Comparison between bonded stem and unbonded types that were classified by different stem surface finishes was carried out on axisymmetric stem-cement-aluminum model of the femoral component of a total hip replacement. Human femur was modeled in non-tapered and tapered(7.5°) aluminum hollow cylinders to emulate the diaphyseal and metaphyseal segments of the femur. We tested aluminum models under axial loading for both cases. To measure the shear stress, three types of strain gage were merged in cement mantle of the constructs. Fatigue test was carried out under axial dynamic compressive loading until failure for both cases. And 2D axisymmetric FE model of the constructs was developed and used to simulate the stress distribution under axial static loading and to check the effect of model sizes.
It was found that unbonded stem was sustained as twice axial load as bonded stem. The unbonded constructs failed in cement mantle under axial compressive load, while the bonded constructs failed in shear at cement-aluminum interface. The unbonded had 60~70% less shear stress in the cement mantle than the bonded. The FE analysis showed that the tendency of shear stress was similar with the experimental results. The unbonded did not make a failure over than 7 million cycles while the fatigue life of the bonded was about 2 million cycles.
These result suggest that polished stem will sustain much higher axial loads than a roughened stem and make more stable cement-bone interface that may promote better osteosythesis around the stem.
