Background: On theoretical grounds mobile bearing total knees should reduce the micromotion of the tibial component relative to the bone.
Patients and methods: We used radiostereometric analysis to measure the three-dimensional micromotio in 42 tibial components during 2 years of follow-up. The patients had been randomized as to whether they would receive a mobile bearing (MB) or posterior stabilized (PS) design. We expected that the MB knee would facilitate dissipation of forces from the prosthesis-bone interface by the motion of the bearing and by load sharing with the soft tissues, leading to less micromotion. In the PS designs, limited free rotation caused by the cam-post articulation might cause additional stress at the bone prosthesis interface.
Results: We found no significant differences between the MB and PS group at the 2-year follow-up evaluation with respect to Knee Society scores and radiographi results. The PS group had a higher variability in subsidence and anterior-posterior tilting of the component than the MB group.
Interpretation: The low variability of the data in the MB knee prosthesis group suggests that this design more predictable and forgiving with respect to micromotion of the tibial component.