Achilles, Pascal, Occam, TUNC and Bonini

In the world of eponyms in surgery - Monteggia, Fournier, Chiari etc - I actually prefer the non-medical ones.

There is, for example, Achilles' Dilemma in hip resurfacing  ("to choose whether to live a short and glorious life or live a long and boring life"); Occam's Razor in diagnostics; Pascal's Wager in avoiding hip dislocation (just use an elevated lip - there is no down side). In case you're wondering, TUNC theory is an acronym not an eponym, for any confused readers.

Here, though, is a new one for me. Bonini's Paradox (courtesy of Kevin Williamson, writing on US politics).

Let us consider hip replacement first. A sphere sits within a hemisphere, as a near perfect fit. It can move freely in any direction for everyday function. It has intrinsic stability by nature of its shape. It is a fairly accurate facsimile of a real hip in many ways. It is a simple design philosophy, if subject to highly sophisticated manufacturing. It is very forgiving in practice, if you put it in slightly askew.

Not so knee replacement. Here there are several radii of curvature in coronal and sagittal planes, not one. A highly complex ligamentous arrangement ensures stability and translational movement. The lateral tibial plateau is smaller in life than the medial side. The lateral plateau has convexity. The shock absorption from the menisci varies between the two sides. Yet the knee prosthesis has a symmetrical biconcave tibial plateau with no difference in impact resistance, no benefit from the cruciates (except in those cruciate retaining cases with a pristine PCL), and relies on a difficult-to-consistently-get-right 'balancing' of the intact collaterals. It is a very unforgiving scenario, so no wonder that pretty much everyone admits these days that knee outcomes are poorer overall than those from hip replacement, particularly in high activity patients.

Attempts to make more complex knee replacements incorporating these issues have generally failed, with either poorer results or at best, unpredictable ones. It's not been for lack of trying by implant companies (1, 2).

Back to Bonini, who is a business professor at Stanford University, looking at reproducing complex systems, for example, a computer model of the brain:

And this is Bonini's Paradox: The less information a model carries about its subject, the less useful it's going to be in helping someone understand that subject. And yet, the more information a model carries about its subject, the less useful it's going to be in helping someone understand any single point of that subject. Any sufficiently detailed map of a region is going to be just as dense and difficult as the region itself. Any sufficiently detailed model of a brain is going to be a brain.

...and any sufficiently detailed model of a knee is going to be a knee. Wikipedia quotes French poet and philosopher Paul Valery, with a similar take, from years earlier in 1937: "Everything simple is false. Everything which is complex is unusable."

Indeed, a hip replacement is undoubtedly 'false', but it nearly always works. A more complex knee replacement is also false, but it just edges a little bit along the spectrum towards unusable.

Another quote from the Gizmodo article already cited: Any model, of anything, is in an act of editing. It picks out what we think is important regarding the subject, and directs our attention to how that important thing can be manipulated.

Even orthopaedic surgeons aren't gods. I suppose

...OK, everybody got that?

The science of walking sticks and related matters

Joe Perry of Aerosmith genuinely uses a walking stick!

A minority of orthopaedic surgeons are entirely at ease with biomechanics, I would say. I am in the slightly perturbed majority. However, all orthopaedic 'exit exams' of which I'm aware will include it, reasonably enough. Drawing free body diagrams, explaining joint reaction forces, discussing the rationale of prosthetic design etc

Happily, as Isaac Newton has amply demonstrated, the principles have been established for centuries, and so the two papers here, from 1959 and 1997, seem bang up to date.

The first, by Robin Denham, one of the relatively unsung heroes of British orthopaedics, is as nice an exposition of basic hip mechanics as you could wish for. He references Blount's classic JBJS article "Don't Throw Away the Cane", and the end of Denham's piece has a great little exposition of the use of a walking stick (cane), which includes why patients will intuitively hold it in the opposite hand from the affected hip. The final 9 points in the summary are in a way all you need to understand for both clinical practice and exams.




The second paper, from Richard Brand in Iowa, is more about osteotomies, and relating the mechanics to the biology. Proximal tibial osteotomy for knee arthritis is definitely on the way up again, and periacetabular osteotomy in adults is an essential part of the hip repertoire now. Proximal femoral osteotomy in adults though is not really on the same page. There seem to be very few patients in whom it would be a better bet than a hip replacement. The last one I did was for a varus proximal femur causing stress fractures, ages ago (it did work). The bottom line is, if you absorb these two papers you know a lot about hip mechanics. if you're like me, and you get most of the content, that's enough to function perfectly well.

If you add in Charnley's stuff on wear and head size, you're almost an expert. Almost

The problem with big heads

*

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.

Understanding knee arthroplasty and the PCL - not as boring as it sounds

Knee arthroplasty is more like hip resurfacing than it is like hip replacement - you really have to understand each stage in a relatively complex sequence, and if you get it wrong, it's not particularly forgiving. Likewise, if you understand each step then you can deal with the unexpected more easily and go off piste with a bit of freehand - if you have to (disclaimer: I am not promoting freehand knee replacement).

There are two keys to this in my view, once you're in the knee. One is understanding the PCL and its role in knee arthritis and knee replacement design. The other is the absolutely fundamental issue of mastering the flexion/extension gap. Wannabe knee surgeons have no excuse for getting the latter wrong.

The first piece is a chapter in a textbook from about 2001, which offers a practical take



As always, you learn more if you get two perspectives on the same topic. I read the second paper here back in the 90's, and I still think it's great, from the excellent AAOS review journal




For the record, I do a PCL sacrificing knee for pretty much ALL primary cases (and most revisions) - it works with almost any deformity and the fully audited long term outcomes are terrific.