Fatigue debonding of the roughened stem-cement interface: effects of surface roughness and stem heating conditions.

The aim of this study was to determine the effects of cyclic loading on the debond process of a roughened stem-cement interface used in total hip arthroplasty. The specific goals were to assess the effects of two surgeon-controlled variables (stem heating and degree of stem surface roughness) and to determine if an independent finite element-based fracture mechanics model could be used to predict the debond response. A clamped cantilever beam geometry was used to determine the fatigue debond response of the stem-cement interface and was created using an experimental mold that simulated in vivo cementing conditions. A second experiment was performed using a torsion-loading model representative of the stem-cement-bone composite. For both experiments, two stem heating (room temperature and 50 degrees C) and surface roughness conditions (grit blasted: Ra = 2.3 and 5.1 microm) were used. Finally, a finite element model of the torsion experiment with provision for crack growth was developed and compared with the experimental results. Results from both experiments revealed that neither stem preheating nor use of a stem with a greater surface roughness had a marked effect on the fatigue debond response. There was substantial variability in the debond response for all cases; this may be due to microscopic gaps at the interface for all interface conditions. The debond rate from the finite element simulation (10(-7.31) m/cycle) had a magnitude similar to the experimental torsion model (10(-(6.77 +/- 1.25)) m/cycle). This suggests that within the context of the experimental conditions studied here that the debond response could be assessed using a linear elastic fracture mechanics-type approach.