RSNA News - May 2005

Arthroplasty Imaging Sheds Light on Particle Disease Process

Everything we know about particle disease is based on x-ray or retrieval patterns. With MR imaging, we’ve learned that it starts much earlier than originally thought.

— Hollis G. Potter, M.D.

Using MR imaging, physicians can detect complications related to joint replacement earlier and more

Hollis G. Potter, M.D. Cornell University Medical College

accurately than when using standard radiography, according to Hollis G. Potter, M.D., chief of MR imaging at the Hospital for Special Surgery and a professor of radiology at Cornell University Medical College in New York City.

This has important implications for a growing number of patients with hip and knee replacements.

“Arthroplasty imaging is something radiologists can do right now to benefit a tremendous number of people,” she said. “It can explain knee pain when the x-ray image and the clinical examination by the orthopedist do not detect a problem. MR imaging offers a noninvasive means to look at the source of that pain,” adding that radiologists should not fear using MR to image arthroplasties.

During arthroplasty, all or part of the joint may be replaced with an artificial device. The National Center for Health Statistics reports that 381,000 total knee replacements and 193,000 total hip replacements were performed in the
United States in 2002.

Particle Disease

Particle disease is the main limiting factor of the longevity of arthroplasties. While clinically asymptomatic, particle disease erodes the bone around the arthroplasty, loosening it and causing failure. “Arthroplasty imaging is changing our concept of the natural history of particle disease,” said Dr. Potter. “Everything we know about particle disease is based on x-ray or retrieval patterns. With MR imaging, we’ve learned that it starts much earlier than originally thought.”

Dr. Potter and her research colleagues found that MR can also quantify the magnitude of disease. “When we studied our patients, we were able to detect and measure the intracapsular burden of particle disease before it eroded the bone,” she said. “This is exciting because it is the first time we have been able to noninvasively monitor these patients to get a sense of the magnitude of disease burden, which can be helpful when applying suitable pharmaceutical intervention.”

Timothy J. Mosher, M.D. Pennsylvania State University

Timothy J. Mosher, M.D. Pennsylvania State University in Hershey, agreed that the technique offers a substantial amount of valuable diagnostic information. “As more arthroplasties are placed, it will be necessary for radiologists to know how to diagnose arthroplastic complications, including particle deposition disease. I think MR imaging can offer a solution to this vexing problem faced by orthopedic surgeons.”

Monitoring particle disease is also of interest to pharmaceutical companies that are developing therapies to delay the degree of bone resorption. A noninvasive method of quantifying the burden of particle disease will be helpful to studying bisphosphonates, disease modifying agents that delay bone resorption by inactivating osteoclasts.

To validate their success with MR imaging, Dr. Potter and her colleagues used a non-clinical, cadaveric model and created known defects. They compared MR to optimized radiographs—the clinical standard—and optimized CT. They found that MR imaging was the most sensitive means by which to detect osteolysis.

Dr. Potter said that MR arthroscopic imaging was done with a 1.5 Tesla magnet, modified commercially available software and pulse sequence parameters modified to reduce the artifact. She said that ultimately, the technique may require proprietary software to reduce the artifact even more.

Dr. Potter and colleagues are launching a prospective study that will correlate with the osteolysis lab, which will look at breakdown products and synovial fluids. This will help correlate imaging with other markers to provide parameters by which the magnitude of the process can be assessed.

T2 Cartilage Mapping Comes of Age

T2 mapping offers a noninvasive method of studying cartilage osteoarthritis. In the spectrum of osteoarthritis, arthroplastiy imaging looks at the last stage of osteoarthritis after everything has failed. In some cases, it is important for physicians to evaluate the status of cartilage health at an earlier time point.

There is some controversy surrounding the application of T2 mapping. Some researchers believe it has a role in basic science while others, including Dr. Potter, think it has important clinical applications. Imaging allows a direct view of the biologic process of disease. T2 mapping detects early changes in cartilage ultrastructure. Clinically this is useful in timing procedures and surgeries.

Dr. Potter offered an example of a 14-year-old girl whose torn meniscus was surgically removed, putting her at risk for developing osteoarthritis. “Eventually a cadaveric meniscus will need to be transplanted, but the optimal timing for that procedure is largely guess work, aided by x-ray and morphologic cartilage imaging on MR. Her physicians really need to know when the cartilage starts to go bad but still retains some thickness,” she explained. “T2 mapping shows tremendous promise for increasing our ability to detect very early cartilage breakdown. With this information, we will know how to better time meniscal transplantation.”

Dr. Mosher said that he sees T2 mapping as a research tool with a wide variety of research applications.

“It can be used to further the understanding of normal cartilage physiology, cartilage damage and how it relates to early osteoarthritis. It is also a useful tool for building a translational bridge between basic science and clinical research,” he said. “On the clinical side, I think T2 mapping is a way of retrieving objective data from subjective interpretations that clinical radiologists have been doing routinely. That’s necessary as a research tool, but maybe not as crucial for radiologists in the day to day practice.”

“Although T2 mapping is a technique that is available at only a few research centers, my sense is that it will become more widely available in the near future,” stated Dr. Potter.

“Imaging is going to change condition outcome. Until now, our perception has been based on how the patient feels and functions. Subjective clinical outcome measurements are very important, but they do not tell us much about the biology of the disease process. Imaging will provide much more information and objective assessment of outcome.”

52-year-old patient with knee arthroplasty and MR imaging evidence of early intracapsular particle disease (arrow). Note also the bone resorption adjacent to the femur (arrowhead). Image courtesy of Hollis G. Potter, M.D.