Biomedical Engineering Reference
In-Depth Information
E
, of the material in question. Since like materials are able to bond easily,
the former factor explains why like pairs of materials tend to show higher
wear rates than unlike pairs. However, the range of moduli is far greater
than the range of surface associations (“stickiness”) in the presence of
lubrication, so modulus effects dominate production of wear debris.
Thus,
in vivo
, we expect that wear will occur preferentially to the
less stiff (possessing lower modulus) member of the articulating pair. In
a MOP pair, the polymer wears almost exclusively, and in a MOC pair,
the metal will wear but will produce much smaller particles than in the
previous case (<0.01-1.0 µm for metal [against ceramic] vs. 1-100 μm for
UHMWPE [against metal]).
These general relationships have been verified clinically, at least with
respect to typical wear debris particle size, through a number of stud-
ies of tissue retrieved from patients during THR arthroplasty revision.
However, such studies have failed to provide reasonable estimates of
either relative wear rates or total volume of wear debris, owing to the
high solubility of fine metallic debris and the increase in phagocytosis
and transport rates with decreasing particle size.
The limit on radiographic determination of dimensional changes associ-
ated with wear
in vivo
is probably
±
0.25 mm/year, although this number has
been criticized as being too small. Thus, most reliable data on wear-creep
rates are obtained from prosthetic components recovered during revision
surgery and, occasionally, at autopsy. The best estimates for
in vivo
volu-
metric wear rates for typical THR designs are given in Table 11.3. There
is general agreement that the rate of 0.15 mm/year (expressed as surface
recession rates rather than volume of wear debris) is a fair estimate of the
true wear rate of conventional UHMWPE/cobalt-base alloy pair in THRs.
Wear-induced osteolysis continues to be a salient concern in large
joint arthroplasty. In general, reducing the volume of wear particles
associated with arthroplasty components should reduce the adverse
biologic responses to them. Concerns over polyethylene wear-induced
osteolysis have spurred interest in alternative bearing surfaces. Volume
Table 11.3
estimated
in vivo
wear rates
Material combination
Material worn
Rate (mm
3
/year)
Metal/polymer (Co-Cr/UHMWPE)
Polymer
40.8
Metal/metal (Co-Cr)
Metal
0.023-6.3
Ceramic (alumina)/polymer (Al
2
O
3
/
UHMWPE)
Polymer
51
Ceramic (alumina)/ceramic
(alumina) (Al
2
O
3
/Al
2
O
3
)
Ceramic
0.004-0.04
Ceramic (zirconia)/ceramic
(alumina) (ZrO
2
/Al
2
O
3
)
Ceramic
0.1
Source:
Data adapted from Vassilou K, Scholes SC, Unsworth A. Laboratory
studies on the tribology of hard bearing hip prostheses: ceramic on
ceramic and metal on metal.
Proc IMechE
,
221 Part H
: J. Engineering
in Medicine 11-20, 2007.
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