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**NEAL R. GROSS**

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"*The physiologic, anatomic, and morphologic characteristics of AAA,*

*with or without iliac involvement*"

By Dr. Hugh Beebe

DR. BEEBE: Okay. Let\'s get started. On the subject of times though, in
some of these sessions, we have got an awful lot to cover, and so we are
going to need to keep one eye on the clock.

John, rather than me saying next slide, if I just point at it, would you
please change it. Thanks. Clearly, morphology, which is \-- we are kind
of setting the stage for getting into this process here by talking about
the standard aneurysm morphology, and how we assess it.

And so that is what Barry and I are going to talk about, and that is a
big deal in all of these clinical areas, but it is also a big deal for
preclinical testing.

And the method of assessing the morphology can be separated from
understanding the data that we have, and Barry is going to address that
subject in a while.

But recognizing that there are some great differences here, and one of
them from my perspective is that we are trying to solve a three
dimensional issue with two dimensional tools in the past, and maybe it
is time that we moved on a little bit.

Rather than going through a kind of tiresome review of what actually are
the morphologic characteristics of aneurysms, which I think everyone in
this room probably knows very well, what I am going to try to do is just
in a qualitative way review some thoughts about morphology to get us
thinking about the difference between clinical and patient selection,
and pre-clinic testing.

They are not always the same thing, and a lot of the morphology data
that we have bears on the clinical issue of patient selection and device
selection for a given patient, but maybe doesn\'t describe very well
what we need to know for pre-clinical testing purposes.

So that the basic thing that we know is to sort of look at, and I have
put some examples, striking examples, that aneurysms come in lots of
shapes and sizes, just as people do.

And I kind of think now that when I look at the classification that we
devised in a consensus and that was published in JVS in 1997 is a little
bit dated, and perhaps a little naive.

That is a personal bias, but I think particularly when you get around to
thinking about pre-clinical testing, this classification system doesn\'t
serve us particularly well. Can you go back one, John.

There was, for example, an enormous preoccupation with the distal neck.
Well, today we don\'t care a whole lot about the distal neck. And there
was a lot of focus on the simplistic, I think \-- and I think we would
all agree \-- notion that there was a meaningful cut point at neck
length.

And now I think we \-- if we knew how to do it better, we would say,
well, it just isn\'t length. It is length plus angulation, plus
diameter, and it is a more complex thing than just simple length. Okay.
Next, please.

And the angulation was described in a way that is a little different
from the way that we usually talk about it today, but it is primarily
the difference between the super renal axis of the aorta, and the neck
end of the body angle, but doesn\'t describe at all the angle within the
aneurysm.

But they have got lots of different angles, and we didn\'t describe
anything. In fact, it is a pretty difficult task to describe iliac
tortuosity. But that is turning out to be a big deal in multiple ways.

And there is nothing in our data, pretty much nothing, about lumen size.
And when you get to thinking about issues that are specific to
pre-clinical testing, maybe this is pretty important. Next, please.

It may be important what is inside this lumen, and devices may behave
differently between the empty aneurysm and what I am calling the full
aneurysm, but most of the time we don\'t pay much attention to this,
unless it is such a small lumen that we are worried about operating a
modular device when it is that small, and that it is so small at the
bottom that it is going to pinch the lumen of the endograft.

This is essentially absent from the literature and so we don\'t have
much data. Well, there are a lot of anatomical factors important in
patient selection, and the question is which of these \-- next please
\-- are important for pre-clinical testing.

And when you boil it down, the meat and potatoes are pretty much
proximal neck factors, and all of these may be, and I think are, we
would agree, very, very, important.

Iliac arteries, we don\'t describe tortuosity very well, and we are not
describing the lumen size very well. Calcification I will say something
more about in a moment.

But all of these are clearly factors that impact on clinical selection,
and which of these need to be taken into account for pre-clinical
testing to be meaningful. That is really the question.

