Effect of protein lubrication on the wear properties of materials for prosthetic joints

The effects of pre-dilution and other modifications of bovine serum lubricants on the wear properties of UHMW polyethylene acetabular cups were evaluated in a hip joint simulator. The wear rate increased, and a nonphysiological type of surface-pitting occurred, when the serum was pre-diluted to 40% or lower concentration. During the wear test, the equilibrium temperature and the precipitation of proteins were substantially greater with zirconia balls than with cobalt-chromium. Protein precipitation, a potential modulator of in vitro wear, was shown to be temperature, concentration, and time-dependent in water-bath tests, which indicated that ball-cup interface temperatures in the simulator must be above 60 degrees C, i.e., well above the bulk lubricant temperature, to account for wear test protein precipitation. Several modifications of serum that were, in part, intended to decrease the tendency for protein precipitation were found to markedly affect the wear properties of the two combinations of materials. In particular, modified serum, which lacked some of the higher molecular weight proteins, produced a much higher wear rate than a control serum with the same initial protein concentration. The results indicated directions for further research to clarify the lubricating properties of serum, and for developing a universal standard test lubricant.