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?