Knee replacements: Pinpointing the cause of infection

TORONTO -- A new study reveals that PET scans accurately detect infections in prosthetic knee joints more than 90 percent of the time, according to researchers at the SNM's 56th Annual Meeting. The findings could represent a significant breakthrough in the treatment of patients who undergo joint replacements. Joint replacements are prone to a number of complications following implantation.

"Infections following joint replacement surgery are very serious, very common and very difficult to distinguish from other problems such as loosening of the prosthetic," said Abass Alavi, professor of Radiology at the University of Pennsylvania, Philadelphia, and the principal investigator of the study. "Our study shows that no other diagnostic tool comes close to PET in accurately diagnosing infection with minimal discomfort for patients. What's more, the broader implication is that PET could eventually be used to successfully detect infections and inflammation for other conditions."

It is estimated that more than one million joint replacements are performed in the United States each year. Infections following implant procedures are difficult to fight because the immune system is unable to destroy bacteria that live on the implanted material. Even after treatment with strong antibiotics, the infections can persist. As a result, it is often necessary to remove prosthetic joints if they become infected.

Accurately determining whether pain in a prosthetic joint is indeed an infection can be difficult. Physicians often have to make difficult decisions to operate without accurate information on whether an infection is occurring. The most common technique used to determine infection is the white blood cell count; however, the test is only about 50 percent accurate in determining infections, is at least two times more costly than PET imaging and takes longer to provide results.

In the study, 80 patients who were experiencing painful knee symptoms underwent FDG-PET imaging after clinical evaluation and laboratory data were inconclusive for infection. Of these patients, 50 were available for full follow-up. Clinical outcomes were determined via surgical, microbiological and other clinical indicators. PET images were interpreted to indicate infection if there was increased FDG uptake at the bone-prosthesis site.

Overall, FDG-PET imaging was 92 percent accurate, correctly diagnosing eight of the nine cases of infection and identifying the absence of infection in 38 of 41 cases.

Scientific Paper 1349: A. Nawaz, J. Parvizi, H. Zhuang, D. Torigian, A. Alavi, Division of Nuclear Medicine, University of Pennsylvania, Philadelphia, and Department of Orthopaedic Surgery, Thomas Jefferson University, Philadelphia, "Diagnostic performance of FDG-PET in differentiating septic from aseptic painful knee prostheses," SNM's 56th Annual Meeting, June 13-17, 2009.

About SNM -- Advancing Molecular Imaging and Therapy

SNM is an international scientific and medical organization dedicated to raising public awareness about what molecular imaging is and how it can help provide patients with the best health care possible. SNM members specialize in molecular imaging, a vital element of today's medical practice that adds an additional dimension to diagnosis, changing the way common and devastating diseases are understood and treated.

SNM's more than 17,000 members set the standard for molecular imaging and nuclear medicine practice by creating guidelines, sharing information through journals and meetings and leading advocacy on key issues that affect molecular imaging and therapy research and practice. For more information, visit www.snm.org.

June 15, 2009