M. S. OZDOGAN et al.: EFFECT OF STRAIGHT AND ANGLED ABUTMENTS ON THE STRAIN ...
251–257
EFFECT OF STRAIGHT AND ANGLED ABUTMENTS ON THE
STRAIN ON A ZIRCONIA CROWN AND IMPLANT IN THE
MANDIBULAR SECOND MOLAR REGION: A FEA-BASED
STUDY
VPLIV POKON^NIH IN PO[EVNIH OPORNIKOV NA
DEFORMACIJE CIRKONSKIH KRON IN VSADKOV V DRUGEM
MOLARNEM PODRO^JU ^ELJUSTI: [TUDIJA NA OSNOVI
METODE KON^NIH ELEMENTOV
Mahmut Sertac Ozdogan
1
, Harun Gokce
2
, Ismail Sahin
3*
1
Ankara Yildirim Beyazit University, Faculty of Dentistry, Ankara, 06220, Turkey
2
TUBITAK Defence Industries Research and Development Institute, Ankara, 06261, Turkey
3
Gazi University, Technology Faculty, Department of Industrial Design Engineering, Ankara 06500, Turkey
Prejem rokopisa – received: 2019-06-07; sprejem za objavo – accepted for publication: 2019-09-30
doi:10.17222/mit.2019.121
In a literature search no study evaluating implant-supported posterior restorations using zircon crowns with straight and angled
abutments was found. In this study, the stress distribution of a zircon-crown restoration that will be prepared by using an angular
and straight abutment in a mandibular second-molar deficiency, under functional forces, stress over zircon crown and
environmental tissues of implant will be evaluated using the finite-element analysis (FEA) method. The FEA result showed that
the use of straight implants contributes to the lowest stresses. The selection of a straight and angular abutment is very important
for the balance of distributed loading.
Keywords: dental implant, zirconia, finite-element analysis
Avtorji ~lanka v raziskavi `e objavljene literature na to temo niso na{li nobene {tudije, ki bi ovrednotila obnovo zob z vsadki, pri
katerih so bile uporabljene cirkonske krone s pokon~nimi ali po{evnimi oporniki. V {tudiji avtorji analizirajo porazdelitev
napetosti v obnovljeni cirkonski kroni, ki je bila stabilizirana z uporabo po{evnih ali pokon~nih opornikov v drugem molarnem
segmentu ~eljusti. S pomo~jo metode kon~nih elementov (FEA, angl.: Finite Element Analysis), so pod obremenitvijo s
funkcionalnimi silami ovrednotili nastale napetosti na cirkonskih kronah in v obdajajo~em se tkivu (dlesnih). Rezultati FEA so
pokazali, da uporaba pokon~nih vsadkov povzro~a manj{e napetosti. Avtorji ugotavljajo {e, da je pravilna izbira pokon~nih ali
po{evnih opornikov zelo pomembna za uravnote`eno porazdelitev obremenitev.
Klju~ne besede: zobni vsadek, cirkonijev oksid, analiza na osnovi metode kon~nih elementov
1 INTRODUCTION
Dental epidemiological studies have shown that
single teeth are common in all age groups.
1
A single
premolar or molar loss is considered to be a common
cause of non-physiological occlusion resulting from the
extrusion and overturning of adjacent teeth and counter-
teeth.
2
For this reason, the continuity of an occlusion is
important. It is controversial whether the missing second
molars should be completed with a prosthetic
treatment.
3–5
For the second mandibular deficiency, there
are several restorative options for a prosthodontic
treatment. A fixed, partial cantilever prosthesis does not
appear to be a preferred treatment, since they have
produced undesirable biomechanical results, and re-
movable partial prostheses could provide disturbing
function and use.
6
Inspite of these arguments, the
negative consequences of failing to compensate for the
mandibular second molars could be the deterioration of a
balanced occlusion, overeruption of the contralateral
tooth, and the incompatibility of the chewing system, in
favor of compensating the missing second molars with a
prosthesis.
