MRI Predicts Failure of Metal-On-Metal Hip Implants

A new study revealed that MRI can be used to identify implants that are going to fail before individuals become symptomatic.

The study’s findings are slated for publication in an upcoming issue of the Journal of Bone & Joint Surgery. “The study shows that synovitis exists in asymptomatic people in a fairly high prevalence,” said Hollis Potter, MD, chief of the division of magnetic resonance imaging at Hospital for Special Surgery (New York, NY, USA). “If that is the case, symptoms alone are insufficient to determine the health of an implant. You can’t wait for people to be sore before we evaluate them for this potential problem.”

The investigators reported that that MRI scans can help identify patients who need revision surgery before tissue sustains additional injury that makes a revision more difficult.

Hip resurfacing, a surgical option to total hip replacement (THR), involves positioning a metal cap over the head of the femur while a corresponding metal cup, similar to that used in a THR, is placed in the pelvic socket. The procedure preserves more of a patient’s thighbone than a conventional hip replacement, but the implants can cause synovitis. Until now, no objective data has existed regarding just how much inflammation these implants can cause. To fill this knowledge gap, investigators set out to assess the ability of MRI to identify and quantify adverse synovial responses in symptomatic and asymptomatic subjects following metal-on-metal hip resurfacing.

The researchers included the first 69 consecutive subjects (74 hips) referred from three surgical practices to the Hospital for Special Surgery for an MRI after a metal-on-metal hip resurfacing arthroplasty. Patients were classified as asymptomatic, having unexplained pain, or symptomatic with a mechanical cause. This latter classification included patients with pain caused by implant loosening, dislocation, periprosthetic fracture, or original faulty positioning of the implant.

All individuals underwent an MRI and then these scans were evaluated by two musculoskeletal radiologists, both of whom were blinded as to which group the patient belonged. X-ray images were also obtained for most patients. Wear of metal-on-metal implant surfaces produces metal wear debris in the form of very small microscopic particles, resulting in metal ion release, which may impact local tissue and other body systems. Therefore, chromium ion and serum cobalt levels measured closest to the date of the MRI scanning were recorded when available. Investigators also gathered demographic data, including sex, age, body-mass index, and length of time since arthroplasty.

Analysis of the MRIs identified synovitis in 68% of asymptomatic hips, 75% of symptomatic hips with a mechanical cause, and 78% of hips with unexplained pain. “What was really interesting about this study was that we found that synovitis was detected in the same amount in symptomatic and asymptomatic patients,” said Dr. Potter.

The average volume of synovitis was 5.0 cm3 in the asymptomatic group, 10.2 cm3 in the mechanical causes group, and 31.0 cm3 in the unexplained pain group. “The data show that there is a high prevalence of abnormal synovial response in both symptomatic and highly functioning, asymptomatic patients who have undergone metal-on-metal arthroplasty, indicating that symptoms alone are insufficient means by which to monitor patients,” Dr. Potter said.

There were no significant disparities in the radiographs or serum ion levels, demonstrating that these could not be utilized to forecast injury. “Many people focus on serum ion levels,” said Dr. Potter. “I think the direction of the pendulum is really changing now, away from serum ion levels and toward imaging, or at least not to focus so much on serum ion levels to predict potential damage. Cross-sectional imaging is the way to go, and specifically, MRI over CT [computed tomography] based on its superior soft tissue contrast.”

Stephanie Gold, BA, and Kevin Koch, PhD, from General Electric (GE) Healthcare (Chalfont St. Giles, UK) also participated in the study, and GE Healthcare provided funding for the scanning of asymptomatic subjects.



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