GSTF International Journal of Engineering Technology (JET) Vol.1 No.1, 2012
Effect of different load conditions on a DHS
implanted human femur
Nooshin S. Taheri, Aaron S. Blicblau, and Manmohan Singh
load, with one of load conditions used being the fall loading
condition [4]. Another study investigates strain distribution
along the proximal femur under six different load conditions
e.g. stumbling or mis-stepping [4]. Lotz et al have considered
fall and gait loading conditions to interpret stress distribution
along the femur in normal and osteoporosis bone [5].
Single leg stance condition is a configuration which is
present in almost every daily activity, therefore by identifying
the stress –strain state of this condition we will be able to have
a better understanding of the implant behavior under such a
load. Impact from a fall on the implanted femur is very crucial,
by investigating this load configuration; we might be able to
prevent the failure caused by impact from a fall on the hip.
Identifying the cause of failure due to load conditions is very
crucial to preclude post-operative fracture or failure of device
and second operation especially when the targeted populations
are the elderly.
In a study comparing the gamma nail with DHS, the result
relating stress distribution established no advantages of one to
the other device [8]. It is worth mentioning that the Gamma
locking nail is more suitable for subtrochanteric fractures
along the shaft of femur. An assessment in performance of a
short plate compression screw with DHS, found that DHS has
better biomechanical function to minimize the credibility of
mechanical failure [9]. DHS proved to offer better stability and
prevention of fixation failure in proximal femur rotational
osteotomy in a finite element study corresponding to the
clinical observations [10].
However, the failure of this device is the main concern in
choosing the proper implant for specific fracture. One of the
complications involving DHS is the cut out of the lag screw
from the head of femur [11]. The main factor of this problem
is known to be the position of lag screw within the head of
femur [12].
According to the research studies, dynamic Hip Screw (
DHS ) is the most commonly used implant for pertrochanteric
fractures [6]. Dynamic Hip Screw is a type of extramedullary
fixation device which is designed to stabilize stable fractures.
There are a number of biomechanical, clinical and finite
element studies which have compared intra and extramedullary
implants [7].
The objective of present study is to establish the role of fall
loading condition on the implanted femur in terms of stress
distribution. This work is a follow up of work conducted on
the selection of materials for a DHS [13] under fall conditions.
Abstract— As a result of improving life expectancy, the
number of elderly people affected by hip fracture is increasing.
Hip fracture treatments are different due to different types of
fractures. Internal fixation is one of the most reliable treatments
for the hip fractures. Nearly half of the fractures are
introchanteric fractures that are best treated by with a Dynamic
Hip Screw (DHS). However, there are concerns in using DHS,
due to the rate of the failure of up to a third of all operations due
to the cut out of the lag screw from the head of the femur.
Revision surgery is essential if the cut out of the lag screw
happens.
This study carried out a finite element analysis to establish the
effects of different load conditions on the DHS implanted bone
system for single-leg stance and fall and the conditions for failure.
Index Terms—hip
configurations; failure
fracture;
dynamic
hip
screw;
load
I. INTRODUCTION
A
S a result of population aging the number of
hospitalization for hip fracture is still rising, according to
data from statistics in Australia 41.9% increase in men and
31.2% in women. As for worldwide figures, it is likely that the
numbers rise from 1.3 million hip fractures in 1990 to 7.3 –
21.3 million by 2050 [1]. Applying loads in specific directions
on the hip joint after surgery would lead to failure of implant,
or in a more traumatic situation often fall would cause postoperative fracture. Falls or stumbling causing hip fractures are
causing 60 % of the injuries [2]. In particular, falls in persons
aged 65 and over are the main source of their death, admission
to hospital and emergency units. Commonwealth and state
health authorities have considered fall prevention programs as
one of their main concern areas in the elderly [2]. We chose
linear analysis on implanted human femur to investigate the
stress distribution under fall load condition and compare it
with the single leg stance case as a reference. Keyak, et al. [3]
performed a finite element analysis on the intact femur to
determine the force directions related to the lowest fracture
Manuscript received May 25, 2012.
