Initial fixation affects ingrowth of uncemented tibial components. Previous studies assessed initial fixation, however most used limited one-dimensional motion measurements. Therefore, a three-dimensional micro-motion and migration analysis was performed to compare initial fixation of four different tibial tray configurations: 1) Keeled component 2) Non-keeled component (post but no keel) 3) Keeled component with screws 4) Non-keeled component with screws.

Osteonics series 7 000 tibial trays (identical to Scorpio design) with and without standard keels were obtained. The 30 mm posts were left intact. The components were implanted without cement into twenty-four fresh-frozen cadaveric tibiae. Specimens were loaded through matched femoral components via two separate loading conditions: 1) sinusoidal medial load from 200 to 2 200 N at 0.5 Hz and 2) sinusoidal torsion load from −5 Nm to +5 Nm at 0.5 Hz with constant 1 200 N axial load. Motion data were collected from reference cubes and transducers rigidly attached to the trays, during 3 000 cycles of loading. Rigid-body mechanics were used to calculate the motion of a wire-frame computer model. Comparisons were made of micro-motion and migration magnitudes.

A keel did not significantly decrease micromotion or migration under medial or torsion loading compared to a post alone (p=1.04). The addition of four cancellous screws decreased motion (p< .05) regardless of the presence of a keel under medial loading. No significant differences were noted under torsional load for any tray configuration.

A keel did not enhance initial fixation compared to screws or a post alone. Screws enhanced fixation under axial offset loading in all constructs. Screw fixation did not affect motion under torsion loads perhaps due to the limited magnitude of torque transmitted across the unconstrained femoral-tibial articulation. Based on these data, initial fixation of uncemented tibial implants should include screw fixation to counter the effects of offset axial loads.