MOBILE BEARING TKA: DO THE POLYETHYLENE BEARINGS ROTATE?

Abstract

The objective was to assess and compare polyethylene-bearing mobility patterns and magnitudes in various total knee arthroplasty(TKA) types of mobile bearing TKA.

In vivo kinematics were determined for 38 subjects implanted with either a PCL-retaining (PCR) mobile bearing TKA, which allows both rotation and antero-posterior (AP) translation (n=20), aposterior stabilized rotating platform (PS) TKA (n=9) or a PCL-sacrificing (PCS) rotating platform TKA (n=9) using video fluoroscopy. Using a 3D model-fitting technique, kinematics were determined during a weight-bearing deep knee bend. The femoral and tibial components and mobile bearing polyethylene insert (implanted with four tantalum beads) were overlaid onto the fluoroscopic images to determine bearing mobility. AP bearing translation was determined for subjects implanted with a PCR mobile bearing TKA. Subjects implanted with PCR and PCS TKA were evaluated at a single interval. Those with a PS TKA were evaluated at two postoperative intervals, (12 months apart) to assess changes in bearing mobility over time.

All subjects experienced polyethylene bearing rotation relative to the tibial tray and minimal rotation relative to the femoral component. The average maximum amount of bearing rotation was 10.3o (3.0o to 20.8o), 8.9o (5.3o to 14.1o), and 8.5o (3.3o to 12.9o) for subjects implanted with a PCR, PS, and PCS mobile bearing TKA, respectively. For subjects implanted with a PS mobile bearing TKA, bearing mobility increased to 9.8o (4.8o to 14.1o) one year later post-operatively. All subject shaving a PCR mobile bearing TKA experienced AP bearing translation, averaging 5.6 mm (1.0 mm to 12.5 mm).

These results demonstrate that the polyethylene bearing is rotating and translating relative to the tibial tray in all subjects. Minimal motion occurred between the femoral component and the polyethylene insert. Magnitude and direction of bearing motion varied among subjects. Paradoxical anterior translation of the bearing during deep flexion was observed in the PCR TKA group. The presence of bearing mobility should result in lower contact stresses, reducing the potential for polyethylene wear.