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Journal of American Science 2016;12(6) http://www.jofamericanscience.org
29
The Effect Of Different Preparation Designs And Cement Type On The Fracture Resistance Of All-Ceramic
Cantilever Anterior Fixed Partial Dentures
Refaie A.M.1, Salah T.2, El-Etreby A.2, Zohdy M. 2
Department of Fixed Prosthodontics, Faculty of Dentistry, Fayoum University, Egypt
Department of Crown and bridge, Faculty of Dentistry, Ain Shams University, Egypt
Ashrafrefaie87@gmail.com
Abstract: Aims: the aim of this study was to evaluate in vitro the effect of different preparation designs and cement
type on the fracture resistance of all-ceramic cantilever anterior fixed partial dentures. Material and methods: 30
all ceramic zirconia cantilever bridges were constructed and divided according to retainer design into three main
groups (10 bridges each). Each group was further divided according to the type of cement: Resin cement and glass
ionomer cement. The fracture resistance of all-ceramic cantilever anterior fixed partial dentures was measured.
Results: Under non-axial loading, the full coverage retainer provided the best fracture resistance, Three quarter
retainer design provides least fracture resistance, No significant difference between resin or glass ionomer
cementation.
[Refaie A.M., Salah T., El-Etreby A., Zohdy M. The Effect Of Different Preparation Designs And Cement Type
On The Fracture Resistance Of All-Ceramic Cantilever Anterior Fixed Partial Dentures. J Am Sci
2016;12(6):29-35]. ISSN 1545-1003 (print); ISSN 2375-7264 (online). http://www.jofamericanscience.org. 5.
doi:10.7537/marsjas12061605.
Keyword: fracture resistance, all ceramic bridge, preparation design, cement type.
1. Introduction
Due to the increasing interest in esthetics and the
high concerns about the toxic and allergic reactions to
certain alloys, patients and dentists have been looking
for metal-free tooth colored restorations. Several
patient reports concerning the replacement of avulsed
primary, congenitally missing teeth, or permanent
missing incisors were published. The three unit fixed
partial dentures, single implant supported crown
restorations and resin- bonded fixed partial dentures
are the treatment modalities recommended for the
replacement of a missing lateral incisor.
Modifications in tooth preparation designs, and
thus different restoration designs were recommended,
but there are still very few studies that evaluated their
effect on the fracture strength of all ceramic
restorations. The cantilever fixed partial denture
(FPD) is a fixed restoration that has one or more
abutments at one end while the other end is
unsupported. This unique arrangement accounts for
the prime disadvantage: the creation of a Class I lever
system. Many dentists have noted a high incidence of
damage with these restorations; consequently, some
are reluctant to prescribe cantilever FPDs for patients.
(1)
For patients with reduced dentition, treatment
with fixed cantilever restorations is a favorable
alternative to treatment with removable partial
dentures. Patient treated using cantilever prostheses
showed better functional conditions, better oral
hygiene, less caries, and less of a need for dental and
prosthetic treatment than patients with removable
partial dentures. (2, 3)
In recent studies, attention has been given to
causes of failure of cantilever restorations. Distinction
was made between biologic causes such as caries, root
fracture, endodontic and periodontal problems, and
technical causes such as loss of retention or prostheses
fracture. (4)
In general the following can be concluded from
these studies; Caries and endodontic problems are the
main causes of failure, (5-7) the frequency of technical
failure is higher when the restorations have a non-vital
abutment tooth, (6, 7) multiplication of the number of
extension units increases the risk of technical failures,
(7) and a healthy supporting periodontium and a strict
recall are a prerequisite for favorable long term
results. (8)
A further key factor in the performance of
cantilever FPDs is the number of abutments. The
creation of a “super abutment” by splinting abutments
together may limit the forces transmitted to the
abutment adjacent to the pontic. However, “double
abutting” in the provision of cantilever FPDs has a
number of disadvantages, including the involvement
of an additional tooth in the prosthesis and possible
periodontal complications. (9)
All ceramic crowns have become a dependable
treatment modality. As a result, there is interest in
expanding this modality to all-ceramic bridgework.
