Biomedical Engineering Reference
In-Depth Information
surface which was similar to that observed clinically. There was a significant
increase in wear rate as a result.
Design issues also arise in the context of wear prevention. Different knee
joint designs can give rise to greatly varying amounts of wear: even quite
subtle design changes can be significant (Mueller-Rath
et al
., 2007). The
history of knee joint design has seen a gradual move away from rather fixed,
rigid designs to looser, more mobile implants which more closely reproduce
the motions of the natural joint. An inevitable effect of this, however, is
an increase in the number and complexity of motions across load-bearing
surfaces. Jennings
et al
. (2007) noticed that, in some artificial knee joints,
the femoral condyles actually lift off the plastic surface of the tibia: simulator
experiments showed that this caused a more than threefold increase in wear
rates.
One simple design change, which at first seems counterintuitive, is the
reduction of wear in the hip joint by reducing the diameter of the femoral
head. This will increase the compressive stress across the joint, but as noted
above, it is the load, not the stress, which controls the wear rate. But reducing
the head will reduce the sliding distance for a given angular motion of the
femur, so we can expect the amount of wear to reduce in proportion to
head size. Of course, if we take this too far we will create other problems
for ourselves. The high local stresses may cause creep in the polymer, for
example; we should always bear in mind that a design change which prevents
one type of failure may encourage another. A recent paper by McEwan
et al
.
(2005) is recommended for a good comparative study of wear in the knee,
showing the interacting effects of design, materials and kinematics.
12.6 Conclusions and future trends
There have been some great successes in the development of implants during
the last few decades: examples are hip joints, dental implants and stents.
These devices all moved from basic concepts to mass-produced products
within a 20-year period. The scientists and engineers who worked on these
developments are certainly to be congratulated for quickly overcoming a host
of problems which had not previously been faced in other medical devices
or in any other kinds of engineering components, for that matter. But many
new challenges lie ahead.
Tissue engineering has been suggested as an approach for the fabrication
of implants, but progress from laboratory studies to clinical products has been
much slower than expected. Certainly the future will see implants which are
more active, which integrate more fully with the body's living system. We
are already familiar with bioactive coatings on implants, and a great variety
of active ceramic and polymeric materials are currently being developed.
However, it is worth remembering that implants are often used because the
