Eric, you wrote:
>What is described is very similar to how Inman described the ankle
> joint.  Inman said that talar trochlear surface wasn't a true cylinder but
> part of a cone because one side of the trochlear surface had a smaller
> radius of curvature than the other side.  If you look at the posterior
> facet of the calcaneus you can also see that portion of the posterior
> facet
> that is closer to the sinus tarsi has a smaller radius of curvature than
> the part farther away.  (When there is a smaller radius of curvature the
> surface will be closer to the axis of rotation.  This is consistent with
> the average position of the STJ axis being angle upward from the
> transverse
> plane.
>
> On the other side of the sinus tarsi the anterior and middle facet, or the
> combined facet, are also part of a cone, but their radius of curvature is
> a
> little more hard to visualize.

The talocalcaneal joint (true STJ-posterior facets only), the talocalcaneal
portion of the talocalcaneonavicular joint (anterior and middle facets
combined), the calcaneocuboid joint, and the talonavicular joint all have
the same type of non-constant radius.  Take a real, disarticulated foot
skeleton and place the talonavicular portion of the navicular head into the
anterior and middle facets of the calcaneus.  It is an exact fit (exact
same, non-constant radius).  It also exactly matches the shape of the CCJ
when you place the head of the navicular into the cuboid.  In other words,
you can find a crossectional view where these articular surfaces are all
identical in terms of their shape, and only the length and width of the
articular surfaces vary.  Now flip the talus upside down and place the
lateral aspect of the talar trochlea into the anterior and middle facets of
the calcaneus.  Exact fit!  Exact same radius.  It makes sense since all
these joints work in unison.  As you may recall, I wrote about this on the
mailbase a few years ago after I made some clay impressions of the joints
and began to compare them.  I wish someone would digitize these surfaces to
prove it conclusively.

Respectfully,
Jeff Root

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