Wear processes in polymer implants - Durability and Reliability of Medical Polymers - page 148

Wear factors for UhmWpE can be in the range 10 -9 to 10 -6 mm 3 n -1 m -1 ,
depending on factors such as the test apparatus, lubricant and speed (Fisher
and Dowson, 1991; Fisher, 1994). The surface roughness of the plate or disc
can affect the wear factor. Lancaster et al. (1997) investigated the wear of
UhmWpE against stainless steel, CoCrmo, alumina and zirconia, using a
pin-on-plate apparatus. The surface roughness values for the different materials
were varied between 0.005 and 0.04 mm. The wear factor of UhmWpE
was found to increase from 7.4 ¥ 10 -9 mm 3 n -1 m -1 ( R a of 0.005 mm) to
16.5 ¥ 10 -9 mm 3 n -1 m -1 ( R a of 0.04 mm). multidirectional motion for
pin-on-plate wear testing was shown to be crucial to produce more realistic
wear rates to those seen in vivo . Using a multidirection apparatus, Saikko
(1998) measured the wear factor for UhmWpE against stainless steel to be
0.8 ¥ 10 -6 mm 3 n -1 m -1 .
a pin-on-plate or disc-on-plate apparatus is useful for screening various
material combinations or investigating factors such as the type of lubricant
or surface roughness, but more realistic loads and motions are required to
attempt to simulate use in the body. For this reason, hip and knee simulators
were developed (Burgess et al. , 1997; Goldsmith and Dowson, 1999a) and
international standards (BS ISO 14242-1, 2002; BS ISO 14243-1, 2009)
published so that comparison can be made between the various designs and
materials combinations available for the implants. For a hip replacement, the
volume of wear generated from an UhmWpE acetabular cup when articulated
against a CoCrmo alloy femoral head is about 48.2 mm 3 per million cycles
(Smith et al. , 2000). a million cycles, on average, represents about one year
in the human body (Shepherd and azangwe, 2007). For zirconia against
UhmWpE, the wear volume is much smaller at 6.3 mm 3 per million cycles
(Goldsmith and Dowson, 1999b). The wear of total knee replacements using
simulators shows the volume of wear to be between 1 and 10 mm 3 per million
cycles (ash et al. , 2000; Flannery et al. , 2008; Utzschneider et al. , 2009).
Flannery et al. (2008) determined the mean wear to be 6.4 mm 3 per million
cycles, which corresponds to a wear factor of 0.033 ¥ 10 -6 mm 3 n -1 m -1 ,
whereas a wear factor of 0.068 ¥ 10 -6 mm 3 n -1 m -1 was found by ash et al.
(2000), with a mean volume of wear of 3.7 mm 3 per million cycles.

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7.4.2 Crosslinked polyethylene
Crosslinked polyethylene was introduced for joint replacement because it
has lower wear rates than conventional UhmWpE. however, there are some
concerns that, although wear rates are reduced, the strength of the material also
reduces, and this may lead to fracture of the polymer implant. The crosslinking
of polyethylene is achieved through high-energy radiation, with the amount
of crosslinking proportional to the absorbed radiation dose. The polymer is
then subjected to a thermal processing step (Kurtz et al. , 2008).