In Vitro Study of Backside Wear Mechanisms on Mobile Knee-Bearing Components

[+] Author and Article Information
S. A. Atwood, F. E. Kennedy, J. H. Currier, D. W. Van Citters, J. P. Collier

Dartmouth Biomedical Engineering Center, Thayer School of Engineering, Hanover, NH 03755

J. Tribol 128(2), 275-281 (Nov 21, 2005) (7 pages) doi:10.1115/1.2162916 History: Received June 24, 2005; Revised November 21, 2005

The long-term success of a total knee replacement depends on the wear performance of a polyethylene bearing that separates a metal femoral component from a metal tibial tray. Although fixed bearing designs secure the polyethylene bearing to the tibial tray, mobile bearing knees allow the polyethylene to move relative to the tibial tray. This study has evaluated the wear performance of an intended articulation on the inferior surface of the LCS®-Rotating Platform mobile bearing by conducting clinically relevant tribological testing and comparing results to retrieved knee bearings. A retrieval analysis leads to the conclusion that third-body particles in the contact produce curvilinear scratches longer than the expected rotation of the knee on both the polyethylene bearing and the CoCr tibial tray. Tribological testing shows that polymethylmethacrylate (PMMA) bone cement particles produce worn surfaces most similar to retrievals. Porous-coating beads and bone debris also have the ability to damage both surfaces. Worn polyethylene surfaces from pin-on-flat tests show scratches longer than the excursion length, and “skipping marks”—pits spaced at smaller rotation intervals along a scratch—as observed in retrievals. These wear features suggest that a ratcheting mechanism, which moves the third-body particles further along the scratch with each cycle, may be responsible for the observed wear.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 6

Test with PMMA particles: 8mm excursion, one-million cycles. Higher magnification micrographs show wear features in greater detail. Top: Surface of worn polyethylene pin. Numbers on higher magnification micrographs correspond to locations shown on central photo of pin surface. Bottom: Surface of scratched CoCr flat specimen.

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Figure 7

Profile of scratch on polyethylene pin using PMMA bone cement (one-million cycle test)

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Figure 8

Profile of retrieved polyethylene tibial bearing

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Figure 9

Scratches on CoCr flat after test with PMMA bone cement. Top: 500,000cycle tests, bottom: one-million cycle tests; right: tests with 8mm excursion, left: tests with 1mm excursion.

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Figure 10

Skipping marks observed on worn surface after tests with PMMA bone cement

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Figure 1

Curvilinear scratching on retrieved UHMWPE bearing back surface (left) and mating CoCr surface (right). The prosthesis had served in vivo for 154months before retrieval. Note that the scratches are longer than the expected arc of rotation (about ±5deg).

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Figure 2

Test configuration in oscillating pin-on-flat tester. The flat-ended polyethylene pin is stationary and is loaded with a static normal load against the oscillating, flat, polished CoCr specimen.

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Figure 3

SEM image of typical PMMA bone-cement particle prior to testing (scale marker=200μm)

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Figure 11

Skipping marks observed on retrieved polyethylene components. Left: Cementless, right: cemented.

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Figure 4

Worn surfaces from test with porous-coating bead: 8mm excursion, 500,000cycles. Micrographs on right are magnifications of central sections of those on left. Top: Surface of worn polyethylene pin with embedded porous coating bead, bottom: surface of scratched CoCr flat specimen.

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Figure 5

Worn surfaces from test with bone chips: 8mm excursion, 500,000cycles. Micrographs on right are magnifications of central sections of those on left. Top: Surface of worn polyethylene pin, bottom: surface of scratched CoCr flat specimen.




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