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The
wear of the ultra-high molecular weight poly- ethylene bearing counterface
is illustrated in the two SEM photographs (20,000 X). The top picture is
the unworn surface, the lower the surface after a wear test against the
cobalt-chromium metal joint.
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Wear
causes surface breakup of the polymer giving a fiber like structure and
small particles. This behavior is caused by adhesion between the polymer
and the metal joint with local fiber drawing.
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As
noted above, the body tries to remove small particles by enzyme action,
however significant collateral damage occurs to the bone in the joint region,
loosening the implant, terminating the useful life of the joint, and requiring
a replacement implant. The local bone damage will have to be removed in
this operation and it may significantly weaken the repaired structure.
In general, only one such revision is possible, and this restricts the
utility of these devices to a 20 year span.
·
The
use of composite materials for this joint application may reduce this fatigue
induced delamination and prolong joint life. |
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