crowns
were designed and milled using CEREC CAD/
CAM system (CEREC MCXL, Dentsply Sirona,
Germany) and divided into two groups (n=40)
according to preparation design (VP or HP).
Each group was sub-divided into four sub-groups
according to ceramic material: (L) lithium
disilicate (IPS e.max CAD, Ivoclar Vivadent,
Schaan, Liechtenstein), (Z) full contour zirconia
(Bruxzir shaded, Glidewell, USA), (C) resin nano
ceramic (CersaSmart, GC,Tokyo, Japan) and
(T) advanced lithium disilicate (CEREC Tessera,
Dentsply Sirona, Bensheim, Germany) (Figure 2).
The monolithic crowns were standardized in the
inlab software (CEREC software version v3.8,
Densply Sirona, Bensheim, Germany). After
milling, each crown was checked on its relevant
epoxy resin die and given a serial number.
L crowns were crystallized and glazed in a
combination ring cycle using IPS e.max CAD
Crystall/Glaze Paste (Ivoclar Vivadent, Schaan,
Liechtenstein), Z crowns were sintered in the
inre HTC speed furnace (Dentsply Sirona) for
90 minutes at a sintering temperature of 1540°C,
C crowns were only polished and T crowns were
glazed and sintered in CEREC speedre (Dentsply
Sirona) at 760°C. The crowns were then cleaned
in ultrasonic water bath to remove any residues
and their t on their relevant epoxy resin dies
was checked again (Figure 2).
The inner surfaces of the crowns were
treated prior to cementation according to the
manufacturer’s recommendations of each ceramic
material and cement type. L groups; were etched
using 9% hydrouoric acid gel (Porcelain etch,
Ultradent Products, United States) for 20 seconds,
then rinsed thoroughly and dried with oil
free air. The surfaces were then silanized by a
primer (Porcelain Silane, Ultradent Products,
United States) and left to react for 60 seconds.
Z Group;
was sandblasted (Renfert basic classic
sandblaster, Hilzingen, Germany) with 50 μm
Al2O3 at 1 bar at 10 mm distance. C group: T Group
Figure 1 - Showing vertical preparation of 0.1 mm feather-edge
design on the left and horizontal 1 mm radial shoulder on the right.
Figure 2 - (A) Illustration of epoxy resin dies with 2 preparations, (B) Illustration of dies with crowns seated in place.
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Nouh I et al.
Mechanical behavior of different machinable ceramic crowns using vertical and horizontal preparations: an in-vitro study
Nouh I et al. Mechanical behavior of different machinable ceramic crowns
using vertical and horizontal preparations: an in-vitro study
was etched using 5% hydrouoric acid gel (IPS
Ceramic Etching Gel, Ivoclar Vivadent, Germany)
for 30 seconds, then rinsed thoroughly and dried
with oil free air. The surfaces were then silanized
by a primer (Porcelain Silane, Ultradent Products,
United States).
Each crown was cemented using self-
adhesive resin cement (RelyX Unicem Aplicap
Capsules, 3M ESPE, Minnesota, United States).
Capsules were activated and mixed in the mixing
device for 10 seconds according to manufacturer’s
instructions. The cement was applied to the tting
surface of the crowns, lightly thinned with air
to ensure its unform distribution. The crowns
were seated on their relevant epoxy resin dies
by static nger pressure then axially loaded with
a 5 kg load using a specially designed loading
device and curing was done for 20 seconds on
the buccal and palatal surface using a light curing
device (Elipar Deep Cure, 3M ESPE, Minnesota,
USA). The crowns were left under the static load
for 10 minutes to ensure cement setting. All the
cemented specimens were left undisturbed for
1 h and then stored in distilled deionized water
for 24 h prior to testing. Specimens were then
subjected to 15,000 thermocycles, temperature
uctuations were 5°C to 55°C with a 60-seconds
dwell time at each temperature.
