Biomedical Engineering Reference
In-Depth Information
difficulties in predicting the performance of custom-made, one-off implants
such as those used for spinal surgery and the repair of complex fractures.
This chapter will begin with an overview of the various types of long-term
failure in engineering components, giving examples of their occurrence in
orthopaedic devices. after a brief note on stress analysis, two case studies
illustrate how particular failure modes can be predicted and prevented. The
chapter will conclude with some general remarks about the application of
failure analysis and prevention methods in the medical field.
12.2 Long-term failure modes
There are essentially four different types of long-term mechanical failure:
fatigue, creep, stress-corrosion cracking and wear. In addition we must
consider time-dependent chemical effects (corrosion and biochemical
reactions) and the functional adaptation reactions of the human body,
especially bone resorption. In this section, each type of failure mode will
be introduced. For a more detailed treatment the reader is referred to one of
the many excellent textbooks on engineering materials, for example ashby
and Jones (1980). The situation is made more complex by the tendency of
these failure modes to interact with each other; thus, for example, wear on
the surface of a component can lead on to the creation of fatigue cracks,
whose growth can be accelerated by corrosion. Much of our understanding
of these interactions has been gained in retrospect, by carefully analysing
failures that have occurred
in vivo
.
12.2.1 Fatigue
Fatigue is the most common type of mechanical failure in engineering
components in general and certainly occurs often in orthopaedic implants.
Fatigue is essentially a cracking process: over a period of time in service,
small cracks form in the material and gradually grow. In some cases, such
as the intramedullary pin shown in Fig. 12.1, a fatigue crack may become
long enough to cause total failure of the component. In other cases the effect
of cracking is to compromise function in some way; for example, multiple
cracking in bone cement causes loss of fixation between implant and bone,
leading to loosening (Jasty
et al
., 1991).
Why do these cracks form and grow? The physics of the process is quite
complex but the essential feature is cyclic stress, that is a stress which changes
periodically with time. For example an implant in the leg experiences a stress
cycle every time the person takes a step during walking. Higher stress cycles
occur less frequently, during running and other strenuous activities and, less
obviously, during stair climbing, rising from a chair and accidental stumbles
and trips (Bergmann
et al
., 1993). Even though the maximum stress in the
