The long-term success of a total knee replacement depends on the wear performance of a polyethylene bearing that separates a metal femoral component from a metal tibial tray. Although fixed bearing designs secure the polyethylene bearing to the tibial tray, mobile bearing knees allow the polyethylene to move relative to the tibial tray. This study has evaluated the wear performance of an intended articulation on the inferior surface of the LCS®-Rotating Platform mobile bearing by conducting clinically relevant tribological testing and comparing results to retrieved knee bearings. A retrieval analysis leads to the conclusion that third-body particles in the contact produce curvilinear scratches longer than the expected rotation of the knee on both the polyethylene bearing and the CoCr tibial tray. Tribological testing shows that polymethylmethacrylate (PMMA) bone cement particles produce worn surfaces most similar to retrievals. Porous-coating beads and bone debris also have the ability to damage both surfaces. Worn polyethylene surfaces from pin-on-flat tests show scratches longer than the excursion length, and “skipping marks”—pits spaced at smaller rotation intervals along a scratch—as observed in retrievals. These wear features suggest that a ratcheting mechanism, which moves the third-body particles further along the scratch with each cycle, may be responsible for the observed wear.