Monday, December 3, 2012

MR Imaging Plays Key Role in Post-Op Metal-on-Metal Hip Arthroplasty

RSNA Nov. 29th, 2012

While metal-on-metal hip arthroplasty became quite popular in the 1990s, and initially reported good outcomes, more and more patients are developing what is called adverse reaction to metal debris (ARMD), and imaging is playing an increasingly important role in managing patients, said the presenter of a Wednesday session.

"Second generation metal-on-metal implants have been complicated by adverse reaction to metal debris," said Alice Ha, M.D., assistant professor of radiology at the University of Washington Medical Center in Seattle., who explained that ARMD occurs when implants shed metal particles into the surrounding anatomy. These metal particles have been shown to cause local and system damage, and have led to some high-profile FDA hip replacement device recalls.

One condition that falls under the umbrella of ARMD is the development of soft tissue lesions or pseudotumors. Another presenter, Eric Chang, M.D., assistant clinical professor of radiology at the University of California, San Diego, Medical Center, presented research showing that clinical symptoms do not help determine the presence of these pseudotumors in patients who have undergone metal-on-metal hip replacements.

This underscores the value of MR imaging in post-operative evaluation, Dr. Chang said during his presentation "MRI after ASR XL Metal-on-Metal Total Hip Arthroplasty: Does Symptomatology Correlate with MR Imaging Findings?" According to Dr. Chang, interest in this area of research began after he and his colleagues noticed that with MR imaging they were finding pseudotumors in patients with metal-on-metal hip replacements even though many of these patients were not symptomatic.

One problem with metal-on-metal hip replacements is that the prostheses have been shown to fail earlier than expected, necessitating orthopedic revisions, Dr. Chang said. These revisions are made more difficult, however, because of the presence of these large complex pseudotumors.

Dr. Chang and colleagues performed MR imaging examinations using the metal artifact reduction sequences (MARS) on 192 hips (175 patients) over a 15-month period. Pseudotumors were found in 69 percent of the cases (132 of 192 hips). Researchers also found that while bone marrow edema and tendon tearing were predictors of patient pain, the presence or the size of the pseudotumors was not, indicating that MR imaging is crucial in evaluating these patients.

Although imaging is an integral part of patient management, researchers are not sure which patients require revision and which patients will be more difficult to revise based on imaging findings, Dr. Chang said.
"We also do not know the natural history of asymptomatic patients with pseudotumors," Dr. Chang said. "Can they improve or do they invariably worsen? If they can improve, are there imaging findings that can help identify these patients? If they invariably worsen, are there imaging findings that can identify which should go to surgery sooner so the operation can be technically simpler? These are questions that we hope to answer to improve patient care."

MR Imaging Helps Assess Muscle Atrophy, Guides Revision Surgery

Shiraz Sabah, B.Sc.
In another Wednesday presentation, "MARS MRI of 200 Metal-on-Metal Hip Arthroplasties: A Prospective Cohort Study," researchers concluded that MARS MR imaging is effective for surveillance of metal-on-metal hip arthroplasties and for diagnosing and characterizing pseudotumors and patterns of muscle atrophy.

Researchers performed MARS MRI on 179 patients who had undergone metal-on-metal hip arthroplasties and had unexplained hip pain (or, for control purposes, had excellent function) after a diagnostic hip arthrogram. Presenter Shiraz Sabah, B.Sc., London, and colleagues discovered that pseudotumors were common (40 percent) following hip arthroplasty and can be symptomatic and asymptomatic. In addition, they found, abductor muscle atrophy can indicate poor function.
"We believe that MR imaging permits better planning and timing of revision surgery, with potential for better outcome for patients," Dr. Sabah said. "MR imaging is particularly useful for assessment of muscle atrophy and can prompt earlier intervention and guide choice of revision prosthesis."

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