The problem with big heads

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The only thing that John Charnley did in orthopaedics that I don’t like is when he designed those big ugly forceps – I prefer a Lane’s any day. All his other instruments convey the message that he must have been a master surgeon and a gifted engineer. When I started orthopaedics his hip was still THE prosthesis in the UK, although back then the Exeter was already catching up. Now the Exeter and its lookalikes rule supreme and Depuy have incredibly pretty much abandoned the original Charnley, the most studied and successful implant of them all.

Back then the McKee had a 35mm head, the Exeter was 26mm (still a pitfall for today’s revision surgeon who hasn’t checked it properly), the dreaded Ring implant was 32mm, there were quite a few 28mm prostheses about, and the Charnley was 22mm. Why 22?

The simple answer is that with the materials available, Charnley saw it as the best trade off between reducing volumetric wear and the associated creation of polyethylene debris in large amounts, and linear wear eroding the superolateral part of the socket.

Not many people now have Charnley’s landmark book Low Friction Arthroplasty of the Hip, currently trading at upward of £150 on eBay, and fewer have actually read his 1969 paper on head size in a bioengineering journal, but here it is! (The second paper is a very handy review from HSS on the modern thinking on head size issues.)

It’s not a great read in a way, but it contains lots of key thinking. I believe that the main problem with those metal on metal articulations which fail in the present era is lubrication failure. Charnley spotted this as a major issue, hence he wanted inherently low friction materials in contact with each other. As he put it in the paper: “in the absence of a fluid film a good theoretical argument can be made out for using the smallest ball which the load bearing capacity of the plastic will tolerate”. Which was obviously not going to be 32mm.

The paper is based on experiments with polytetrafluoroethylene (PTFE), commonly called Teflon (as in various dodgy politicians), which is not the still useful ultra high molecular weight polyethylene (UHMWPE). This stuff seemed to behave differently in the lab compared to in the human body. It famously failed badly in a large early series of Charnley’s patients which in the current climate would have ended with a visit to the GMC and we would never have enjoyed the amazingness of total hip replacement. Possibly.

But I digress. Remarkably, even as early as 1969, Charnley had intuited that it was the microscopic plastic wear particles that were creating a biological reaction and implant loosening, and it was therefore his mission to reduce the overall volume of wear as much as possible. He ended up with the view that a head diameter half of the outer diameter of the socket was the right balance – see his relatively simple calculations. If the average socket is around 52mm, that makes the Exeter cult about right with 26mm. Which is now virtually abandoned too. As the great man admitted however “little is lost if the diameter lies between 21 and 31mm” 

The old myth that 22mm was chosen for manufacturing reasons is not borne out. It was purely to do with reducing volumetric wear, and Charnley reckoned that going below 22mm meant too high a risk of dislocation, and of ‘boring’ into the socket. The truth is, 22mm worked brilliantly.

The current vogue for large heads of 36 and 40mm is genuinely worrying. There could still be lubrication concerns and idiosyncratic wear problems, however low friction the coupling theoretically is with the latest hard bearing materials.

Finally, all the best orthopods come from the North West of England.