CAD use helps AF bodybuild

An Air Force hospital is using computer-aided design and manufacturing to create 3-D
models of human body parts so surgeons can practice complex surgical procedures before
cutting real patients.

Wilford Hall Medical Center at Lackland Air Force Base, Texas, isn’t the only
medical facility to use stereo lithography, a CAD process of producing physical models
from medical imaging. But it is the only medical center in the world, military or
civilian, that has its own in-house stereo lithography system.

The process, developed by the automotive and aerospace industries for rapid
prototyping, has been around for more than a decade, but Wilford Hall has been using the
technique for only three years.

“It’s a real interesting use of computers,” said Col. William Moore,
director of Wilford Hall’s stereo lithography lab. “It’s basically the same
thing Boeing Co. or Chrysler does if they want to design a new part, only instead of
building you a fender, we can build you a model of your skull.

“We’ve done kidneys, bones, skulls and jawbones,” Moore said.
“Instead of generating a CAD-CAM file through manual drawing we take medical imagery,
input that into the computer and generate a 3-D, resin model from the data.”

The stereo lithography process has two components. The first step is to take medical
imagery and convert it into a stereo lithography data file. The second step is to take the
CAD-CAM or stereo lithography file and construct the 3-D image of the body part.

Digital images of a body part taken from a CT scan or magnetic resonance imager are
stored on a magnetic optical disk and fed into a processing program called Mimics from
Materialized Software Inc. of Brussels, Belgium. The program converts the data into a
CAD-CAM format.

The CAD-CAM digital file is then downloaded into a stereo lithography system, which
consists of a server or workstation attached to a laser system that can recreate digital
images stored in the server. The workstation, using the digital dimensions in the CAD-CAM
file, runs a laser that traces the image of the body part into the plastic and creates the
model in a vat of resin. Layers of resin are folded over the model until it attains the
required thickness.

“If I were to hand you one of these skulls it would look just like the
patient’s real skull,” Moore said. “All the internal anatomy and all the
fine bone structure is present in the skull. We’ve done studies, and it is an
extremely accurate technique.”

The models are made of light, hardened Epoxy resin. A simple model can be built in
eight to 10 hours, but a full skull can take up to 48 hours and a hip can take as long as
72 hours, Moore said.

Surgeons used the technology to separate Bethany and Hannah Rainey, 6-month-old Siamese
twins who were joined at the hip, during a 1995 operation at Wilford Hall. By practicing
on a lab-built model of the twins’ hip, surgeons found a way to separate the two
without leaving one of the twins without a thigh bone.

Both girls, now almost 4 years old, are getting around just fine with the help of
prosthetics, Moore said.

The system also saves time in the operating room and decreases blood loss during
operations, Moore said.

The medical center has built body part models for patients at Brook Army Medical Center
at Fort Sam Houston, Texas, and the University of Texas Health Care Science Center in San

Medical imaging can be performed outside of Wilford Hall and sent to the medical center
on disk or digital tape. Data can also be sent from remote terminals through a T1 link via
the Internet; lab officials hope to eventually receive imagery via satellite.

“That’s our long-term goal,” Moore said. “What we’d like to do
is access this data from a remote site like Bosnia, where they have field CT-scanners and
uplink that data to us so we would have models built by the time the patient arrives
stateside for the surgery.”

“But right now we’re stuck with the land lines,” he said.

The Wilford Hall lab bought the system from 3-D Systems Inc. of Valencia, Calif., for
$500,000. Mimics was originally written as a DOS program. But Wilford Hall is now
beta-testing the software on a Hewlett-Packard Co. PC running Microsoft Windows NT.  


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