Eric, you wrote:
> 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.

Eric and Kevin,

If you bisect the posterior aspect of the calcaneus (the bone itself, not a
living heel), you will see that when you place the bisection vertical, the
anterior and middle facets both incline and the division between them occurs
at the bottom of the valley.  The anterior facet is everted and the middle
fact is inverted.  Therefore abduction of the talar head will act on the
everted, anterior facet resulting in an inversion moment or torque on the
calcaneus.  Conversely, when the talar head adducts it contacts the
inverted, middle facet, causing an eversion moment or torque to the
calcaneus.  When the facets are continuous, you lose the distinction between
them but the functional significance of the orientation of the facets
remains the same.  When the facets are continuous they may not be distinct
and separate, but the orientation of the apex of the valley and the slopes
still exist.  In other words, the facets, be they fused or not, each have a
different orientation relative to the cardinal body planes and can be
differentiated simply by their orientation.  I believe this relationship is
what results in the different quantities of motion observed in the frontal
plane when the STJ becomes supinated or pronated.

I have been playing a bit of the devils advocate in my postings.  I do
believe that the STJ supinates and pronates.  When you hold the calcaneus
still and move the talus through its ROM the head of the talus describes an
arc, albeit a slightly imperfect arc.  This suggest an relatively constant
axis of motion or a very tight bundle, if in fact a bundle exists.  The STJ
works much like a gear mechanism.  Looking at the calcaneus, the posterior
facet is the male portion of the gear and the anterior and middle facets
combine to form the female portion of the gear.  Conversely, the posterior
facet of the talus forms a female portion of the gear and the inferior
aspect of the head of the talus forms a male portion of the gear.  Hence,
the STJ is essentially a two toothed gear.

What is interesting in terms of STJ function is that we can produce
seemingly large ranges of frontal plane motion during the open chain,
non-weightbearing examination, but we don't seem to see the same ROM during
weightbearing activities.  Is the larger ROM reserved for activities that
demand extreme motion?  There are no muscles attached to the talus and it is
largely covered with cartilaginous, joint surfaces.  It has two, complex
curved joints on the inferior surface and a similar, singular contour at the
ankle joint.  The double jointed interior surface would seem to resist
motion in that compression should return the joint to the point of maximum
surface contact.  Therefore, perhaps only when sufficient motion can't be
achieved at the proximal ankle joint or at the distal MTJ, does the STJ
provide the extra motion required by uncoupling and then recoupling.
Perhaps the talus and calcaneus move as unit far more than we actually
appreciate.  Certainly the MTJ would seem to permit the rearfoot unit to
move due to the complex motion available at the MTJ.  After heel lift,
reduced compression force will likely occur at the STJ but it will likely
increases at the MTJ.  Perhaps the fuller range of STJ motion occurs during
swing phase as there should be less compression at that time.

Respectfully,
Jeff Root

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