Analysis of deformable bodies during the gait cycle

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Κοκκόρη, Γιαννούλα

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Abstract

Total hip replacement (THR) is one of the most successful orthopedic interventions with over 100 years of operative history. People who suffer from severe chronic pain diseases such as osteoarthritis have various disabilities in daily routine. Replacing the hip joint with this effective procedure, the patients relieve the pain and get back to normal life. The continuous development of technologies and computers provide us various ways to perform a preoperative planning for the THR procedure. This thesis analyzes the models of the human femur bone and the assembly of the femur bone with the THR implant. The study focuses on the application of loads during the gait cycle and the finite element analysis of the models. The relative results relate to the mechanical behavior of the models and more specifically the Von Mises stresses and deformations due to the applied loads. In detail: Chapter 1 provides a brief reference to total hip arthroplasty, the historical background, the surgical procedure and the risks that it poses as well as the anatomy of the femur bone and the biomechanics of the hip joint that is of major importance for the static analysis of it. Still there is a description of the gait cycle and the stress shielding phenomenon that occurs after THR during walking. Chapter 2 refers to the biomaterials that are used in total hip replacement. Various materials such as metals, polymers and ceramics are used in the different components of the THR implant and the main of them are described in detail. Chapter 3 deals with the basic mechanical theory that every structure is based on. Stresses, strains and the Hooke’s law are presented with emphasis in 3D case. The Von Mises criterion and the static analysis are also mentioned. Chapter 4 analyzes the finite element method that is used in the thesis. A brief historical overview and the basic steps of the analysis (preprocessing, processing and postprocessing) are presented. Then, as the processing phase is performed by the computer, the sequence of mathematical equations and their way of solution was also mentioned. Finally, there is a reference in the finite element software programs and an analytic description of the relative steps in ANSYS software. Chapter 5 presents the simulation process. First, a section was made in the appropriate point of the femoral bone and the THR implant was inserted in the right position with the assistance of the CAD software package SolidWorks. Then, the ANSYS software generated the appropriate mesh for each model and applying the proper material properties, loading and boundary conditions we proceeded to the analysis. Chapter 6 finally shows the finite element analysis results of the femoral assemblies as obtained using the ANSYS software program. The results refer to the relative Von Mises stresses, Von Mises strains and deformations due to the applied loads of the gait cycle. Finally, the conclusions and the prospects for further studies are presented. Regarding on the results, we concluded that on the calcar region of interest occurs the “stress shielding” phenomenon where the implant “shields” the bone and does not allow it to grow properly. Moreover, after the implantation we observed that the maximum stress occurs on the neck of the hip implant for all studied the materials. Finally, Ti alloy and Co-Cr alloy on anterior and stem tip regions represent similar results and thus have little effect on the femur bone.

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Finite Element Analysis (FEA), Femur bones, Hip joints, ANSYS, Implants, Biomechanics, Total hip replacement

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