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  #1  
Old June 1st 04, 05:58 PM
Brian Grimley
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Doug Turner wrote:

"I have never seen any reference to "unravelling" of unloaded knots
nor any standard way to measure the propensity for this to occur."

"I would be grateful for any advice or pointers on this."

Doug, what follows is just a simple "thought experiment" to try to get
an understanding of the situation you describe.

First, what is holding a knot together? I would suggest that the
answer is friction. To have friction there must be some forces or
loads in the knot and the material must have some coefficient of
friction. (I assume there is no "glue" and the material does not bond
together at a molecular level)

Where can the load, that creates friction, on the knot originate? I
look at the knot used by a surgeon as a binding knot. Further, I see
it as having three parts. The first would be the loop holding or
pinching the tissue together. The second could be the knot itself. The
third is the loose ends. The load must come from one (or more) of
these parts.

First, the "loop". We can look at an old rule of thumb. If you are
binding hard materials together, e.g. steel pipes, use a rope that
stretches. The stretch in the rope creates and maintains a force that
helps create the friction in the knot that holds the knot together.
If you are binding a "soft" material together, e.g. a rug, you can use
a rope with little stretch. The force, to help create friction in the
knot, is created by the rug wanting to expand. I assume with tissue
(soft?), there is an initial force caused by the tissue wanting to
separate. As the tissue heals, I would guess that initial force
decreases, perhaps to zero. I would also guess the "stretchiness" of
the thread might contribute a force. But, depending on the tissue, as
the tissue heals that force might also be lost. I would guess that the
"loop" does not contribute to knot's friction and stability after a
"short" intial period.

Second, the knot itself. The knot is tied, dressed and set. It is the
setting or tightening of the knot that will stretch the "rope" within
the knot. That stretch, within the knot, will create and maintain a
force, that will create and maintain the friction to help hold the
knot together.

Third, the loose ends. No force, no help.

If the above simple model has any validity, I might say the following.
You want a binding knot that can be tightened to pull tissue together.
That "pulling of the tissue together" creates a force that helps stop
the knot from unravelling. However, that "pulling of the tissue
together" will decrease over time to zero. As a result, you also want
a knot that will not unravel when shaken that has loose ends (standing
parts). We might call it a bend. Now I understand some of the multiple
"knot knots" I have seen in pictures of knots used by surgeons!

Elsewhere you mention a "knots-in-a-sponge - warm rinse cycle". I like
it! As a "rough and tumble" first look at the question of unravelling,
I would tie a knot through the sponge and over a rod so that the rod
can be removed after the knot is set. The rod's removal should set the
force on the knot from the loop to zero. I would tie a particular knot
several times for each material. I would also try to tie each knot
with the same tension. I would try to obtain the "stretch" and
coefficent of friction of the materals and see if there is any
correlation between them and the results of the "knot-in-a-sponge -
warm rinse cycle".

In a specific material, it would not surprise me that the resistance
of the knot to unravelling was related to the tension used to set the
knot. However, one would need to measure the tension used. With a
range of tensions with a specific knot and specific material, one
could repeat the "knot-in-a-sponge - warm rinse cycle" and see if
there is a threshold. Similarily, different materials may have a
requirement for a much higher tension when setting a knot to obtain
resistance to unravelling than other materials.

As a "knot tyer", I now have another criteria to evaluate knots - a
"secure and stable" bend that can be tied as a binding knot. I will
leave the "tied in thread using forceps to you! :-)

I hope the above thoughts and the simple model is a help and has some
validity! I am looking forward to your comments.

All the best,
Brian.
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  #2  
Old June 1st 04, 11:09 PM
Doug Turner
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(Brian Grimley) wrote in
om:

Thanks for your comments Brian.

... I would tie a particular knot
several times for each material. I would also try to tie each knot
with the same tension.


We repeated the experiment 7 times for each of 11 different suture
materials. The final 2 throws were tied with a standardised tension of
500gms.

I would try to obtain the "stretch" ....


This might be a factor. Some of the materials (particularly the
monofilaments) stretch but don't seem to recover. Ductile might be the
term I think. We did not "prestretch" to remove this ductility. Maybe
that would make a difference.


coefficent of friction of the materals and see if there is any
correlation between them and the results of the "knot-in-a-sponge -
warm rinse cycle".


I'm sure this is a factor. Some threads (particularly the braided ones)
are deliberately coated so they slip through the tissue easily but the
down side is the knot insecutity. The coatings vary between
manufacturers but commonly soap teflon, or other polymers are used.


