This document summarizes a study that used finite element analysis to model a leg brace and examine stress distributions. A 3D model of a leg and brace was created. The model included bones, soft tissues, and a brace structure. Materials were assigned to components and the model was meshed and loaded. Stress distributions were analyzed to identify pressure points and tissue deformation. The goal was to optimize brace design to minimize forces and prevent soft tissue damage or bone deformation from long-term brace use.

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MACROJOURNALS
The Journal of MacroTrends in
Health and Medicine
Numerical predicting of contact and pressure sore
of lower extremity parts caused by prosthetic and
orthotic
Reza Fakhrai, Bahram Saadatfar, Mohammad Reza Shah Mohammadi
Department of Energy Technology, Royal Institute of Technology KTH, Stockholm, Sweden
Abstract
One of the major problems of using external skeleton or brace is the wounds, and
injury in muscle, due to increasing collagen content, with a long-term use of orthosis.
Biological soft tissues of all kinds are viscoelastic. Due to the muscle weakness, the
main structural muscles make deformation under the weight of body and compression
of the bone's microvasculature, potentially leading to severe pressure in a sole and
pain. Moreover, after continual use of this brace, the bones, especially the tibia, will
be deformed. The several important points are proposed depends on walking. A three
dimensional (3D) model of the human foot and a leg brace structure, which is used to
support the weak muscles during walking are created. Real model is the left foot of a
person who has a weakness in his muscles. Then Finite element model was developed
by commercial software, to evaluate the pressure area and validation of the proposed
points. The effects of the contact areas between the brace and leg are studied and
analyzed in order to identify pressure hotspots on skin and soft tissues and tissue
deformation as well as the degree and probability of deformation of bones. . The two
major steps including distributions of stress and strain as well as displacement were
analyzed. Then, the bone structure and joints are considered in the main model to
investigate a model which is similar to the real case. The state of the art of this study
is to to model the connective skeletal muscle tissues and identify the most important
contact points. The brace shape was optimized to support efficiently and to minimal
force or compression with less soft tissue damage, wounds and also prevent
deformation in the bones.
Keywords: leg brace; pressure and stress sore; breakage depth; stress; strain; finite element

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1. Introduction
Many researches have been conducted on the biomechanical device for supporting or
rehabilitation. Many mechanical and electromechanical devices with many components is
designed and developed to make it easier for disable people step the stairs up and down or
walking more stable. The simple pneumatic gait controller with mimicking the functions of
physical therapists is studied by Joonbum et al. [1]. Mohad Aliff et al. [2] proposed the new
control and learning algorithm. In the many structure, the devices are heavy, expensive and
useless for short period care [1–3].
One of the most common devices is a leg brace, which is cheaper and lighter than the
electromechanical and external skeleton. Leg brace is a simple structure, which can be made of
plastic or light composite material to cover important parts of foot like knee and ankle.
Moreover, this frame can keep the normal form of leg more appropriately. Efficient and
optimized design of a good brace makes the person more comfortable and do not harm any
part of the body. The leg brace not actually hold the person up, but it helps to keep weak joints
straight so the bones can support the body weight. There is different kind of braces, which the
design is depended on the weak part. If the ankle is weak, then it is better to use under a knee
short brace and if the knee is weak, the full-leg brace is used.
Many studies have been done about different structural analysis of leg braces with different
supports. Ewing [4] did the research on the Long Leg Brace Modification. In 1972, Brat [5], made
different structure and examined new measuring as well as molding methods. He also tested
new fraction for making composite material, which is used in lateral side and hinges and covers.
Chen et al. [6] the stress distribution in sole was investigated, and different material was studied
to reduce the stress in the flat and hallux region full covered foot wear. Cheung et al. [7] did the
same study with improvement of material properties and considered nonlinear and hyper-
elastic behavior of soft tissues and orthosis in their finite element (FE) analysis. Sun et.al [8],
studied the relation between arch height ratio and stress distribution inside the bones. The
results showed that with increasing in arch height, the pressure will be increased inside the
bone connections. During the last decade, many studies have been conducted, investigating the
foot and foot wear interaction and stress distribution in bones, sole, and other parts.
The review shows that although different studies have been done on different parts of leg with
different goal for designing a foot wear or modification of an external skeleton, still, there is
problem in the simple leg brace comfort. The current study focuses in both brace structure and
material. Both linear and nonlinear material behavior is studied for body and structure to make
it more comfortable. Moreover, the pressure and stress distribution is studied in important
joints and sole. Moreover, a modification on crank place and structure design will be proposed
to reduce the stress and also deformation of main bone like tibia.

