Tissue Damage from Metal-on-Metal Hip Implants Appears Before
Pain Symptoms Appear
MRI Useful for Predicting Failure of Metal-on-Metal Hip Implants
New York – May 15, 2013
Metal-on-metal hip implants can cause inflammation of the joint
long before symptoms appear, and
resonance imaging (MRI) can be used to identify this inflammation, according
to a new study by researchers at Hospital for Special Surgery. The
study, which appears in an upcoming issue of the Journal of Bone & Joint
Surgery, demonstrates that MRI can be used to identify implants that are
going to fail before people become symptomatic.
"The study shows that synovitis exists in asymptomatic people in a fairly
high prevalence," said
Potter, M.D., chief of the
Magnetic Resonance Imaging at Hospital for Special Surgery, in New York
City. "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 researchers say that MRIs can help
identify patients who need revision surgery before tissue sustains further
damage that makes a revision more difficult.
Hip resurfacing, a surgical alternative to total hip replacement (THR),
involves placing a metal cap over the head of the femur while a matching metal
cup, similar to that used in a THR, is placed in the pelvic socket. The
procedure preserves more of a patient’s thigh bone 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 evaluate the ability of MRI to
detect and quantify adverse synovial responses in symptomatic and asymptomatic
subjects following metal-on-metal hip resurfacing.
The investigators 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 category 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 to. X-rays 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, serum cobalt and chromium ion
levels measured closest to the date of the MRI were recorded when available.
Investigators also collected demographic data, including age, sex, 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
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 differences in the X-rays or serum ion levels,
demonstrating that these could not be used to predict damage. "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 based on its
superior soft tissue contrast."
Other Hospital for Special Surgery investigators involved in the study
include the lead author, Danyal Nawabi, M.D.; Catherine Hayter, MBBS;
Giorgio Perino, M.D.; and Stephanie Gold, B.A.; Kevin Koch, Ph.D. from
General Electric (GE) Healthcare also participated in the study, and GE
Healthcare provided funding for the scanning of asymptomatic subjects.