InMotion Invests In RSA Research

By Scott Shepard | The Daily News | November 21, 2007

Memphis orthopedic engineers soon will be able to answer questions in a matter of months that used to take decades. Questions like: 'Did my hip modification work?' Or, 'How did my new polyethylene compound endure?'

That will come when the InMotion Musculoskeletal Institute becomes just the second center in the United States to offer Radio Stereometric Analysis (RSA) for clinical research.

"There are other places in the country with RSA, but they're not using it for solving patient problems," said Ruxandra Marinescu, manager of the Medtronic Biomechanics Library at InMotion. "Only in Boston are they using RSA for clinical research."

The technology is not new and has been available in Europe more than 20 years. The difference is cultural; American academic research is very fluid, with people moving about to work on new projects. It encourages innovation and creativity but, at least in the case of RSA, has been a barrier to forming critical mass.

"It takes about six critical pieces that need to come together at the same time," Marinescu said. "A place might have RSA equipment but maybe they don't have the right surgeon, the technician or the support people. We're seeing everything come together in Memphis."

A hair's breadth

RSA begins with an orthopedic surgeon with both an interest in hips and a passion for research. Surgery includes the time-consuming process of planting metal markers made of tantalum, which is similar to titanium. X-rays from two different angles, taken simultaneously, can see these markers. Implanted joints perform best when stable. Subsequent X-rays are compared to the first set to check for either a shift in position or abnormal wear of components.

RSA, Marinescu said, operates on the same principle mariners have used for 250 years for navigating the open sea. Stars in the sky are like the tantalum markers, with very specific locations. RSA operates as a sextant, aligning the markers. RSA can accurately detect deviations as tiny as 1/200th of an inch: less than the diameter of a human hair.

"We test everything in the lab in a simulator, running it through millions of cycles, but in the human body it's always different than the lab," said surgeon Peter Heeckt, chief medical officer of Smith & Nephew Orthopaedics. "The difference with RSA is that in humans you cannot take out an implant and measure it, unless it fails."

Quicker results

Typically, engineers and surgeons had to go to Europe, Canada or Australia to ask early questions about displacement or wear, or wait 20 years for an artificial joint to fail so they could examine it after replacement. Heeckt said that instead of waiting half a career for the answers of a lifetime, with RSA they can peer inside someone's body and learn how to improve their product.

"We can wait 15 years and see how many implants fail to do an outcomes study, but it takes forever and those studies are never completed because people move," he said. "With RSA we can look at wear as early as two years."

Heeckt is a long-term user of RSA in his own research, and has an ongoing study into the micromotion of implants after surgery. Smith & Nephew now works with the RSA lab managed by orthopedic surgeon Robert Bourne of the Department of Orthopaedic Surgery at London Health Sciences Centre, part of the University of Western Ontario.

Of particular interest with hips is the plastic cup placed in the pelvis, which bears the pressure of the metal ball protruding from the leg. The molecular structure of these polyethylene cups is always subject to improvement, and with RSA it's possible to get earlier reports on how a new material fares.

The rubbing of metal on plastic can release microscopic particles into the body. These can act like sandpaper in the joint, wearing it out faster.

"If we see a particular material has tremendous wear in a few years, we can stop production, and if it is wearing well, we can learn from that," Heeckt said. "An implant can show wear long before it fails."

A worthy investment

InMotion is still on the hunt for a second X-ray unit to complete the RSA system, but already has begun the first research study in which orthopedic surgeon John Crockarell Jr. of Campbell Clinic is comparing the wear patterns of two different knee designs. InMotion is also gearing up to conduct studies on knees and wrists.

One major reason Marinescu joined InMotion, she said, was the opportunity to develop an RSA service. In September she and lab assistant Haden Janda trained with the people considered the world's leading experts in RSA: Henrik Malchau and Charles Bragdon, of Massachusetts General Hospital in Boston, an affiliate of the Harvard Medical School.

The Crockarell study is the template for further studies: Patients who are enrolled in a clinical study will receive regular exams, including X-rays, at Campbell Clinic in Germantown. At the InMotion headquarters, part of the University of Tennessee-Baptist Research Park Downtown, Marinescu and Janda have the high-horsepower software to evaluate the images. InMotion also has the clinical support staff and statistical analysis to make such research valid.

A first-rate RSA lab can do an entire, valid study with just 20-30 participants, Heeckt said, compared to drug studies that can involve thousands of patients at dozens of centers around the nation. It means an RSA study is not only fast, but can be contained within one city.

InMotion is taking a crucial step in expanding Memphis' position as a center of orthopedic innovation.

"To be at the forefront of research it's important to have the proper people and software," Heeckt said. "It's a great development that they've invested in RSA."