Wear of Mobile Bearing Knees: Is It Necessarily Less?

Some well-known mobile bearing designs have had truly excellent long-term clinical results. Their lower constraint and ability for some self-alignment might have helped reduce the shear forces and torques transmitted to the prosthesis–bone interface, thereby lowering the risk of loosening. However, the most commonly assumed benefit of mobile bearings is the reduction in wear due to less contact stress and reduced cross shear. In a rotating platform, wear can be reduced because the rolling/sliding motion is separated from the transverse rotational motion, which reduces cross-shear. Although it has not been categorically proven clinically, such lower wear expectations with mobile bearings might have influenced the thinking of some total knee replacement (TKR) designers and test engineers. This paper amalgamates invitro TKR wear results from two separate laboratories (in Nebraska and Germany) to present the largest data set ever published on wear, across the widest variety of fixed and mobile bearing TKR designs. Many hundreds of TKR samples were tested with largely similar methodologies using the ISO 14243-1 force-control method. These tests covered 133 different fixed and mobile bearing designs and materials, in total (bicondylar) and unicompartmental forms, and of a wide range of sizes. Clear differences in wear resulted with known superior bearing materials. This illustrates how sensitive and capable of discriminating between low and high wearing implants the force-control wear testing methodology is. However, between both labs, and across all tests, no statistically significant differences were found in wear overall between fixed and mobile bearings. Therefore, the wear of mobile bearing knees is not necessarily less than that of fixed bearings. In both, it depends on the detailed design and materials of the TKR. Testing appears to be necessary with all implant designs, regardless of the history of clinically successful predicates of seemingly similar generic design.