The effect of thread design on stress distribution in a solid screw implant: a 3D finite element analysis.

Abstract

The biomechanical behavior of implant thread plays an important role on stresses at implant-bone interface. Information about the effect of different thread profiles upon the bone stresses is limited. The purpose of this study was to evaluate the effects of different implant thread designs on stress distribution characteristics at supporting structures. In this study, three-dimensional (3D) finite element (FE) stress-analysis method was used. Four types of 3D mathematical models simulating four different thread-form configurations for a solid screw implant was prepared with supporting bone structure. V-thread (1), buttress (2), reverse buttress (3), and square thread designs were simulated. A 100-N static axial occlusal load was applied to occlusal surface of abutment to calculate the stress distributions. Solidworks/Cosmosworks structural analysis programs were used for FE modeling/analysis. The analysis of the von Mises stress values revealed that maximum stress concentrations were located at loading areas of implant abutments and cervical cortical bone regions for all models. Stress concentration at cortical bone (18.3 MPa) was higher than spongious bone (13.3 MPa), and concentration of first thread (18 MPa) was higher than other threads (13.3 MPa). It was seen that, while the von Mises stress distribution patterns at different implant thread models were similar, the concentration of compressive stresses were different. The present study showed that the use of different thread form designs did not affect the von Mises concentration at supporting bone structure. However, the compressive stress concentrations differ by various thread profiles.

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ORIGINAL ARTICLE
The effect of thread design on stress distribution in a solid
screw implant: a 3D finite element analysis
Oğuz Eraslan & Özgür İnan
Received: 16 December 2008 /Accepted: 8 June 2009 /Published online: 20 June 2009
# Springer-Verlag 2009
Abstract The biomechanical behavior of implant thread
plays an important role on stresses at implant-bone
interface. Information about the effect of different thread
profiles upon the bone stresses is limited. The purpose of
this study was to evaluate the effects of different implant
thread designs on stress distribution characteristics at
supporting structures. In this study, three-dimensional
(3D) finite element (FE) stress-analysis method was used.
Four types of 3D mathematical models simulating four
different thread-form configurations for a solid screw
implant was prepared with supporting bone structure. V-
thread (1), buttress (2), reverse buttress (3), and square
thread designs were simulated. A 100-N static axial
occlusal load was applied to occlusal surface of abutment
to calculate the stress distributions. Solidworks/Cosmos-
works structural analysis programs were used for FE
modeling/analysis. The analysis of the von Mises stress
values revealed that maximum stress concentrations were
located at loading areas of implant abutments and cervical
cortical bone regions for all models. Stress concentration at
cortical bone (18.3 MPa) was higher than spongious bone
(13.3 MPa), and concentration of first thread (18 MPa) was
higher than other threads (13.3 MPa). It was seen that,
while the von Mises stress distribution patterns at different
implant thread models were similar, the concentration of
compressive stresses were different. The present study
showed that the use of different thread form designs did
not affect the von Mises concentration at supporting bone
structure. However, the compressive stress concentrations
differ by various thread profiles.
Keywords Implantthreadformdesign.Dentalimplants.
Biomechanics.Finiteelementanalysis.Stressdistribution
Introduction
Dental implants function to transfer load to surrounding
biological tissues [12]. Thus, the primary functional design
objective is to manage (dissipate and distribute) biome-
chanical loads to optimize the implant-supported prosthesis
function [12]. Implant thread configuration is an important
objective in biomechanical optimization of dental implants
[2, 7, 15, 20, 21]. The implant-bone interface can be easily
compromised by high stress concentrations that are not
dissipated through the implant configuration [8]. It is
necessary that biomechanical concepts and principles are
applied to thread design of dental implant to further
enhance clinical success [8].
Thread geometry includes thread pitch, depth, and shape
[7, 12]. Although thread pitch and depth could affect the
stress distribution, traditionally, the manufacturers have
provided implant system a constant pitch and depth [8]. So,
for commercial implant system, a better design of thread
configuration is emphasized [8].
Threads are designed to maximize initial contact,
enhance surface area, and facilitate dissipation of stresses
at the bone-implant interface [10]. Thread shapes in dental
implant designs include square, V-shape, and buttress [12].
In conventional engineering applications, the V-thread
design is called a “fixture” and is primarily used for
fixating metal parts together, not load transfer [17]. The
buttress thread shape was designed initially for and is
Clin Oral Invest (2010) 14:411–416
DOI 10.1007/s00784-009-0305-1
O. Eraslan (*):Ö. İnan
Department of Prosthodontics, Faculty of Dentistry,
University of Selcuk,
42079 Kampus,
Konya, Turkey
e-mail: oguzeraslan@selcuk.edu.tr