7
As a result, the use of single implants for
second-molar restoration seems to be a preferred
treatment. The use of dental implants in cases of partial
toothlessness has been accepted as a modern clinical
method with long-term success.
8
In order to achieve this
success, technical and biomechanical decisions must be
made carefully.
9
As a result of a study conducted by M. S. Kim et
al.,
10
more than 90 % of patients stated that they were
functionally satisfied with a second-molar single-implant
treatment. These findings suggest that a second-molar
single implant can increase the chewing function. The
ultimate goal of the implant treatment is that it is
functional, aesthetic and easily applicable.
11,12
For this
purpose a correct implant placement is essential. Precise
radiographic images of the potential recipient sites and
Materiali in tehnologije / Materials and technology 54 (2020) 2, 251–257 251
UDK 620.174.2:616.314-089.843:669.296 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(2)251(2020)
*Corresponding author's e-mail:
isahin@gazi.edu.tr (Ismail Sahin)

appropriate surgical guidelines are necessary to place the
implants in their predetermined positions.
13
The implant
placement is usually not ideal due to the morphology of
the existing bone. One solution to this clinical problem is
to use angled abutments.
14
The anatomy of the jaws and the morphology of the
edentulous crest determine the orientation and angle of
the implant placement. Similarly, the position and mor-
phology of the teeth are determined by aesthetic and
functional considerations. In most cases there is a diffe-
rence between the long axis of the implant and the long
axis of the planned dental implant. The main factor lead-
ing to long-term failure in implant-supported restorations
is the lack of understanding of biomechanical concepts.
15
Ideally, the implants should be positioned parallel to
the adjacent teeth and aligned vertically with the axial
forces. However, it may not be possible for anatomical
reasons. The mandibular canal position, crest anatomy,
proximity to sinuses are other factors that can affect im-
plant placement.
16–18
The angle of the abutment used for the prosthetic
restoration is one of the important factors affecting the
stress distribution in the implant bone interface. Studies
show that the use of angled abutments can cause a stress
reduction. K. Tian et al.
19
who examined the tension in
tissues surrounding the implant with the use of angled
abutments on one-unit dental implants in their FEA
study, reported that the use of angled abutments reduced
the stresses and showed a better stress distribution at the
implant-bone interface. On the other hand, N. L.
Clelland et al.
20
in their study using photoelastic stress
analysis reported that angled and straight abutments
cause an increased stress in the bone and prosthesis, but
this is within the physiological limits.
N. T. Inijan et al.
21
reported that in their FEA and a
natural mastication cycle study in implant abutment
structures at different angles, the maximum stress can be
minimized by changing the implant angle in the cervical
region of the implant. Nowadays, in the construction of
implant-supported restorations, especially in the aesthe-
tic area of the dental arch, high-strength, all-ceramic
materials are used.
22
The mechanical properties of
zirconia allowed them to use in posterior fixed partial
prostheses and significantly reduce the thickness of the
cores.
23
In a literature search there was no study evaluating
implant-supported posterior restorations using zircon
crowns with straight and angled abutments. In this study
the stress distribution of a zircon-crown restoration that
will be prepared by using angular and straight abutments
in mandibular second-molar deficiency, under functional
forces, stress over the zircon crown and environmental
tissues of implant are evaluated by the FEA method.
2 MATERIALS AND METHODS
This study was designed to examine the tension
occurring on the implant placed in the buccal direction
using a 20-degree abutment and on the zircon crown due
to the anatomical imperfections of the implant placement
in the lower molar region. In the present study, the
finite-element method (FEM) is used to find different
stresses and strains caused by different angulated
abutments of an implant prosthesis that is subjected to
constant load.
