Background. The purpose of this study was to investigate the kinematics of the polyethylene insert in two designs of mobilebearing total knee arthroplasty, using a six-degrees-of-freedom knee simulator. It was consequently not clear whether the motion of the polyethylene bearing in mobile-bearing total knee arthroplasty could be demonstrated during the gait cycle or more rapid movement. Methods. A mobile-bearing knee (Zimmer) and a low contact stress rotating-platform design (Depuy) were mounted on a simulator which was regulated by the kinematic data of gait. The simulating test was conducted under a static condition as well as under dynamic conditions of 0.5 Hz and 1.0 Hz. We recorded the motions of the implants with two charge-coupled device (CCD) cameras, and the positions of the insert were calculated. Results. In spite of the same relative motion between the femoral component and the tibial tray, the polyethylene insert showed unique relative motion according to the given condition. The motion of the insert during the dynamic conditions was considerably decreased in comparison to the static condition in both mobile-bearing designs. In addition, the insert showed a smaller amplitude and frequency of rotations under increasing speed in the low contact stress rotating-platform design. The low contact stress rotating-platform design showed a larger amplitude and frequency of rotations than the mobilebearing knee. Conclusions. Despite the mobility of the insert in the mobilebearing total knee arthroplasty, the motion of the insert was decreased during dynamic conditions because of the disruption of full contact between the femoral component and the polyethylene insert. Differences in the rotation between the mobile-bearing knee and the low contact stress rotatingplatform design were due to the fixed axis of the internalexternal rotation in the low contact stress rotating-platform design. The theoretical advantages for the mobile-bearing design over the fixed-bearing design were not demonstrated in this study.
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