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View Full Version : ACL reconstr. mech prop (summary)



Marco Viceconti
09-21-1995, 11:16 PM
Quite a long ago I posted a question about tensile properties of ACL
reconstruction. A few colleagues replied and we discussed a little. I
held this summary because it seemed that nobody had any conclusive evidence
to support or to refuse my hypothesis. But the time did not help, so I am
posting it now.
As Joseph Crisco pointed out, "At least I have never seen any discussion
stating that the viscoelastic properties of thick [fibrils] were different
from thin". However, the idea was that fibrils thickness distribution
could influence not the viscoelastic properties but the strength and the
failure mode of the ACL. Another point is that we were referring to the
failure LOAD not to the failure STRESS, thus we can have also structural
effects. If somebody else can say something about it, we start up again
the discussion!

Since the question was cross-posted to BIOMCH-L and BIOMAT-L also the
summary will be cross-posted; sorry for those signed to both.

Marco Viceconti


*************************ORIGINAL POSTING*************************
To: biomech-l_forum, biomat-l_forum
From: lk1boq74@icineca.cineca.it (Marco Viceconti)
Subject: ACL reconstruction mech props
Cc:
Bcc:
X-Attachments:

Dear Colleagues,
we've just completed a study on tensile strength of sheep ACL
reconstruction done using the central third of the patellar tendon, at
different stages of healing.
Our methods were very similar to those used in other studies (i.e. Amendola
et al, Am J sports Med 20-3 1992, pp:336-346) but with one substantial
difference; the load rate. In fact our test were quasi-static (100 N/min)
were the results obtained in literature are usually obtained at high load
rate.

The results we found tend to support the hypothesis of almost complete
recover on mechanical strength after about 12 months; this finding seems to
be in contrast with other studies; the same paper of Amendola reports an
identical study on sheep where after 12 months the max. tensile load was
still 44% of that of the intact ACL. Again, these finding are based to
tests done at a much higher load rate than ours.

How should we read these results? Can the load rate explain the difference
between our and other data?

Any opinion or comment will be welcome.
*************************END OF ORIGINAL POSTING*************************

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X-Sender: Joseph_Crisco_III@postoffice.brown.edu
Mime-Version: 1.0
Date: Thu, 22 Jun 1995 07:48:04 -0500
To: Marco Viceconti
From: Joseph_Crisco_III@brown.edu (Trey Crisco)
Subject: Re: ACL reconstruction mech props

Marco,
Very interesting. I do not have Amendola's paper, but I would be
very suprised if their rate of loading was "high". To the best of my
knowledge there have been no studies on ligaments at rates that we would
expect to occur during in vivo injury production. The reason for this is
the history of testing. Material testers have been adopted by
biomechanical engineers, and most of these testers are slow. The fastest a
ligament has been pulled to failure is about 1 meter / second. Note that
most previous work has been displacement rate controlled rather than force
rate controlled, as in your work.
Regardless, ligaments may not be that sensitive to rate; the
material properties are more affected by age. I would doubt that the
difference you measured is due to rate, since all the rates are all quite
slow.
Is there a difference in the age of your sheep vs. others?
What about activity restrictions after surgery?

Trey Crisco, Ph.D.
josep_crisco_iii@brown.edu

To: Joseph_Crisco_III@brown.edu (Trey Crisco)
From: lk1boq74@icineca.cineca.it (Marco Viceconti)
Subject: Re: ACL reconstruction mech props
Cc:
Bcc:
X-Attachments:

Joseph,
thanks for your notes. No, the average age of the sheep is
comparable to that of similar studies. Also the post-surgery mobility
program was quite similar.

Probably you're right but consider that 1 m/s is 1000 mm/s Vs the 100 N/min
(0.0167 mm/s considering a stiffness of 100 N/mm in the linear portion);
thus my rate is many order of magnitude lower than those you 're referring
to.

Let me know what do you think.

ciao

Marco
X-Sender: Joseph_Crisco_III@postoffice.brown.edu
Mime-Version: 1.0
Date: Thu, 22 Jun 1995 08:27:25 -0500
To: lk1boq74@icineca.cineca.it (Marco Viceconti)
From: Joseph_Crisco_III@brown.edu (Trey Crisco)
Subject: Re: ACL reconstruction mech props

Marco,

>Probably you're right but cosider that 1 m/s is 1000 mm/s Vs the 100
>N/min (0.0167 mm/sec considering a stiffness of 100 N/mm in the linear
>portion); thus my rate is many order of magnitude lower than those you 're
>referring to.