N.S.Taheri was a graduate student at Swinburne University of Technology,
faculty of Engineering and Industrial Sciences, Hawthorn, Australia
A.S.Blicblau is with Swinburne University of Technology, faculty of
Engineering and Industrial Sciences, Hawthorn, Australia (e-mail:
ablicblau@swin.edu.au).
M.Singh is with the Swinburne University of Technology, faculty of
Engineering and Industrial Sciences, Hawthorn, Australia.
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© 2012 GSTF
GSTF International Journal of Engineering Technology (JET) Vol.1 No.1, 2012
In this paper we have considered implanted femur instead of
the intact or fractured femur to investigate the effect of force
direction in both single –legged stance and fall loading
conditions. In no case have an implanted femur under fall
loading condition been investigated in the literature. In
particular we addressed the following tasks: identification of
the location of peak stresses within the implanted femur in
single-legged stance and impact from a fall; assessment of the
effect of direction of load on the implanted femur under
different load conditions; and analysis of the biomechanical
behavior of dynamic hip screw during walking and fall.
structural integrity of different implants against each other
helps to give guidance to surgeons to make the right choice [9,
12, 15].
A three- dimensional FE model of the right femur to include
DHS implant was considered, as shown in Fig 1 to assess the
level of stress developing in the implanted bone. The model of
DHS was created by means of Unigraphics using the geometry
and dimensions from the DHS manufacturer, SYNTHES [16].
The implant has four distal screws which are in different
planes to have better biomechanical performance compared to
the previous designs. The angle of 135⁰ between the plate and
the lag screw is there to ensure stability in fractured femur.
The length of the plate is about 38 mm. DHS device generates
minimum irritation to the soft tissue [7]. The geometry of a
standard femur has been created from a series of CT images.
The femur model was accurately modeled for screw
dimensions and orientation of the DHS in the Pro Engineering
program. Then the 3-D model of featured bone and DHS as an
assembly was imported into an ANSYS pre-processor.
Lotz et al have applied a modulus of 17 GPa to thickest
layer of the bone and 0.45 as Poisson’s ratio [5]. In another
study a modulus 14.217 GPa was chosen for the cortical bone
and 0.10 GPa for trabacular bone, again 0.3 was employed for
the Poisson's ratio [17]. Brown [12], similarly used a modulus
of 17 GPa for cortical bone and 1.3 GPa for cancellous bone in
the femoral head, as well as 0.3 for Poisson ratio. In work on
using a Gamma nail for implants, the properties of the femur
were Young’s modulus of 17.0 GPa for cortical bone and 0.3
for the Poisson`s ratio [18].
Although there are some other alternatives for the material
of the implant, stainless steel was appointed to the DHS device
for its material properties, with the value of 2l0 GPa for its
modulus of elasticity and 0.3 for its Poisson`s ratio. In order to
simplify the model and have a better mesh for the implant, the
threads and the tips of implant screws were not modeled.
The 3-D model of femur was considered homogeneous,
linear, and elastic exhibiting isotropic properties. The loading
simulation of the bone implant system was performed using
ANSYS (version 11.) computer software.
ANSYS provides a list of various contact types for such
assemblies like femur-implant systems. Fully bonded contact is
chosen for this case to allow load transfer between the plate
and the femur model. The contact area of the bone DHS
implant assembly body is shown in Figure 2.
For convergence of the model, with mesh refinement, the
von Misses stresses differed by less than 5%. Using this
approach, the two requirements for the consistency of the
model, compatibility, and stability are defined. According to
Lax-Wendrof theorem, consistency and stability imply
convergence [19].
