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A DNA test promises to transform knee replacement surgery for patients with metal allergies. YKC’s Jim Newman identifies a few issues with the science.

“DNA test could spare thousands of patients pain after knee surgery.” So ran The Telegraph’s headline recently, in response to the news that a new genetic test is promising to reveal the metals to which patients may be allergic. The thinking here is that if you know a patient has a specific metal sensitivity you can choose an implant that doesn’t use that particular metal.

Sounds reasonable enough. It is true that some people are sensitive to/allergic to some metals like nickel, cobalt or chromium, although that’s generally a contact (skin-based) allergy. And it is true that a small percentage of patients will feel knee pain after their knee replacement.

There’s just one problem (well, several problems actually) with connecting post-op knee pain with a metal allergy:

1. The connection between skin and implant-related metal allergies is unclear

If you believe a metal allergy lies at the root of your knee pain, that’s probably because you’re sensitive to certain metals touching your skin. It follows that you might then be interested in this DNA test.

Yet the current advice from the British Orthopaedic Association is as it has been for several years: having a skin allergy to metal does not necessarily mean that a person will experience an adverse reaction to a metal implant.

That’s because a skin allergy occurs when metal comes into direct contact with the skin and activates the immune system through the skin’s specialised immune cells. In contrast, metals in a knee implant are located deep within the body, where they interact with a very different set of tissues and immune pathways.

2. The data is old

Articles like the one referenced above often roll out the same stats about the number of patients experiencing post-op pain. But most of those studies are old and (certainly in the case of Yorkshire Knee Clinic’s surgeons) the results bear no relation to reality.

It’s also the case that as knee replacement techniques, implants and instrumentation continue to improve, the old data becomes even less relevant.

3. The issue isn’t the same for hips

Hip implants use the same metals as knee replacements, yet we don’t see the same level of metal allergy-based complaints in hip surgery. That would suggest the issue actually lies with the fact that the knee is an incredibly complex joint (far more so than the hip) rather than the implant being used.

4. There’s no cause and effect

What we appear to have here is a DNA test in search of an application. It can identify metal sensitivities and makes the leap that it could be of value to patients who believe a metal allergy lies at the root of their knee pain.

That’s very different from, for example, testing patients who have all had their knee implants revised and monitoring the outcomes, which would give us some rather more rigorous science to work with.

5. The solution may not be better

Let’s suppose for a moment that the test really does shape the choice of implants for patients. That only presents another issue. Because while it is possible to choose implants that are free from certain metals, they are – generally speaking – far more expensive, far less commonly used, far less well understood and backed by much less data than the most successful implants.

Asking a surgeon to use an unfamiliar implant in an extremely complex joint risks replacing one problem with another.

So where does that leave us? In truth, I’m not sure this test helps at all. The jury is very much out as to the issue of metal-related post-op pain because there are so many other elements, from the complexity of the joint to mental health, that could also be contributory factors. And as noted above, alternative options aren’t entirely appealing either.

What is clear, however, is that the vast majority of patients find that a knee replacement dramatically reduces pain and increases mobility, and remains the best choice for addressing advanced osteoarthritis.

To explore your treatment options, please contact us, or phone us on 03453 052 579.

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