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Terminology
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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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* ==== |
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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 |
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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. |
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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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