Abstract
Some mobile bearing knee replacement designs have shown truly excellent long-term clinical results. The higher laxity of a mobile bearing helps reduce the shear forces and torques transmitted to the prosthesis-bone interface, and this could only help reduce the risk of loosening. Some argue that self-alignment of a mobile bearing rotationally can produce more central patellar tracking. However, the most commonly assumed benefit of mobile bearings is the reduction in contact stress, which is typically expected to reduce fatigue and wear. In a rotating platform TKR for example, wear is also expected to be less because the rolling/sliding motion is separated from the transverse rotational motion onto two separate articulating surfaces, thus less cross-paths and less wear. Such expectations may have dominated the thinking and perhaps even clouded the expectations of TKR wear test engineers. Such wear reduction however has not really been categorically proven clinically.
This paper combines in-vitro wear results from two separate laboratories, one in Nebraska USA and one in Germany. These two (industrially unattached labs) possess between them a very large set of in-vitro wear testing results across the widest variety of fixed and mobile bearing TKR designs. Fortunately, the wear testing methodology using the force-control regime used in the two labs was largely similar, and was highly consistent within each lab. The fixed and the mobile bearings were subjected to the exact same force fields, allowing their Anterior-Posterior translation and internal-external rotation kinematics to vary based on the individual TKR design.
Tens of implant designs have been tested, both fixed and mobile, in total (bycondylar) form and unicompartmental, of various sizes. Some mobile bearings had rotating platforms and some were rotating-translating. Some of the tests specifically compared mobile to fixed bearing tibial components using identical femoral components. Between both labs, and across all tests, no statistically significant difference resulted in wear between fixed and mobile bearings. Yet, such differences did clearly feature with known superior bearing materials (for wear) and other favored design features. Also, generally, the force-control test methodology has proven highly discriminatory in its simulation and measurement of wear as a potential clinical failure mode.
The take home message to test engineers is to expect the wear of both mobile and fixed bearings to depend more on the detailed design and materials of the TKR than on the mobility of the bearing. The results of this study re-confirm the need for wear testing to be performed prior to any clinical use on all implant designs, despite seemingly similar predicates or success of some mobile bearings.
