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Research Papers

Field Variable Associations With Scratch Orientation Dependence of UHMWPE Wear: A Finite Element Analysis

[+] Author and Article Information
Matthew C. Paul

Department of Orthopaedics and Rehabilitation, and Department of Biomedical Engineering, University of Iowa, Iowa City, IA 52242; Wright Medical Technology Inc., Arlington, TN 38002

Liam P. Glennon

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

Thomas E. Baer

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

Thomas D. Brown1

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

Typical diameters used for ultra-high molecular weight polyethylene (UHMWPE) pin-on-flat specimens are on the order of 15mm. A larger diameter was chosen here to minimize the influence of edge effect. In a pilot contact finite element analysis (FEA) study, pin force/radius combinations were parametrically varied to find a size consistent with a nearly uniform contact stress distribution.

Marathon® UHMWPE is cross-linked by the cold irradiated subsequent melt (CISM) process. The UHMWPE is subjected to a moderate 50kGy(5Mrad) dose of gamma radiation, then is remelted at a high temperature, above 135°C, to reduce free radicals and decrease the potential for oxidation. Finally, the material is gas plasma sterilized.

1

Corresponding author.

J Biomech Eng 130(6), 061019 (Oct 24, 2008) (10 pages) doi:10.1115/1.2939273 History: Received January 25, 2005; Revised April 24, 2008; Published October 24, 2008

Scratches on the metal bearing surface of metal-on-polyethylene total joint replacements have been found to appreciably accelerate abrasive/adhesive wear of polyethylene, and constitute a source of the considerable variability of wear rate seen within clinical cohorts. Scratch orientation with respect to the local direction of relative surface sliding is presumably a factor affecting instantaneous debris liberation during articulation. A three-dimensional local finite element model was developed, of orientation-specific polyethylene articulation with a scratched metal counterface, to explore continuum-level stress/strain parameters potentially correlating with the orientation dependence of scratch wear in a corresponding physical experiment. Computed maximum stress values exceeded the yield strength of ultra-high molecular weight polyethylene (UHMWPE) for all scratch orientations but did not vary appreciably among scratch orientations. Two continuum-level parameters judged most consistent overall with the direction dependence of experimental wear were (1) cumulative compressive total normal strain in the direction of loading, and (2) maximum instantaneous compressive total normal strain transverse to the sliding direction. Such stress/strain metrics could be useful in global computational models of wear acceleration, as surrogates to incorporate anisotropy of local metal surface roughening.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

(Top) Laser scanning microscopy image of custom scratch profile created on 316L stainless steel. (Note the scale differences, which accentuate the scratch for visual emphasis.) (Bottom) Scratch profile (cross section) employed as the counterface surface in the finite element model.

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Figure 2

(a) Pin-plate articulating couple used in polyethylene-stainless steel reciprocating wear tester. The parallel scratches on the surface of the metal platen are spaced 150μm apart. (b) The corresponding polyethylene continuum mesh (white) and analytical scratched stainless steel surface (gray) utilized in the finite element model. (c) Enlarged view illustrating the spatial refinement of the polyethylene mesh in the region used for data registry.

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Figure 3

Contour plot of instantaneous longitudinal normal stress during passage of a scratch oriented at 45deg. Note the edge effect near the sides of the block. Fiducial nodes for stress registry were therefore located along the block centerline.

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Figure 4

The two surrogate mechanical stimuli judged to best resemble the scratch lip direction dependence of experimental wear. “Total strain” refers to the sum of elastic and plastic strain in the specified component direction.

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Figure 5

Statistical fit distribution for all 2054 comparisons of surrogate candidates to experimental wear. Selected cases (red dots) are illustrated in Fig. 6, in order of decreasing quality of fit. Note that after the worst-fitting candidate listed above (No. 1310, quality of fit=0.046), for administrative/procedural reasons, all remaining candidates involved incomplete datasets, and were assigned a quality of fit=0.

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Figure 7

UHMWPE debris particles collected from the experimental apparatus following articulation. (a) Particles generated by a scratched surface were typically orders larger than the most biologically reactive submicron debris. Here, a particle of cross-linked UHMWPE is presented, produced by a scratch orientation of 5deg. (b) A smooth articulation couple produced debris that were of submicron- or micron-order size, similar to the overwhelming volumetric fraction of particles found in tissues surrounding an implanted total joint in vivo. Further details regarding the relationships between local stress fields and sliding parameters are reported elsewhere (26).

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