Personally, I am very interested in (and supportive of) rate dependence and
we are actually studying the effects of "impact" loading (neither force nor
displacement control) on ligament properties.

Consider the following from Woo et al. J Ortho Res 1990 (8):712-721.
Their displacement rates ranged from 0.008 mm/s to 113 mm/s. With this
increase in displacement rate, failure load increased from 54 N to 124 N
and stiffness increased from 24 N/mm to 38 N/mm.

So clearly rates in this range have some effect (note that this range is
still very slow compared to would we would expect in vivo). BUT. I am
assuming that you have normalized your results to the controlateral intact
tendon. If so, the effect of rate should be normalized.

There is however, one assumption that may not be valid. That assumption is:
scar tissue and normal tendon have the same response to changes in rate.
If scar tissue was orders of magnitude more sensitive to changes in rate
than normal tendon, than this might explain your findings. However, I
think this is a new concept and clearly needs further investigation.

Cheers, Trey

Trey Crisco, Ph.D.
joseph_crisco_iii@brown.edu

To: Joseph_Crisco_III@brown.edu (Trey Crisco)
From: lk1boq74@icineca.cineca.it (Marco Viceconti)
Subject: Re: ACL reconstruction mech props

>assuming that you have normalized your results to the controlateral intact
>tendon. If so, the effect of rate should be normalized.

Yes we have normal tendon data; however, we've normalised with respect to
the intact ACL (this is a quite clinically oriented study, thus we car
about how much weaker is the reconstruction with respect to the normal ACL.

>There is however, one assumption that may not be valid. That assumption is:
>scar tissue and normal tendon have the same response to changes in rate.
>If scar tissue was orders of magnitude more sensitive to changes in rate
>than normal tendon, than this might explain your findings. However, I
>think this is a new concept and clearly needs further investigation.

That is my problem actually; using TEM we found that there is a substantial
difference between a normal sheep ACL and a reconstruction with tendon
after one year of healing: the second has much less thick collagen fibrils.

"In a normal tendon we found a bimodal distribution with two peaks at 64
and 130 nm of diameter. For the normal ACL we found a three modal
distrib., with peaks at 52, 120, and 187 nm.
For the grafts at 12 months we found a unimodal distrib. with the mean at
66 nm. It seems like the thicker fibre just disappeared; however, consider
that, cross section and fibrils organisation is approximately unchanged.
Now, if I do a static test, I let each fibre to elongate during the test,
and the effective cross section is quite close to the nominal one. But if
a go faster usually I get failure of a few bundles before the others; thus
in the first case, the cross section is the control factor, is the second
the chance of having thick collagen fibrils is driving the result."
Do you think the reasoning makes some sense? Before writing this in a
paper I'd like to have colleagues feedback on it!

Thanks for your attention.

X-Sender: Joseph_Crisco_III@postoffice.brown.edu
Mime-Version: 1.0
Date: Thu, 29 Jun 1995 07:42:00 -0500
To: lk1boq74@icineca.cineca.it (Marco Viceconti)
From: Joseph_Crisco_III@brown.edu (Trey Crisco)
Subject: Re: ACL reconstruction mech props

Marco,
The above variance in distribution may explain some differences in
stress-strain, but I do not see how it relates to the original difference
you presented which was a rate dependent difference. At least I have never
seen any discussion stating that the viscoelastic properties of thick were
different from thin.

I hope this is of some help, Trey
*************************************************

From: Lisa Bellincampi
Subject: ACL reconstruction mech props
To: lk1boq74@icineca.cineca.it
Date: Thu, 22 Jun 1995 12:38:43 -0400 (EDT)
Mime-Version: 1.0

Hello. I received your question on the ACL mechanical properties through
the Biomaterials Mailing List. You're results are interesting. But
before we comment, may I ask you:
What were your controls in this experiment? Did you also test
a bone-ACL-bone complex at this low strain rate, or are you
relying completely on the literature for this comparison?

and

What was your reason for testing at this lower rate?

I would be very interested in comments from other colleagues on this
matter. If you do not plan to post other comments to the list, would you
drop me a line? Thank you.

Lisa D. Bellincampi (bellinld@umdnj.edu)
University of Medicine and Dentistry of New Jersey
Department of Orthopaedic Surgery
Date: Fri, 23 Jun 95 09:41:20 SOL

To: Lisa Bellincampi
From: lk1boq74@icineca.cineca.it (Marco Viceconti)
Subject: Re: ACL reconstruction mech props
Cc:
Bcc:
X-Attachments:

Lisa,
of course, as far I'll get a few answer I'll post the usual summary.
No, we did test intact BACLB and reconstructed ones; the changes I was
referring are with respect of the normal BACLB of the sheep we measured.

