271
The
technology
for fashioning
joints
in medieval suits of armor developed over
centuries. The technology for creating
the
prostheses
(artificial joints) used
in medicine today developed, in relative
terms, in a flash—less than 60 years.
Unlike the joints in medieval
armor
,
which was worn outside the body, today’s
artificial joints must function inside the
body. The history of joint prostheses dates
to the 1940s and 1950s, when World War
II and the Korean War left large numbers
of wounded who needed artificial limbs.
Today, nearly 1 million Americans per year
receive a total joint replacement, mostly
because of the destructive effects of
osteoarthritis or rheumatoid arthritis.
To produce durable, mobile joints
requires substances that are strong,
nontoxic, and resistant to the corrosive
effects of organic acids in blood. In
1963, Sir John Charnley, an English
orthopedic surgeon, revolutionized the
therapy of arthritic hips with an artificial
hip design that is still in use today. His
device consisted of a metal ball on a stem
and a cup-shaped polyethylene plastic
socket anchored to the pelvis by methyl
methacrylate cement. This cement proved
to be exceptionally strong and relatively
problem free. Hip prostheses were
followed by knee prostheses, but not until
10 years later did smoothly operating total
knee joint replacements become a reality.
Today, the metal parts of the prostheses
are strong cobalt and titanium alloys, and
the number of knee replacements equals
the number of hip replacements.
Replacements are now available for
many other joints, including fingers,
elbows, and shoulders. Total hip and knee
replacements last about 10 to 15 years
in elderly patients who do not excessively
stress the joint. Most such operations are
done to reduce pain and restore about
80% of original joint function.
Replacement joints are not yet
strong or durable enough for young,
active people, but making them so is
a major goal. Another problem is that
the prostheses work loose over time,
so researchers are seeking to enhance
the fit between implant and bone. One
solution is to strengthen the cement that
binds them (simply eliminating air bubbles
from the cement increases its durability).
Another solution is to use a cementless
prosthesis, which allows the bone to grow
into its surface, fixing it in place. In order
for this to happen, a precise fit in the
prosthesis and the bone must be achieved,
something at which surgical robots such
as ROBODOC excel.
Dramatic changes are also occurring
in the way artificial joints are made.
CAD/CAM (computer-aided design
and computer-aided manufacturing)
techniques have significantly reduced the
time and cost of creating individualized
joints. Fed the patient’s X rays and medical
information, the computer draws from
a database of hundreds of normal joints
and generates possible designs and
modifications for a prosthesis. Once the
best design is selected, the computer
produces a program to direct the
machines that make the prosthesis.
Joint replacement therapy is coming of
age, but equally exciting are techniques
that call on the ability of the patient’s own
tissues to regenerate.
Bone marrow stimulation: Small holes
poked through to the bone marrow
allow mesenchymal stem cells from the
Joints: From Knights in Shining Armor to Bionic Humans
bone marrow to migrate into the joint
and produce new cartilage.
Osteochondral grafting: Healthy bone
and cartilage are removed from one
part of the body and transplanted to
the injured joint.
Autologous chondrocyte implantation:
Healthy chondrocytes are removed from
the body and seeded onto a supporting
matrix of tissue-engineered collagen.
When subjected to mechanical pressure
in the lab, the cells produce new
cartilage, which is then implanted.
Mesenchymal stem cell regeneration:
Undifferentiated mesenchymal cells
are removed from bone marrow and
placed in a gel, which is packed into an
area of eroded cartilage.
These techniques offer hope for
younger patients, since they could stave
off the need for a joint prosthesis for
several years.
And so, through the centuries, the
focus has shifted from jointed armor to
artificial joints that can be put inside the
body to restore lost function. Modern
technology has accomplished what the
armor designers of the Middle Ages never
dreamed of.
A
C L O S E R
LOOK
A hip prosthesis.
X ray of right knee showing total knee
replacement prosthesis (co-designed by
Kenneth Gustke, M.D., of Florida Orthopedic
Institute).
8
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