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Marco Viceconti
07-03-1994, 11:41 PM
Enclosed you'll find the summary of the responses I collected to the
following question:


**********ORIGINAL POSTING*********
Dear Netters,
I've just finish to test the tensile properties of sheep ACL. The
elastic part of the force-elongation curve is sensibly non linear and any
tentative to calculate the Young module was driving to absurd results. I am
wondering if it would make sense to report the tangent module at the foot
and at the yield point or if it would be better try a non linear modeling.
My problem is to report someway the elastic behaviour in the most standar
way.
Personal opinions and/or references are welcome.
**********ORIGINAL POSTING*********

As you can see the few opinions I collected are quite different; so, any
further contribution would be welcome.

Sincerely

Marco Viceconti



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Marco,

It is my understanding that the calculation of Young's Modulus for a
non-linearily elastic system is to use the most linear portion of the
stress-strain curve above the "toe" region and below the maximum tensile
stress. You can probably find a reference for this in Y.C. Fung's book on
biomechanics (I think that is the title).

Good Luck,
Matt Kay
North Carolina State University
Raleigh, NC USA

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Dear Marco,
I am a Ph.D. student at the Univ. of Connecticut (USA) in Polymer Science.
I have previously worked for the Davis & Geck R&D Div. of American Cyanamid
Co. where I had been involved in quite a few projects in Orthopaedic device
devel- opment, including ACL properties and reconstruction. While it has
been several years (5-6) since involved in ACL work, I have always been
interested (as well as vested interest due to a knee injury) in ACL
research. [Most of the data and references I used are still with the
company.]
Is there a discussion group focused in ACL research, either in the Biomch-L
or Biomat-L internet groups? Is there some recent publications where I can
up- date myself on research in this area in the last 5-6 years? Lastly, has
there ever been any measurements with ACL's (such as sheep) to measure the
dynamic mechanical properties?-This would give information on the
viscoelastic nature of the (tissue) material and can be performed at
varying frequencies. Please let me know if I can be of assistance.
MARK J. STACHOWSKI
STACHOWS@UConnvm.UConn.Edu

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Marco Viceconti -
I am involved in a similar project at the Pennsylvania State University,
Center for Locomotion studies. The format you use to present your data will
depend on its intended use. We use a look - up - table so anyone
referencing it can make their own inferences. A curve fit would also
represent your data, but a couple of tangents will not do so adequately.
Basically, you have derived an interesting curve, and should do your work
justice in presenting it. Let anyone make their own approximations as
necessary. Well, that's my opinion.

Terrence E. George
TEG102@PSUVM.PSU.EDU

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Marco,

If you have any desire to try and apply some nonlinear modeling, I have
some techniques that maybe useful to you. Especially when trying to model
systems that can not be physically interpreted! If you are interested in my
help, let me know. I'll be out of circulation for the next couple of weeks
(on vacation) and won't be responding to my mail, but I'd be happy to
discuss your situation upon my return.

Ciao for now,
Michael


.................................................. ............................
Michael D. McPartland, Ph.D.
804 Furnas Hall
Department of Mechanical & Aerospace Engineering
State University of New York at Buffalo
716-645-3538
FAX 645-3875
.................................................. ............................

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Dear Marco,
Try looking at the references by David Bul Try looking at the references by
David Butler, et al. They do, in my opinion, the most compreshensive workd
on the ACL. To solve your problem in the long run, try a simple exponential
fit to the stress strain curve, e.g. stress=A*exp{strain*B) - 1, where A,B
are constants. B is a measure of the stress-strain curve's steepness. The
Derivative of this function can also give you the tangent modulus any point
along the stress-strain curve. The fit should only include up to the
pre-yield region. Good luck
Michael Sacks
Biomedical Eng.,U. Miami



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MARCO VICECONTI

Laboratorio di Tecnologia dei Materiali tel. 39-51-6366864
Istituti Ortopedici Rizzoli tel. 39-51-6366861
via di barbiano 1/10, 40136 - Bologna, Italy fax. 39-51-6366863

APPLELINK: ITA0940 INTERNET: lk1boq74@icineca.cineca.it

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