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optimized for pullout loads [12]. The square thread
provides an optimized surface area for intrusive, compres-
sive load transmission [12].
A key factor for the success or failure of a dental implant
is the manner in which stresses are transferred to the
surrounding bone [22]. Shear loading is reported as the most
detrimental loading profile for bone [12]. The reduction in
shear loading at the thread-to-bone interface provides for
more compressive load transfer, which is particularly
important in compromised D3 and D4 bone [12]. The finite
element analysis (FEA) allows researchers to predict stress
distribution in the contact area of implants with cortical bone
and around the apex of implants in trabecular bone [8]
without the risk and expense of implantation [5]. FEA is
utilized at current study to evaluate the effects of different
implant thread designs on stress distribution characteristics at
supporting structures. The null hypothesis was that different
implant thread shape does not affect the stress distribution
within supporting structures.
Materials and methods
The study was conducted using a 3D FE method and the
Solidworks 2007 9.0.3 structural analysis program (Solid-
works Corporation, Concord, MA, USA). An ITI solid
cylindrical screw implant, 3.8-mm diameter, 10-mm bone
sink depth (Straumann AG, Waldenburg, Switzerland) was
modeled. The implant system was modeled as a single body
unit with its abutment. Cortical and cancellous bones are
also modeled representing the cross-section of the posterior
human mandible. Two-millimeter thickness of cortical bone
was modeled around the cancellous bone and implant neck.
The 3D solid screw implants were modeled with similar
conditions of same thread number, position, height (D), and
pitch (P) (Fig. 1). Initially, cross-sections of structures
included in mathematical model were sketched at front and
right planes separately for each unit at computer environ-
ment. Coordinates of the contouring points were then
entered as border nodes of mathematical models. These
nodes were joined to form each structures 3D volume that
together defined the final geometry of FE model.
In order to better understand the stress distribution, four
different thread-form configurations are compared: V-
thread, buttress, reverse buttress, and square shape thread
forms of 0.4 mm thread width (L; Fig. 2). The geometric
models were meshed with tetrahedral quadratic elements
(Fig. 3). Each mathematical model included approximately
186,000 nodes and 133,000 solid elements. The bottom
exterior nodes of the alveolar bone in the FEM models were
fixed in all directions as the boundary condition (Fig. 3). A
100-N static axial occlusal load was applied to occlusal
surface of abutment to calculate the stress distributions
(Fig. 3).
Materials used in study were assumed to be homogenous
and isotropic. Elastic properties of materials (Young’s
modulus (E) and Poisson’s ratio (μ)) were determined from
the literature and given in Table 1. The FE modeling was
accomplished with the Solidworks software program, and
analyses were run at Cosmosworks software program,
which is integrated with Solidworks.
Results
Results were presented by considering von Mises criteria
[1, 13, 18, 19, 25], and principal compressive stress field.
Calculated numerical data were transformed into color
graphics to better visualize mechanical phenomena in the
models. Both 3D whole model view and mesiodistal cross-
sectional views were presented for each thread type. All
stress values were indicated in mega pascals (MPa).
The analysis of the von Mises stress values revealed that
maximum stress concentrations were located at loading
areas of implant abutments for all models (Fig. 4). Also,
high stress values were located at cervical cortical bone
regions adjacent to implants at all models. It can be seen
that the stress concentration at cortical bone structure was
higher than that of spongious bone at 3D whole model view
(Fig. 4).
When the mesiodistal cross-sectional views of bone
structures (Fig. 5) were evaluated, it was seen that the stress
distribution patterns at different implant thread models were
similar. However, the maximum von Mises stress values at
different models were not similar; 17.7 MPa for V-thread,
18.3 MPa for buttress thread, 18.2 MPa for reverse buttress
thread, and 17.3 MPa for square thread type. These
maximum values were observed at cervical cortical bone
regions adjacent to implants and at bone structure adjacent
to first thread which was located at cortical bone structure.
Also, from this cross-section view, it was clearly seen that
Fig. 1 Schematic illustration of thread profile properties used in
study. Threads pitch (P), height or length (L), depth (D)
412 Clin Oral Invest (2010) 14:411–416

