Friction and wear - Orthopaedic Biomaterials in Research and Practice - page 270

Motion

Released
fragment

Metal

Vertical
fissures

Horizontal
cracks

Polymer

FIGUre 11.11 fatigue “wear” of polymer.

processes also play a role in OA, secondary to stiffening of subchondral
bone (owing to sclerosis or microfracture) and thinning of cartilage.
Small degrees of incongruence in hard-on-hard bearings such as
metal-on-ceramic (MOC) (e.g., CoCr-Al 2 O 3 ), ceramic-on-ceramic, and
metal-on-metal (MOM) bearing pairs can produce extreme elevations
of local stress leading to high wear and failure. Thus, very precise tol-
erances are necessary for successful manufacture of such component
pairs.

Other wear
phenomena

Wea r is a complex process, with many features of specific materials
pairs and designs affecting the outcome. Some important additional pro-
cesses, which may occur independently or together, are as follows:

Corrosive wear . When sliding takes place in a corrosive environment,
wear processes may disrupt the passivation layer on the metal part
of the pair, producing accelerated material loss. This is discussed
further in Chapter 12.
Fretting . When relative motion is possible, but over a very restricted
range, such as between screw head and hole in a screw plate inter-
nal fracture fixation device, significant localized wear, called fret-
ting , can occur. The amount of debris produced is often amazing,
producing extensive tissue discoloration in vivo even under condi-
tions that do not favor corrosion processes. In vitro studies demon-
strate that serum proteins act as lubricants, radically reducing the
effects seen in saline solutions.

Wear rates

There have been many efforts made to measure the rate of wear debris
production in the laboratory. In general, the results depend on the geom-
etry of the test, on the lubricant selected to simulate synovial fluid, and,
to some degree, on the experimenter. There have been great difficulties
encountered in reproducing in vitro experimental results. However, all of
these studies agree in that both the size of the individual particle and the
number of particles produced in a given situation vary directly with the
ability of the surfaces to “stick” and inversely with the elastic modulus,