May 6, 1998 St. Louis, April. 30, 1998 -- About 200,000 Americans undergo hip replacement surgery each year according to the American Academy of Orthopaedic Surgeons. And studies suggest one in five patients will need to have their artificial hip joints replaced within 15 years due to complications. A new study at Washington University School of Medicine in St. Louis suggests that 3-D models of the pelvis work better than X-ray images in preparing surgeons to revise hip replacements. Douglas D. Robertson, M.D., Ph.D., lead investigator of the study and an assistant professor of radiology and orthopaedic surgery, says that surgeons benefit from using 3-D models because they think in three dimensions. "There's something about holding a 3-D model in your hand," Robertson says. "There's a whole depth of information that's lacking when you look at flat X-ray images."
This added information can help in the crucial planning stage for complicated, technically difficult surgeries, says Charles J. Sutherland, M.D., who took part in the study while at the School of Medicine. Sutherland, now an orthopaedic surgeon at The Toledo Clinic Inc. and an associate clinical professor of orthopaedic surgery at the Medical College of Ohio, says, "It's like taking a trip. You need to have a map and be very familiar with it in order to handle any problems that come up."
The study involved 19 people with failed total hip replacements and severe bone loss from the socket, or acetabulum, of the pelvis. This socket normally holds the door-knob shaped top of the femur bone of the leg and allows for smooth leg movement.
The researchers estimated the amount of lost bone using either X-ray images or 3-D models, which they generated from computed tomography (CT) images. Sutherland then checked these estimates against those he made during surgery for 15 of the patients.
The X-ray images underestimated socket bone loss by at least 20 percent. In contrast, the physical models appeared to accurately portray pelvic damage and were used by Sutherland as the basis of all 19 surgeries. The results are published in the May/June issue of the Journal of Computer Assisted Tomography.
"With X-rays, you can make subjective calls about how much bone is missing, but you can't get accurate measures," Robertson says.
Computed tomography provides images that look like slices through a structure. Robertson used a computer to stack the slices into a 3-D image of the entire pelvis. Computer-aided design and rapid prototyping converted the images into 3-D polyurethane models.
The models helped Sutherland visualize the surgery much better than the CT scans. "You've got all these different computed tomography pictures that you have to integrate mentally to figure out where the bone defects are," he says.
He also used the models to rehearse surgery. The models of deformed bones were shaved down with surgical tools to determine whether the remaining bone could support implants and what size and type of artificial joints should be used.
In all but one instance, Sutherland was able to use the models to preselect the implants and accurately practice the steps of surgery. The exception involved a hip bone that was more fragile than expected, requiring the use of alternative implants.
The 3-D models can cost several thousand dollars and take several weeks to manufacture. However, Robertson recently purchased a 3-D modeling machine that should reduce costs and produce plastic or wax models within a few days. The Mayo Clinic in Rochester, Minn., is the only other U.S. institution to have such a machine.
As well as helping hip surgeons, the model also could provide guidance on the surgical removal of tumors or complex surgeries of the bones of the wrist, face and other sites, Robertson says.
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