Dany Audish, Enhanced Failure Modelling of Human Femur, 2012
(MSc. Project in collaboration with ORL-UMC St. Radboud)
The femoral bone quality is severely deteriorated in patients with metastatic bone disease. Unfortunately, fracture risk assessment in current clinical practice is inadequate and often leads to under- or overtreatment of patients. Consequently, there is an urgent need for more accurate predictions of fracture risks of metastatic femurs.
Patient-specific non-linear Finite Element Method (FEM) based on Quantitative Computed Tomography (QCT) has shown promising predictions of femoral fracture risks under uniaxial compression. Therefore this numerical model was set in development at the Orthopaedic Research Lab in Nijmegen. The current model is accurate in the simulation of femoral compression tests but the model should also be valid when more complex loading cases are applied. Therefore torsion experiments were also validated. The current model lacks a spiral fracture in the femoral shaft, which is frequently seen in the torsion experiments. Furthermore, the prediction of the load-deflection graph is incorrect including the failure load. Consequently, the current model fails in the prediction of femoral torsion strength.
An essential phenomenon in femoral bone fracture is the brittleness. Bone fracture is predominantly a result of localised cracking that causes a brittle fracture. The representation of material behaviour in the current model though does not account for these localisations. The smeared crack approach can simulate a spiral fracture as a result of a torque load. This damage model was analysed and optimised in order to reduce the corresponding numerical challenges.
After an implementation of the smeared crack model in the femoral model, the extended model was tested against torsion experiments of intact and metastatic cadaveric proximal femurs. For the validation of the model in another load-configuration, one compression experiment was also simulated.
A spiral fracture can be obtained by the addition of the smeared crack model. The fracture pattern in the compression simulations improves as well. The strength prediction of the metastatic femurs under torsion is very accurate in the new model. For further research, there is still room for improvements, especially on the failure modelling of intact femurs. Significant improvements can be obtained if the Partial Volume Effect is reduced and the anisotropy is taken into account.
Another extension was made by using the arc-length method in the solution procedure of FEM, this increased the efficiency of the model considerably.
To conclude, the obtained model has improved the strength prediction of metastatic femurs by representing the failure in a more realistic approach.