Metal on Metal Hip Implants

The hip joint is known as a 'ball and socket' joint, where the rounded head of the femur, that long bone in the thigh, fits into a cup in the pelvis.  The joint needs to be both strong and mobile, and takes a lot of wear over a lifetime.  Small wonder, then, that hip replacements are so very common, especially as obesity increases and wears it down sooner, and we are living longer, so more people need joint replacement.  As I question in this week's YouTube, if you were an engineer, what materials would you use to construct a strong, durable joint?  Metal on metal seems logical, for both ball and socket, but in this week's podcast we look at a study in the Lancet, showing that these devices are turning out to be problematic.

The study examined data from the National Joint Registry of England and Wales on total hip replacements in over 400,000 people in the time period from 2003 to 2011.  About 8% of those implants were the stemmed metal on metal variety, and this type was much more likely to require revision (read that 'reoperation') than other types.  Factors associated with a need for revision included a larger head size and female gender aged 55, as compared with women age 70. Common reasons cited underpinning a need for revision included loosening of the prosthesis and pain.  Infection rates related to these implants were also slightly higher than in people with other types of prostheses.

Other concerns about some metal on metal prostheses include the leaking of chromium and cobalt into the bloodstreams of patients as the surfaces wear.  These metals are well known to cause toxicity in the brain, kidneys and lungs, and also impact on DNA, so may be related to cancer development.  Clearly, as the authors of this paper suggest, these implants should be removed from the market.  Those who already have them should undergo annual clinical and radiologic examination to assess the implant.

One observation that should prove helpful in designing prostheses in the future include the fact that larger head sizes seem to resist dislocation better.  An eye to the future also begs the question of prospective studies on new prostheses before they come to market. As I suggested earlier, metal on metal stands to reason as a practical and durable choice for prosthesis construction, but has turned out to be disasterous. My heart goes out to the folks who will need another operation, more physical therapy and downtime, yet still not be able to remove damaging metal ions from their bodies.  Would increased oversight help?  Rick opines, as a member of one of the FDA advising committees on cardiac devices, that yes, a slightly more prolonged and systematic approach would likely benefit most.  As regulations in the UK and elsewhere worldwide are more lax than in the US, that's also a place needing closer scrutiny.

Other topics this week on PodMed include the best treatment for cryptogenic stroke in NEJM, the smoking gun in atherosclerosis and heart disease, also in the Lancet, and costs versus outcomes in healthcare, in JAMA.  Until next week, y'all live well.

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