Finite element analysis of stress and strain in human knee replacement prosthesis

Dissertation for obtaining a master's degree in the field

Mechanical engineering with applied design orientation

Abstract

The experience of losing a member is very painful and bitter. The full understanding of this feeling will be possible only when a person loses a limb. Obesity and longer life expectancy have increased the number of knee replacement implants [1]. Due to the increase in life expectancy and also the decrease in the age of people who accept these prostheses, the expected life of these prostheses should be increased. Therefore, in order to improve their quality, many researches are being conducted in order to increase their useful life so that the patient does not need repeated surgeries to replace and repair the damaged prosthesis. In this thesis, the performance of a real example of a knee replacement prosthesis removed from an old woman's leg has been investigated. Due to its geometric complexity, the desired prosthesis has been modeled by a coordinate measuring device [2], which is an example of three-dimensional contact scanners, by creating cloud points with high accuracy and edited with Solidorex commercial software [3]. The desired model has been simulated by the finite element method and with Ansys Workbench software with different boundary conditions and subjected to nonlinear static analysis. The comparison of the matching results has shown the simulation results for two different boundary conditions. In order to reduce the convergence time, the assumption of linearity of the material is used in the simulation and the results of linear and non-linear mode are compared. These results have been analyzed in various loading situations and in the most critical situations of walking, climbing stairs, getting up from a chair, standing on two knees and squatting, to evaluate the mechanical performance of the prosthesis. It has been observed that the linear and non-linear results are in good agreement due to not leaving the tibial elastic state[4] of the knee prosthesis. Therefore, the assumption of linearity is used in the continuation of the investigation. Assuming linearity, for the positions of walking, climbing the stairs, getting up from the chair, standing on two knees and squatting, and using the first type of boundary condition, the simulation results showed that the squatting position exerts the most stress on the knee prosthesis and is the most prone to failure (plastic deformation) in the knee prosthesis. Therefore, the patient is required to follow the rules of using the prosthesis. In this regard, by presenting a revised design, an attempt has been made to reduce the equivalent stress in this critical situation. The simulation results with this modified model indicate a significant reduction in the maximum applied stress.

Key words: simulation, analysis, mechanical behavior, loading, knee replacement prosthesis

1-1-Preface

The experience of losing a member is very difficult and bitter. The full understanding of this feeling will be possible only when a person loses a limb. A prosthesis is a device that is designed and used to replace the missing limb of a disabled person. Obesity and longer life expectancy have increased the number of knee replacement implants [1]. The important point in the design and manufacture of prosthesis is to ensure the feeling of comfort for the person, the reliability and durability of the prosthesis. The reason why prostheses are superior to each other can be found in how these factors are met, because the inappropriate design of the prosthesis can damage the remaining limb or other organs. Due to the increase in life expectancy and also the decrease in the age of people who accept these prostheses, the expected life of these prostheses should be increased. Therefore, in order to improve their quality, many researches are being carried out in order to increase their useful life so that the patient does not need repeated surgeries to replace and repair the damaged prosthesis.

Given the importance of increasing the life of knee replacement prostheses, in this thesis, the stress on the knee on a real sample of the replacement prosthesis removed from an old woman's foot is considered in order to determine the efficiency of the prosthesis. He studied the desired knee in various situations of daily activities.In this regard, the 3D model of the desired prosthesis will be prepared using modeling software (CAD) and then it will be simulated by the finite element method and analyzed with the Ansys Workbench software and subjected to non-linear static analysis (due to the non-linear behavior of the materials used to make the prosthesis). In this simulation, the mechanical function of the prosthesis will be investigated in various loading situations and in the most critical state of daily activities.

1-2-Anatomy of the knee joint

The knee joint is the largest and at the same time the most complex joint in the body. This joint consists of four bones, which are:

Thigh

Thigh

Kneecap

The back bone of the tibia Fibula [2] (thin back bone of the leg)

Two main joints in the knee include the joints It is leg-thigh [3] and kneecap-thigh[4].

1-1-1-leg-thigh joint[1]

In the knee, the end part of the femur has a spherical part that is similar to a horse's leg, and its protruding end is in front of the femur. The two ends of the femur are stretched backwards and are called medial and lateral vertebrae. These surfaces are articulated with the vertebrae of the leg and cause knee flexion and extension. Two soft cartilaginous discs (meniscus) are also placed between the vertebrae of the leg and femur.

1-1-2-knee-thigh joint [2]

The kneecap (patella) slides up and down in front of the bone surface and causes the knee to bend and extend. The connection of the kneecap and the hip joint forms a joint. The patella is held in place in front of the knee joint by the following as shown in Figure 1-2. The quadriceps muscle is located at the front of the thigh and is used to extend the leg. The quadriceps tendon connects the quadriceps above the knee to the knee. Ligament[4] (to connect the bone to other parts) of the kneecap, which connects the distal corner of the pelvis to the tibia[5] (the large bone of the lower leg).

The stability of the knee joint is achieved with the help of a smart system of ligaments, powerful muscles and a strong and elastic joint compartment. Four ligaments connect the tibia and femur to each other. On the sides of the joint, there are medial and lateral ligaments, abbreviated as Mcl and Lcl, which act as a stabilizer for side-to-side movements of the joint. Figure 1-3 shows the ligaments of the knee. In front of the center of the joint is the anterior cruciate ligament (Acl for short). This ligament is a very important member for stabilizing the thigh bone on the leg bone and also keeps the leg bone from rotating and sliding movements during the process of walking, jumping and activities with fast movement. Just behind the Acl is the posterior cruciate ligament (Pcl), which prevents the tibia from sliding backward. The quadriceps muscles in the front of the thigh are connected to the kneecap by the quadriceps tendon, and its continuation passes over the kneecap and connects to the front of the leg bone. style="direction: rtl;">The experience of losing a part of the body is very painful. The perfect realization of this sense is available only when a person loses one of his organs. The fatness and longer lifetime enlarges the need to implant total knee replacements. Due to increasing the expectancy of life and decreasing the age of persons who need these prostheses, the lifetime of the total knee replacements must be enhanced. Therefore, in order to improve the quality of them, many researches were done to increase the effective lifetime of the prostheses to reduce the need of next frequent surgeries for replacing or repairing the destroyed prostheses.