2.1. Creation of 3D model
In the present study, a 3D model of an implant
prosthesis, which was used in place of a tooth, was
generated using SLICER 4.11 Software. This software is
creating a STL model from tomography results. It also
supports different data formats like BRE, STL, PLY and
VRML. In order to obtain a 3D model whose point cloud
data (STL) were generated, Quick Surface Reconstruc-
tion and Digitized Shape Editor modules of CATIA
V5R2015 software were utilized. The point cloud data
transferred to the CATIA V5 software is in Stereo
Lithography (STL) format. The number of points
generated is 230077.
A mesh model needs to be created between the points
for a re-modeling over the point cloud. For this purpose,
the mesh model is generated, with the help of the Mesh
Building Wizard, based on the maximum and minimum
length data between the dimensions of the mesh and the
points. The structure of the mesh was designed so that
M. S. OZDOGAN et al.: EFFECT OF STRAIGHT AND ANGLED ABUTMENTS ON THE STRAIN ...
252 Materiali in tehnologije / Materials and technology 54 (2020) 2, 251–257
Figure 1: a) 0-deg 3d model cross-section, b) 20-deg 3d model cross-section

the maximum dimension of the mesh was 1.5 mm and
the maximum distance between the points was 0.1 mm.
It is very important to minimize the amount of
deviation between the 3D model generated and the
point-cloud data. For this purpose, a deviation analysis
needs to be conducted for all the surfaces generated
through zone definitions. The deviation analysis is
studied taking into account the dimension of the part, the
running accuracy and the regional stability variables. The
maximum allowable deviation for this model is 0.05 mm.
For this reason, an analysis was performed on every
surface generated. The implant, abutment and crown
models of which cross-sections are presented in Fig-
ure 1.
Table 1: Number of elements of all the constituent parts
Model name Number of nodes Number of elements
Cortical bone 146.245 96.882
Trabecular bone 121.632 68.548
Gingiva 135.227 88.167
Crown 28.414 18.380
Cement 6.358 3.241
Implant 63.918 38.491
Abutment 9.963 5.768
Abutment screw 638 342
2.2. Computational analysis
ANSYS V17.2 FEA software was used to determine
the stress distribution on the implant. The 3D solid
model obtained with CATIA V5 was transferred to
ANSYS Design Modeller and a 3D solution mesh was
created with Mesh Generation. The FEM is a numerical
method that allows us to obtain information about the
structure by dividing the structure into a finite number of
small elements and by solving a finite number of
equations instead of an infinite number of equations. For
this reason, the established solution mesh is vital for the
calculation result. An adaptive mesh is applied in the
FEM. The total number of solution meshes used in the
constituent parts is given in Table 1.
Two different FEAs were performed for the abutment
geometry at 0° and 20° angles. The same boundary
conditions were determined for all the solutions and a
250 N load was applied
40
on the functional cusp in the
vertical axis direction through the coating material on the
implant placed on the lower-right second. However, the
surface of the bone is fixed in all directions from the
base. It is accepted that the implants are 100 %
bone-implant contact (100 % bone-implant contact). The
following figure shows the boundary conditions used in a
linear static analysis.
Static results were obtained for the two different
abutment geometries using the ANSYS V17.2 FEA
software. In the creation of the model, the lengths are
given as μm units and the modulus of elasticity is given
by N/μm
2
unit. In the model; the height of the crown is
7000 μm, the width of the crown is 5000 μm, the
thickness of the adhesive is 50 μm and the height of the
implant is 4950 μm. The usability of the model was
controlled by ensuring that the sum of the forces with
astatic equilibrium is equal to zero (the resultant force is
equal to zero).
M. S. OZDOGAN et al.: EFFECT OF STRAIGHT AND ANGLED ABUTMENTS ON THE STRAIN ...
Materiali in tehnologije / Materials and technology 54 (2020) 2, 251–257 253
Table 2: Properties of materials used in the FEA
Model name Material Young’s modulus (GPa) Poisson’s ratio (u) References
Cortical bone – 18.6 0.30 K. S. Lee at al.
