Mechanical properties - Orthopaedic Biomaterials in Research and Practice - page 70

2. Even if the forces produced by aversion or impact do produce
the fracture, the resultant stresses may be far below σ u , so that it
can more properly be said that the device was essentially “time
expired” and that any slight overload would have produced failure.
In this case, the overload event should be considered as contribut-
ing to failure, but the primary cause of failure was the preexisting,
ongoing fatigue process.

Static fatigue All materials are subject to fatigue, with metals and
polymers displaying the features discussed here. Ceramics at body tem-
perature do not possess significant ductility and thus do not show fatigue
striations. However, when subject to high static or peak dynamic loads,
especially in chemically active environments, they may fail suddenly,
producing fracture surfaces not distinguishable from those of single-
cycle failures. Since ceramics are rarely deliberately designed to sustain
high tensile cyclic loads, these failures are often called static fatigue
fractures. The detailed mechanism is unknown. As mentioned earlier,
composites also display fatigue behavior, but the damage appears to be
primarily a progressive failure of adhesion between the various compo-
nents of the material rather than fatigue fracture of any one material.

Clinical observations of fatigue failure Fatigue fractures of ortho-
paedic devices usually initiate where expected, in regions of high tensile
stress:

On the lateral faces of femoral stems or at the proximal limit of distal
support (Figure 3.12)

Knot

Bend

Peak
stress
(lateral
pace)

Nick

Lateral
limit of
distal
support

FIGUre 3.12 Points of fatigue failure initiation.