Abstract
Introduction
Loading of the implant/cement bond during polymerization is possible when a joint is put through passive range of motion shortly after implantation. This may adversely affect the integrity of the cement – implant interface. The aim of this study was to evaluate the effect of implant motion during cement polymerization on the mechanical properties of the cement – implant interface.
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.
Results
Failure of all implants was due to implant pullout, specifically implant/cement bond failure. Groups with motion at 3 minutes showed similar mechanical testing results for rotation (729±179), micromotion (756±161), and combined loading (708±254). For tests at 6 minutes, however there appeared to be decreased properties when rotated at this late time point. The maximum tensile force for implants with axial micromotion at 6 minutes was 719±166N while those with rotation were 559±163N and with rotation and micromotion were 556±163N. ANOVA results did not indicate significant differences between any of the groups.
Discussion
Although the differences did not reach statistical significance, a trend towards decreasing implant/cement properties with late rotational motion was present. This is of relevance as it relates to passive motion immediately following implantation of cemented knee components.