41
Trabecular Bbone – 13.7 0.30
Gingiva – 10.0 0.40
Crown Zirconia 205.0 0.19
Cement Resin adhesive cement 18.6 0.28
Abutment Titanium (grade IV) 110.0 0.35
Implant Titanium (grade IV) 110.0 0.35
Abutment screw Titanium (grade IV) 110.0 0.35
Figure 2: a) Relationship between maxillary and mandibular arches, b) 3D mesh model

3 RESULTS
In Figure 3 a 250-N load is applied on the crown
surface in the directions of Y and Z axes and the total
deformations on the crown are shown. When the
deformation solutions formed on the crown are
examined, it is observed that the solutions made in the
model using an angled abutment give the lowest values.
Figure 4 shows the highest stresses on the abutment.
It was seen that the highest stresses were on the tooth
tops, which were the connection surface with the im-
plant. Stresses in the straight and angled abutment
models were determined at similar points. When the
results were examined, it was revealed that the use of
straight abutments provided the lowest stresses.
The lowest stresses on the implant were examined. It
was observed that the lowest stresses were formed on the
axial surface of the implant and the highest stresses were
on the tooth surfaces with the abutment. It has been
shown that the use of straight implants contributes to the
lowest stresses.
When the von Misses stress distribution is shown in
Figure 6, it is a very critical region because the highest
stresses are at the junction of the crown and the adhesive
cement layers and also on the outer surface as a geo-
metric settlement. Considering the properties of the
material, this stress distribution in general, the formation
of cracks in the model and separation between layers can
be estimated. When we look at the von Misses stresses, it
M. S. OZDOGAN et al.: EFFECT OF STRAIGHT AND ANGLED ABUTMENTS ON THE STRAIN ...
254 Materiali in tehnologije / Materials and technology 54 (2020) 2, 251–257
Figure 3: Total deformation on zirconia crown with: a) straight 0°,
b) angled 2°
Figure 5: Maximum and minimum principal stresses on the implant with: a) straight 0°, b) angled 20°
Figure 4: Maximum and minimum principal stress on the abutment with: a) straight 0°, b) angled 20°

was determined that the use of straight implants showed
better results than other applications (Figure 7).
4 DISCUSSION
There are three different ways in which stress anal-
yses can be performed: analytical, experimental and
computational. In fact the most important one is the
computational, in which the stress analysis would seem
to be the most natural way to approach this problem.
With an increase in the computational power, the use of
the FEM can help to simulate with a great accuracy the
functioning of the human tooth.
24
Numerical and experimental studies have shown that
the increased abutment angle results in more stress on
the prosthesis and surrounding bone than straight
abutments. However, a prosthesis with angled abutments
did not show a decrease in lifetime. There was no more
bone loss than with straight abutments.
25
There may also
be anatomical reasons that prevent the implant’s place-
ment in a second molar region. Anatomical constraints
sometimes require surgical positioning of the implants at
angles that are not optimal for prosthetic restorations.
The width, height, angle, presence of the bone undercut,
the shape of the arch and the relationship between the
jaws are the considerations for implant placement.
For example, there may be insufficient bone height or
width to place an implant without the use of a bone graft,
or the position of the inferior alveolar nerve may impede
the placement of the second molar implant without per-
forming bone augmentation.
18
Another possibility of
implant placement in this region is the use of angled
implant abutments. There are angled abutments at speci-
fic separation angles; furthermore, a customized angled
abutment can be prepared for an acceptable prosthesis
treatment.
26
Historically, the need to change the angle of
the abutment has been understood as a result of the diffe-
rence between the bone present for implant placement
and the long axis of the planned restoration. However,
there are concerns about the negative effects of non-axial
forces on the survival of implants.
27
The preferred inclination of implants is a common
problem that can jeopardize the aesthetics and function
of implant-supported fixed prostheses.
28
The buccolin-
gual folding of the mandibular posterior toothless region
can affect the conditions of prosthetic loading, thus
causing high-stress areas that can easily cause stress.
