Biomedical Engineering Reference
In-Depth Information
will be subjected to in service and for what durations: this is known as
the stress history of the component. The general situation with regard to
stress analysis for engineering components has been greatly aided by the
development of computer simulations, especially finite element analysis
(FEA) and multi-body analysis. Thanks to these tools and to the increasing
power of computers, we can estimate with reasonable accuracy the stresses
that arise in many engineering structures, such as a girder in a bridge or a
suspension component in a car. The same technology can also be applied
to predict the stress histories of orthopaedic devices, but not with the same
level of accuracy.
There are a number of reasons for this. First, a typical implant has several
mechanical features which, whilst they can be simulated using FEA, create
problems which reduce accuracy. These include the presence of interfaces in
contact, in conditions in which the properties of the interface (e.g. the friction
coefficient) are not well known, and the existence of materials which have
anisotropic properties (such as bone) and time dependent properties (such as
bone cement and soft biological tissues). Second, the exact geometry of the
body components (e.g. the diameter of the medullary cavity of the bone) is
poorly known, difficult to measure and varies from one person to another.
Third, there is considerable uncertainly about the boundary conditions of
the model, for example the forces provided by the various muscle groups
surrounding a joint. Finally, unlike the engineering components mentioned
above, there is limited scope for conducting experimental measurements that
would act as validations of the computer simulations. Significant progress in
this area has been made by Bergmann and co-workers who have provided
data from instrumented implants
in vivo
(Bergmann
et al
., 1993).
all this is not to say that computer simulations are useless, far from it.
They have provided us with important insights, helped us to understand
failures after they have occurred and prevented mistakes being made in new
designs. But we should regard these simulations as providing useful general
indications of the stresses and strains that arise in components, rather than
precise, quantitative results.
The stress history of a component will also tell us about the number of
cycles it experiences. of course this is something which varies considerably
from person to person, depending on activity level and lifestyle. For general
design purposes, some workers have defined typical activity levels in terms
of the number of load cycles per day for walking, running and so on. For
example, Table 12.1 gives some data from Whalen
et al
. (1988). We might
reasonably assume that a person who has received a joint implant such as
an AHJ would be content with a sedentary lifestyle, consisting mainly of
low-stress cycles arising from slow walking. However, this author has come
across one person who continued working on his farm after receiving an
AHJ and another who was very fond of Irish dancing: in fact both of these
