Effect of low protein concentration lubricants in hip simulators

Proteins play an important role as boundary lubricants in vivo and in vitro. Hip simulator studies have tested various protein concentrations of lubricants. Several groups reported that nonphysiological pits were created with low protein concentrations. Our study showed details of wear findings with low protein concentrations. This may be the first mapping of the run-in wear morphology on ultra high molecular weight polyethylene (UHMWPE) cups.

The UHMWPE cups used were holding cobalt chrome (CoCr) balls. This study was run on orbital-type hip simulators at up to 1.0 million cycles (Mc). The lubricant was bovine calf serum (0, 3, 5, 7, 10, and 1 mg/ml protein concentrations). Two volumes of lubricant were used (40 and 16 ml). Volumetric wear rates were calculated and the cup surfaces were observed using reflected light microscopy (RLM) and scanning electron microscopy (SEM).

The 40-ml volume showed a lower wear rate than the 160-ml volume. The RLM findings showed that the machine marks gradually disappeared up to 1.Mc, but there were no obvious pits on the polyethylene surface. The SEM findings showed nonphysiological wear phenomena in the 0-mg/ml protein concentration and physiological wear phenomena in the low protein concentrations. In the main bearing wear area, many nodules were observed with fibrils. In the peripheral bearing wear area, mainly ripples were observed. At the nonbearing wear area, folds were observed.

Our study showed the microwear findings with low-protein serum: First, folds were formed, followed by ripples, and finally nodules accompanied by fibrils. During the run-in wear phase, nodules and fibrils had already appeared in the main bearing wear area. Thus, we need to investigate more details of the wear process when folds or ripples change to nodules at the main wear area in the run-in wear.