David, Craig, Susan, and all,
In a message dated 98-07-05 09:48:03 EDT,
David Wrote:
>
> A rather broad question (or questions):
>
> For those of you out there who use pressure sensing devices in your
clinical
> (or research) practices, I'd like to know how you believe both impulse
> (force x loading time) and pressure time integral (force x loading
> time)/area play a role in your care of the patient. Would you abandon a
> device or treatment that has reduces pressure very well but causes a high
> impulse at footstrike? How does this impact rigid vs. semi-rigid vs. low
> durometer orthoses? Low-dye straps? Do these aforementioned values have any
> correlation with symptomatic relief? Has any one looked into this yet?
A couple of questions in return.
How are you using pressure time integral. Are you looking at discrete areas
or the entire foot. Which discrete areas are you looking at? The reason I
ask is that different things will effect different areas of the foot. If you
were concerned about ankle arthritis then a short duration high amplitude blip
on the force time curve woulud be bad news. (Is this what you mean by high
impulse at heel strike?) However, if you were worried about a forefoot ulcer
I would be relatively unconcerned. Usually, the blip on the force time curve
occurs before the forefoot hits if there is the usual heel toe gait. This
really well documented in running, but less so for walking.
Bobbert MF. Yeadon MR. Nigg BM. Mechanical analysis of the landing phase in
heel toe running. Journal of Biomechanics. 25(3):223 34, 1992 Mar.
Bobbert MF. Schamhardt HC. Nigg BM. Calculation of vertical ground reaction
force estimates during running from positional data. Journal of Biomechanics.
24(12):1095 105, 1991.
The interesting conclusion from those articles was that the impact forces
(blip on the force time curve) was related to the vertical velocity just
before heel strike. This is something that a person "chooses" in their
running style. Perhaps a device that reduces pressure allows the person to
choose a higher impact and still feel comfortable. I believe Craig once
talked about the idea of loss of protective sensation in relation to soft
shoes and tried to applly that idea to diabetics at one of the Weed seminars.
I believe Craig referred to an article by Robbins. The two above articles
are an outgrowth of this article that looked at various hardnesses of shoe
midsoles.
Nigg BM. Bahlsen HA. Luethi SM. Stokes S. The influence of running
velocity and midsole hardness on external impact forces in heel toe running
Journal of Biomechanics. 20(10):951 9, 1987.
This article found little correlation with impact forces (blip) and midsole
hardness. I would assume that this finding would apply to rigid and soft
orthoses. The softness of the device may change how you choose to land. One
of the things that was mentioned in those studies was a change in the ankle of
the knee at heel strike. (This was in running.) The knee was more flexed
with the harder midsoles. When the knee is in a more flexed position it will
be a "softer spring" than when it is more extended. I am not aware of any
studies that attempt to correlate the angle of the knee at contact with knee
pain. An interesting study would be to look at several people with knee pain
and change the durometer of their shoes and see if their angle at contact
changes and the knee pain decreases. Although the study I am suggesting is an
outcome study, it is still important to understand how the variable you
measure effects mechanical stress on the structure you are concearned with.
Craig replied
>
> Not directed related to the above but two staff here (Anita R and Lesley N)
> did some research on the pressure distribution by custom made insoles and
> looked at the above variables - without going into details (they can do
> that !!), but I quote from the conclusion: "...Peak vertical pressure, the
> pressure/time integral and the total contact surface area were
> statistically significant for the insole versus no insole comparison.
> Interestingly, there were large differences between participants in the
> amount of pressure redistribution that occured with the insole, which in
> some cases was negligible...." Apparently this "negligible" pressure
> reduction was sufficient for ulcer to remain healed, at least for the
> duration of the study. Which raises the periennial issue that I suspect we
> all know about .... just how important are the above variables? - or is it
> all due to shear stress and Davis's "wrinkled carpet" effect?
>
> (Ref is: Raspovic A, Newcombe L, Lloyd J, Dalton E: The Effect of
> Customised insoles on vertical pressures in sites of previous neuropathic
> ulceration in the diabetic foot. In: Keenan AM and Menz H (eds) 18th
> Australian Podiatry Conerence Proceedings. Australian Podiatry Council
1998)
>
Just a thought on shear. Isn't the location of the highest amount of shear
going to be the location of the highest amount of pressure? When you rub your
fingers on a table the harder you press the more shear you feel. Craig, in
the study you site you mention a decrease in peak pressure. Do you remember
if they looked at peak pressure at specific locations.
Cheers,
Eric Fuller
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