Specimens were then subsequently loaded
to fracture in a universal testing machine (LRX-
plus; Llyod instruments Ltd., Fareham, UK)
using a stainless-steel bar with a 3 mm diameter
ball end centralized along the long axis of the
samples. 0.3 mm thick tin foil was placed between
the loading stamp and crowns to achieve a
homogenous stress distribution. Load was applied
using a preload of 5 N and a cross-head speed of
1mm/min until failure. Failure was determined
when a sharp drop in the load to displacement
curve took place or an audible sound was heard or
when the curve went from straight to an irregular
pattern. Data was recorded and analyzed with
controlling computer software (Nexygen-MT
4.5.1; Llyod instruments, Ametek, West Sussex,
UK). The collected data was revised, coded and
tabulated using Statistical Package for Social
Sciences (SPSS) version 20 and Microsoft Ofce
2013 (Excel). Data was presented and suitable
analysis was done according to the type of data
obtained for each parameter. Shapiro-Wilk test
was used to assess data normality and data was
assumed normally distributed. Two-Way ANOVA
test was used to examine effect of preparation
design and material and on the fracture resistance.
P-value is the level of signicance, if
P
> 0.05:
Nonsignicant (NS),
P
≤ 0.05: Signicant (S).
RESULTS
Two-way ANOVA revealed a significant
inuence of the preparation design and ceramic
material on the fracture resistance of monolithic
crowns. The fracture resistance of the VP
design (1344 + 118 N) was signicantly lower
(p=0.0153) than the HP design (1646 + 191 N.
regardless of the ceramic material. Within the
vertical preparation design, Tukey’s post hoc
test showed that translucent zirconia had higher
fracture resistance mean values (2708 + 266 N)
than advanced lithium disilicate (1112 + 119 N)
followed by lithium disilicate crowns (869 + 51 N)
and resin nanoceramic (687 + 36 N) and the
differences were statistically signicant (p<0.001).
However, within the horizontal preparations,
Tukey’s HSD showed that translucent zirconia had
signicantly higher fracture resistance mean values
(2977 + 495 N) than advanced lithium disilicate
(1434 + 132 N) and statistically significantly
higher than lithium disilicate (1098 + 53 N) and
Resin Nano ceramic (1078 + 87 N) which were
not statistically different.
DISCUSSION
Being minimally invasive while respecting
biology and obtaining high esthetics requires
materials that has both optimum esthetic and high
mechanical properties when used in minimum
thicknesses [17,18]. The increasing popularity
of the BOPT concept in the recent years raised a
question whether different machinable ceramic
materials available in the market can be used
in minimum thickness in vertical preparations.
Hence, this in-vitro study aimed to test the
mechanical behavior of ceramic materials from
different compositions; resin nanoceramic, lithium
disilicate, advance lithium disilicate and zirconia
in vertical preparation designs and compare it to
the gold standard horizontal preparations.
Horizontal preparation as radial shoulder
and deep chamfer nish line preparation with a
thickness of 1-1.5 mm all around at the cervical
margin is the gold standard for all-ceramic crown
preparations and the one recommended by the
manufacturers of all ceramic material [19,20].
Tooth preparations are based on maximum
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Nouh I et al.
Mechanical behavior of different machinable ceramic crowns using vertical and horizontal preparations: an in-vitro study
Nouh I et al. Mechanical behavior of different machinable ceramic crowns
using vertical and horizontal preparations: an in-vitro study
preservation of dental substance, especially in the
cervical area where the pulp preparation distance
is of vital importance, both for the strength of
the abutment and to reduce the onset of pulp
complications. The increased mechanical properties
of machinable ceramics allowed clinicians to
consider using minimal preparations as the vertical
preparations. However, the literature contains
an abundant amount of criticism of such vertical
preparations, mainly because of the presence of
an over-contoured restorations and the consequent
fragility of the crown, determined by the limited
thickness of the restoration in the cervical area,
also difculty in processing accuracy and clinical
chipping fractures [21]. However, recently the
literature has been recommending this minimal
preparation for conservation of tooth structure and
better health for the gingival tissues [1,22].