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2. Geometry and Material
The geometry is obtained from CATIA 5V20 software. The model is made for a left leg of a male
(age: 40-years-old, height: 170 cm, mass: 75 kg) who use the leg brace. Freestyle designing is
used to design the leg with CATIA. More than 10 cross sections are used to model it precise and
all important dimensions like length of leg, the ratio between ankle and knee, knee and thigh
are kept. Then the main bones of leg are modeled to give the model real composite
characteristics of the real foot. The geometry of the examined case is shown in Fig. 1 and Fig. 2.
Fig. 1 Illustration of designed leg in CATIA environment: a) side view, b) front view
Fig. 2 Representation in details of heel, sole and ankle design
Then the 3D FE model is created with importing the model to ANSYS 14 software. All soft tissues
and bones are simulated with SOLID 45 as standard and proper element, which has three
degrees of freedom and can identify the plasticity, creep, stress stiffing, large deflection, and
large strain. The soft tissue was bonded to the bony structure. The interaction between insole
and foot was under a contact situation using contact elements. The joints are coupled together
with frictionless contact. In relation to the contact stiffness, the factor of normal penalty
stiffness was 0.1. The contact algorithm was the augmented Lagrange method.
One of the most important issues is material selection. As human living tissue, it is significant to
measure all characteristics as precise as possible to simulate damages properly. To simplify the
analysis, all tissues were idealized as homogeneous, linearly elastic and isotropic. Table 1, shows
the element type and material properties used in the FE model.

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Component
name
Element
type
Modulus of elasticity
(MPa)
Poisson’s
ratio
Bone Solid 45 7300 0.3
Soft Tissue Solid 45 0.15 0.45
Table 1 Element type and material properties that is used in FE model
3. Grid Design
The improvement of the simulation result depends on the mesh generation. Initial mesh design
is generally based on certain assumptions regarding to the exact solution which will be
examined in the post processing phase to make sure of reliability and accuracy of computed
data from the finite element solution. The tetrahedral mesh type is used on foot. However, in
the irregular shape parts the automatic tetrahedral meshes are generated and also the bony
and encapsulated soft tissue structures were meshed with 4-node tetrahedral elements.
Meshes are refined in the interested under pressure conjectured points like heel, knee, and
tibia bone. In Fig. 3, the FE model of the leg is shown. The Fig. 4 and Fig. 5 show the refined
meshing system.
Fig. 3 FE model

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Fig. 4 Presentation of mesh refinement on ankle and heel
Fig. 5 mesh refinement in tibia bone
4. Loading and Boundary Conditions
A foot can be exposed to various loading conditions under different ambulatory activities. In the
current study, the only considered load is the normal gait during mid-stance phase. The forces
and moments applied on the plantar surface of the foot during gait can be assumed as a static
force. The static force is applied on leg during mid-gait balanced standing. The subject weigh is
750 N; hence, the weight, in the standing position was regarded as reaction force on single foot.
This reaction form results in the symmetrically expansion of soft tissues around thigh and tibia
bone, which is structurally limited by brace body. This expansion and limitation during long term
is the most important reason of wounds. In terms of boundary conditions, side surfaces of the
tibia, thigh, sole, and around the ankle are covered, and soft tissues are fixed (Fig. 6 and Fig. 7).
The Fig. 8, shows a real leg brace.
Fig. 6 The area covered with brace, the tibia side and thigh side

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Fig. 7 the distribution of load on sole
Fig. 8 The structure of leg brace
5. Results
Strain and Stress distribution based on static load is one of the major characteristics of a brace,
which cause to impose on the soft tissue and bony part of foot. Among different stresses, three
different stress, shear, buckling, and von-misses are more important. The shear stress is a result
of the angle of every bone with joints or in general the longitudinal plane, and it causes of the
most if the pains in bones. Buckling effect is very important and effective because usually, loads
are damps in muscles. However, in this case with consideration of weak muscles, all loads
directly are applied along the tibia and thigh bone. The deformation of tibia bone in real cases is
observed. Moreover, von-misses stress is a presentation of maximum of stress distribution in
the simulation. Basically, the wounds on soft tissue are results of these strains.
In the first case, the foot without bone is modeled and different stress and strain is simulated on
foot. There is some area like below the hip and also gastrocnemius which the edge of the frame
has been overlapping contact with these areas, so the strain is more important here as far as
one of the serious wounds is happened there.

