I wrote:
>I find this most interesting in terms of the talocalcaneal articulations
> since the "STJ" is said to have a single axis and the other joints would
> seem to be multiaxial or have a shifting axis.  How is it that the "STJ"
> can
> be uniaxial and have the same non-constant contour as the ankle joint, the
> CCJ, and the TNJ?<<<

Simon replied:
>The STJ is not uniaxial; it has bundle of non-constant axes throughout the
ROM of the joint.<


The key to a successful fishing trip is to know what bait to use and where
to cast it.  I knew I would catch something when I threw my STJ axis "line"
in this pond!  If we really want to appreciate how the STJ (a misnomer at
best) functions, we can't stop asking more questions.  Whether the STJ has a
bundle of axes is of little value until we can establish why, and how it is
significant functionally.  How does the STJ axial bundle relate to the
morphology of the joint?

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?  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).

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!

Respectfully,
Jeff Root

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