Confirmation of long-term in vivo bearing mobility in eight rotating-platform TKAs

Abstract

Background: Posterior-stabilized rotating-platform prostheses for TKAs were designed to improve contact mechanics at the femoral-polyethylene (PE) interface. Short-term followup studies have shown that the PE bearings rotate with respect to the tibia but might not necessarily track with the femur. It is important to know how kinematics in these designs change owing to long-term in vivo use.

Questions/purposes: We asked whether there is a significant change in the in vivo kinematic performance of a posterior-stabilized rotating-platform prosthesis at as much as 10 years postoperative. We specifically examined (1) relative femoral component-PE bearing and relative PE bearing-tibial tray motion; (2) relative AP motion of the femoral condyles with respect to the tibial tray; and (3) relative femorotibial condylar translations.

Methods: In vivo three-dimensional kinematics were evaluated for eight patients at 3 months, 15 months, 5 years, and 10 years after TKA with primary implantation of a posterior-stabilized rotating-platform prosthesis. Each patient performed deep knee bend activity, and three-dimensional kinematics were reconstructed from multiple fluoroscopic images using a three-dimensional to two-dimensional registration technique. Once complete, relative component axial rotation patterns, medial and lateral condyle motions throughout flexion, and the presence of femoral condylar lift-off were analyzed.

Results: Overall, tibial bearing rotation was maintained at 10 years postoperatively. There was no statistical difference between postoperative periods for any kinematic parameter except for femoral component-PE bearing axial rotation, which was reduced at the 10-year evaluation versus other assessment periods (p = 0.0006). The lack of statistical difference between postoperative evaluation periods indicates sustained overall implant kinematic performance.

Conclusions: Our study showed that PE bearing-tibial tray mobility was maintained and that femoral component-PE bearing rotation was reduced at the 10-year followup. This suggests that the overall kinematic performance of this mobile-bearing implant is not negatively affected 10 years postoperatively.

Level of evidence: Level III, retrospective study. See the Instructions for Authors for a complete description of levels of evidence.

Fig. 1

The mean (± SD) of the variations in axial rotation patterns of the polyethylene (PE) bearing with respect to the tibial tray as a function of flexion angle are plotted for eight patients at the four followup times. External rotation of the PE bearing with respect to the tibia is presented as positive.

Fig. 2

Individual patient axial rotation data from full extension to maximum knee flexion are shown for the 10 year followup. External rotation of the superior component with respect to the inferior component is positive.

Fig. 3

The mean (± SD) of the variations in axial rotation patterns of the femoral component with respect to the polyethylene (PE) bearing as a function of flexion angle are plotted for eight patients at the four followup times. External rotation of the femoral component with respect to the PE bearing is positive.

Fig. 4

The mean (± SD) of the variations in the average femoral lateral condylar motion with respect to the tibial tray as a function of flexion angle are plotted for eight patients at the four followup times.

Fig. 5

The mean (± SD) of the variations in the average femoral medial condylar motion with respect to the tibial tray as a function of flexion angle are plotted for eight patients at the four followup times.