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RESEARCH PAPERS

The Quantification of Physiologically Relevant Cross-Shear Wear Phenomena on Orthopaedic Bearing Materials Using the MAX-Shear Wear Testing System

[+] Author and Article Information
Matthew R. Gevaert, Martine LaBerge, Jennifer M. Gordon

Department of Bioengineering, Clemson University, Clemson, South Carolina 29634 Phone: 864-656-4178, Fax: 864-656-4466

John D. DesJardins

Department of Bioengineering, Clemson University, Clemson, South Carolina 29634 Phone: 864-656-4178, Fax: 864-656-4466jdesjar@ clemson.edu

J. Tribol 127(4), 740-749 (Jun 01, 2005) (10 pages) doi:10.1115/1.2000272 History: Received February 24, 2004; Revised June 01, 2005

Background: The occurrence of multi-directional sliding motion between total knee replacement bearing surfaces is theorized to be a primary wear and failure mechanism of ultra-high molecular weight poly(ethylene) (UHMWPE). To better quantify the tribologic mechanisms of this cross-shear wear, the MAX-Shear wear-testing system was developed to evaluate candidate biomaterials under controlled conditions of cross-shear wear. Method of approach: A computer controlled traveling x-y stage under a 3 degree-of-freedom statically loaded pin is used to implement the complex multi-directional motion pathways observed during TKR wear simulation. A MHz collection of dynamic x-y friction was available on all six environmentally controlled stations. The functionality of this testing platform was proven in a 100,000 cycle, 11.6 MPa, wear test using 15.0 mm diameter polished stainless steel spheres against flat GUR4150 UHMWPE. A five-pointed star wear pattern was used to incorporate the physiologically relevant cross-shear sliding conditions of stop/start, 50mms entraining velocity and five crossing angles of 72°. Using normalized volumetric reconstruction of the resulting surface damage, a direct quantitative relationship between linear and cross-shear surface damage intensity was obtained. Results: Cross-shear surface damage volume loss was found to be 2.94 (±0.88) times that associated with linear sliding under identical tribologic conditions. SEM analysis of linear wear damage showed consistent fibril orientation along the direction of sliding while cross-shear wear damage showed multi-directional fibril orientations and increased surface roughness. Significant increases in discrete crossing-point friction coefficients were recorded throughout testing. Conclusions: This scientific approach to quantifying the tribologic effects of cross-shear provides fundamental wear mechanism data that are critical in evaluating potential biomaterials for use as in vivo bearings. Relevant multi-axis, cross-shear wear testing is necessary to provide quantifiable measures of complex biomaterials wear phenomena.

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Copyright © 2005 by American Society of Mechanical Engineers
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Figures

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Figure 1

The MAX-Shear wear testing system

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Figure 2

A close-up view of one wear testing station showing the pin and frictional force load cell array configuration

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Figure 3

Five-pointed star wear testing pathway

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Figure 4

Star-shaped damage track on UHMWPE sample, shown magnified at 4×. All crossing locations within the star experience similar damage, as did all five star points. Infrequent closed-loop trajectory corrections during testing caused star-point to star-point profile corrections, resulting in the effect of a pentagram formed by the points of the star.

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Figure 5

Surface profilometry scan (r) showing typical area of cross shear damage (profilometry magnification at 5×; FOV 0.93×1.25mm)

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Figure 6

Sample friction trace showing stop / starts (# indicates one of five regions) and crossing points (× indicates one of five sets of double peaks created by cross pathway transients) at 0.2 km cycles

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Figure 7

SEM images of UHMWPE specimens: orientation of linear damage track(s) shown by white arrow. Orientation A: Cross Shear at low (40×) magnification. B: Fibrils aligned parallel to damage track (3,000×). C: Fracture within wear track (10,000×). D: Cobblestone wear within cross shear, close to transition zone (5,000×). E: Smearing of larger fibrils within center of cross shear zone (10,000×).

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Figure 8

Summary of wear test methods. Symbols ∘∕• indicate absence / presence of shear.

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Figure 9

Circularly translating pin-on-disk motion, adapted from Saikko (16). The net effect of the translation is a circular wear track on the disk, represented on the far right.

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