MRI Predicts Failing MoM Hip Implants

MedPage Today—May 17, 2013

Synovial volume as measured by MRI is highly predictive of metal-on-metal (MoM) hip implant failure in both symptomatic and asymptomatic patients, a study has found.

The mean volume (and standard deviation) of synovitis among hip replacement patients referred for evaluation was 5 ± 7 cm3 (95% CI 1.2-8.7 cm3) among asymptomatic patients, 10 ± 16 cm3 (95% CI 1.4-19.4 cm3) among patients who were symptomatic with a mechanical cause, and 31 ± 47 cm3 (95% CI 11.5-50.5 cm3) among patients who had unexplained hip pain, according to Hollis Potter, MD, chief of the Division of MRI at New York City's Hospital for Special Surgery, and colleagues.

The coefficient of repeatability between the examiners was 1.8 cm3 for measurement of synovitis, the researchers wrote in the May issue of the Journal of Bone & Joint Surgery.

The findings suggest MRI may be useful for identifying patients who need revision surgery long before symptoms occur and significant tissue damage takes place, they said.

Implantation of metal-on-metal hip replacements sometimes results in failures that are attributed to adverse local tissue reactions, the authors noted in their introduction.

"Patients with adverse local tissue reaction present with periprosthetic fluid collections, which have been termed 'pseudotumors' and typically are not detectable on radiographs or computed tomography," they wrote. "Patients with adverse local tissue reaction may present a challenge as a result of tissue necrosis at revision surgery and poor outcomes following revision."

Although MRI is "ideally suited" for evaluating hip replacement patients "because of its high soft-tissue contrast and lack of ionizing radiation," a lack of quantitative data makes it difficult to determine if there is a difference in the degree of synovial reaction between symptomatic and asymptomatic patients, the authors continued. "Therefore, MRI showing an abnormal synovial reaction has limited utility for a surgeon attempting to decide whether to proceed with revision in an asymptomatic patient."

To add to the available data, Hollis and colleagues examined the ability of modified MRI to detect and quantify synovial responses following MoM hip replacement in 69 patients (74 hips) who had been consecutively referred for evaluation.

The patients were divided into three groups: asymptomatic (22 hips); symptomatic with a mechanical cause, such as implant loosening, dislocation, periprosthetic fracture or faulty positioning (20 hips); and symptomatic with no known cause (32 hips). The three groups had similar ages, BMIs, and implant durations.

Synovitis was detected in 15 asymptomatic hips (68% of patients in that group), 15 (75%) symptomatic hips with mechanical causes, and 75 (78%) hips with unexplained pain. Among the subset of 13 subjects who underwent revision surgery, the volume of synovitis seen on MRI was higher in patients who experienced an adverse local tissue reaction, they noted.

"To our knowledge, this is the first study to quantify synovial reactions around metal-on-metal hip implants," the researchers wrote. They concluded that longitudinal studies are needed to better understand the value of synovial fluid as a test to identify candidates for revision surgery.

Such a test is badly needed as federal regulators continue to struggle with how to best advise people who have received MoM devices, including whether to recommend routine metal ion testing.

A recently published study found that metal ion levels in the blood have limited value for identifying MoM hip implant recipients who are likely to require revision surgery.

In an interview with MedPage Today, Potter said MRI analysis of synovial fluid is highly predictive of abnormal synovial response, unlike serum ion testing, radiographic analysis or even patient symptoms.

The long-term clinical relevance of synovitis in asymptomatic people with MoM hip implants has not been proven, but Potter said the evidence suggests that it is highly meaningful for predicting implant failure.

In as-yet-unpublished work, Potter's research team identified synovial thickness as an even more important predictor of inflammation and patient outcomes than synovial volume.

When the researchers examined 13 variables seen on MRI and then compared them with tissue damage scores following revision surgery, they also determined that synovitis type -- whether the synovitis was solid or liquid -- was highly relevant.

The study was funded by grants from the National Institutes of Health and General Electric Healthcare. General Electric Healthcare provided funding for the scanning of asymptomatic subjects.

Potter reported receiving a grant from General Electric Healthcare. A co-author reported a link with Smith & Nephew.

Read the full story at medpagetoday.com.