Hi Jeff and all,

Jeff wrote:
>A few years ago I did some drawings from calcanei and projected the
>rectangular body of the calcaneus (it is rectangular except for a small
>portion of the medial tubercle and the entire projection of middle facet,
>and it resembles a rectangular beam when you lop off a few of the
>distracting features, but I digress!) onto a two dimensional sheet of
>paper and I superimposed the calcaneal facets.  An axis bisecting the
>posterior facet and extended between the apex of the valley between the
>anterior and middle calcaneal facets seemed to closely parallel the
>average axis of the STJ relative to the sagittal plane.  If in fact this
>is true, then the talar head would logically increase a pronation moment
>when it applied an increased vertical load on the middle calcaneal facet
>which is medial to the STJ axis (eversion torque on the calcaneus via
>middle facet).  Conversely, increasing the vertical load on the anterior
>facet which is located lateral to the STJ axis would then create a
>supination moment (inversion torque on the calcaneus).  That's my take on
>it, although I really haven't given it much thought before now.  ;-)  Just
>kidding!


Jeff, are you familiar with Cahil DR. The Anatomy and Function of the
Contents of the Human Tarsal Sinus and Canal.  Anat. Rec. 153 p1-18.
1965.  In this article he describes much of what you are talking
about.  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.  Actually the shape we are talking about
here is two cones with their points connected together and the axis of the
cones is the axis of the STJ.  The STJ axis enters the calcaneus between
the posterior and middle facets.  So as you look at the top of the
calcaneus and think of the two cones, the posterior facet is the top
surface of the proximal cone and the anterior and middle facets are on the
top surface of the distal cone.

So, if the anterior facet faces upward, the axis will be above it.  Jeff,
in your post you mentioned that more force from the talar head on the
calcaneus would cause pronation because this surface was medial to the
axis.  However, the axis will lie in the middle of the joint surface.  It
is the shape of the joint surfaces that most likely determine the location
of the STJ axis.



Cheers,

Eric Fuller

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