Early cement damage around a femoral stem is concentrated at the cement/bone interface

This study aimed to improve understanding of the mechanical aspects of cemented implant loosening. After aggressive fatigue loading of stem/cement/femur constructs, micro-cracks and stem/bone micro-motions were quantified to answer three research questions: Are cracks preferentially associated with the stem/cement interface, the cement/bone interface or voids? Is cement damage dependent on axial position? Does cement damage correlate with micro-motion between the stem and the bone? Eight Charnley Cobra stems were implanted in cadaveric femora. Six stem/cement/femur constructs were subjected to “stair-climbing” loads for 300 kcycles at 2 Hz. Loads were normalized by construct stiffness to avoid fracture. Two additional constructs were not loaded. Transverse sections were cut at 10 mm intervals, stained with a fluorescent dye penetrant and examined using epi-fluorescence stereomicroscopy. Crack lengths and cement areas were recorded for 9 sections per specimen. Crack length–density was calculated by dividing summed crack length by cement mantle area. To isolate the effect of loading, length–density data were offset by the baseline length–density measured in the non-loaded specimens. Significantly more cracks were associated with the interdigitated area (35.1%±11.6%) and the cement/bone interface (31.0%±6.2%) than with the stem/cement interface (11.0%±5.2%) or voids (6.1%±4.8%) (p<0.05). Load-induced micro-crack length–density was significantly dependent on axial position, increasing proximally (p<0.001). Micro-motions were small, all stems rotated internally. Cement damage did not correlate with micro-motion.