II. MODELLING AND ANALYSIS
A. Modelling
Investigating biomechanical behavior of the femur to better
understand its properties is a hard task since the femur is very
complicated in shape, material properties, geometry, porosity,
density and furthermore femur is a live part which is changing
all the time. Among all methods finite element analysis is most
reliable and its validity is supported by comparisons by other
researchers’ work [7,8,9,17,18]. Experimental methods face
shortcomings of the specimen and different types of analysis
on different parts of femur which will cause specific limitation.
To assess different material behavior like stress, strain and
total deformation at the same time on one single specimen is
only possible in finite element analysis. Finite element could
also deal with complicated material properties linearity and
nonlinearity of the analysis.
Fig. 1. 3-D model of femur included DHS
Cheal et al [14] have performed a finite element analysis to
establish the role of loads and prosthesis material properties on
the proximal femur in total hip replacement. In most
comparative studies using finite element to investigate the
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GSTF International Journal of Engineering Technology (JET) Vol.1 No.1, 2012
B. Loads
Two load regimes were considered to analyze
biomechanical behavior of implanted femur. Load condition 1
simulating single-legged stance phase of gait, borrowed from
literature [10]. In all daily activities stance phase of the gait
cycle is most seen, like stair climbing, stair descending,
running ,walking and jogging; this is the main reason most
studies consider single legged stance as a basic and important
loading condition [20]. The load direction in the single leg
stance case is shown in Figure 4.
During the gait cycle, every muscle is active at a specific
period. The muscle active in single-legged stance phase of gait
is the Gluteus medius. The joint contact resultant force and
muscular load are applied on two different locations. The
muscular load created by the Gluteus medius muscle is applied
on the great trochanter area and joint contact force is applied
on the femoral head. The direction of two forces are opposite
with an angle to the femoral shaft. (Table 2)
Load condition 2 represents impact from a fall on the side
from standing. The direction of loads to replicate impact from
a fall is a medially directed load applied to the great
trochanteric and an equal and opposite laterally directed load
applied to the femoral head. This specific fall load condition
was used by Backman to create fractures to represent real
cases of fractures in vitro [21]. The fall is assumed to be from
standing height and was carried out such that the lateral aspect
of the hip came in direct contact with the floor. The average
force evaluate over the great trochanter after experiencing fall
from standing was nearly 11 times body weight (considering
7350N in this case) [5]. However, since our work is
considering the implanted femur after the operation, we
assumed the impact from fall to be 4.5 times body weight, with
resulting failure of the implant, or the fracture of the femur.
Another study collected similar values for the load of impact
from a fall, under better conditions, which suggest that impact
force for fall from standing height is in the range of 10 to 17
times body weight [22].
In our work, a global Cartesian axis set was defined with a
z- axis parallel to the long axis of the femur shaft and implant
plate, the x- axis along the anterial to posterial of the femur
and the y-axis in medial to lateral directions. In Table 2 the
direction and magnitude of loadings are shown.
The femur is restrained at the distal end. The best result is
achieved when the distance from the trochanter is about 38
mm. The boundary condition is defined in simulation by fixed
support [13].
Fig. 2. The contact area of bone implant assembly
A complicated geometry like femur will require a fine mesh
prior to solving to verify mesh control setting after the
convergence 3 mm size element was applied. We chose fournoded tetrahedral elements throughout the assembly. In total,
for the model of the bone DHS implant system, the number of
nodes of the DHS and the femur assembly were 45935 with
27147 elements, as shown in Figure 3.
TABLE 2
FORCE APPLIED ON MODEL (N)
Direction of loads
A-P
M-L
Fig. 3. View of mesh on the bone implant system
Moreover, the number of elements used for the intact bone
and the DHS implanted bone model in the FEA simulation is
listed in table 1
CASE
S-I
Stance phase1
1
Joint
211
453
2165
2
Trochanter
195
485
-1384
Impact from fall2
3
Joint
1690
-2700
1150
4
Trochanter
-1690
2700
-1150
1
stance phase adapted from [10], 2fall data adapted from [5]
M–L Medial to lateral, A–P anterior to posterior, I–S inferior to superior
TABLE 1
NUMBER OF ELEMENTS AND NODES FOR BOTH 3D MODELS
Item
Intact bone
Implanted bone
Nodes
25544
62424
Element
14768
37061
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© 2012 GSTF
GSTF International Journal of Engineering Technology (JET) Vol.1 No.1, 2012
close. The deformation of the proximal femur during fall on
the side of hip was much higher. In intact bone during fall, the
deformation is 23.503 mm. If someone experiences a fall after
the operation of internal fixation, total deformations would be
around 54.794 mm in head of femur region, which might seem
unrealistic.
