During functional activities, mobile bearing total knee replacements have been reported to have little or no motion o f the bearings. Understanding the conditions under which component motion does or does not occur is seemingly important to the success of these devices. The purpose of this study was to investigate the mobility and contact mechanics of two rotating platform mobile bearing total knee systems, under representative functional loading conditions.

Experimental and finite element analyses were performed, with loading conditions similar to those of activities of daily living. Parameters that varied were axial load, condylar load allocation, flexion angle, and axial rotation load application (static vs. dynamic). Similar results of the physical model and finite element model in areas common to both lend credence to the validity of the findings. The relationship between resisting torque and axial load was approximately linear. A t four times body weight axial load, peak resisting torque measured was 9.47 ± 0.61 and 5.51 ± 0.38 Nm, for static and dynamic torque (torque required to initiate rotation and sustain rotation), respectively. Medially biased load allocations had little effect on resisting torque. For all practical purposes, the polyethylene insert and the femoral component rotate together simultaneously.

For functional simulations of in vivo performance, phasic waveforms (axial load, axial torque, flexion angle, and anteroposterior forces) from a standardized walking cycle were used as input for the finite element models. Simulated soft tissue constraints and friction levels were varied to characterize the effect of these changes. Reducing friction had the most significant effect on articulation mechanics, by increasing axial rotation, reducing rollback, reducing liftoff and reducing contact stresses. Axial rotation ranged from 3.5° to 9.5°.

With a highly conforming rotating platform knee, mobility is imperative for longterm success. The high levels of torque required to axially rotate these components may be the reason that bearing non-motion was observed in some patients. The lull walking cycle simulations produced about half of the axial rotation range of motion seen in a normal knee. The clinical implications of limited bearing motion include potentially adverse consequences in terms of accelerated wear and/or loosening.