All ceramic can be used when the cantilevers length is
not more than the mesio-distal dimension of a
Journal of American Science 2016;12(6) http://www.jofamericanscience.org
30
premolar tooth and metal ceramic restorations can be
used in longer situations. (10)
Zirconia is a polymorphic material that occurs in
three temperature-dependant forms that are:
monoclinic (room temperature to 1170 °C), tetragonal
(1170 °C–2370°C) and cubic (2370°C – up to melting
point). The tetragonal-to-monoclinic phase (t-m)
transformation occurs below 1170°C and is
accompanied by a 3-5% volume expansion which
causes high internal stresses. This transformation is
reversible and begins at 950°C on cooling, unless
stabilizing oxides are added. (11, 12)
When stabilizing oxides such as magnesia, ceria,
yttria and calcium are added to zirconia, the tetragonal
phase is retained in a metastable condition at room
temperature, enabling a phenomenon called
transformation toughening to occur. (13)
Yttrium-oxide (Y2O3 3% mol) is added to pure
zirconia to control the volume expansion and to
stabilize it in the tetragonal phase at room
temperature. This Yttrium-oxide partially stabilized
zirconia (Y-TZP) has high initial flexural strength
(900 to 1200 MPa) and fracture toughness (9 to 10
MPa x m1/2). Tensile stresses at a crack tip will cause
the tetragonal phase to transform into the monoclinic
phase with an associated 3-5% localized expansion.
The volume increase creates compressive stresses at
the crack tip that counteract the external tensile
stresses and retards crack propagation. In the presence
of higher stress, a crack can still propagate. The
toughening mechanism does not prevent the
progression of a crack it just makes it harder for the
crack to propagate. (14, 15)
Advances in CAD⁄CAM technology has made it
possible to more readily use zirconia in dentistry. (15)
There are two types of zirconia milling processes
available: soft-milling and hard-milling.
The bonding of zirconia substructures should be
based on both micro-mechanical and chemical
bonding since the micro-mechanical retention
supports chemical bonding and if bonding is based
only on chemical compounds some de-bonding might
happen in moist environments.
Airborne-particle abrasion has been shown to be
a good method for cleaning and roughening the
zirconia surface after clinical try-in procedures, since
the contamination with saliva is known to decrease the
bond strength. (16, 17) Tribochemical silica coating is a
system where a silica layer is formed on the surface of
zirconia ceramic by airborne-particle abrasion with
silica coated alumina particles. MPS silane can be
used on the surface of silica coated zirconia
substructure and efficient chemical bonding can be
achieved. However, tribochemical silica coating seems
to produce durable bonding with some zirconia
ceramics, whereas silica coating particles do not
provide adequate bond to the surface of denser
zirconia ceramics. (18)
Several factors affect fracture resistance such as
modulus of elasticity of the supporting substructure,
properties of the luting agent, loading condition,
surface roughness, residual stress, artificial aging,
tooth-preparation design and restoration thickness. (19)
2. Material and Methods
A right maxillary lateral incisor was removed
from the upper arch of an acrylic typodont*.the socket
of the upper lateral was closed by pink modeling wax
to stimulate an edentulous area of missing lateral
incisor bounded by maxillary right central and canine
acrylic teeth.
Three identical maxillary right canine acrylic
teeth were selected from the refill teeth of the
typodont. Each tooth was prepared to receive a
different all ceramic retainer design; a full coverage,
three quarter and a modified three quarter all ceramic
retainer (fig 1).
Figure 1: abutments preparation
A total of 30 all ceramic zirconia cantilever
bridges were constructed and divided according to
retainer design into three main groups (10 bridges
each). Each group was further divided according to the
type of cement: Resin cement and glass ionomer
cement.
