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

Imaging is playing an increasingly important role in managing metal-on-metal hip arthroplasty patients who have developed adverse reaction to metal debris.

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, according to research presented at RSNA 2012.

“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 Y. 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. 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.

While 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

MARS MR imaging is effective for surveillance of metal-on-metal hip arthroplasties and for diagnosing and characterizing pseudotumors and patterns of muscle atrophy, according to another RSNA 2012 presenter.

Shiraz Sabah, M.B.B.S., B.Sc., London, and colleagues performed MARS MR imaging 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. Researchers 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.”

DECT Screens Patients with Retained Ballistic Projectiles

Winklhofer

Dual-energy CT (DECT) can be used to distinguish between ferromagnetic and non-ferromagnetic ballistic projectile remnants in patients who have suffered firearm wounds and consequently can determine which patients are suitable candidates for MR imaging, according to research presented at RSNA 2012.

Despite the idea that patients with firearm wounds are unsuitable for MR imaging if they have retained bullet fragments, recent research has shown that most bullets are safe to scan with MR imaging, said Sebastien Winklhofer, M.D., of the Department of Radiology at University Hospital Zurich, and University Zurich, Institute of Forensic Medicine. For example, a study presented at the American Academy of Orthopedic Surgeons Annual Meeting demonstrated that MR imaging can safely scan patients with bullet fragments that are non-ferromagnetic.

“The main problem is that most patients do not know whether their retained projectile has ferromagnetic properties,” Dr. Winklhofer said. “Conventional radiographs have the potential to detect any metallic object within the patient; however, they are unable to provide any information on the composition of the object.”

DECT, on the other hand, is used in clinical radiology to assess the composition of different objects and materials. Dr. Winklhofer and colleagues investigated whether DECT had the ability to distinguish between ferromagnetic ballistic projectiles and those that consist of different materials, such as brass or lead.

Researchers examined 11 ballistic projectiles (nine bullets and two shotgun pellets, 5 ferromagnetic and 6 non-ferromagnetic) in an anthropomorphic model of the thorax using a 128-section dual-source CT scanner. Tube voltages were set at 80 kVp, 100 kVp, 120 kVp and 140 kVp and two readers independently assessed CT numbers on images reconstructed with an extended CT scale. Dual-energy indices (DEI) were calculated from both 80/140kVp and 100/140 kVp pairs.

According to Dr. Winklhofer, the single-energy analysis showed no significant difference regarding CT numbers in the comparison of ferromagnetic and non-ferromagnetic projectiles at any tube voltage. However, the dual-energy analysis showed that for both 80/140 kVp and 100/140 kVp pairs, DECT was able to discriminate between ferromagnetic and non-ferromagnetic with 100 percent accuracy.

“Dual energy CT appears to have the potential to screen patients with retained ballistic projectiles prior to MRI and distinguish between those who are suitable to undergo MRI and those who are not,” concluded Dr. Winklhofer. He cautioned his study was carried out in a model and is not yet ready for clinical application; he and his colleagues plan to scan a larger number of objects in the future.