Numerical predicting of contact and pressure sore of lower extremity parts caused by prosthetic and orthotic

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1 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 121

2 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 hyperelastic 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. 122

3 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. 123

4 3. Grid Design Component Element Modulus of elasticity Poisson s name type (MPa) ratio Bone Solid Soft Tissue Solid Table 1 Element type and material properties that is used in FE model 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 124

5 Fig. 4 Presentation of mesh refinement on ankle and heel 4. Loading and Boundary Conditions Fig. 5 mesh refinement in tibia bone 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 125

6 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. 126

7 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. 127

8 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. 128

9 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. 129

10 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 MPa for sole and the shear stress on tibia bone. The current simulation with the rough modeling predicted this value around 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 130

11 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 References [1] Bae J, Tomizuka M. A gait rehabilitation strategy inspired by an iterative learning algorithm. Mechatronics 2012;22: [2] Aliff M, Dohta S, Akagi T, Li H. Development of a Simple-structured Pneumatic Robot Arm and its Control Using Low-cost Embedded Controller. Procedia Eng 2012;41: [3] Mohamaddan S, Komeda T. Wire-driven mechanism for finger rehabilitation device IEEE Int. Conf. Mechatronics Autom., IEEE; 2010, p [4] Ewing MB. LONG LEG BRACE MODIFICATION. Arch Phys Med Rehabil 1963;44: [5] Titus BR. A modification of the VAPC PTB brace. Artif Limbs 1971;15:68 9. [6] Chen W-P, Ju C-W, Tang F-T. Effects of total contact insoles on the plantar stress redistribution: a finite element analysis. Clin Biomech (Bristol, Avon) 2003;18:S

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