Assessing the risk of future femur fracture is an important step in preventing this serious and common injury. The current methods to predict the fracture risk are typically based on bone mineral density measurement, used as a surrogate for femoral strength. An alternative to this which has been shown to give more accurate information on the strength of a femur is subject specific finite element analysis, generally in a loading position that approximates a sideways fall since this is how most femur fractures occur.
Most such models are static or quasi-static, these provide an estimate of the force needed to fracture the femur under a constant or slowly increasing load. They do not however model the actual impacts under which the femur would break in a sideways fall and are thus limited in their ability to predict its behavior under such conditions. In this project a subject specific dynamic finite element modeling procedure solved with an explicit solver was developed. Models of 8 femurs from 22 to 88 year old women were created and the results of these models were evaluated. The meshes of these models were based on the geometry of the subjects bone found from CT-images and use element specific tangent stiffness and density which were also based on CT images of the femur. Additionally the models take into account the strain rate dependence of the stiffness of bone. The femur is given an initial velocity such that it hits a rigid surface.
The results of these models show good agreement with the results expected based on theory and previous tests of these femurs. Most femurs were predicted to fracture in the lateral femoral neck as expected when compared to previous quasi static loading tests of the same femurs. The impact forces at which the femurs would be expected to fracture during an impact were predicted based on the forces under which they fractured during quasi static loading and an experimental relationship between these. Comparing these expected fracture forces to the force at time of fracture from the simulations showed good agreement between the two. The major differences between the expected fracture forces and the ones predicted by the dynamic simulations were occurred in models of femurs much weaker than those used in the experimental study which found the relationship between the fracture forces for quasi-static and impact situations. The results seem promising for future work, such as adding soft tissue to the model, since they do appear to accurately capture the mechanical properties of each femur.