Wear processes in polymer implants - Durability and Reliability of Medical Polymers - page 147

and failure. It is often necessary therefore to employ statistical methods for
wear prediction (Bhushan, 2002).
The paris law of crack propagation can be used to show that, once again,
the fatigue wear rate is strongly dependent on normal load and to a lesser
extent on surface roughness. It is worth considering, that all of the other
wear mechanisms discussed above require not only a normal load but also
physical contact between the surfaces. however, if the surfaces are separated
by a lubricant film, adhesive and abrasive wear are virtually eliminated but
fatigue wear can still occur (hutchings, 1992).

7.4 Polymers
There are a variety of polymers used in joint replacement implants that can
be subject to wear. In this section UhmWpE, crosslinked polyethylene,
poly(ether ether ketone) (pEEK), silicone and polyurethan are discussed. Some
of the mechanical properties for these polymers are shown in Table 7.1.

7.4.1 Ultra high molecular weight polyethylene
Ultra high molecular weight polyethylene (UhmWpE) has been the mainstay
of joint replacement implants for over forty years. There are many studies that
have undertaken laboratory investigations looking at the wear of UhmWpE
against various counterface materials. most screening work involves the
use of a pin of UhmWpE that is loaded against a plate or disc made from
stainless steel, CoCrmo alloy, alumina or zirconia. The plate or disc is then
moved relative to the pin to create wear debris. The use of lubricants is
crucial in the wear testing of materials used for joint replacement implants
to attempt to simulate the synovial fluid found in the natural synovial joint.
The lubricant used in wear testing is typically bovine serum that is diluted
with distilled water (pylios and Shepherd, 2008) although in some tests
distilled water is used on its own.

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Table 7.1 Typical mechanical properties of polymers used for joint replacement
implants

Material Young's modulus

(GPa)

Ultimate
tensile strength
(MPa)

Reference

Crosslinked polyethylene 860 29.3 Lewis (2001)

PEEK 3.6 93 Brown (2006)

Polyurethan 0.02 10 Scholes et al . (2006)

Silicone 0.03 7 Brown (2006)

UHMWPE 915 48.7 Lewis (2001)