The tests specimens were constructed using the
inlab system, which was compromised acquiring an
optical impression with the Cerec -3 acquisition units,
Journal of American Science 2016;12(6) http://www.jofamericanscience.org
31
designing the restorations with the inlab 3.1X software
and finally milling bridges in the inlab MC XL milling
unit.
When inLab restoration data have been captured,
a dialog box appeared where the material we want to
use for milling the restoration can be chosen. VITA
YZ cubes with appropriate size were selected from the
dialog box. The restorations would be milled with an
oversize of approx.25%to be shrunk subsequently to
the exact fitting final contour in sintering process. The
exact shrinkage data of the respective block are stored
in a barcode on the block itself, which was
automatically read prior to the milling process by the
built-in laser scanner inside the MC XL unit.
Finally, the machining icon was selected and YZ
ceramic block was placed in the milling chamber of
the MC XL unit. Appropriate grinding instruments for
VITA In-Ceram YZ (Step Bur 20 and Cylinder
pointed Bur 20) were selected and mounted in the
milling machine. Clicking OK on the screen starts the
machining (fig2).
Figure 2: YZ block mounted in inlab MC XL milling
chamber
The bridge substructures were placed on their
labial surfaces into the sintering bowl of the high
temperature furnace. The entire surface of the
substructure was supported by glass pearls or beads
for support to avoid deformation. The sintering firing
was then started by pressing the "START" key.
The sintering program was then started to run
automatically; the duration of the program run was
approx.7.5 hours including the cooling phase to 200
°C .the substructures were sintered at a sintering
temperature of 1570°C .
After the sintering process the fit of the
substructure was checked on the die. Ideally, after the
sintering firing no more adjustments should be made
by grinding. If any necessary minor adjustments were
needed, they were done by high speed diamonds under
copious coolant, and then a thermal treatment
(regeneration firing) of the substructure was done in
order to reverse any phase transformations which may
have taken place at the surface. Any micro cracks
which have arisen cannot be regenerated.
Predrying temperature was 500°C.heating up was
5 minutes to reach a firing temperature of 100 °C at a
rate of 100°C/min. the holding time was 15 minute
without vacuum(fig3).
Figure 3: labial view of sintered YZ cantilever
bridges
The YZ bridges were veneered with VITA VM 9
in a ceramic firing furnace fine structure veneering
ceramic. In order to achieve good bonding between
the YZ substructures and VITA VM 9 a base dentine
wash firing was performed according to the following
firing cycle: Predrying temperature was 500°C with a
2 minute time. Heating up time was 7.27 minutes to
reach a firing temperature of 950°C at a rate of 60°C
/min. the holding time was 1 minute with vacuum.
The entire surface of the bridges was covered
with VITA AKZENT Glaze.
The fitting surfaces of the retainers were
sandblasted with 50 µm Al2O3 at a maximum pressure
of 2.5bar for 30 seconds at an approximate distance of
2 cm. the fitting surface was not touched after that
until cementation to avoid contamination with
impurities.
Figure 4: Epoxy cast mounted in custom made jig
Journal of American Science 2016;12(6) http://www.jofamericanscience.org
32
Addition polymerization silicon impression
material was used for taking impression for each
prepared canine tooth after being placed in its socket
of the typodont. The impression included the prepared
canine tooth, the upper right lateral incisor edentulous
area and the upper right central incisor. Impressions
were poured after one hour according to the
manufacturer instructions with Epoxy resin.
The samples were subjected to lateral loading
using universal testing machine. Fracture resistance
test was conducted, in order to identify the site and
mode of failure for each retainer design under the
investigated load direction (fig4).
Load was applied with a custom made load
applicator (steel rod with round end 3.4 mm diameter,
placed palataly 2mm below the incisal edge of the
pontic) attached to the upper movable compartment of
the machine until the very first discontinuity resulting
from an early crack, debonding or catastrophic failure
of bridge and/or die was detected. Then the results
were tabulated and statistically analyzed (fig5).
