The plastic portion of a tibial component is usually fixed to the metal tibial base plate. In some knee designs , the plastic component is allowed to "float" on the metal tray. There is a hope that these types of implants may last longer before the plastic is worn out. However , despite heavy advertising there is currently no convincing evidence that these are more durable or allow greater range of motion than a fixed bearing total knee replacement. Despite the media hype as "new" implants , these implants have been around for many years. In fact , I was involved in a study of the Depuy LCS mobile bearing knee versus the Howmedica PCA fixed bearing knee while I was a resident at Johns Hopkins Hospital. We found no significant differences in the complications , performance , or durability of these two implants at four years follow up after surgery. However , I continue to be intrigued by the design concepts of a mobile bearing and hope to start a comparative study of fixed versus mobile bearings in the future.