Anisotropic finite element models of the proximal femur using CT- and anatomical slices (1999 - 2003)


In the framework of this project a numerical simulation tool was developed, which enables a simulation of the adaptive bone reaction on physiological and non-physiological stress.The main goal of the project was to predict the reorganisation of bone resulting fromfemoral endoprosthesis implantation in order to provide an optimal individual operationplanning. Moreover the system should support the surgeon by performing the preoperativplanning of repositioning osteotomies preserving the joint. In this cases, the stress andload-bearing of the femoral bone depends on the modification of the joint geometry. In theframework of this project, the Helmholtz-Institut developed a FE-network generator to create the networks for further simulations.

Fig. 1: Generation of an isotropic FE- model from the CT-slices

Generation of anisotropic FE-Networks

The generation of the anisotropic finite element network based on CT-slices as well as onanatomical cuts. In a first step, the geometrical reconstruction of bone is generated bythresholding segmentation from the CT-dataset. To determine the osseous parameters aliterature review has been performed by the Orthopaedic-Clinic. This investigation showsan unambiguous correlation between the mechanical parameters of the bone and the X-raydensity. Moreover, data from literature were collected to characterice cortical andspongious bone structures by Young's modulus, Poisson's ratio and shear modulus.On this basis it was possible to make a valid assignment of the specific material valuesand the HU density values of the CT-image data (see fig. 1).

The essential information about the cortical and spongioustrabecular structures, which were needed to calculate the anisotrop model, are generated by investigation of anatomical cuts of the femur. Therefore cuts in transversal and sagittal plane were made, where the directions of the orthotropie axis could be plotted(see fig. 2). Through a computer aided merging of the cutting data and the CT- data it waspossible to generate a FE Model in which each finite element is assigned to a spatialorthotropie axis. The output of the FE-models data format could be imported by theCAD-System ABAQUS. On the basis of this system, the institute of "AllgemeineMechanik" performed FE-simulations for the stress-strain behaviour as well as bone adaption depending on different stress situations.

Fig. 2: Anatomical cut planes of the femur for determination of the orthotropie axis