And we need to remember that this is unstable anatomy. It does vary by
gender and by age. The older the patient, we are starting with generally
a larger and different anatomy than a younger patient\'s. Next, please.

Now, there has been proposed a classification method for an aneurysm
neck shape. Next please.

But most of the literature ignores this. Here is an example of why we do
so at our peril. This is a cone shaped neck, but both the top and the
bottom of the neck were within the diameter limits of inclusion and
exclusion criteria for a clinical trial.

We put an endograft in this patient, and 12 months later, it has
migrated down 20 millimeters. We were within the perimeters of the
trial, but it was a poor patient selection in my opinion. Next, please.

When we talk about diameter and I think Barry will probably get into
this a bit more, we have been fooling ourselves by measuring diameter
simplistically by simply taking the least dimension on an axial slice as
recommended in the literature.

But if you use 3-D methods and reformat the slice, you find out that
these necks are not all round. In fact, they are clinically significant.
Here as an example is 6 millimeters. Actually, this is the true shape of
this lumen.

It is not a round neck. So sometimes we are mistaken, and calcification,
we don\'t have a classification system that works in my opinion, and
this circumference thing is whacko \-- next, please \--because they are
not lined up.

The volume averaging effect of the CT slice is misleading us, and we
have developed a kind of quirky classification system there that is
based on an artifact.

Neck filling defects. They come in lots of different sizes and shapes.
Next please. Papers have been written about how they don\'t matter. A
lot of people think they do matter. For sure, it is not normal aortic
wall. Next, please.

So what do we do about this in pre-clinical testing? Do we ignore it or
what is the deal here. We really don\'t know. Next, please.

Type II endoleaks, do they affect device performance, and if they do,
how, and should this be taken into account. This is quite different from
patient selection. I think that most of us tend to ignore the patent
lumbars, and perhaps patent IMAs, and wait to see what happens. Next,
please.

Well, enough about anatomy. A few thoughts about physiology. Next,
please. A long time back, the 19th century, the idea that the aorta is a
wind kessel. It is kind of a neat concept, but there is a huge elastic
recoil phenomenon and reflected ways that accompany the original left
ventricular ejection wave.

And these tend to summate where? Right where we are trying to put in
endografts, folks. It is in the infrarenal aorta. As a matter of fact,
in an experimental animal, with short acting beta blockers, you can move
where those waves summate up and down the aorta almost at will by
tytrating the animal. It is very interesting. Next, please.

If you look at the some of the experimental work that has come from
Stanford, both in vitro and more recently exercise in vivo data, you
find out that there is some very interesting things that happen in the
infrarenal aorta.

Here, if you can make it out, with exercise and the solid line is
resting data, this is bicycle ergometry on an MR. Look at how aortic
flow changes with really not heavy exercise.

Most of the endograft patients are not doing heavy exercise, but a lot
of them are more active than we tend to think. These are not just people
that sit in a chair. They are out doing stuff.

And the flow rates change dramatically with very significant changes in
shear stress. Next please.

So this is a complex stress strain curve in the infrarenal aorta, and
there are a lot of factors that impact on it, and which of these are
important, and how to take them into account is a big challenge. But to
ignore them is no longer tenable. We just can\'t blow it off. Next,
please.

So in conclusion, I would say that there are many characteristics of
aortoiliac morphology and physiology that you can measure. There is tons
of stuff.

The existing classifications maybe aren\'t quite suited, both to patient
selection, but certainly to preclinical testing how. Next, please. So
that the preclinical testing methods must select out the really valuable
anatomical and physiologic factors that are going to influence the
failure mode, inducers and promoters.

And we probably should be taking into account from the beginning the
usefulness of 3-D methods if we are ever going to get to the point of
using computer simulation to help in preclinical testing, and graft
performance predictions.

So that is my remarks. Now, Barry is going to heighten our awareness
about how the imaging data that we are using sometimes needs a lot of
interpretation.