Wear rate and surface morphology of UHMWPE cups are affected by the serum lubricant concentration in a hip simulation test

The wear rate and surface morphology of both cross-linked and non-cross-linked UHMWPE cups against cobalt–chromium alloy balls were evaluated when lubricated with bovine serum diluted to 90 or 25% of its original content in a laboratory hip simulation. The wear rates for the cross-linked polyethylene (PE) lubricated with 90 and 25% bovine serum were 20.4±0.1 and 34.1±2.6 (mg/million cycles), respectively. The wear rates for the non-cross-linked PE lubricated with 90 and 25% bovine serum were 43.9±1.8 and 57.8±0.6 (mg/million cycles), respectively. Similar morphology, i.e. in the form of generally smooth surfaces with many fine scratches, was observed under light microscopy for both materials lubricated with 90% serum. In contrast, roughened and irregular regions were on the articular surface of the non-cross-linked PE cups lubricated with 25% serum after 1 million cycles and remained throughout the remainder of the test. The cross-linked PE tested with 25% serum did not develop the surface appearance until 3.5 million wear cycles. For comparison purposes, PE cups that were revised due to PE wear were also examined. The cup surfaces appeared generally smooth and covered with scratches. However, no irregular surface was found as in our simulated cups lubricated with 25% serum. The results from this study support that cross-linked PE has better wear resistance than non-cross-linked PE. Serum lubricant with higher protein concentration provides better lubrication and caution must be taken when using serum with lower protein concentration, as it may create non-clinically relevant wear morphology.