Hi all,  Jeff responded to Stanley,

>You wrote:
> >> I thought that all joints have a non constant radius based on the
> Fibonacci
> > curve. The advantage of a non constant radius is that the pressure is
> > spread over a greater area during movement of the joint.
>The joints that I described all have the same, non-constant radius, not
>just a non-constant radius.  In other words, if you look at the long axis
>of the joint surfaces in question, there is one particular plane in which
>they all have the same, complex contour or radius.  The total width and
>length of the joint all vary, but they have a common contour over a
>portion of the joint surface.  Some of the joints are wider and longer,
>but they all reflect an identical contour which suggest common motion.  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?


Certainly the experimental results (Van Langelaan E. J. van, A Kinematical
Analysis of the Tarsal Joint an Radiographic Study.  Acta Orthopaedica
Scandinavica Suppl #204. 1983.  Vol. 54 ) show that the STJ has a bundle of
axes.  That is the axis in a slightly different position as the STJ moves
through its range of motion.  However, this change in position of the axis
may not be clinically significant and the assumption of single axis may be
a good approximation.  (If anyone has the van Lanaalan paper easily
available we should look at what was found for the ankle joint axis.  Was
there a bundle there too?)  I don't find it surprising at all that the
opposite sides of the STJ have the same contour.  The motion of the talus
and calcaneus is very likely to be tightly constrained by the interoseous
ligaments and joint surfaces.

Jeff, I think my description of the double cone surfaces explains how these
joint surfaces could produce a bundle of joint axes that are almost similar
enough to be considered a single axis.


Cheers,

Eric Fuller

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