The effects of material combination and lubricant on the friction of total hip prostheses

Polymeric wear debris produced by articulation of the femoral head against the ultra-high-molecular-weight polyethylene (UHMWPE) socket of a total hip replacement has been implicated as the main cause of osteolysis and subsequent failure of these implants. Potential solutions to this problem are to employ hard-bearing surface combinations such as metal-on-metal or ceramic-on-ceramic prostheses. The aim of this study was to investigate the difference in lubrication modes and friction of a range of material combinations using synthetic and biological fluids as the lubricants. The experimental results were compared with theoretical predictions of film thicknesses and lubrication modes. A strong correlation was observed between experiment and theory when employing carboxy methyl cellulose fluids as the lubricant. Under these conditions, the ceramic-on-ceramic joints showed full fluid film lubrication while the metal-on-metal and metal-on-plastic prostheses operated under a mixed lubrication regime. With bovine serum as the lubricant in the all ceramic joint however, the full fluid film lubrication was inhibited due to adsorbed proteins. In the metal-on-metal joints, this adsorbed protein layer acted to reduce the friction while in the ceramic coupling the friction was increased. The use of bovine serum as the lubricant also significantly increased the friction in the metal-on-plastic joint. Therefore, the simple calculations using non-biological lubricants should not be relied upon in the design of orthopaedic bearing surfaces.