In vitro fatigue failure of cemented acetabular replacements: a hip simulator study

Although hip simulators for in vitro wear testing of prosthetic materials used in total hip arthroplasty (THA) have been available for a number of years, similar equipment has yet to appear for endurance testing of fixation in cemented THA, despite considerable evidence of late aseptic loosening as one of the most significant failure mechanisms in this type of replacements. An in vitro study of fatigue behavior in cemented acetabular replacements has been carried out, utilizing a newly developed hip simulator. The machine was designed to simulate the direction and the magnitude of the hip contact force under typical physiological loading conditions, including normal walking and stair climbing, as reported by Bergmann et al. (2001, Hip 98, Freie Universitaet, Berlin). A 3D finite element analysis has been carried out to validate the function of the hip simulator and to evaluate the effects of boundary conditions and geometry of the specimen on the stress distribution in the cement mantle. Bovine pelvic bones were implanted with a Charnley cup, using standard manual cementing techniques. Experiments were carried out under normal walking and descending stairs loading conditions with selected load levels from a body weight of 75-125 kg. Periodically, the samples were removed from the test rigs to allow CT scanning for the purpose of monitoring damage development in the cement fixation. The hip simulator was found to be satisfactory in reproducing the hip contact force during normal walking and stair climbing, as reported by Bergmann et al. Finite element analysis shows that the stress distributions in the cement mantle and at the bone-cement interface are largely unaffected by the geometry and the boundary conditions of the model. Three samples were tested up to 17 x 10(6) cycles and sectioned post-testing for microscopic studies. Debonding at the bone-cement interface of various degrees in the posterior-superior quadrant was revealed in these samples, and the location of the failures corresponds to the highest stressed region from the finite-element analysis. Preliminary experimental results from a newly developed hip simulator seem to suggest that debonding at the bone-cement interface is the main failure mechanism in cemented acetabular replacements, and descending stairs seem to be more detrimental than normal walking or ascending stairs with regard to fatigue integrity of cement fixation.