Biomedical Engineering Reference
In-Depth Information
Contact stress
w
r
i
r
0
Congruent
r
0
=
r
i
t
Increasing
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0
/
r
i
σ
c
w
r
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r
0
Non-
congruent
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>
r
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σ
o
r
0
/
r
i
= 1
0
0
t
FIGUre 11.10
effect of congruence on contact stress.
fitting THR combination or in the articulation of a total knee
replacement (TKR) or, for that matter, in most natural joints.
2. If the softer material, usually the socket because of creep consid-
erations, is too thin (“t” too small), then the contact stress is addi-
tionally increased. This occurs since the “substrate” or supporting
material, whether in a natural joint or in an artificial replacement,
is much stiffer than either articular cartilage or a typical bearing
polymer, respectively. One may easily appreciate this effect by
comparing the feeling of standing barefoot on concrete or on soft
earth; the relative sensations are a direct consequence of the dif-
ference in average contact stress imposed by the metatarsals on the
soft tissues of the mid- and forefoot. (This is, unfortunately, not a
pure example since contact areas may also differ.)
These effects may combine to produce increases of up to 10 times
normal in contact stress in noncongruent thin polymeric components
such as thin metal-backed tibial plateaus in TKRs. For the typical radial
dimensions encountered in THR and TKR designs, these effects dimin-
ish and approach a lower limit for polymer (UHMWPE) thicknesses of
7-10 mm and are unimportant at greater thicknesses.
The consequences of such high local contact stresses in polymers are
first a localized surface cracking called “mud-caking” because of the
similarity of appearance to the surface of dried mud. As the process
progresses, fatigue cracks extend parallel to the surface, releasing frag-
ments and producing accelerated wear, through both material loss and
third-body wear (Figure 11.11). This has been observed in late failure
of early metal-backed acetabular components with thin polymer shells;
evidence of the early stages has also been found in more conventional
implants retrieved after long periods (7-10 years). The similarity between
these defects in polymeric components and those observed in articu-
lar cartilage in osteoarthritis (OA) has led to suggestions that fatigue
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