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Research Papers

Sliding Direction Dependence of Polyethylene Wear for Metal Counterface Traverse of Severe Scratches

[+] Author and Article Information
Liam P. Glennon1

Department of Orthopaedics and Rehabilitation,and Department of Biomedical Engineering, University of Iowa, Iowa City, IA 52242

Thomas E. Baer, James A. Martin

Department of Orthopaedics and Rehabilitation, University of Iowa, Iowa City, IA 52242

William D. Lack2

Department of Orthopaedics and Rehabilitation,and Department of Biomedical Engineering, University of Iowa, Iowa City, IA 52242

Thomas D. Brown3

Department of Orthopaedics and Rehabilitation, and Department of Biomedical Engineering, University of Iowa, Iowa City, IA 52242tom-brown@uiowa.edu

Under certain conditions (17), cross-linked polyethylene performs comparably to—or perhaps even worse than—conventional polyethylene, when articulated against badly damaged counterfaces.

1

Present address: Zinsser North America, Northridge, CA.

2

Present address: School of Medicine, Harvard University, Cambridge, MA.

3

Corresponding author. Present address: Orthopaedic Biomechanics Laboratory, University of Iowa, 2181 Westlawn, Iowa City, IA 52242.

J Biomech Eng 130(5), 051006 (Jul 14, 2008) (7 pages) doi:10.1115/1.2947157 History: Received August 08, 2005; Revised April 17, 2008; Published July 14, 2008

Third-body effects appear to be responsible for an appreciable portion of the wear rate variability within cohorts of patients with metal-on-polyethylene joint replacements. The parameters dominating the rate of polyethylene debris liberation by counterface scratches are not fully understood, but one seemingly contributory factor is the scratch’s orientation relative to the direction of instantaneous local surface sliding. To study this influence, arrays of 550 straight parallel scratches each representative of the severe end of the clinical range were diamond stylus-ruled onto the surface of polished stainless steel plates. These ruled plates were then worn reciprocally against polyethylene pins (both conventional and highly cross-linked) at traverse angles varied parametrically relative to the scratch direction. Wear was measured gravimetrically, and particulate debris was harvested and morphologically characterized. Both of the polyethylene variants tested showed pronounced wear rate peaks at acute scratch traverse angles (15deg for conventional and 5deg for cross-linked), and had nominally comparable absolute wear rate magnitudes. The particulate debris from this very aggressive test regime primarily consisted of extremely large and elongated strands, often tens or even hundreds of microns in length. These data suggest that counterface damage regions with preferential scratch directionality can liberate large amounts of polyethylene debris, apparently by a slicing/shearing mechanism, at critical (acute) attack angles. However, the predominant manifestation of this wear volume was in the form of particles far beyond the most osteolytically potent size range.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Rakelike scratching apparatus, having 11 spring-loaded scribes, positioned atop a lapped/polished plate. (b) Close-up of a diamond-tipped scribe. Tip radius is 50μm.

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Figure 2

Zeiss LSM 510 confocal microscope reconstruction of a typical scratch profile. Note that each axis is on a different scale, which exaggerates scratch severity for purposes of visual emphasis.

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Figure 3

Custom-built Scotch yoke pin-on-plate fixture. The articulating couple (25.4mm diameter UHMWPE pin above, scratch-ruled plate below) was housed within the fixture.

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Figure 4

Wear rate versus angle between scratches and motion direction, for cross-linked polyethylene versus CPE. (Dispersion bars indicate standard deviations, for three replicate tests.) Statistically significant wear rate differences (p<0.05, by paired t-test) were achieved only for 5deg for HXPE and only for 15deg for CPE.

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Figure 5

(a) Typical particle from a 15deg CPE ruled-plate wear test. Note the curling and twisting of polyethylene. (b) Ribbonlike particle recovered from 15deg CPE ruled-plate wear test. (c) 90deg CPE scratch orientation ruled-plate wear test debris. (d) Flakelike particle recovered from a 15deg CPE ruled-plate wear test. (e) Debris from a nonroughened plate wear test using CPE. (f) 180-grade emery-roughened plate wear.

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Figure 6

Comparison between ruled-plate and emery-roughened plate particulate frequency, and associated aspect ratios

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