> What was your reason for testing at this lower rate?

That's a good question. Two reasons: as a material engineer I do prefer to
know static properties before furthering the study. But most important:
after an operation of ACL reconstruction, in non-professional sportsmen
usually you don't have another trauma in the first year (except you're
really unlucky!!); thus having info on the effect of healing on static
properties could be a one boundary scenario (the other one is that given by
data collected using dynamic loads).

Hope is clear enough;

Ciao

Marco
*************************************************

From: "Frieder Grieshaber"
Organization: University Stuttgart / Germany
To: lk1boq74@ICINECA.CINECA.IT, fg@bmt.uni-stuttgart.de
Date: Mon, 26 Jun 1995 17:33:25 CET+100
Subject: Re: ACL reconstruction mech props
Priority: normal


Marco,

we have researched not only the tendons of lower leg but
also the nervous, venal, arterial tibialis and other
tissues. With muscles, we performed quasi-static tests and
dynamic tests with rates of deformation up to 12 m/s. We
have found that the max strength strongly depends on the
rate of deformation. So, at a rate of 9 m/s, muscles have a
max strength which is 4 times higher than with quasi-static
loads. These results will be published in a few months. So
long.

Best wishes
frieder
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Date: Tue, 27 Jun 1995 11:38:37 -0400 (EDT)
From: COLLEEN1@vms.cis.pitt.edu
Subject: Re: your posted question
To: lk1boq74@icineca.cineca.it
MIME-version: 1.0

Mr. Viceconti,

One of my co-workers alerted me to your posted question concerning ACL
reconstruction in sheep. There was a paper published by our laboratory which
may help you try to work this out. It is M.I. Danto and S.L-Y. Woo, "The
Mechanical Properties of Skeletally Mature Rabbit Anterior Cruciate Ligament
and Patellar Tendon over a Range of Strain Rates" published in Journal of
Orthopaedic Research, 11:58-67. Central thirds of rabbit PTs were tested at
various strain rates, the slowest of which may be more or less comparable to
your strain (or loading) rate. However, the results of this study were that
slower strain rates led to decreased modulus. Although the numbers are not
specified in the paper, the stress strain curve tends to indicate that the slow
rate also resulted in higher strains. Little could be said about ultimate
tensile strength as all but 1 PT specimen failed by avulsion.

Colleen Weaver
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From: T.J.Lawes@bristol.ac.uk (TJ. Lawes)
Subject: Re: ACL reconstruction mech props
To: lk1boq74@ICINECA.CINECA.IT
Date: Tue, 27 Jun 1995 17:18:36 +0100 (BST)
MIME-Version: 1.0

Marco, Its very evident that all collagenous tissues (including bone,
tendons and ligaments) are not pure springs. They exhibit both hysteretic
behaviour and, more importantly, have a large component of internal
damping in their material properties. Hence their load, stress and strain
behaviour is very dependant on the rate of any mechanical test.

The faster loading occurs, the less the material is able to deform and as
is well known in fractures of bone, the more energy is stored in the bone
before breaking. In other words, as the loading rate increases, then
the higher the breaking stress (or applied force) in BONE.

In ligamentous tissue, whose damping properties are dominant, then the
tissue will be more rigid at higher loading rates and therefore it is
likely that problems such as equalisation of stress throughout the
material will occur. The result is that there are regional starins higher
than the bulk material strain and therefore the material may fail at a
lower maximum. I am not familar with the particulars of the paper you
refer to, but the problem can be described as strain rate dependant
embrittlement. In other words, the material becomes more brittle at
higher rates. This can be due to either sliding of fibres over one
another during the loading process being inhibitted or soley to the
material properties of the fibrous, ground substance and cellular
components of the matrix.

I hope that this ("little bit") is of some help to the list !!!!!!!

Tim Lawes
University of Bristol
Dept of Anatomy EMail: t.j.lawes@bristol.ac.uk
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-------------------------------------------------------------------
MARCO VICECONTI
(lk1boq74@icineca.cineca.it)
Laboratorio di Tecnologia dei Materiali tel. 39-51-6366865
Istituti Ortopedici Rizzoli fax.
39-51-6366863
via di barbiano 1/10, 40136 - Bologna, Italy

Tiger! Tiger! Burning bright in the forest of the night,
what immortal hand or eye could frame thy fearful symmetry?
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