A perfect fit

Surgeon develops method to ensure accurate hip replacement

Dr. James V. Bono likes to compare hip replacement surgery to taking a road trip.

"You look at the map, and your destination appears to be only two inches away, so you look at the mileage scale and calculate it will take two hours to drive, but it actually takes three or four hours," says Bono, associate clinical professor of orthopedic surgery at the School of Medicine. Like the roadmap, Bono says, the blueprints developed for hip surgery often fail to reflect accurately the real measurements of the situation.

dr. bono

Dr. James V. Bono has made it his personal crusade to ensure that replacement hips are accurately sized so patients don't have to have repeat surgery. © Mark Morelli

While an incomplete reading of a map can mean arriving late, an inaccurate selection of a prosthetic hip joint can mean a significant difference in leg length and stability. These conditions account for at least some of the revisions (the word of choice for additional surgery) required in about 20 percent of artificial hip joints within 20 years of the initial surgery.

In pursuit of accuracy
That's why Bono, in his position at New England Baptist Hospital, made a "personal crusade" of developing an algorithm to increase accuracy in prosthesis sizing.

Software developed by Philips Electronics around that algorithm that is being produced by the Swedish firm Sectra is expected to go to market this year. In 100 clinical trial surgeries, Bono says he has had 83 percent accuracy in selecting the exact size joint components and in measuring leg discrepancy within 2 millimeters. "It used to be that half an inch was acceptable," he notes.

The problem lies in the nature of imaging. Because patients come in various sizes, it means their hips are at different distances from the X-ray beam and therefore appear larger or smaller than they actually are. "It's like holding a flashlight in a dark room and bringing your hand up and down toward the beam, creating a giant hand shadow and then a small one," says Bono, who never seems to run out of analogies.

Under-magnification in small patients can mean the chosen artificial joint is too small, which leads to a loose fit of the new joint. Over-magnification can lead to a too-large replacement, risking a split in the femur.

The issue was addressed by placing a millimeter scale on the same plane as the patient's hip during the X-ray. But the marker also suffers distortion in the X-ray, throwing off the final measurements, Bono says.

hip replacement parts

Hip replacement parts

The situation got worse when hospitals began to move their radiography to computers. "New England Baptist was in the forefront of going to digital images," says Bono, who has been doing joint replacement surgeries there for nine years. "We moved from X-ray viewbox to computer screen in October 1995."

Enter the algorithm
Orthopedic surgeons, however, continued to print out the digital image so they could prepare templates for artificial hip joints. The image suffered further distortion in the printing process. Bono says he noticed an increase in leg-length discrepancies after hip surgery that relied on digital images.

Eventually, a digital template was developed for the computer, but, Bono says, it inadequately addresses the X-ray magnification distortions, continuing to skew the scale. His algorithm allows the user to import a template into the computer that calibrates digitally to scale and prints out the proper size of components—the socket, ball and shank—as well as a diagram showing their position. He says the process takes about 10 seconds and eight clicks of the computer mouse.

There are about 250,000 joint replacements done in the United States each year, and Bono considers all of those procedures to be at risk for inaccurate sizing. "Right now, it's as if an architect designs a dream house and gives it to a contractor, who makes everything 5 to 10 percent off-scale. Everything is out of whack."