.... range of tensions with a specific knot and specific material, one
could repeat the "knot-in-a-sponge - warm rinse cycle" and see if
there is a threshold. Similarily, different materials may have a
requirement for a much higher tension when setting a knot to obtain
resistance to unravelling than other materials.


I think this might be a fruitfull avenue to explore. Intuitively I
think that with failure under load the tension when tying the knot might
not matter all that much, where as for unravelling when not loaded (I
still think there needs to be a generally accepted term for this) it
might be a more important factor.

Doug Turner
  #3  
Old June 2nd 04, 08:56 PM
Dan Lehman
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Brian Grimley wrote:

First, what is holding a knot together?
I would suggest that the answer is friction.

Duck tape--the universal answer.

To have friction there must be some forces or loads in the knot
and the material must have some coefficient of friction.

Actually, one can form, e.g., a SquaREef Knot with stoppers in the ends
(and SParts anchored) such that the knot itself is l o o s e but won't
come undone, the stoppers preventing the ends from working through (so,
only SO loose). Incorporating Overhand stoppers into some surgical
knots might be an answer (OH stoppers can sometimes be worked tight
snug against something--not so with e.g. a Fig.8 stopper).

Doug Turner wrote:

We repeated the experiment 7 times for each of 11 different suture materials.
The final 2 throws were tied with a standardised tension of 500gms.



And what was the knot that had a "final 2 throws"?
--sounds ominously large & repetitive! (Maybe there is a better structure.)

I would try to obtain the "stretch" ....


This might be a factor. Some of the materials (particularly the
monofilaments) stretch but don't seem to recover. Ductile might be the
term I think. We did not "prestretch" to remove this ductility. Maybe that
? would make a difference.


As for "stretch" being much help in knotting, consider ropes of high-modulus
materials (such as Vectran or HMPE, with elasticities 4% at rupture): one
often will have scant or nil stretch by manual tying. (And, in the case esp.
of HMPE, not much help in coef. of friction.)
Absent much stretch, the slightest loss of binding tension can lead to a loose
knot.

.... range of tensions with a specific knot and specific material, one
could repeat the "knot-in-a-sponge - warm rinse cycle" and see if
there is a threshold. Similarily, different materials may have a
requirement for a much higher tension when setting a knot to obtain
resistance to unravelling than other materials.


I think this might be a fruitfull avenue to explore.


Although to me it sounds bothersomely complex for pratical circumstances.
(Esp. when occasionally we learn of operations to e.g. the wrong leg or with
wrong blood type--now, for doctors to mind matching tensions & fibers?)

Intuitively I think that with failure under load the tension when tying


In what might be one of the few scientific investigations of knotting,
Stanley Barnes found that maximum strength for angling knots was achieved
with about a 50-60% tensile load on setting (which equates to maybe 80+%
of knotted break point). Simply, the shape of knot can be affected (and
maybe this partly explains lower breaks in some common knots in rope,
where getting 50% load isn't achievable w/o some leveraging device.

the knot might not matter all that much, whereas for unravelling when
not loaded (I still think there needs to be a generally accepted term
for this) it might be a more important factor.


"passive loosening"?
Certainly, the tighter one sets a knot, often the harder it is to untie.
(And perhaps a jammed overhand knot is the ultimate challenge.)

--dl*
====
  #6  
Old June 3rd 04, 09:35 AM
O J
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On Thu, 03 June, Karl Pollak wrote:


Would it not make much more sense to simply use a Surgeon knot?
You knw, like surgeons actually use.


LOL!! Good one Karl. And here I thought you had no sense of
humor. Live and learn.

Regards,
O J
  #8  
Old June 4th 04, 07:07 PM
Brian Grimley
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Doug Turner wrote:


.... range of tensions with a specific knot and specific material, one
could repeat the "knot-in-a-sponge - warm rinse cycle" and see if
there is a threshold. Similarily, different materials may have a
requirement for a much higher tension when setting a knot to obtain
resistance to unravelling than other materials.


I think this might be a fruitfull avenue to explore. Intuitively I
think that with failure under load the tension when tying the knot might
not matter all that much, where as for unravelling when not loaded (I
still think there needs to be a generally accepted term for this) it
might be a more important factor.


Continuing with the "thought experiment" ...

Another variable to interpret the "knot-in-a-sponge - warm rinse
cycle" would be the "spring" in the thread. By "spring" I mean the
force of the thread to straighten itself when bent. This spring would
act against the frictional force stopping the knot from unravelling.
It would be another force, in addition to the forces created by
shaking, that the tension used to set each part of the knot would have
to overcome. If there is a tension threshold for the resistance to
unravelling when not loaded, then it would nicely mark some complex
relationship between coef. of friction, stretch, spring, environment,
perhaps the thread's diameter and other unidentified variables. If you
do the "rough and tumble method" experiment and there appears to be a
threshold, I would very grateful if you would let us know!

