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Dan Barker
07-12-2004, 12:02 PM
I write in relation to Marco Viceconti's excellent and relevant posting in
relation to the FE models in orthopaedic biomechanics.

I am too concerned that FE models are presented with ever increasing
complexity without a basic understanding of the underlying model parameters.

This may appear inflammatory and this is not my attention but I challenge
the more experienced FE modellers of this list to show cases where FE
analysis has led to actual change in clinical practice for orthopaedic
surgeons.

The basis of my question/challenge comes from experience in submitting FE
papers to clinical journals where the objective has been to look at stress
shielding of bone. Reviewers have often asked what the clinical realities
are for say a 20% decrease in strain at a particular site in bone, and
reject the paper, perhaps correctly i must admit, because we the authors
cannot provide the answer. When i pose this questions to clinicians,
clinical researchers, bone researchers etc nobody has the answer. They often
say they believe stress/strain shielding is important but do not have the
foggiest what quantitative values are important. Yet FEA models quantify
answers to sometimes 3 decimal places and I am as guilty of this as anyone.
Furthermore bone remodelling algorithms are included in FE models with "time
constants" so that the simulated remodelling agrees with retrospective
clinical observation.

I also point to the rough versus polished surface finish debate for cemented
hip stems which an enormous computational effort was devoted to. Some
studies said that a polished surface increased tensile hoop cement stresses
because of stem subsidence and would lead to early failure whereas others
said the polished stem subsided and encouraged compressive stresses at the
bone-cement interface rather than shear and loaded the bone more
physiologically and this was a good outcome. These computational approaches
were rarely accompanied by any in vitro or in vivo data. In reality, only a
national outcomes database with many thousands of arthroplasties such as the
Swedish model has the statistical power to differentiate and even then may
only be relevant to one implant shape.

Furthermore, we still have a scant understanding of the loads, boundary
conditions, material properties etc making up our models.

I wonder if it is necessary to channel the funding directed towards
computational efforts back into basic lab and clinical research until we are
more confident to argue to our orthopaedic colleagues the meaning of our
analyses. Perhaps the horse needs to be put back in front of the cart! One
may look at tools such as RSA as an example of "real world" research that is
now providing great insight into the behaviour of artificial joint
constructs.

I could not agree more with the comments of Anders Eriksson who wrote

"Assuming that you have good knowledge of the basic building blocks included
in your simulation package, you can very easily vary assumptions and
parameters in the simulation model, to see how results are affected by these
assumptions, thereby allowing some conclusions to be drawn, when these
simulations are compared to experiments (or general knowledge of behaviour)
This is, but only when you have the underlying knowledge about the modelling
assumptions, something very different and much more powerful than regression
of parameters in a fixed basic model, as it allows a much wider space of
assumptions to be tested."

Some may argue the objective of FEA is not to influence clinical practice
directly, but is an illustrator of the physics involved in orthopaedics and
I wonder if this is more the truth of the matter. Perhaps the stochastic FE
approach must be the goal of FE researchers in orthopaedics biomechanics
with a removal of the N = 1 approach to reflect the true realities of our
models,

Yours Respectfully

Dan Barker, PhD.




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