Jeff,
You wrote :
and the talonavicular joint all have the same type of non-constant radius.
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.
Respectfully,
Stanley
At 08:49 AM 11/25/04 -0800, you wrote:
>Eric, you wrote:
>>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.
>
>The talocalcaneal joint (true STJ-posterior facets only), the talocalcaneal
>portion of the talocalcaneonavicular joint (anterior and middle facets
>combined), the calcaneocuboid joint, and the talonavicular joint all have
>the same type of non-constant radius. Take a real, disarticulated foot
>skeleton and place the talonavicular portion of the navicular head into the
>anterior and middle facets of the calcaneus. It is an exact fit (exact
>same, non-constant radius). It also exactly matches the shape of the CCJ
>when you place the head of the navicular into the cuboid. In other words,
>you can find a crossectional view where these articular surfaces are all
>identical in terms of their shape, and only the length and width of the
>articular surfaces vary. Now flip the talus upside down and place the
>lateral aspect of the talar trochlea into the anterior and middle facets of
>the calcaneus. Exact fit! Exact same radius. It makes sense since all
>these joints work in unison. As you may recall, I wrote about this on the
>mailbase a few years ago after I made some clay impressions of the joints
>and began to compare them. I wish someone would digitize these surfaces to
>prove it conclusively.
>
>Respectfully,
>Jeff Root
>
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