The
last thing you expect an assistant professor of aerospace and
mechanical engineering to show you is an X-ray, and the one in
Ryan Roeder’s binder of overhead transparencies holds surprises
of its own.
It shows a girl’s arm with something obviously missing
— her humerus, the large bone normally found in the upper
arm. The girl was born without most of the bone. Other X-rays
on the transparency show the results of a surgical procedure to
replace the missing bone segment with one taken from a cadaver.
As is evident from the pictures, the operation was a success.
Years later the arm looks perfectly normal.
A miracle? Hardly. It’s actually old news, according to
Roeder: The procedure was performed back in the 1950s. Roeder
uses the X-rays to show how well a young and growing body can
fix itself with implanted bone, whether taken from the patient’s
own body (the crest of the hip is a popular spot for harvesting
small amounts) or taken from a bone bank of body parts donated
to medical science.
Surgically implanting natural bone isn’t always practical,
however. Not everyone is healthy enough to undergo the pair of
surgeries required to move a bone from one part of their body
to another. The problem with donated bone is that supplies are
limited. And there’s always the possibility of disease contagion
when you put someone else’s bone — or even an animal
bone — into your body.
For these and other reasons Roeder is among a number of bioengineers
trying to develop synthetic bone. It’s a task more difficult
than one might imagine. Literally at their core, bones are blood
factories, and science is nowhere near to replicating the manufacturing
methods of bone marrow. For now Roeder is content with developing
materials that behave mechanically like natural bone. Rather than
replace an entire fibia or tibia, such materials would be used
to patch the cortical or hard outer layer of damaged or diseased
bones.
As Roeder explains, one of the difficulties in engineering a
material like bone is that, unlike fiberglass or steel, bone is
a living material. It’s composed of tiny, tightly bundled,
blood-fed fibers with cells that are constantly being born and
dying. The fibers are wound into bundles that are wound into other
bundles and so on as in nautical rope. The vertical alignment
of the fibers makes bones stronger lengthwise than side to side.
Roeder is a materials engineer whose earliest research involved
a ceramic insulating material used in computer chips. As he explains,
bones are a composite of a natural polymer, collagen, reinforced
with a ceramic known as bone mineral. His proposed synthetic bone
material is made up of high-density polyethylene (the raw material
of milk jugs) reinforced with hydroxyapatite, the synthetic ceramic
most similar to bone mineral.
His first success has been in getting the ceramic fibers to
line up in the polyethylene the way the mineral crystals do in
bone. That allows the synthetic to mimic the mechanical properties
of the real thing. His next challenge will be to make the material
porous like natural bone.
Roeder’s work is part of a growing body of research at Notre
Dame involving biomechanics and biomaterials. The projects unite
engineers, biologists and other faculty with researchers at private
companies and medical schools. Notre Dame is ideally situated
to enter into orthopedics research collaborations because six
of the world’s largest orthopedic implant manufacturers
are located in Warsaw, Indiana, less than an hour from campus.
Roeder envisions the development of not just one bone substitute
but a range of materials to suit different parts of the body and
patients of different ages. Synthetic bone, he says, may eventually
serve as a kind of scaffold on which genetically engineered bone
or other tissue could be grown in a lab. Implanted at the site
of damage, the scaffold would give the tissue an advanced starting
point for growing into the natural skeletal parts. Ideally the
scaffold would then be remodeled by the body over time.
