MoM Hips: Unexplained Pain Probably Tissue Damage

Hospital for Special Surgery (HSS) researchers have revealed the results of their new study showing that the cause of unexplained pain among metal-on-metal hip implant patients is more likely to be tissue damage than wear of the implant. The study was performed by Dr. Danyal Nawabi, an orthopedic surgery fellow, along with research collaborators from HSS.

The research group, which did not have any ties to hip implant manufacturers, was led by Douglas Padgett, M.D., chief of the Adult Reconstruction and Joint Replacement Division and chief of the Hip Service at HSS, and Hollis Potter, M.D., chief of the Division of Magnetic Resonance imaging (MRI). They compared 50 patients who came to HSS for revision surgery because of unexplained pain, to a control group of 48 patients who came to HSS for revision surgery because of loosening, malalignment, infection or fracture.

The investigators combined results from clinical examinations, MRIs, wear analysis studies on the removed implants, and pathology studies, including the degree of aseptic lymphocytic vasculitis-associated lesions (ALVAL), a sign of adverse tissue reactions to metal debris.

Thirty patients with unexplained pain (60% of the group) had an ALVAL score of at least 5 on a 10-point scale, indicating moderate to high adverse tissue reactions; 12% of patients had some buildup of metal deposits in their soft tissue. The average synovial thickness was three times higher in the unexplained pain group compared to the control group, and the average synovial fluid volume was five times higher in the unexplained pain group compared to the control group. Ten times as many patients in the unexplained pain group had high-grade tissue damage scores compared to the control group. Researchers found no differences between the unexplained pain group and the control group in terms of age, sex, body mass index, length of implantation, or size or positioning of the implants. Implants in both groups showed similar signs of wear.

“We found that some patients had a significant amount of tissue damage but not a lot of wear, ” Dr. Wright said, “suggesting that factors other than wear are contributing to the problem regardless of whether the patients have pain. We have used the information from our study to develop guidelines for patients and surgeons.”

Asked what work remains to be done and how it should be approached, Dr. Padgett told OTW, “The real underlying question that remains is as follows: Are there unique patient characteristics that make some more susceptible to this reaction as opposed to many others who seem to have no problem with metal-on-metal bearings? Our research group is pursuing this line of investigation by looking at possible genetic factors. This work is made possible through our tissue registry at HSS. This is an exciting next step in understanding patients’ response to prosthetic implants.”