In this in-vitro study, our results showed that
fracture resistance values of vertical preparations
were significantly lower than horizontal
preparations. This could be due to better load
distribution on the 1 mm horizontal margin, the
presence of a denite margin which was capable
of counteracting forces directed to it and more
material thickness. However, the mean fracture
loads for the VP (1432 + 809 N) although it
was less than HP, it still exceeded the maximum
chewing forces reported in the literature in the
posterior region (300 – 850 N) [23]. Accordingly,
this means that vertical preparation design
could be an alternative to the less conservative
chamfer and shoulder margin designs and can
be used in certain clinical situations, such as
severely damaged teeth, endodontically treated
teeth, periodontally affected teeth and implant
supported restorations where margin needs to
be placed deep subgingival following the BOPT
concept [24,25].
Our results agreed with several
studies [19,26-28], which reported that a
1 mm deep shoulder finishing line with a
rounded internal line angle has better fracture
strength for all ceramic crowns and provide more
even stress distribution than other preparation
design. However, other studies reported good
success rates for vertical preparations [29-35] as
Reich et al. [34] who reported knife edge nish
lines had 38% higher fracture load values than
chamfer nish lines [34].
Monolithic zirconia crowns showed
the highest fracture resistance mean values
regardless the preparation design. This could be
attributed to the superior mechanical properties
of zirconia restorations, especially transformation
toughening and resistance to crack propagation
in comparison to silica-based ceramics; lithium
disilicate and advanced lithium disilicate and resin
based nanoceramic. Lithium disilicates however,
had higher fracture resistance mean values than
the posterior bite force and has well documented
long term success when used in feather edge
margin designs [12,36-39]. This justifies the
possible use of advanced lithium disilicate in
vertical preparations as they have shown higher
fracture resistance than LD. This is due to its
unique microstructure of 0.5 μm long lithium
disilicate crystals embedded in a glassy matrix
together with 0.2–0.3 μm platelet like lithium
alumino silicate crystals (Li0.5Al0.5Si2.5O6),
known as virgilite [15]. As for resin nanoceramic,
they had the lowest fracture mean values due to
their weaker resinous matrix [14].
Although vertical preparations provide
more conservative preparations and biological
response to tissues as proven by the literature.
A recent study in the US in 2018, showed
that most dentists still prefer chamfer/heavy
chamfer margin designs, followed by shoulder
preparations [40]. This reflects on the lack
of sufcient data awareness and education of
possible other techniques that can be used by
dentist in different situations.
The choice of machinable ceramic to be
used not only should depend on its mechanical
and esthetic properties but also their biological
compatibility and soft tissue response. Zirconia
has an advantage in terms of biological properties
with soft tissues as it encourages broblasts cell
attachment and has a low surface energy and
hence has low bacterial colonization on its surface
when used as a polished surface in the subgingival
areas [41,42]. This makes it from the authors
opinion one of the most recommended materials
in the subgingival areas of the BOPT. Additional
laboratory and clinical studies are needed to
study the effect of using different machinable
ceramics on the soft tissue response and material
survival in BOPT.
CONCLUSION
Within the limitations of this study the
following can be concluded:
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Nouh I et al.
Mechanical behavior of different machinable ceramic crowns using vertical and horizontal preparations: an in-vitro study
Nouh I et al. Mechanical behavior of different machinable ceramic crowns
using vertical and horizontal preparations: an in-vitro study
1- The use of vertical preparation is a successful
type of preparation design which can preserve
more tooth structure than traditional
horizontal preparations while providing
good biological seal with the surrounding
soft tissue.
2- All tested ceramic materials showed fracture
resistance within the clinically acceptable
range.
Author’s Contributions
IN: Conceptualization. NR: Methodology.
IN, KE: Writing – Original Draft Preparation. NR:
Writing – Review & Editing. KE: Supervision.
Conict of Interest
The authors declare no conict of interest.
Funding
No funding was received.
Regulatory Statement
Authors declare that no ethical committee
approvals were needed for this study.
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Kamal Ebeid
(Corresponding address)
Ain Shams University, Faculty of Dentistry, Fixed Prosthodontics Department, Cairo,
Egypt.
Email: kamal_ebeid@dent.asu.edu.eg Date submitted: 2022 Nov 22
Accept submission: 2022 Nov 30