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Fig. 9 the strain along the soft tissue in boneless simulation
The strain in a foot that simulated boneless case is presented in Fig. 9. The simulation clearly
shows the imposed strain. It shows the areas below the frame, are under force, which will affect
the soft tissue after a short period to get scratched.
The high sensitive areas above the hip are shown in Fig. 10. Weight of foot by itself as well as the
load of total weight will cause of expansion of soft tissue in size. Due to this expansion, the
frame experiences tighten and the leg tolerates against more stress and strain.
Fig. 10 Illustration of strain along sensitive parts in detail
Another important studied parameter is stress. Stresses along the foot as well as the sole are
important. Although the method of simulation is different and also modelling of foot and bone
has been done by MRL system, the results can be reliable reference compared with. The shear
stress and Von-Mises stress are illustrated in Fig. 11 and Fig. 12 respectively.

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Fig. 11 Shear stress distribution
Fig. 12 Von-Misses distribution along soft tissue
As it can be seen the minimum shear stress occurs in the fixed support areas, where the places
are under the cover, with a maximum near the edge of these bands. This shear stress causes
wound happening to these sensitive skins, which are soften during time. The stress distribution
on whole sole is placed on the average amount of stress gradient, and this gradient is a little bit
higher on the heel.
In the next phase of the study, the bony structures within the foot model the bone was
modeled. The dimensions of tibia and thigh bones are considered. The first simulation was for
prediction of important areas and examined information about a soft tissue behavior, which has
been considered with two non-linear damping coefficients. The strain imposed to the foot with
bone is presented in Fig. 13. The illustration shows that the strain is applied mostly on the frame
cover similarly happened on the tibia behind area and under the hip. The other important is the
pain in the sole, especially in the heel and interior side of the sole. In the Fig. 14, the strain
distribution is presented.

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Fig. 13 Strain illustration along soft tissue in the simulation with internal bone
Fig. 14 Strain on heel and interior side of sole
It is clear that the area, where tolerate the weight has more strain and regard to this load the
deformation and pain are more than other parts.
The shear stress is shown in figure 14. The figure presented shear stress starting from the thigh
bone and continuing to the tibia bone. The maximum of the value occurred in the tibia bone,
where there is a deformation in the real case. This shear stress usually is one of the reasons in
bone pain in the leg. Moreover, the back side of the thigh bone also tolerate too much of shear
stress.

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Fig. 15 Shear stress distribution along the bone
Cheung et al. [9] did the measurement of the average of the shear stress with precise
equipment and created a model. They reported the value about 0.057 MPa for sole and the
shear stress on tibia bone. The current simulation with the rough modeling predicted this value
around 0.054 MPa. The predicted shear stress in this study shows about 5% difference. The
difference might be due to assumptions in the model. In order to find overall stress distribution
in bones, the Von-Mises stress gradient along the bone is also extracted.
Fig. 16 Von-Mises stress in tibia and thigh bone
In the other hand, the Von-Mises stress does not have the maximum number in tibia. However,
the thigh bone shows the maximum point of pressure, about 2 MPa. The Von-Mises stress
quantity for tibia and thigh bone can be predicted from Von-Mises data in [9], which reported 7
MPa for sole. However, it can result that the stress is less than 3 MPa.
6. Discussion
In this study, a computational approach using the finite element method was proposed for
investigating of the effects of stress redistribution on foot structure especially the main bones
like tibia and thigh and also the soft tissue which are covered by brace structure. The results

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showed that with utilizing the brace both the peak and the average shear and equivalent stress
placed in the mid-tibia and end-up of thigh bone. Moreover, most pressure regions which are
extracted as strain were redistributed on interior side of foot and heel. This distribution result in
the pain during a short period.
The deformation solution in this simulation shows the common brace structure needs
structurally to be modified in order to have less pressure and load on both living tissue and
bones.
7. Conclusions
Current study shows the common brace structure has great impact on soft tissue and also bone
formation. The visual experiment proved that the user of this brace always has a problem with
the pain in tibia, heel, thigh bone and burning the skin under the brace cover. Moreover,
Solutions shows that the static load, which are worse during walking in the dynamic phase with
the impact effect.
Acknowledgements
The authors would like to acknowledge the financial support of Promobilia that made the project
possible. The Foundation's purpose is to promote the development of technical aids for the disabled
so that they can have a more active life. Promobilia supports research and development of technical
facilities and ensures that they come into production and reach those in need. Foundation gives grants
primarily for the development of aids for disabled handicapped but has also supported the research of
understanding of reading and writing. The Foundation has also supported research on various diseases
that can cause serious operating difficulties
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