Biomedical Engineering Reference
In-Depth Information
leading to so-called corrosion fatigue which is a much more ubiquitous
phenomenon than SCC.
Many new materials like bioceramics have indeed been deliberately
designed to interact with the body. These are often used as coatings, which
work by encouraging bone formation on the surface of an implant; in some
cases the original coating is designed to be resorbed by the body and replaced
by living tissue. another example of a reactive material which has a long
history is bone graft: a material designed to temporarily fill a defect such as
a hole in a fractured bone. Bone graft materials include ground-up pieces of
human bone and artificial substitutes based on calcium compounds similar
to the body's own, such as hydroxyapatite or tricalcium phosphate. These
materials have very inferior mechanical properties but, provided the defect
is not too large, they can support the structure for long enough (a few weeks
or months) until they are replaced by living bone.
The body's ability to create new bone to fill a defect is just one example of
the more general phenomenon of functional adaptation, whereby the structure
and properties of bone, and indeed of almost all living tissues, change in
response to the mechanical environment. This important phenomenon will
be dealt with in another chapter of this topic; clearly it must be taken into
account when predicting the long-term integrity of implants. one example,
which will be discussed below, is the resorption of bone that can occur close
to a joint implant as a result of 'stress shielding' by the rather stiff metallic
material.
Finally, mention should be made of the important interactions between
wear and physiological reactions, because they are the cause of some modes
of long-term failure in joint implants which are becoming increasingly
important as the lifetime of these implants increases. as mentioned above,
wear processes create wear debris: small particles removed from the surfaces
of the interacting materials. In joint prostheses, polymer wear particles
are created, not only on the polyethylene bearing surfaces, but also from
wear in the PMMA bone cement. These particles are very mobile and can
very quickly find themselves in other parts of the joint, for example on the
interface between bone cement and bone, where they can initiate adverse
reactions, leading to the formation of a layer of soft tissue which threatens
the fixation of the implant (Ingham and Fisher, 2005).
12.3 Stress analysis, simulations and other
design methodologies
Before moving on to some case studies, it is worth reflecting on what will be
needed in order to use the above information in a practical way. assuming
that we have access to material data from the various tests mentioned above,
this will only be of value if we also know what stresses our materials
