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dhrassoulian64
07-11-2007, 06:58 PM
Just a quick comment which maybe more thought provoking than helpful!

I think if you measure the ankle axis, say for plantar flexion, you can do
it in-vivo by having the subject's leg hanging off a table so that the
action is done in non-weight-bearing conditions or you can have them rising
up to their toe which will be weight bearing condition i.e. you have
bone-on-bone forces pushing talus into tibia.

Under non-weight bearing you are likely to have the axis of rotation all
over the place but under weight bearing it is fairly consistent with the
inter-maleolar line.

As for cadavers, you need to decide if the action was done under weight
bearing or non-weight bearing conditions. I assume most cadaver studies can
only do it under non-weight bearing conditions and that might explain the
large variations in the results.

Also, what most people ignore is the effect of the soft tissue, and I mean
tissue other than the tendons and ligaments.


--
Dr. H Rassoulian BSc, MSc, PhD, CEng, FIMechE
Clinical Scientist
Southampton General Hospital
Tremona Rd.
Southampton SO16 6YD

Tel: +44 (0)23 80 79 69 45
Fax: +44 (0)23 80 79 41 17
Alternative Email:
Hamid.Rassoulian@suht.swest.nhs.uk


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Quoting "van den Bogert, Ton" :

> Steve Piazza wrote:
>
> > We studied cadaver specimens with
> > markers mounted on bone pins, so skin movement was not an
> > issue. We still saw errors in axis orientation that were
> > between 20 and 40 degrees, errors that did not result when we
> > tested an anthropomorphic mechanical linkage. This suggested
> > to us that the cadaver joints exhibited deviation from their
> > mechanical analogues that made the problem ill-posed.
>
> Could it be that the cadaver joints are somewhat "loose" when you move
> them passively? I did my in vivo measurements with the subject moving
> their foot with their own muscles, without ground contact. The slight
> compression of the joint may have caused the motion to be more
> consistent with the two-hinge model.
>
> With in vivo data I estimated the error being much less than 20-40
> degrees. Yes, the minimum was shallow but quite robust, given enough
> data over the entire range of motion. With weightbearing though, the
> results were all over the place.
>
> Ton van den Bogert
>
> --
>
> A.J. (Ton) van den Bogert, PhD
> Department of Biomedical Engineering
> Cleveland Clinic Foundation
> http://www.lerner.ccf.org/bme/bogert/
>
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