Biomedical Engineering Reference
In-Depth Information
forced the U.S. government to react, and it entered into a deal with some military companies
to improve prosthetic function. This agreement paved the way to the development and
production of modern prostheses, and aluminium and plastic composites and other new
materials led to the development of lighter and stronger devices. However it was still some
time before the basic articulated hook was replaced by something more advanced.
The problem with prosthetics research has always been the small size of the market.
Notwithstanding increased use of improvised explosive devices (IEDs) and antipersonnel
mines in war zones, and of course the proliferation of higher-speed motor cars in both
the First and Third World, there are still insufficient amputees for industry to recover
investment in expensive research and development (R&D).
In the conflicts in Afghanistan and Iraq up until February 2009, 862 U.S. troops be-
came amputees, of whom 186 had lost arms. In the entire United States, there are fewer
than 100,000 arm amputees. Fortunately, public opinion has influenced the U.S. govern-
ment, and in 2007 the Defence Advanced Research Projects Agency (DARPA) began the
Revolutionising Prosthetics program, the cost of which is now close to $100 million.
The first phase of the project headed by DEKA Research and Development Corp was
a 2-year program to make an advanced prosthetic arm using the world's best existing
technology. The 2009 program is driven by Johns Hopkins University's Applied Physics
Laboratory, and its mandate is to create a prosthesis that would be “mind controlled” and
restore the patient's ability to feel heat, cold, pressure, and even surface texture (Kuniholm,
2009).
Though these projects have been moderately successful, as reported later in this chap-
ter, their outputs are still experimental, and at the time of writing the amputee public at
large has yet to benefit significantly from the research.
10.2
STRUCTURE OF THE ARM
Arm mechanisms consist of up to three articulated joints and the structures between them:
the wrist, elbow, and shoulder separated by the forearm and the upper arm segments. The
hand and trunk are each considered to be single segments, as shown in Figure 10-7.
10.2.1 Wrist
The wrist is one of the most complex joints in the body, combining high mobility with the
ability to carry heavy loads. It provides two degrees of freedom (DoFs): flexion-extension
and abduction-adduction. Flexion of the wrist allows the palm to rotate toward the forearm
up to a maximum of about 90
◦
, whereas extension is rotation in the opposite direction up
to about 80
◦
. Abduction is wrist rotation toward the radius and is limited to about 15
◦
,
whereas adduction is rotation toward the radius and can reach between 20
◦
and 30
◦
in a
normal human arm.
A prosthetic wrist unit provides a means of orientating the terminal device in space.
This can be positioned manually or by cable operation and, once positioned, is held in
place by friction or with a mechanical lock. Wrist flexion is also sometimes provided as it
improves an amputee's performance of midline activities such as shaving or manipulating
buttons.
