I believe that Dr. Kirby has nailed the biomechanic
etiology of the CC joint subluxation. I think that
another very interesting an valuable question to
answer is: why does the plantar fascia rupture more
often in those patients suffering from diabetic
neuropathy?
From my own experience, I have several observations.
I have noticed that in those patients developing
peripheral sensory neuropathy with greater deficit on
one side than the other, the plantar pressures of the
forefoot (measured with F-scan) are increased
(compared contralaterally) during late midstanceon on
the side with greater deficit. My hypotheses for this
phenomenon are two:
1) I believe that the gradual loss of proprioception
causes the patient to "overload" the forefoot in an
attempt to create a more stable platform from which to
propulse. It may be that the patient has to create
much greater pressure to illicit a "normal"
proprioceptive sense.
2) Without (pain) sensory feedback from the plantar
forefoot and from the overly stressed tendon and
ligament structures, the patient will reach higher
peak pressures and tensions than otherwise. This is
analogous to the loss of "protective sense" that we
look for to predict risk of plantar ulceration.
Furthermore, in advanced diabetes, we see
glycosylation of both tendon and ligament causing a
shift in the Blix curve of these tissues. This tissue
compromise certainly would increase the chance of
rupture or "creep" in light of increased load. I
should clarify that these are just my observations and
I have not yet collected enough data on the subject
for a strong study.
Sincerely,
Jay
--- Kevin Kirby <[log in to unmask]> wrote:
> Bruce, Craig and Colleagues:
>
> Bruce wrote:
>
> <<So, my question is why does Charcot subluxation,
> tend to occur at the CC joint? Why does the cuboid,
> "drop down" in these patients. Any thoughts on the
> musculature mechanics, as well as the joint
> mechanices??>>
>
> I would contend that the cause of the cuboid
> "dropping down" and the rupture of the plantar
> fascia are more specifically due to another
> mechanism. In the late midstance phase of gait, the
> strong upward tensile force of the Achilles tendon
> combined with the strong downward compression force
> on the superior talus from the tibia tends to cause
> a very strong rearfoot plantarflexion moment. This
> strong rearfoot plantarflexion moment tends to also
> cause a large forefoot dorsiflexion moment due to
> ground reaction force (GRF) pushing vertically
> upward on the distal forefoot, resisting the
> plantarflexion motion of the forefoot. The
> combination of the strong rearfoot plantarflexion
> moment along with the strong forefoot dorsiflexion
> moment tends to the force the calcaneo-cuboid joint
> (CCJ) in a plantarward direction.
>
> These rotational forces on the rearfoot and forefoot
> during the latter midstance phase of walking gait,
> if not resisted or counterbalanced by adequate
> rearfoot dorsiflexion moments and forefoot
> plantarflexion moments from:
>
> 1. plantar intrinsic muscle contractile activity;
> 2. peroneus longus, flexor digitorum longus,
> posterior tibial and flexor hallucis longus muscle
> activity
> 3. short and long plantar ligament integrity
> 4. plantar aponeurosis integrity
>
> will cause a tendency for plantar ligamentous
> rupture and/or lengthening (i.e. creep response).
>
> Therefore, if the question is :
>
> Why does Charcot subluxation, tend to occur at the
> CC joint? Why does the cuboid, "drop down" in
> these patients?
>
> the answer then becomes:
>
> because the rotational forces acting across this
> joint tend to cause plantarflexion of the rearfoot
> and a dorsiflexion of the forefoot unless these
> movements are resisted by the passive and active
> tensile forces of #1-4 above.
>
> In other words, unless these rotational forces were
> present in the first place, then neither cuboid
> "drop down" or plantar fascia rupture would occur.
>
> Cheers,
>
> Kevin
>
>
****************************************************************************
> Kevin A. Kirby, DPM
> Adjunct Associate Professor
> Department of Applied Biomechanics
> California School of Podiatric Medicine at Samuel
> Merritt College
>
> Private Practice:
> 107 Scripps Drive, Suite 200
> Sacramento, CA 95825 USA
>
> Voice: (916) 925-8111 Fax: (916) 925-8136
>
****************************************************************************
>
>
>
>
>
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