Patients who received metal-on-metal hip replacements are at no greater risk of developing cancer after a seven-year period than those with alternatives or the general population, according to a study.
A longer-term review is needed of the effect of metal particles leaking from the implants, because some tumors take many years to develop, according to the research published in the British Medical Journal today. Investigators looked at the incidence of malignant melanoma as well as hematological, prostate and renal tract cancers, which are associated with metal ions in the body.
The study comes after the BMJ and the British Broadcasting Corp. said Feb. 28 that a joint investigation showed patients with the metal-on-metal implants may have been exposed to toxic metals that can cause long-term disability. The devices were more likely to fail than others and should be banned, U.K. researchers wrote in the medical journal The Lancet on March 12.
“There isn’t an increase in cancer, and that’s reassuring,” Ashley Blom, head of the orthopedics department at the University of Bristol in the U.K. and co-author of today’s BMJ study, said in a telephone interview. Blom also contributed to The Lancet article. “I think a lot of the things that have come out in the BMJ and the news recently have been frightening.”
The study looked at 40,576 patients with metal-on-metal hip implants and 248,995 patients with alternative bearings and their incidence of cancer based on hospital admissions. Patient data came from the National Joint Registry of England and Wales, which collects information about hip, knee and ankle joint replacement procedures, and the U.K.’s National Health Service. The registry funded the BMJ study.
Using computer models, the study found that patients with metal implants had a lower rate of cancer than their non-metal counterparts and the general population, though the difference was so small it wouldn’t influence decisions about which implant to use, Blom said.
Manufacturers of the devices include Johnson & Johnson, Zimmer Holdings Inc. and Smith & Nephew Plc.
There are some weaknesses in the study, aside from the short time period, Blom said. Patients who receive hip implants tend to be healthier than the general population because people who are ill or have a short life expectancy don’t receive them, Blom said. Metal-on-metal implants were given to younger patients, who tend to have lower rates of cancer, because metal lasts longer, Blom said. Poorer people, who statistically have higher cancer rates, are less likely to be offered joint replacements, the authors said.
The long-term biological effects of implants made from metals such as cobalt and chromium are unknown, Blom said. Metal ions, which can cause irreversible damage to DNA in cells, have been found in many organs following hip replacements, including marrow, blood, liver, kidneys and bladder, the study said.
“It’s a question that people have asked for a number of years,” Blom said. For a comprehensive study “you need really big numbers and ideally you would want a 20- to 30-year follow-up.”