Rotating platform versus fixed-bearing total knees: an in vitro study of wear.

One of the assumed benefits of mobile bearings is the reduction of UHMWPE wear. However, to date, such benefit has not been categorically proven. To test the hypothesis that rotating platform total knee arthroplasty would have less wear than a fixed-bearing of the same design, this in vitro study compared the wear and kinematics (which influence wear) of one type of mobile with fixed-bearing tibial components of otherwise identical design. We tested four fixed bearing (FB) and four rotating platforms (RP) on force control knee simulators using identical ISO standard force inputs and simulated soft tissue restraint for 6 million walking cycles. The internal/external rotations peaked just before toe off, reaching an average maximum of 7 degrees internal (tibial rotation) in the RP, 1.5 times that of the FB, which peaked at approximately 4.5 degrees internally. Two of the RP specimens showed infrequent and mostly temporary dislocations of the UHMWPE insert. The wear rate for the FB averaged 8.14 +/- 2.63 mg/million cycles and the RP averaged 6.78 +/- 1.74 mg/million cycles. Both were very low wear rates compared with most other implants tested similarly in the same laboratory. We concluded polyethylene wear was similar for both designs.