Biomedical Engineering Reference
In-Depth Information
two interesting non-metallic candidates: a carbon fibre composite material
made using long, continuous fibres and a particulate-reinforced composite
material which is interesting because its young's modulus is similar to that
of bone itself.
In theory, all of these materials are suitable for this application because all
the factors of safety are greater than unity. However, we should remember
that, as discussed above, the accuracy of this analysis is quite low, so materials
which give a factor of safety less than 2 should certainly be treated with
suspicion, which rules out the particulate composite. The most interesting
result from this analysis is that the titanium alloy, though it does not have
the highest fatigue limit, does have the highest safety factor, by virtue of its
relatively low Young's modulus. The fibre composite material seems to be a
plausible candidate, especially because a low young's modulus is attractive
for other reasons (see below), and indeed some AHJs have been made from
carbon fibre composites (Christel
et al
., 1987). The problem is that, in order
to get a reasonably high fatigue limit, it is necessary to have long fibres and
to arrange them so that they are aligned with the principal stress axes, not
an easy task in this rather complex, three-dimensional component.
almost all cemented hip implants use the same bone cement material, a
rather poor-quality version of PMMA. As Table 12.3 shows, this has a low
safety factor which accords well with the fact that cement failure is a major
cause of loosening. one might consider improving the cement by adding
reinforcing fibres and this indeed has been attempted (Pilliar
et al
., 1976).
However Table 12.3 would predict no improvement in the fatigue situation
and possibly even a slight disimprovement, because the increase in fatigue
limit is less than the increase in stiffness. rubber would seem to be a much
better material and, indeed, with regard to fatigue it would be; however rubber
would be unsuitable because it would creep, leading to massive subsidence
of the prosthesis. Attempts to improve PMMA by adding particles of rubber
have encountered the same type of problem. Given these findings, it is
perhaps not surprising that the bone cement of choice for most orthopaedic
surgeons has not changed significantly in the last 20 years.
Table 12.3
Safety factors for various AHJ cement materials (using a prosthesis
stem with
E
= 200 GPa)
Material
Young's
Fatigue
Maximum stress Safety factor
modulus (GPa)
limit (MPa)
in stem (MPa)
Standard PMMA
2.3
12
8.4
1.4
PMMA with short
5.5
16
13
1.7
carbon fibres
added
Rubber
0.1
30
0.4
75
