CANCELLOUS BONE STRESSES SURROUNDING THE FEMORAL COMPONENT OF A HIP-PROSTHESIS - AN ELASTIC-PLASTIC FINITE-ELEMENT ANALYSIS

The cancellous bone stresses surrounding the femoral component of tota l hip replacement were investigated using the finite element method. F our versions of a certain femoral hip prosthesis (the Freeman, Corin M edical), cemented. HA coated, press-fit, and press-fit with ridges wer e analysed. Each model was subjected to two separate load cases with t he cancellous bone modelled as an elastic perfectly plastic material. The effect of bone quality was investigated by varying the cancellous bone stiffness. The resulting cancellous bone stress distributions wer e compared to that of the intact femur. The results were also compared to clinical subsidence data (published elsewhere) for the Freeman fem oral prothesis to determine if the initial cancellous bone stress dist ribution could be used to predict the migration of the various version s of this prosthesis. The results showed that the press-fit designs of prosthesis generated substantially higher cancellous bone stresses th an the cemented and HA coated designs, and that these stresses were up to 6.5 times higher than found in the intact femur. For all forms of fixation that cancellous bone stress distribution was found to be inse nsitive to changes in the trabecular bone stiffness: thus poor quality cancellous bone is more likely to promote 'plastic' deformation, and therefore subsidence of the prosthesis. Comparison with the clinical m igration data showed a good correlation and revealed that it may be po ssible to use the calculated initial cancellous bone stresses to predi ct the migration of the implant, and hence that probability of early a nd mid-term aseptic loosening. This is the first time, to the authors' knowledge, that the 'plastic' deformation of cancellous bone has been reported as a contributing factor to the subsidence and failure of pr oximal femoral prostheses.