A suggestion for a generally accepted term for unravelling when not
loaded is UNL (Unravelling when Not Loaded). I think UNL trips lightly
from the tongue and acronyms are always fun and popular. :-)

I was interested to see that in some climbing ropes a few meters of
each end of the rope were manufactured to have diffent
characteristics. Those characterists, which were not acceptable for
the rope as a whole, recognised that the ends typically had knots tied
in them.

As a "brain storming" suggestion, perhaps the suture should be
manufactured so that the needle and the first 1/2 of the suture are
friendly to piercing tissue. The second half changes (perhaps with
colour for identification) to have properties to reduce UNL.

If this isn't already the case, and if you mention this to a suture
manufacturer, please let us know their response. If their response to
you is, "when you learn to tie knots properly, you won't have this
problem", I will have to say "Dem's the breaks". If you are Doug
Turner, the "orthopod", I expect, when you read this, to hear your
groan where I live - 1/2 way around the world.

Best wishes - Brian.
  #9  
Old June 5th 04, 06:28 AM
Doug Turner
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(Brian Grimley) wrote in
om:

.....
Another variable to interpret the "knot-in-a-sponge - warm rinse
cycle" would be the "spring" in the thread. By "spring" I mean the
force of the thread to straighten itself when bent.......


Hi Brian, I think this might be another term for "memory". A rubber
band (besides being very stretchy) has a lot of memory. You can bend or
kink it as much as you like and it will resime its initial shape.
Stainless steel wire on the other hand has little memory. Once bent or
deformed it retains that shape. This tends to make knots more secure.


... A suggestion for a generally accepted term for unravelling when not
loaded is UNL (Unravelling when Not Loaded). I think UNL trips lightly
from the tongue and acronyms are always fun and popular. :-)


Sounds good too. But what really surprises me is there is not an
already accepted term.

...As a "brain storming" suggestion, perhaps the suture should be
manufactured so that the needle and the first 1/2 of the suture are
friendly to piercing tissue. The second half changes (perhaps with
colour for identification) to have properties to reduce UNL.


Mostly a long thread is used to tie multiple sutures like you see in a
repaired skin wound. Each suture is tied near the end of the long
thread, cuting the tail off near the skin and re-using the now shortened
thread until none is left.


...If you are Doug Turner, the "orthopod", I expect, when you read
this, to hear your
groan where I live - 1/2 way around the world.


'Tis me. ...I give up...how did you find me?

Doug Turner
  #10  
Old June 6th 04, 04:48 PM
Karl Pollak
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x-no-archive: yes
Doug Turner wrote:

Most surgical knots are formed by passing a length of suture material
around some tissue then tying the ends together with a 2=1=1 knot. This is
what most surgeons call a "Surgeons Knot". To make them more secure
surgeons will often add more throws eg 2=1=1=1 etc. Trouble is they still
come undone some times.


Doug, I take it that you are talking about silk sutures, rather than
sythetic ones. Unfortunately, the advantage of natural fibers (being
soluable) become also its disadvantage, if it disolves too soon. The only
suggestion of a layman I can make for situations where such is likely, to
simply double up on the thread.

If it is the knot itself that becomes unraveled on its own, yes you do have
a problem. Most likely the type of material you use is given by the
situation in which it is being used. The kind of knot you use is usually
given by the material you have at hand and the purpose for which it is
used.

So you are left with finding a different knot. Personally, I don't think
this is the place to find it. We're used to play with fishing lines and
cables and ropes and all sorts of other fun things, but I doubt there is
one amongst us who had ever tested his knotting skills inside somebody
else's body.

A surgical forum would probably yield better results.

As a last resort. What you are desctribing does not sound to me like the
knot we knot tyers call "surgeon's knot". I am sure that tehre will be
somebody on the net who does have an illustration of it on his website, if
you would Google the term "surgeon's knot". In the remote possoibility
that even the serach gives no joy, Somebody here will gladly scan and
e-mail you an illustration.

Essentially surgeon's knot is a variation of the reef or square knot in
which the second time you wind the two different ends around each other,
you do 3 or 4 passes instead of a simple one. The additional friction is
believed to hold the knot more secure. The reef knot on its own is not
intended for holding too well without some load and untying it
easily/quickly is considered one of its advantages.

--
Karl Pollak, Richmond, British Columbia
Sea Scouting in Canada at http://www.seascouts.ca/
 




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