Lubrication and Wear of Artificial Knee Joint Materials in a Rolling/Sliding Tribotester

This paper describes the influence of lubrication on wear during testing of materials for artificial knee joints in a rolling/sliding tribotester built to simulate contact conditions in a total knee replacement. The test configuration consists of parallel cylinders (pucks) of ultrahigh molecular weight polyethylene (UHMWPE) and polished cobalt-chrome alloy in oscillatory rolling/sliding contact in a bath of dilute (25%) bovine serum. Wear tests of three different UHMWPE materials were run under constant load at 40% sliding for 1.5 million oscillation cycles at 1.5 cycles/second. Wear of the UHMWPE was determined by measuring the profile of the cylindrical contact surface of the puck before and after each test. Profile measurements were repeated after at least fourteen days to account for creep. Differences between initial and final profiles were attributed to wear of the UHMWPE. It was found that the largest wear depth occurred near the ends of the oscillatory contact area, with the most heavily irradiated material showing the least wear. Analysis of the elastohydrodynamic lubrication in the rolling/sliding contact was carried out assuming a line contact situation. The time dependent modified Reynolds equation and the elasticity equation with initial conditions were solved numerically using a multigrid technique with full approximation scheme, and using a Newton Raphson method to solve the highly nonlinear system of equations. The thickness of the lubricating film of bovine serum was determined for points along the length of the wear track. It was found that the smallest film thickness (h_min) occurs very close to the location in the oscillating contact where the greatest wear occurs, owing to the very low entraining velocity near the ends of the oscillation cycle. The coefficient for wear of the UHMWPE (K) was found to be relatively constant over the central section of the oscillatory motion, but increased to a higher value where h_min decreased to near zero. Thus, the important influence of lubrication on wear of artificial knee bearings was demonstrated.