Three dimensional shape reconstruction and finite element analysis of femur before and after the cementless type of total hip replacement.

Computerized tomography was used to reconstruct a shape, and stresses in three-dimensional objects were analysed. The human femur, which has a very irregular shape, was chosen as an object. CT image data of a cadaver femur were transferred to a computer, and an edge extraction program generated the cross-section of bone by specifying a range of CT values for each slice. Pixel data from the CT scan are converted into a vector of points (x, y, z) which can specify the boundaries of bone. Lateral surfaces are defined by stacking up the slices and making use of the vectorized data. Intermediate and oblique cross-sections can be obtained by an interpolation technique. The constructed model was used as input data for the finite element analysis. To understand the stress distributions before and after the cementless type of total hip replacement, a three-dimensional finite element stress analysis of the bone-implant system was carried out, assuming micromotions between the stem and the femur. The analysis was done for both frictionless and friction cases, modelling the contact point with a gap element having isotropic friction. The analysis shows that the stress is not concentrated on the femoral calcar when the friction coefficient is large.