We changed the direction of load on the head of the femur
to the z direction only, the displacement of the femur
decreased by 4.17 % as a value of 13.011 mm. The result
shows the direction of load has a significant role on the
displacement of the femur compared to its magnitude. (Fig5).
The yield criterion chosen was the von Mises criterion
assuming isotropic behavior of cortical bone. The von Mises
stress gives an indication of the elastic shear strain energy in
element considered for elastic breakdown [12].
The implanted femur with DHS device is considered under
the fall loading and stance phase to investigate the role of DHS
in these conditions after the surgery to establish the strength of
the implant and the level of stress it tolerates.
Fig. 4. Load direction in single- legged stance case
Fig. 5. von Mises stress in intact femur in fall case 3
III. RESULTS
TABLE 3
A. von Mises Stress Distribution
The von Mises stress distributions on the surface of the
implanted femur are shown in Fig.5. The results show that the
peak stress levels are on the DHS device: 128.05 MPa in
stance and 1127.8 MPa in fall. It shows that DHS as a bearing
load device carries the load in the system. Most of the load is
carried by the DHS plate responding to the force transmission
between the DHS devise and the femur at the contact point.
Further analysis revealed that high stresses were located in
the region of posterior – anterior of the neck as well as the
distal end of the femur on impact from a fall. On the contrary,
during stance phase of gait, the base of the neck and the area
of the superior neck were stressed. The patterns of high stress
in the femur increased as the axial plane moved distally along
the femur in impact from a fall in intact bone. Interestingly, in
the implanted femur system trend was towards high local stress
in the area of the lateral plate of DHS and less at the distal end.
COMPARISON OF THE VON MISES STRESS AND PRINCIPAL STRESSES
Case1
Case2
Case3
Case4
von-Mises
78
154
554
923
Maximum
principal
-18
to52
-5 to
89
-102
to482
-33 to
398
Minimum
principal
-67
to12
-58 to
16
-602 to
60
-924 to
60
Stress (MPa)
C. Principal Stress Distribution
The principal stresses for all cases are shown in Table 3. In
stance case, the principal stresses were concentrated within the
intertrochanteric region of the femur. In contrast on impact
from a fall maximum principal stress are located in contact
area of the implant and bone as shown in Fig 7.
However, during fall, intact bone experienced maximum
principal stress around base of the neck and intertrochanteric
region of the bone that makes sense, since most fractures are in
that area. During fall, compressive stresses are also around
anterior –superior area of the neck
B. Deformation of the femur
Representative deformation patterns of system and
undeformed model are shown in figure 6. In case of stance
phase of gait loading the overall deflection had a value of
3.607 mm in the implanted femur. The deflection of intact
femur under the same loading was 2.911 mm, which are very
D. Stress Distribution on Screws
Lag screw insertion hole was heavily stressed during stance
loading. 52.389 MPa, which is under the yield strength of the
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GSTF International Journal of Engineering Technology (JET) Vol.1 No.1, 2012
material. As for the distal screws, the topmost screw was under
the highest stress when compared with the other screws.
However, during fall the distal screws were under heavy
stress: 281.36 - 462.76 MPa. Because of the load direction,
stress distribution was the highest in anterior positioned
screws. The lag screw hole and the lateral side of the plate had
the peak stress values
critical and weak parts of DHS are fourth distal screw and
plate. The maximum principal stresses are in contact area of
the plate and bone is shown in Figure 7. Lateral side of the
plate generates the highest stress concentration especially in
model-SS. [23].
