Hi all,
  Jeff wrote:

>The head of the talus is governed by the anterior and middle calcaneal
>facets.  If you fix the calcaneus in space and move the talus while
>observing the path of the talar head anteriorly, starting from the fully
>pronated position with the talar head resting on the lateral aspect of the
>anterior (supination) facet, the talar head adducts and plantarflexes until
>it reaches the bottom of the valley formed by the anterior and middle
>facets.  Once it reaches the bottom of the valley it continues to adduct as
>it pronates (?) but it then begins to dorsiflex.  Does that mean the STJ
>actually pronates and supinates, or not?


Jeff, I think you are making this more difficult than it is.  As a point on
the talus revolves around the axis (the imperfect hinge: several axes) it
will follow a circular path.   That point on the talar head will, as you
say, go down in a valley and then up the other side.  The posterior half of
the talus is moving in the opposite direction because it is on the other
side of the axis of rotation.  Yes, it pronates and supinates around an
axis (an axis that is somewhat variable.)

>Perhaps the motion of the foot to
>the leg is pronation and supination but perhaps it is not actually occurring
>entirely at the STJ!?  Perhaps it depends on your point of reference.  It is
>impossible to determine the relative motion of the talus to the calcaneus
>unless you exclude the other bones of the foot and leg and only model the
>two bones in question (tough to do given that the other bones influence the
>spatial relationship of the talus and calcaneus).

Good questions Jeff.  What I've seen some call subluxation of the STJ, I
would call midtarsal joint motion.  Anatomically, it is impossible to
sublux the STJ without ripping ligaments or breaking bones. So, I would
agree with you Jeff that we do have to look at all of the joints of the foot.



>Here is another interesting question.  Why is the middle facet longer (and
>larger) than the anterior facet if there is roughly twice as much supination
>to pronation, when you measure supination and pronation in the frontal
>plane?  My answer is that the anterior (supination) facet is oriented to
>produce more frontal plane motion of the calcaneus but the middle
>(pronation) facet must allow as much or more motion but it is less evident
>in the frontal plane.  So, how is the STJ axial bundle related to the
>morphology of the joint?  Time to bait my hook again!

I agree with Kevin's point that it if you assign a function to the anterior
facet that is distinct from the middle facet you have a problem in those
feet where the middle and anterior facets are combined.  I would also agree
with you earlier point that when the talus moves relative to the calcaneus,
there will be motion at all of the facets.  All the facets will guide the
motion all of the time.


Cheers,

Eric Fuller

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