Biomedical Engineering Reference
In-Depth Information
joints, though thankfully they are less common today. Another mechanism
of wear - common in engineering components though not so much in
orthopaedic applications - results from local adhesion between the two
surfaces, producing wear debris by local fractures when motion occurs. This
adhesive wear mechanism usually occurs if the two materials are similar,
for example stainless steel screws used to attach a stainless steel fracture
plate.
Failures that occur after relatively long periods of time, in good-quality
materials chosen for their ability to resist mechanisms such as abrasive
and adhesive wear, tend to occur via a fatigue mechanism. Near-surface
material is cyclically loaded, for example on each walking step, leading to
the creation of small fatigue cracks, usually lying just below the surface.
These cracks can grow up to the surface, causing a large piece of material
to spall off; alternatively they can grow down through the part, leading to a
fatigue failure. Figure 12.5 shows an example of the latter: a knee joint in
which the polyethylene component was completely worn through, causing
contact between two metal components (the tibial tray and the femoral
condyles). A shiny wear patch is visible, from the edge of which a fatigue
crack has formed, causing part of the tibial tray to break off. This synergistic
combination of wear and fatigue is called fretting fatigue; more details of
this kind of failure can be found in specialized engineering publications, for
example Lindley (2004).
12.5
Fretting fatigue in the tibial tray of a knee joint, seen after
removal of the polymer liner. Massive wear of the polymer (as
illustrated in Fig.12.4) has resulted in metal-on-metal contact,
forming the bright wear patch on the metal surface. Fatigue cracking
at the edge of the wear patch has caused part of the tibial tray to
detach.
