Mobile-bearing total knee arthroplasty: do the polyethylene bearings rotate?

In vivo kinematics were determined during a weightbearing deep knee bend in 39 patients implanted with four different designs of mobile-bearing total knee arthroplasty to assess polyethylene bearing mobility patterns and magnitudes. The femoral and tibial components and mobile polyethylene insert (implanted with four tantalum beads) were overlaid onto the fluoroscopic images using a three-dimensional model-fitting technique to determine bearing mobility. Three of the four designs were tested at a single time interval while one was evaluated at two postoperative intervals, (12 months apart) to assess changes in bearing mobility with time. All patients had polyethylene bearing rotation relative to the tibial tray and minimal rotation relative to the femoral component. The average maximum amounts of bearing rotation ranged from 8.4 degrees to 10.3 degrees (range, 3.0 degrees -20.1 degrees). In patients evaluated at two time intervals, the average maximum bearing rotation increased from 8.5 degrees (range, 3.4 degrees -15.5 degrees) at 3 months to 9.8 degrees (range, 48 degrees -14.1 degrees) 15 months postoperatively. The presented data demonstrates polyethylene bearing mobility occurs following mobile-bearing total knee arthroplasty and mobility is maintained during the time interval tested. The presence of bearing mobility should result in lower contact stresses reducing the potential for polyethylene wear.