Finite Element Analysis of Hip Joint Contact
|Bearbeitung:||Prof. Dr.-Ing. Udo Nackenhorst, M.Sc. Kristin Fietz, M.Sc. Andre Lutz|
Starting from CT data the geometrical models of pelvis and femur are extracted using segmentation techniques, which is shown in video1 and figure 1. With this data solid CAD models of pelvis and femur are generated using digital shape reconstruction. In this approach the cartilage of acetabulum and femur are modelled as one single body which is shown in yellow in figure 2. Based on this accurate solid CAD model the finite element model is created.
The contact layer (shown in green in figure 2), which is used to evaluate the contact pressure, is situated between the cartilage body and the femur head and is generated fully automatically. It consists of linear wegdes. Supports in the pubic symphysis and in the sacroiliac joint are described by Dirichlet boundary conditions as displayed in figure 3. The system is loaded by a single force acting in the centre of the femoral head. To simulate physiological correct hip behaviour, the material properties related to the bone structure are used. Therefore the Hounsfield units are mapped onto the finite element model and converted to bone mass density information. The elastic moduli are calculated by a quadratic constitutive law.
|figure 1: segmentation data and surface mesh on CT data||figure 2: finite element model||figure 3: boundary conditions|
The contact layer consists of contact elements which only have normal and inplane shear stiffness. These are 3D small deformation contact elements, that form a one-element-layer between both contact partners. Therefore no contact search is needed and the evaluation of the contact pressure is very efficient. The movement situations analysed are chair down/up, knee bend, stairs down/up & walking slow/normal/fast, which were measured with instrumented hip implants at Charité Berlin and are provided by the OrthoLoad database. As example the gait analysis of walking in normal speed is shown in video2. The loads are applied inverse on the femur as shown in figure 3. For every movement situation 30 timesteps of about 0.04 sec are evaluated.
In video 3 a the pressure distribution during walking in normal speed is displayed. To get a more general idea of the demand of the hip, contact maps for all load steps and movements are created. In figure 4 elements that encountered pressure values higher than 1 MPa are displayed, which is an indicator for regions with higher demand of the acetabulum. This is in quite good accordance with figure 5, where the real hip cartilage geometry called facies lunata is shown. In figure 6 the number of contact detections for all loadsteps and movements is depicted. The cranial ventral part undergoes the most contacts. Oberlaender measured in 1977 the thickness of the facies lunata was measured and the thickest regions correlate good with those here identified.
|figure 4: contact map with treshold of 1 MPa||figure 5: facies lunata||figure 6: summation of contact detections|
Because the contact maps, as a measure for the demand of the hip, match the hip cartilage geometry and also agree with the thickest regions, the quite small collection of load sets assumed seem to be representative to describe daily motions. Even though the whole motion range of the hip could not be considered due to a lack of data.