Stress analysis of femur and femoral stems for hip arthroplasty

Abstract

Osteoarthritis is the major reason that causes hip problem. According to Cristofolini (1997), there are more than 800,000 hip replacements being implanted worldwide annually. It is important to know the performance of hip prostheses especially the stability and the longevity. In this project, numerical simulation based on finite element method is used to analyze the mechanical behavior of femur-implant system. Finite element analysis is carried out on three-dimensional model of a human femur on both full and half models. This is to investigate the behavior of an intact femur under loading. Then, the analysis is repeated for an Anatomic Medullary Locking (AML) hip prosthesis, which is of one type of cementless hip prosthesis, implanted inside the femur. This is only done on half femur model. Both the stem and the head are made by Cobalt Chromium Molybdenum (CoCrMo). After that, the analysis is carried out on a cemented hip prosthesis. The cement is made by polymethylmethacrylate (PMMA), which is of flexible polymeric cement. The hip prosthesis model used for analysis is of Charnley type. The study on the stem length effect is then done. Lastly, the analysis is repeated for cancellous with different density. The cortical, cancellous, metal and cement are assumed to be linear, elastic, isotropic and homogeneous. Linear elastic analysis is adapted and maximum principal stress/strain and von Mises stress are the criterions that are of concern. Results show that both full and half femur modeling give similar stress distribution. Besides, the treated femur is always understressed at the upper most region of the femur. Cemented type of total hip replacement (THR) gives a better stress distribution on the femur compared to cementless type. In addition, hip prosthesis with shorter stem induces the stresses more evenly on the femur. Also, different cancellous density does not significantly affect the stresses.