Mechanical properties of cancellous bone from the acetabulum in relation to acetabular shell fixation and compared with the corresponding femoral head.

To gain initial stability for cementless fixation the acetabular components of a total hip replacement are press-fit into the acetabulum. Uneven stiffness of the acetabular bone will result in irregular deformation of the shell which may hinder insertion of the liner or lead to premature loosening. To investigate this, we removed bone cores from the ilium, ischium and pubis within each acetabulum and from selected sites in corresponding femoral heads from four cadavers for mechanical testing in unconfined compression. From a stress-relaxation test over 300 s, the residual stress, its percentage of the initial stress and the stress half-life were calculated. Maximum modulus, yield stress and energy to yield (resilience) were calculated from a load-displacement test. Acetabular bone had a modulus about 10-20%, yield stress about 25% and resilience about 40% of the values for the femoral head. The stress half-life was typically between 2-4 s and the residual stress was about 60% of peak stress in both acetabulum and femur. Pubic bone was mechanically the poorest. These results may explain uneven deformation of press-fit acetabular shells as they are inserted. The measured half-life of stress-relaxation indicates that waiting a few minutes between insertion of the shell and the liner may allow seating of a poorly congruent liner.