Biomedical Engineering Reference
In-Depth Information
example, beyond the polymer yield point, the hardening or cold drawing portion
behaviour in uniaxial tension is sensitive to the molecular weight. Under
biaxial drawing conditions of the small punch test,
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the large deformation
mechanical behaviour of polyethylene also displays strong molecular weight
dependence.
8.2.5 Wear of UHMWPE in total joint replacements
the generation of wear debris is a key issue in the longevity of total joint
replacements, owing to the subsequent tissue reaction to such debris. Particulate
debris of UHMWPe generated by wear, fragmentation or fretting induces the
formation of an inflammatory process in order to resorb the polymer particles.
this process promotes a foreign-body granulation tissue response that has
the ability to invade the bone-implant interface. it results in a progressive
local bone loss that compromises the fixation of the device.
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Wear involves the erosion and loss of material in particulate form as a
consequence of relative motion between two surfaces.
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the particles (wear
debris) may be lost from the system. there are three principal wear processes:
(1) adhesive wear is also known as scoring, galling or seizing and it occurs
when two solid surfaces slide over one another under pressure; (2) abrasive
wear occurs when material is removed by contact with hard particles; (3)
surface fatigue by which the surface of a material is weakened, and ultimately
subsurface cracks will appear and propagate to form particles that are shed
form the surface, by cyclic loading and unloading.
in general, in the most popular joint replacement (cobalt-based alloy head
and UHMWPe cup), the UHMWPe component wears almost exclusively.
the wear rate of this couple is generally in the order of 0.1 mm/year, with
particulate generation in the order of 1 ¥ 10
6
particles per step or per cycle.
in clinical studies, the wear rate of UHMWPe implants has been found to
increase with the following processes: (1) physical activity and weight of the
patient; (2) size of the femoral head and (3) oxidation of the UHMWPe.
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8.2.6 New UHMWPEs: highly cross-linked UHMWPEs
As it has been mentioned earlier in the text, the cross-linking of UHMWPe by
gamma irradiation or other methods leads to a decrease in polymer wear with
potential for less wear debris and therefore less bioreactivity/osteolisis.
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three important processing steps are necessary to produce highly cross-linked
polyethylene for hip bearings. these steps are an irradiation step to promote
cross-linking, an intra or post-irradiation thermal processing step to increase
the level of cross-linking and remove residual stress and a sterilization step.
in the irradiation step, gamma radiation produces free radicals (unpaired
electrons) in the polyethylene, which in secondary chemical reactions
