Bone loss and subsequent implant failure are well known complications of metal-on-metal (MoM) hip implants. Researchers now think they know why. In an article published recently in the journal Biomaterials, German investigators say metal debris from the implants may affect the body’s ability to form new bone.
A metal-on-metal hip implant consists of a ball and cup made of a cobalt and chromium alloy. Originally developed as a more durable alternative to implants with ceramic or polyethylene (plastic) components, MoM implants proved to be the opposite. In general, they have a much higher failure rate than implants made of other materials – a problem attributed mainly (but not solely) to bone erosion in the hip joint, which leads to the artificial joint loosening and the need for revision surgery.
Minute chromium and cobalt particles (ions) produced when the metal ball rotates against the metal socket are known to cause inflammation in the tissues around the implant. But the researchers suspected metal debris might also affect mesenchymal stem cells (MSCs) in nearby bone marrow. MSCs have the ability to turn into several types of specialized cells, including bone-building cells called osteoblasts. If MSC function is impaired, they reasoned, new bone cells can’t form.
Tracking the path of metal particles
The researchers began by testing the tissue, blood, bone marrow and joint fluids of 18 patients undergoing revision surgery for a failed MoM implant. They found high levels of dissolved metal ions in all these areas. (Dissolved metals have the ability to travel through the bloodstream and cross cell membranes.) The researchers say theirs is the first study to look at metal debris in multiple areas, and it shows that MoM wear generates “vast amounts” of metallic particles.
The next step was to analyze MSCs isolated from the affected bone marrow. The stem cells showed a significantly reduced ability to turn into osteoblasts, although their ability to become cartilage and fat cells remained unchanged. The researchers then tested their findings in the laboratory by exposing cell cultures from healthy volunteers to the same levels of chromium and cobalt found in MoM patients. The results were the identical – the cells lost their ability to turn into bone cells.
Study author Carsten Parka, MD, medical director of the Center for Musculoskeletal Surgery at Charité – Universitätsmedizin in Berlin, says understanding the effects of the materials used in implants is essential to ensure that artificial hips are safe and long-lasting. Like his colleagues, he thinks the harms of metal-on-metal hips clearly outweigh their benefits and that their use should be carefully considered.
What happened to metal hips?
In the United States, all-metal implants are no longer used for total hip replacement, but all-metal components are still used in hip resurfacing, according to Edwin Su, MD, a hip and knee surgeon at Hospital for Special Surgery in New York City and an expert on hip resurfacing. (Unlike total hip replacement, the head of the thighbone isn’t removed in hip resurfacing; instead, it’s reshaped and capped with a metal covering.) There have been fewer issues with the metal components in hip resurfacing, “so they continue to be used, though very selectively,” he says.
Dr. Su, who was not involved in the study, explains that Johnson & Johnson’s DuPuy metal-on-metal implant, which was pulled from the market in 2010, had an exceptionally high failure rate due to the excessive production of metal debris. Although other MoM hips have not had as many problems, surgeons have moved away from them in favor of safer options.
The researchers note that although metal debris plays a central role in the failure of MoM hips, other factors are involved, too, including the biomechanics and anatomy of individual patients.
Dr. Su agrees, saying, “I think the paper is an important one in helping us understand why some of these hips fail; however, as with anything in medicine, there is a spectrum of responses by different patients. Some patients may react to the metal debris and demonstrate this issue; others probably do not. All of the patients studied in this paper were failures of MoM hips, thereby selecting the patients who are likely to have this problem.… [But] there are many patients who have had a successful result for 15 years and counting.”
Most experts say patients with metal-on-metal hips should be monitored at least annually by their surgeon. (If you are not sure what kind of hip implants you have, ask your doctor.) Patients should immediately report any new or worsening symptoms, including trouble walking or pain, inflammation or numbness around the hip joint. Some doctors recommend regular surveillance of ion levels in MoM patients. The Food and Drug Administration, which intensified its crackdown on metal hips this year, suggests that patients tell their doctors about any changes in their overall health, as well, because metal ions can potentially cause problems in other parts of the body, including the heart.
Author: Linda Rath for the Arthritis Foundation