Figure 5: lateral loading (proximal view)
3. Results
The mean values and standard deviation of
fracture resistance (N) as function of preparation
design and cement are summarized in table (4) and
graphically drawn in figure (50).
Table 1: Fracture resistance results (Mean values± SDs) as function of preparation design and cement type
Variables
Cement
GIC
Resin
Mean
±
SD
Mean
±
SD
Preparation design
Partial coverage
327.07
23.10
195.58
2.94
Modified p. coverage
411.11
83.30
334.27
60.58
Full coverage
336.12
15.11
420.64
57.36
Different letter in the same column indicating statistically significant difference (p < 0.05)
*; significant (p < 0.05) ns; non-significant (p>0.05)
Figure 1: Histogram of fracture resistance mean
values as function of preparation design technique and
cement type
4. Discussion
This study was performed to investigate the
fracture resistance of yttrium stabilized zirconia all
ceramic cantilever anterior bridges as influenced by
the of preparation design and cement type.
Yttrium stabilized zirconia has been selected
because of high initial flexural strength ad fracture
toughness. Zirconia has mechanical properties similar
to that of stainless steel. Its resistance to traction can
be as high as 900-1200 MPa and its compression
resistance is about 2000 MPa. [20,21]
In order to produce a ZrO2 core for a prosthetic
Restoration. It is necessary to use a computer aided
design / manufacturing (CAD/CAM) system that can
deal with zirconia and create a fitting framework.
Various production techniques have been developed
for enhancing the fabrication of consistent and
predictable restorations in terms of strength, marginal
fit, and esthetics. The Cerec system (Sirona,
Bensheim, Germany), was selected since it is the most
famous system that has been marketed for several
years (since 1986). [20,22]
All ceramic anterior cantilever bridges retainer
and the adjacent pontic may be subjected to various
forces during function. In this study force directed 45 °
on the palatal surface of the pontic to subject the
restoration to torque force to evaluate the résistance,
as stresses that affect all-ceramic cantilever anterior
bridges during mastication and protrusive mandibular
excursions are complex and not usually directed
parallel to the long axis of a tooth. [23,24]
Investigating test specimens on human teeth has
however a touch of realism, but they are often
0
100
200
300
400
500
GIC Resin
Cement
Failure Load (N)
Partial coverage
Modified partial coverage
Full coverage
Journal of American Science 2016;12(6) http://www.jofamericanscience.org
33
replaced by the easily available metal alloy or resin
materials this is because, such materials in contrast to
human teeth ensure that the studies can be carried out
using identical tooth dimension. [25] The difficulty in
standardizing natural abutment teeth is due to the large
variation in their size and condition which depends on
their age and individual structure. Investigations that
considered the fracture pattern of natural teeth showed
a different fracture mode of the extracted tooth
compared with teeth in vivo, implying that the dentin
thickness remaining after preparation influences the
fracture strength. [26]Human and bovine teeth were
also avoided because of the variability in their
mechanical strengths resulting from material
anisotropy and the variable pulp size. [27,28]
Before cementation the fitting surfaces to be
cemented were sandblasted with 50µm Al2O3 at a
pressure of ˂ 2.5 bar. The dual cure resin cement (G-
CEM Capsule) and Chemical cured glass ionomer
(GC Fugi I Capsule) were used for cementation of the
test specimens without the need for silication and
silanization of the surfaces. [20, 29]
All ceramic anterior cantilever bridges retainer
and the adjacent pontic may be subjected to various
forces during function. In this study force directed 45 °
on the palatal surface of the pontic to subject the
restoration to torque force to evaluate the résistance,
as stresses that affect all-ceramic cantilever anterior
bridges during mastication and protrusive mandibular
excursions are complex and not usually directed
parallel to the long axis of a tooth. [23,24]
The Effect of Preparation Design
In some investigations, steel or resin dies have
been used for fracture testing of all-ceramic crowns
and bridges. [30,31] It can be argued that a standard steel
or resin die, enforces consistent preparation shape and
identical physical quality of the abutments under
loading; however, steel or resin abutments do not
reproduce the actual force distribution that occurs on
crowns cemented to natural teeth. [32]
