A 57-year-old patient with rheumatoid arthritis showed posterolateral impingement after total knee arthroplasty. The radiographs showed bone cement extrusion posterolateral to the tibial tray. Arthrotomy through a posterolateral approach revealed that the impingement was caused not only by cement extrusion against the fibular head but also by proximal tibiofibular joint instability. It was speculated that rheumatoid arthritis had caused proximal tibiofibular instability, active knee motion had caused fibular head shift by tension of biceps femoris and the fibular head had been impinged on the extruded cement. In cementing the tibial tray, especially in a rheumatoid patient, it is of paramount importance to take caution against posterolateral cement extrusion in order to minimize the risk of fibular head impingement during total knee arthroplasty.