Methods

Simulated titanium tibial trays (15 mm dial tray, 15 mm keel) were used in this study and implanted in cellular rigid polyurethane foam (12.5 pcf) (Sawbones Vashon, WA, USA). Surface roughness (Ra) of implants was verified as 3.60μm with a 2μm tip at 0.5 mm/s over a length of 1.6 mm (SurfAnalyzer, MAHR Federal Inc., Providence, RI, USA). Palacos cement (Heraeus Medical, Wehrheim, Germany) was mixed for 2 minutes followed by implantation and one of 3 motion regimes at two time points. Six groups were tested. Motion was applied at three minutes for three groups. This motion was 1)axial micromotion for 20 cycles at 100 microns and 0.5 Hz, 2)rotational of 20 cycles at +/− 1.5 degrees and 0.5 Hz, or 3)both motions sumultaneously. An additional three groups were tested at 6 minutes under the same conditions. Motion was applied using calibrated mechanical testing equipment (MTS systems, Eden Prarie, MN, USA).

Implants were tested in tension to failure at 0.5 mm/min, 24 hrs after implantation. The peak load, stiffness and energy were determined for each sample. Data was analysed using an Analysis of Variance and a Games Howell post hoc tests where appropriate.