Rosentritt et al. [33]: investigated the influence of
the abutment material on the fracture resistance of all-
ceramic (FPDs) using human, polymer and alloy
abutments. The application of the alloy abutments lead
to an overestimation of the fracture resistance of the
all-ceramic (FPDs). However, studies using resin
material for the abutment teeth reported similar
fracture forces for zirconia-based (FPDs). [34]
In the present study to better simulate clinical
conditions, epoxy resin material are selected as die
material on which the test specimens are cemented for
investigating their fracture resistance since its
modulus of elasticity is similar to the reported
modulus of human dentin and bone(18.3GPA). [35,36]
Three preparation designs were investigated
during this study. The full coverage, as a standard
preparation, that provides the ultimate resistance and
retention to restorations. The conventional three
quarter preparation, a well suited design for short span
anterior bridges as it provides good mechanical and
esthetic properties as well as being conservative. [37,38]
The obtained results showed that under 45° load
the full coverage resist rotation, the modified partial
coverage design exhibited the statistically significant
higher mean of fracture resistance than the partial
coverage design, this may be due to that, under 45°
load, the 2 mm incisal reduction done as a preparation
modification had increased the resistance to rotation
than that exhibited by the conventional partial
coverage design.
This justifies the claim that appropriate crown-
preparation design is necessary if retention and
resistance of cantilever FPDs are to be maximized.
[39,40]
It has been claimed that the tooth furthest from
the pontic especially should be extremely retentive to
resist dislodgment. [39]
The effect of cement type
Zirconia surface cannot be etched due to the
absence of the glassy component in its structure, thus
its surface was only sandblasted with 50μm Al2O3 at
less than 2.5bar according to manufacturer's
instructions to create a roughened surface and increase
micro-mechanical retention. [41]
Glass ionomer cement was chosen over other
conventional cements because of its bacteriostatic
effect through fluoride release properties, chemical
adhesion to tooth structure, low coefficient of thermal
expansion to maintain that bond and good physical
and mechanical properties. [42] Its use is recommended
by manufacturer to cement zirconia restorations which
have high strength. [43]
Self-Adhesive Resin cement was chosen as it
eliminates the need of pretreatment of the abutments.
(44) It also contains phosphate monomer which has a
high affinity to zirconia and enters into a durable
chemical bond with the sandblasted surface of zirconia
forming low soluble stable phosphate compounds
without the need for silication and silanization of the
surfaces. It was dual cured as the thickness of the
crown might not allow the light to penetrate through
its full thickness. [45,46]
However, there was no significant difference of
fracture resistance mean values between resin cement
and glass ionomer. [47]
The correlation between preparation design and
cement type:
The clinical performance of all-ceramic
cantilever FPDs is challenging, because available data
for maximum clinical forces and in particular on
cantilever FPDs vary widely for cantilever FPDs,
maximum local forces between 150N anteriorly and
Journal of American Science 2016;12(6) http://www.jofamericanscience.org
34
700N posteriorly were reported. [48] (According to the
present study , stress under 45° load they range from
195.5 N to 420.6 N which was well beyond the 150N
required [49], predicting promising prognosis of such
designs.
In case of full coverage retainer design: resin
group recorded statistically non-significant higher
fracture resistance mean value (420.64N) than GIC
group (336.12N).
In case of Partial coverage group: It was found
that GIC group recorded statistically significant higher
fracture resistance mean value (327.07 N) than resin
group (195.58 N).
In case of Modified partial coverage group: It
was found that GIC group recorded statistically non-
significant higher fracture resistance mean value
(411.11 N) than resin group (334.27 N).
Conclusions:
Within the limitations of this study the following
conclusions could be obtained:
1- Under non-axial loading, the full coverage
retainer provided the best fracture resistance.
2- Three quarter retainer design provides least
fracture resistance.
3- No significant difference between resin or
glass ionomer cementation.
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