A frictional study of total hip joint replacements

Polymeric wear debris produced by articulation of the femoral head against the ultra-high-molecular-weight polyethylene socket of a total hip replacem ent has been implicated as me main cause of osteolysis and subsequent failu re of these implants. Potential solutions to this problem are to employ har d bearing surface combinations such as metal-on-metal or ceramic-on-ceramic prostheses. The aim of this study was to investigate the difference in lub rication modes and friction of a range of material combinations using synth etic 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 (CMC) fluids as the lubricant. Under th ese conditions the ceramic-on-ceramic joints showed full fluid film lubrica tion while the metal-on-metal, metal-on-plastic, diamond-like carbon-coated stainless steel (DLC)-on-plastic and ceramic-on-plastic prostheses operate d under a mixed lubrication regime. With bovine serum as the lubricant in t he all ceramic joints, however, the full fluid film lubrication was inhibit ed due to adsorbed proteins. In the metal-on-metal joints this adsorbed pro tein layer acted to reduce the friction while in the ceramic coupling the f riction was increased. The use of bovine serum as the lubricant also signif icantly increased the friction in both the metal-on-plastic and ceramic-on- plastic joints. The friction produced by the DLC-on-plastic joints depended on the quality of the coating. Those joints with a less consistent coating and therefore a higher surface roughness gave significantly higher frictio n than the smoother, more consistently coated heads.