Our results relating to the high stresses in lateral side of
DHS plate is in consistent with the findings of other
researchers [7, 24, 25]. The high stresses at the distal end of
the intact femur and implanted femur observed in our study is
similar to findings of other similar studies [14]. Our study
revealed that around fourth distal screw the highest
concentration of stress are developed which is in consistent
with other DHS studies [24,25].
These results show the significant role of directions and
magnitudes of loads applied on the implanted femur after
internal fixation during daily activities. Better knowledge of
the biomechanical factors that rules the risk of hip fracture and
implant failure Identifying weak areas of implant under
important load conditions also assists in developing enhanced
and improved design and materials of implants.
Fig. 6. Deformation patterns of system and undeformed model.
V. CONCLUDING COMMENTS
The main objective of this study was to determine stress
distribution in two different load conditions on femur-DHS
system, during gait and impact from a fall. Results indicate
that there are significant differences between gait and falls in
both magnitude and pattern of stress in both intact and
implanted femur. During gait, the stress patterns are within the
infer-medial neck. On impact from a fall, however, the stress
patterns were very different to the gait; it appears the principal
source of strength is in sub-capital region of neck.
In case of impact from a fall on implanted femur, it seems
that the peak stresses are located mainly in the contact area of
the DHS and the shaft of the femur less in the neck. This could
mean the failure of the implant or even a second fracture,
which we were not able to confirm as limitation of software is
most likely.
Our study faced potential limitation, which our finite
element model was restricted to linear, isotropic material
behavior. The data show significant post-yield non-linear
performance for both cases, which this implicates failure of the
implant or second fracture for the femur implant system. In
intact bone, cases are comparable with the literature, which
were used as reference, and our data was in agreement with
their results.
Fig. 7. Maximum principal stresses are in contact area of the plate and bone.
IV. DISCUSSION
This study provides a mechanical evaluation before and
after internal fixation by means of finite element analysis and
application of stance phase of gait and stair descent loading
conditions. The results indicate the significant changes in
biomechanical behavior of the proximal femur after internal
fixation. During stance, stress rise is observed after internal
fixation, however, maximum stress values are still below the
yield stress values of implant and bone materials.
Nevertheless, the stance case induces more stress on the
implanted femur. The stress rise in implanted femur during
stance is nearly more than that during stair descent replication.
There are few studies relating to simulation of stair descent
loading condition on the implanted femur with DHS. The
present results can be used as a reference for the implanted
femur with DHS for stair descent. As a result of an impact, the
implanted femur experiences very high magnitudes of stress
particularly in the DHS parts. The results reveal that the
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Nooshin S. Taheri (ME SUT) . Obtained her first degree in mathematics in
Iran. She then completed a M.Eng at Swinburne University of Technology
where she specialized in in the biomechanical behavior of human femur. Her
areas of interest include mathematical modeling, finite element analysis and
their applications.
Aaron S.Blicblau (BE Monash, ME UNSW). Received his first degree in
materials engineering and his second degree in civil engineering materials.
He is currently a senior lecturer in the faculty of Engineering and Industrial
Sciences at Swinburne University of Technology. His research interests are
in Mathematical modeling of engineering processes, failure of engineering
materials, engineering education, active learning, project based learning, and
syllabus and curriculum development. He is a chartered member of both
IE(Aust)and the IMMM well as a member of ASEE
Manmohan Singh (MSc Roorkee, PhD Roorkee). Received his MSc and
Phd from University of Roorkee in Applied Mathematics. He is currently a
senior lecturer in the faculty of Engineering and Industrial Sciences at
Swinburne University of Technology.
His research interests are
Mathematical Biology, Numerical Methods, and Theoretical Astrophysics.
He is a member of the Australian Mathematical Society, and the Society for
Mathematical Biology.
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