Fatigue Analysis of a Reconstructed Femur. Effects of Prosthetic Materials

Total Hip Replacement has been reported as the most successful surgical procedure in the previous century. However, long-term studies of cemented total hip replacements have shown that failure often occurs ten years postoperatively and mostly at the bone-cement interface. The aim of this thesis is to investigate the effect of different materials of the prosthetic stem on long term stability of a reconstructed femur. The focus is put on the femoral component, while the pelvic component is out of scope. In an intensive literature research the anatomy of the hip joint, material properties of bone and bone cement, loading conditions and daily activity profiles of total hip replacement patients are investigated. Anatomically accurate 3D models are created based on CT-scan data. A sensitivity analysis is carried out on different methods of modelling and assigning heterogeneous anisotropic material properties to the bones. Static finite element analyses are performed to investigate the stress distribution in the reconstructed femur and to evaluate the effect of different stem materials. Real loading conditions during a normal gait cycle and ascending stairs are simulated in dynamic transient analyses, where hip contact forces and muscle forces are implemented in the model. Long term stability of bone cement, which is the weakest part in the reconstructed joint, is calculated in basic and multi-axial fatigue analyses. Material behaviour of bone cement and several influence factors of fatigue lifetime are taken into consideration. The effect of cement mantle thickness is examined and suggestions of increasing longevity of bone cement are given. Stainless steel performs best of all stem materials in reference to longevity of bone cement. A fatigue lifetime of 9.47 million duty cycles is found before the crack initiation in bone cement starts. A decrease in the Young’s Modulus of the stem results in a reduction of the long term stability. If titanium is used instead of stainless steel, the fatigue lifetime would decrease dramatically to 3.83 million load cycles, which is a reduction of 59.6 percent.