Friction and wear - Orthopaedic Biomaterials in Research and Practice - page 273

Contribution of
creep to wear

Materials subject to creep, such as UHMWPE, may show local defor-
mation that may be mistaken for wear. This is easily appreciated in
polymeric prosthetic components for tibial plateau replacement, in
which peak stresses may exceed 10 MPa, and has come to be called
“cold flow” in the clinical literature. At more moderate stresses, such as
the 3-4 MPa experienced by the typical acetabular cup, it may not be
obvious but has been estimated to account for as much as one-fourth to
one-half of the apparent wear observed in retrieved UHMWPE cups.
This type of deformation depends on geometric stress, so that heavier
patients or those whose femoral prostheses have relatively smaller heads
will experience greater creep. In addition, there is believed to be some
slight increase in creep rate over time in vivo , owing to absorption of
low-molecular-weight species that can act as plasticizers, making creep
easier.
The limiting creep rate for a typical patient for an acetabular cup
of at least 10 mm wall and 32 mm internal diameter is believed to be
between 0.03 and 0.1 mm/year. Composite materials, such as carbon
fiber-reinforced UHMWPE, may be expected to show lower creep rates
in vivo . Metals and structural ceramics essentially do not creep at body
temperature.

examples of wear Wear produces changes in surface appearance, particularly since bear-

ing surfaces are usually highly polished. The most frequent change
in appearance of metals associated with wear is a “frosted” or matte
appearance. This is due to an increase in surface roughness, secondary
to the inhomogeneous nature of wear. When relative motion is oriented,
parallel lines or score marks occur, as seen in the hinge pin of a fully
constrained stainless steel TKR retrieved after 7.5 years (Figure 11.12).
Evidence of wear of metal components in MOP devices is usually less
obvious. However, exceptional cases may occur, as seen in the Ti6A14V
femoral head retrieved after 15 months shown in Figure 11.13. The origin
of such extreme wear in titanium-base alloy components is not known.

FIGUre 11.12 Wear of hinge pin of tKr (stainless steel).