Polymers - Orthopaedic Biomaterials in Research and Practice - page 131

CHaPTer SIX
Polymers
The selection and use of implant materials involve important prospec-
tive decisions. Each material has specific combinations and ranges of
chemical, mechanical, electrical, thermal, and biologic performance
characteristics. Design requirements dictate materials choices; how-
ever, once materials choices are made, they strongly affect the design
process in both positive and negative ways. If there were a single “best
implant material,” then all devices would be made from it. There is no
such thing; to make all devices from the same material would be like
asking a painter to work with only one color of paint. There are classes
and groups of materials with similar properties, and within each group
different choices may be made of composition and of processing tech-
niques, to yield different combinations of final properties.
Design is a creative process of compromise. Suppose a designer
begins to reduce an idea for a new configuration of cemented femoral
total hip replacement component to an engineering design so that trial
parts may be made. Key or critical design features are identified, such as
the desire for the femoral stem to be able to sustain large reversing loads
without fatigue failure. However, having done this, the designer can-
not reach for his materials handbooks and specifications and select the
material with the highest endurance limit. The same material must meet
other requirements, such as single-cycle fracture strength, smoothness,
corrosion resistance, and so on, and it must be able to be fabricated into
the desired shape (Figure 6.1).
Thus, the design is refined by developing the shape of the stem in
detail. This leads to calculation of the peak magnitude of the reversing
stress that might be expected in the “average” 70 kg patient with an aver-
age lifestyle and in turn to specification of a minimum endurance limit,
with a suitable safety factor, which is permissible if the stem material
is to survive the expected fatigue stresses. All materials that meet this
minimum are then candidates for use, and further steps in the design
process result in choices being made between them on the basis of other
criteria, until a clear final choice of material composition and processing
emerges for the fabrication of the femoral stem.