Local Head Roughening as a Factor Contributing to Variability of Total Hip Wear: A Finite Element Analysis

[+] Author and Article Information
Thomas D. Brown, Kristofer J. Stewart, Douglas R. Pedersen, John J. Callaghan

Department of Orthopaedic Surgery, University of Iowa, Iowa City, IA 52242

John C. Nieman

Department of Biomedical Engineering, University of Iowa, Iowa City, IA 52242

J Biomech Eng 124(6), 691-698 (Dec 27, 2002) (8 pages) doi:10.1115/1.1517275 History: Received June 01, 2001; Revised June 01, 2002; Online December 27, 2002
Copyright © 2002 by ASME
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Element zoning for the sliding distance-coupled finite element model. The direction of maximal wear depth (dashed arrow) is denoted in terms of its polar coordinates (θ,ϕ) relative meridionally to the pole of the cup (Z-axis), and circumferentially to a datum line (X-axis) in the equatorial plane of the cup. The solid arrows labeled with α values denote the centroids of femoral head regions used to study the effects of roughening site location in Series 3, where α=0 denotes the site on the head passed-through by the resultant joint load vector at the instant of peak force during the stance phase of normal level walking.
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Computed UHMWPE wear depth distributions after 106 cycles for two different wear coefficients (k=baseline, A, and k=100× baseline, B) for a roughened region of 122 mm2 located at the α=0° site on the femoral head. Besides the much greater depth for the k=100× case, note that the individual wear depth contours are appreciably distorted compared to the quasi-circular depth contours seen for the test-tube-like wear tract produced for k=1×.
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Cross-sectional profiles of the wear front as a function of roughening severity (Series 1). For comparison, corresponding profiles from idealized spherical wear fronts of equivalent maximum depth are superimposed (dashed lines). Note that the computed wear profiles depart progressively from sphericity (become increasingly more centrally protuberant) as roughening is increased.
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Volumetric wear rate versus time for two wear coefficient cases (solid line: k=baseline; dashed line: k=250× baseline). For the roughened case, the initial wear rate is approximately 17 times that of the non-roughened case. After approximately 1 year (106 cycles) of simulated in vivo service, this elevated wear rate decays to a lower, steady-state value, due to removal of a “trough” of polyethylene apposing the motion locus of the roughened head region. This steady-state wear rate for k=250× baseline is nevertheless approximately four times that of the non-roughened case.
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Contact stress distributions for the initial situation (A) of smooth hemispherical acetabular surface, and (B) after adaptive remeshing to account for material removal in the wear “trough” caused by overpassage of the roughened head region. Note the shift of contact stress predominantly to the lesser-worn polyethylene shoulders adjacent to the trough. Since these shoulder regions articulate only with non-roughened head regions, there is a diminished rate of computed polyethylene wear in the next simulation cycle. Physically, creep of polyethylene would tend to further prolong this computed transient.




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