Tribological conditions in mobile bearing total knee prostheses

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This paper describes the in-vivo contact conditions that occur in total knee prostheses, with focus on mobile bearing designs. As background, typical loading and kinematic conditions occurring in the human knee joint are briefly reviewed, for normal walking as well as for other activities including running/jogging. The loads and relative motions are then used, along with typical properties of knee joint's synovial fluid, in a transient elastohydrodynamic analysis of the fluid film lubrication conditions at the articulating tibio-femoral interface of mobile bearing total knee prostheses. Most of the analyses in this paper are for the most common material combination used in total knee prostheses, cobalt–chrome–molybdenum femoral component in contact with ultra-high molecular weight polyethylene (UHMWPE) tibial bearing. The transient fluid film thickness and the pressure within the contact area on the implanted knee prostheses are determined during typical walking and jogging cycles. It is found that the lubricating film is so thin that the contact is within the mixed lubrication condition much of the time, and at times descends into the boundary lubrication zone. Therefore, wear of the contacting materials, particularly the UHMWPE, is expected to occur nearly continuously. Examination of retrieved mobile tibial bearings confirmed that wear of the articular bearing surfaces is consistent with predicted contact conditions; wear is most severe where contact pressure is highest and lubricant film thickness is lowest. The effects of different prosthesis materials (e.g., ceramic femoral component) on the resulting minimum film thickness and pressure are determined and the implications for implant wear are discussed.