Photo by Craig Coleman A medical engineer with Walter Reed’s 3-D Medical Applications Center hold a model of a patient’s skull. The red is an exact representation of the blood vessels in the patient’s brain. Surgeons successfully removed an anuism.

Sponsored by the WRAMC departments of radiology, surgery, orthopaedics and rehabilitation, the center is the only military facility of its kind, and one of only a few in the world. It creates between 250 and 400 3-D models each year. Surgeons use the models for pre-surgical planning of complex cases. They see exactly what the area where they will be working inside the body looks like, possibly saving hours in the operating room.

Dr. Stephen Rouse, 3-D Medical Applications Center director, likens 3-D modeling to a navigation system for surgeons. The familiarity with a route a navigation system approximates can make driving someplace new an easier experience by increasing the driverís confidence.

ìIn most cases, particularly the most complex cases, the doctors wonít even do the surgery without the model,î Rouse said. ìEven if all they do is have the model in their hand and look at it, and have it on the table when theyíre in the OR, what they tell us is, ëWe were expecting this to be a really difficult, and it was like, weíve been there before.î

Peter Liacouras, the centerís senior medical engineer who holds a doctorate in bioengineering, discussed a virtual model displayed on his computer monitor. ìThis was a child with a tumor in the knee area, very close to a nerve and an artery,î Liacouras explained. A large boney area on the side of the knee contained a tumor with nerves in front and an artery behind. ìBasically, this doctor was fearful of this surgery, that he would hit either the artery or the nerve. When he went into surgery he put this model into a sterile bag right on the sterile table. He aligned his osteotome with the model, went back to the patient and aligned his osteotome with the patient, and was able to chisel off this nerve without disrupting the tumor or the artery.î

For cranial and facial cases, Rouse and his staff combine the imagery obtained from thin-slice computed tomography (CT) scans and the three-dimensional 3-D light photography combining the facial appearance boney destruction underneath it. ì[We] utilize the modeling to define which bones can be salvaged and which ones you just canít worry about,î Rouse said. ìWe either make duplicate models so they can do surgery on the model itself, or we can digitally move the bones on the model and re-approximate in the positions they would be during surgery. We can also either create a new model with the bones in the right position or make the implant or fixation device the surgeon needs to put the bones back in their proper position.î

In many cases, the decision on where to make incisions are based on the model, because it gives the surgeon better orientation. Doctors also show the models to patients, explaining what their physical problems are and what the surgeons will do to repair the damage. ìWhen you come to talk to the patient about what youíre going to do, you bring the model and the patient sees what youíre going to do, too,î Rouse said. ìSo informed consent is a whole lot more informed.î

Key to the model-making process are thin-slice computed tomography (CT) scans. The images are imported into computer software and digital models are created. Sometimes previous surgeries involving metal fixations can make the CT scans inaccurate for the purpose of modeling.

ìWhen you acquire a CT and the beam hits a metal, there is scatter radiation on the film,î said Carlos Villamizar, a medical engineer with the center. ìThose artifacts have to be removed.î

According to Liacouras, the centerís senior medical engineer who holds a doctorate in bioengineering, software helps create a virtual model from the CT scans. Sometimes, because the model will be three dimensional, space between parts has to be created.

ìWith a knee, for instance, with space between the femur and the tibia, weíll connect those feature with (virtual) cylinders.î Surgeons can tell the cylinders from anatomical features in the models by their perfect spherical shapes. They also use cylinders to approximate the boney growth that can result from amputation. ìWeíll also connect those with cylinders so they can get the general location of the [boney growth] in relation to the bone,î Liacouras said.

After the virtual model has been perfected on the computer, it is taken to a special printer that creates the model. A scribe head traces each layer, creating an exact three dimensional copy of the virtual model.

Building a model can take as little as eight hours, or as long as 18 hours for a full cranial model. Much depends on the condition of the initial scans. ìIf we have something like a blast injury to a lower limb, and they have placed external fixation, pins and apparatus to stabilize this in transit, and theyíve used titanium pins, then weíre generally in pretty good shape,î Rouse said. A couple of hours of work and itís ready to go,î Rouse said. ìIf theyíve used steel pins ó that plays absolute havoc with the X-rays. You get a lot of artifact on the CT scan.

ìIn some, like spinal injuries, where youíve got previous fixation in place, we can spend 12 hours just cleaning the scan up to where we have a model ready to build.î The time spent modeling saves time in the operating room, which reduces cost.

The center has the capacity to make more models and encourages military physicians, surgeons and dentists to use their services. Call the center at (202) 782-7765/6 for specific guidelines.