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stress concentration at cortical bone structure (17.3–
18.3 MPa) was higher than that of spongious bone (11.7–
13.3 MPa). Also, the stress concentration of bone structure
adjacent to first thread which was located at cortical bone
structure was higher than that of other threads which were
located at spongious bone.
When the compressive stress values around the implants
were evaluated (Fig. 6), maximum compressive stress value
was 18 MPa, and this value was observed at all four thread
types. Nevertheless, the intensity (area covered) of this
maximum stress value was different among the different
thread types. The maximum stress intensity area covered
the biggest area at reverse buttress thread type. It was
similar at buttress and square thread type, and it was lower
than reverse buttress thread type. The minimum stress
intensity area was observed with V-thread shape.
Discussion
This FEA-utilized study showed that while the von Mises
stress distribution characteristics at supporting structures
were similar at four different implant thread designs, the
distribution of compressive stresses were different, and
these results were similar to a study [8] that compared the
square and V-thread types and stated that thread form
configurations does not greatly affect the stress distribution
in cortical bone. Current study adds the buttress and reverse
buttress thread shapes to this findings. Based on these
results, the null hypothesis that the thread shape would not
affect the stress distribution at supporting bone structure
was rejected for compressive stresses and accepted for von
Mises stresses.
It was reported in the literature [4, 12] that stress
(compressive) was more evenly distributed in the case
when the implant thread shape was square than V-thread
shape. Findings of current study were in accordance with
them; however, current study adds buttress and reverse
buttress thread type to comparison and especially the
reverse buttress thread shape seems to be more advanta-
geous if compressive types of stresses are desired to occur
at thread-bone structure interface. In contrast, Hansson and
Werke [9] reported that the thread profile has a profound
effect upon the magnitude of stresses in the bone; in the
present study, stress distribution patterns at different
implant thread models were similar. The reason for this
opposition could be that they assumed the implant to be
infinitely long and embedded in a cortical bone cylinder.
Also, that implant was not having neck-abutment parts, and
implants were assumed rotationally symmetric. At current
study, threads were modeled with revolution downwards as
it is commercially available in the market.
Previous researches reported that the stress was con-
centrated in the cervical cortical bone region [6] and the
highest stress concentration occurs at the region in jaw
bone adjacent to the first thread of implant. Current FE
study confirmed that the stress was concentrated at the
cervical region and first thread. It has been suggested that
compressive stresses may act as a bone maintenance
Table 1 Mechanical properties of investigated materials
Material Elastic modulus (E; GPa)Poisson’s ratio (μ)
Titanium [16]
Cortical bone [23]
Cancellous bone [23]
110
13.7
1.37
0.35
0.30
0.30
Fig. 3 Illustration of 3D FE model, load application, and boundary
condition. Pink arrow represents the load application, and green area
is assumed as fixed as boundary condition
Fig. 2 Solid screw implants modeled in study that has four different
thread profiles. V-thread (a), buttress (b), reverse buttress (c), and
square shape (d) threads
Clin Oral Invest (2010) 14:411–416 413

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Fig. 4 Distribution of von
Mises stresses (MPa) at main
models. V-thread (a), buttress
(b), reverse buttress (c), and
square shape (d) threads. Blue to
red colors represents stress
values from lower to higher,
respectively
Fig. 5 Distribution of von
Mises stresses (MPa) at mesio-
distal cross-section view of bone
structure. V-thread (a), buttress
(b), reverse buttress (c), and
square shape (d) threads. Blue to
red colors represents stress
values from lower to higher,
respectively
414 Clin Oral Invest (2010) 14:411–416

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stimulus [12]. Also, high shear stress concentration at
implant-bone interface [12] and insufficient mechanical
stimulation (compressive forces) have been suggested to
be a major etiological factor [24] behind the marginal
bone loss often observed at dental implants. At current FE
study, high von Mises stress concentrations and lack of
compressive stresses were observed at cervical bone
region. Thus, thread designs like reverse buttress, which
form more compressive stresses, may be considered for
bone stimulation.
As mentioned before, thread pitch and depth also could
affect the stress distribution [8]. This topic was analyzed in
literature and effects of thread pitch, depth, and width were
reported [9, 11]. Also, it was noticed that a systematic
analysis of the effect of different thread profiles upon the
bone stresses has not yet been made [9]. Thus, it was aimed
to compare four different thread profiles at current study.
However, as a limitation of current study, analyzed implants
did not have threads at neck region. Implants that have
threads at neck region are commercially available, and this
configuration type can be compared at a further study.
The size of occlusal force is selected as 100-N value.
However, it is not necessary for this force to match the
reality exactly because standardization between conditions
has been ensured in the current study, and the conditions
have been compared qualitatively with each other. Chen
and Xu [3] have emphasized that the value of FEM
modeling is in relative values calculated at distribution
pattern.
The FEM results are presented as stresses distributed in
the investigated structures. These stresses may occur as
tensile, compressive, shear, or a stress combination known
as equivalent von Mises stresses. Von Mises stresses
depend on the entire stress field and are a widely used
indicator of the possibility of damage occurrence [13, 14].
Thus, von Mises and compressive stresses were chosen for
presentation of results.
As with many in vitro studies, it is difficult to
extrapolate the results of this study directly to a clinical
situation. The model used in this study implied several
assumptions regarding the simulated structures. The
structures in the model were all assumed to be homoge-
neous, isotropic, and to possess linear elasticity. The
properties of the materials modeled in this study, partic-
ularly the living tissues, however, are different. Also, it is
important to point out that the stress distribution patterns
may have been different depending on the materials and
properties assigned to each layer of the model and the
model used in the experiments. Thus, the inherent
limitations in this study should be considered. Further
studies that better simulate the oral environment and
including fatigue loading are recommended.
Conclusion
Within the limitations of this study, the following con-
clusions were drawn:
1. The different implant thread forms do not affect the von
Mises stress distributions at supporting bone structure.
2. Different implant thread forms produce different com-
pressive stress intensities at bone structure.
3. Cortical bone and bone structure adjacent to first thread
bears more both von Mises and compressive stresses
than spongious bone.
Conflict of interest
Counsil of the University of Selcuk. The authors declare that they
This study is funded by the Research Projects
Fig. 6 Distribution of compres-
sive stresses (MPa) at mesiodis-
tal cross-section, zoomed view
of bone structure adjacent to
cervical implant region.
V-thread (a), buttress (b),
reverse buttress (c), and square
shape (d) threads. Blue to red
colors represents stress values
from lower to higher,
respectively
Clin Oral Invest (2010) 14:411–416 415