(PreventDisease) – Hip replacement surgery has skyrocketed in the last few decades and surgical intervention to replace the arthritic ball and socket joints has used some questionable materials. Scientists have now discovered that the inflammation in tissue surrounding failing chromium-cobalt metal-on-metal (MOM) implants is caused by the release of cobalt ions from metal debris that wears away from replacement joints. These ions are known to cause genetic damage and will eventually lead to further medical complications.
Most hip replacements consist of cobalt and chromium alloys, or titanium. Stainless steel is no longer used. All implants release their constituent ions into the blood. Surgeons have been telling patients for years that these ions are excreted in the urine, but in certain individuals the ions can accumulate in the body. In implants which involve metal-on-metal contact, microscopic fragments of cobalt and chromium can be absorbed into the patient’s bloodstream causing a host of illnesses.
Tens of thousands of patients have had cobalt-chromium alloy MOM hip implants. The rubbing between the components causes nanoscopic metal debris to be released into surrounding tissue, causing chronic inflammation and loss of mobility in patients.
Now, researchers from Imperial College London and Ohio State University have used a new approach that combines high resolution X-ray and electron microscopy to determine the cause of the chronic inflammation in tissue samples from affected patients. They discovered that residual chromium is oxidised and cobalt ions are released as the nanoparticles corrodes in the tissue, which is the cause of the inflammation. Previous studies have shown that cobalt ions are genotoxic, which could potentially damage DNA and lead to further long-term medical complications in patients.
The study, published online this month in the journal Chemical Communication, is one of the first to look at the effects of nano-particles in humans and raises questions about how materials are tested before they are used as implantable materials.
Dr Mary Ryan, co-author of the paper from the Department of Materials at Imperial College London, says:
“We were able to meet patients who had these failing implants and we could see first-hand the chronic inflammation, pain and loss of mobility they experienced. Even though a huge number of patients have benefited from replacement surgery, we still don’t fully understand the long-term impacts that implantable materials have on our bodies. Our work is one of the first to study these nanoparticles and the effects that they have on damaged cells and tissue. This has enabled us to understand in much more detail the side effects that these materials may have in patients.”
Dr Alexandra Porter, co-author also from the Department of Materials at Imperial, adds: “There is a double edged sword to these findings because on the one hand, we’ve found a root cause of inflammation, which may lead to better intervention therapies for patients. On the other, although we still need to do more work to understand the full impact, our results suggest that these nano-particles may have a long-term genotoxic impact on patients.”
The researchers in the study found the nano-particles accumulate in white blood cells, whose job it is to clean up debris in the body. Here they undergo a corrosion process where the cobalt dissolves rapidly and is released into the surrounding tissue and blood stream. The less soluble chromium forms a solid residue that remains in the tissue.
By 2010, reports in the orthopaedic literature have increasingly cited the problem of early failure of metal on metal prostheses in a small percentage of patients. Failures may relate to release of minute metallic particles or metal ions from wear of the implants, causing pain and disability severe enough to require revision surgery in many patients.
Metal-on-metal designs have not fared as well, where some reports show 76% to 100% of the people with these metal-on-metal implants and have aseptic implant failures requiring revision also have evidence of histological inflammation accompanied by extensive lymphocyte infiltrates, characteristic of delayed type hypersensitivity responses.
The team needed to understand the chemical make-up of the nanoparticles in the tissue as well as structural information about their size and distribution. They took tissue samples to the Canadian Light Source — Canada’s national synchrotron research facility and one of the few facilities in the world where materials can be studied at a high enough resolution to see the nano-particles. They used the high resolution X-ray microscopes to scan the tissue to determine the chemistry of the nano-particles. The researchers then analysed the same samples in the Titan microscope at Imperial in the UK, which uses electrons to image the size of the nanoparticles. This two-step process, which hasn’t been used before, enabled the researchers to build a highly detailed picture of how the nano-particles corroded in the tissue samples.
The next step will see the team carrying out further research to understand why the nanoparticles are corroding while the bulk of the alloy in MOM hip implants is corrosion resistant. The researchers also aim to use this correlative approach to explore other diseases where nanoscale materials may have an impact on human health, such as Alzheimer’s disease.
Jonathan Hamilton is a researcher and author with an interest in chemical toxicity, sustainability and natural living.