Fatigue of bone cement with simulated stem interface porosity

Cracks in bone cement have been observed in carefully examined post-mortem preparations of cemented stems. These cracks were probably caused by fatigue, and frequently appeared to initiate at pores. Ubiquitous porosity, occurring preferentially at, or near, the stem, is most likely caused by polymerization shrinkage. Preparation of air-free cement has only a marginal influence on the interface porosity, but pre-heating the stem in order to reverse the direction of polymerization can reduce or eliminate it. To estimate the impact of interface porosity on the fatigue strength of bone cement, test plates for this study were cast in a steel mold without release foils, and with one side of the mold warmer. Sample plates so prepared from chilled, partial vacuum-mixed PALACOS, have one face essentially pore-free and the other porous, the extent and morphology of the porosity being very similar to that observed on the stem-cement interface. Four-point bending fatigue strength, determined after 60 d conditioning in Ringer's solution at 37 degrees C, was only 20 MPa (at 10(6) cycles, with the porous side under tension) compared to 30 MPa for conventionally prepared, pore-free material. This corresponds to a 10-100 fold reduction in cycles to failure in the range of stresses predicted to occur in vivo.