29
A. Ellakwa et al.
30
showed that using angled implants
reduces the crown fracture resistance on the implant. The
results of the present study demonstrate that when the
deformation solutions formed on the crown are exam-
ined, it is observed that the solutions made in the model
using an angled abutment give the lowest values. This
result can be thought of as the imposition of implants on
the buccal and the application of loads on the functional
cusp.
X. A. Saab et al.
31
showed the effect of the abutment
angulation on the strain in the anterior maxilla by the
FEA. There was no difference in the pattern of the strain
distribution predicted using a straight or an angled
abutment in the anterior maxilla. J. Cavallaro et al.
25
and
B. Rohit et al.
32
evaluated five abutment divergences (0°,
15°, 25°, 30°and 35°). On the basis of available data in
the literature, they concluded that though the com-
M. S. OZDOGAN et al.: EFFECT OF STRAIGHT AND ANGLED ABUTMENTS ON THE STRAIN ...
Materiali in tehnologije / Materials and technology 54 (2020) 1, 251–257 255
Figure 7: von Misses stress on bone: a) straight 0°, b) angled 20°
Figure 6: von Misses stress on abutment and implant with: a) straight 0°, b) angled 20°

pressive and tensile stresses generated through axial and
oblique loading increase as the abutment angulation
increases, they are within the tolerance limits of the
bone.
Z. Arsalanloo et al.
16
found that by increasing the
abutment angulations, the amount of strain in the implant
and cortical bone could be higher. T. Brosh et al.
33
and
T. Begg et al.
34
also verified that vertical loads applied in
angled abutments produced higher stresses at the coronal
zone of the implant compared with straight abutments.
To avoid high stresses at that location, the implant
should be planned to be subjected to vertical forces
during function.
35
There seems to be no difference in the
survival of implants based on the use of angulated
abutments ranging from 0° to 45°. T. J. Balshi et al.
36
have also demonstrated that the survival of implants
loaded via 30-degree abutments is not significantly
different from implants loaded via straight abutments.
When the stresses on the abutments were examined, it
was determined that the stresses occurred in the straight
and angled abutment models at similar points. When the
results were examined, it was revealed that the use of
straight abutments provided the lowest stresses.
With the clinical loading of implants restored using
angled abutments, lateral occlusal forces can increase. N.
L. Clelland et al.
37
found a statistically significant in-
crease in the stress and strain with an increase in the
abutment angulation when evaluating 0-°, 15-°, and 20-°
abutments, but the principal strains were considered to
be within the physiological zone for bone. R. Celletti et
al.
38
observed no adverse effect on the surrounding bone
with straight or pre-angled abutments in monkeys. Based
on a histological examination at 1 year after loading,
excellent osseo-integration was reported.
In the present study, when the stresses on implants
were examined, it was determined that the highest and
lowest stress zones are at the same points. It has been
shown that the use of straight implants contributes to the
lowest stresses. Increases in the abutment angulation can
increase the principle strains (compressive and tensile) in
the bone around the implants, as shown by in-vitro
strain-gauge studies.
36
Whether or not a second molar should be replaced
after its removal is debatable. The data indicated that
replacing a second molar provides some increased masti-
catory performance, but first-molar occlusion facilitates
a 90 % chewing efficiency. It was concluded that after a
patient/dentist discussion regarding second-molar
replacement, it is the patient’s preference that usually
dictates the decision.
39
5 CONCLUSIONS
Within the limitations of this study, the following
conclusions can be drawn:
• The FEM is the nearest-possible method available
today to simulate the oral cavity in vitro.
• FEA is a numerical method for addressing mecha-
nical problems and it is a powerful contemporary
research tool.
• FEA results have shown that the use of straight
implants contributes to the lowest stresses.
• Similar to the literature, the selection of straight and
angular abutments is very important for the balance
of a distributed loading.
• This will continue to be studied due to the limitations
of the tooth’s anatomy.
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