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TECHNICAL PAPERS

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

[+] Author and Article Information
Francis E. Kennedy, Douglas W. Van Citters

Thayer School of Engineering,  Dartmouth College, Hanover, NH 03755

Khanittha Wongseedakaew, Mongkol Mongkolwongrojn

Department of Mechanical Engineering, ReCCIT, Faculty of Engineering,  King Mongkut’s Institute of Technology Ladkrabang, Bangkok, Thailand

J. Tribol. 129(2), 326-335 (Nov 14, 2006) (10 pages) doi:10.1115/1.2464130 History: Received March 25, 2006; Revised November 14, 2006

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 1.5cycless. 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 53days to eliminate the contribution from creep. Differences between initial and final profiles were attributed to wear of the UHMWPE. It was found that the largest wear depth in the lubricated tests occurred near the ends of the oscillatory contact area, while dry (unlubricated) tests of the same materials showed a peak wear depth near the center of the contact area. In the lubricated tests, the worn depth was lowest for the most heavily irradiated material. Analysis of the elastohydrodynamic lubrication in the rolling∕sliding contact was carried out assuming a line-contact situation with smooth cylindrical surfaces. 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 (hmin) 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 (K) for wear of the UHMWPE was found to be relatively constant over the central section of the oscillatory motion, but increased to a higher value where hmin decreased to near zero. Thus, the important influence of lubrication on wear of artificial knee bearings was demonstrated.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Multistation rolling sliding tribotester

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Figure 2

Contact conditions in one station of rolling sliding tribotester

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Figure 3

Plot of roll and slide velocities along wear track on polyethylene specimen

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Figure 4

Wear scar profile for unirradiated polymer surface after 21,600cycles of dry (unlubricated) roll∕slide test

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Figure 5

Typical profile of the wear scar on an unirradiated polymer surface after 1.5 million cycles of lubricated roll∕slide test

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Figure 6

Worn surface profiles for polymer materials that had been irradiated with either 29kGy or 65kGy. Roll-slide test duration: 1.5 million cycles

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Figure 7

Plots of pressure profile (dimensionless) within contact area at four different points along wear track (X=ξ∕R1A1)

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Figure 8

Plots of film thickness profile at four different points along wear track (X=ξ∕R1A1)

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Figure 9

Plot of minimum film thickness hmin along wear track

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Figure 10

Dimensionless sliding distance per cycle (Δs∕a) encountered by a point on the polyethylene wear track

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Figure 11

Plot of wear coefficient (K) along wear track for the three UHMWPE materials

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