Материал: surgical knot tying manual covidien
knot slippage |
Knot slippage is counteracted by the frictional forces of the knots. The |
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degree to which a knot slips can be influenced by a variety of factors |
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including the coefficient of friction of the suture material, suture diameter, |
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moisture, knot type and final geometry. Knots of the granny type (crossed) |
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usually exhibit more slippage than do knots with a square-type (parallel) |
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construction. |
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With each additional throw, incrementally greater forces are required for |
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knot untying. After a specified number of throws, failure will occur by knot |
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breakage, after which the knot breakage force will not be enhanced by the |
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addition of more throws. Consequently, these additional throws offer no |
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mechanical advantage and represent more foreign bodies in the wound that |
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damage host defenses and resistance to infection. |
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The human element in knot tying has considerable influence on the |
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magnitude of knot slippage.19 The amount of tension exerted by the |
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surgeon on the “ears” of the knot significantly alters the degree of slippage. |
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The careless surgeon who applies minimal tension (10% of knot break |
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strength) to the “ears” of the knot constructs knots that fail by slippage. |
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Knot slippage can be minimized by applying more tensions (80% of knot |
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break strength) to the “ears” of the knot. Another serious error often made |
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by the inexperienced surgeon is not to change the position of his/her hands |
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appropriately during construction of square and/or granny type knots. The |
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resulting knot, a sliding or slip knot, will become untied regardless of the |
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suture material. The risk of forming a slip knot is greatest when tying one- |
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hand knots and/or with deep seated ligatures.20-23 |
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When enough force is applied to the tied suture to result in breakage, the site of |
knot breakage |
disruption of the suture is almost always the knot. The force necessary to break |
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a knotted suture is lower than that required to break an untied suture made |
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of the same material.19 The forces exerted on a tied suture are converted into |
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shear forces, by the knot configuration that break the knot. The percentage loss |
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of tensile strength, as a result of tying a secure knot, is least with mono-filament |
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and multifilament steel.25 This relationship between the tensile strength of |
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unknotted and knotted suture which is designated knot efficiency, is described |
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in the following equation: |
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Knot efficiency (%) = tensile strength of a knotted suture |
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tensile strength of unknotted suture |
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Regardless of the type of suture material, the efficiency of the knot is enhanced |
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with an increasing number of throws, although only up to a certain limit. |
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The type of knot configuration that results in a secure knot that fails by |
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breaking varies considerably with different suture material. The magnitude of |
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force necessary to produce knot breakage is influenced by the configuration |
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of the knotted suture loop, type of suture material, and the diameter of the |
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suture.3 The tissue in which the suture is implanted also has considerable |
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influence on the knot strength of suture. In the case of absorbable sutures, a |
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progressive decline in knot breaking strength is noted after tissue implantation. |
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In addition, the magnitude of knot breakage force is significantly influenced by |
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the rate of application of forces to the “ears” of the knot.23 When a constant |
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force is applied slowly to the knot “ears,” the knot breakage force is significantly |
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greater than that for knots in which the same constant force is applied rapidly |
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to the “ears.” The latter knot loading rate is often referred to as “the jerk at the |
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end of the knot,” especially when the knotted suture breaks. |
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suture cutting tissue
Suture failure also may occur if the knotted suture loop cuts through the tissue. The type of tissue has considerable influence on the magnitude of force required to tear the suture through the tissue. Howes and Harvey26 reported that the forces required to tear gut sutures through canine fascia was the greatest followed by muscle, peritoneum and then fat. Using cadaver specimens 6 to 93 days after death, Tera and °Aberg27 measured the magnitude of forces required for suture to tear through excised musculoaponeurotic layers of laparotomy incisions. The rationale for this study was that the forces required to tear sutures through a musculoaponeurotic layer would provide a basis for the choice of a suture whose strength is at least as strong as the forces required to tear the suture through the tissue. When the suture was passed lateral to the transition between the linea alba and the rectus sheath, the force required to tear the suture through the tissue was greater than that for any other musculoaponeurotic layer tested; the paramedian incision required the lowest forces to pull sutures through its sheaths. When they recorded the
forces needed for sutures to tear through structures involved in the repair of inguinal hernia, the structures making up the conjoined tendon and Cooper’s ligament were the strongest and exhibited twice the resistance to suture tearing than those of the other structures.
As expected, the force required for sutures to tear through tissue changes during healing. Aberg28 reported that the forces needed for sutures to tear through the aponeurotic muscle layer reduced significantly during the first week of healing. When the wound edges were approximated by suture tied tightly around this aponeurotic muscle layer, the reduction in force needed for the suture to pull through this tissue persisted for two weeks.
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Mechanical trauma to the suture by surgical instruments can also result in suture failure. Nichols et al29 cautioned surgeons about the handling of sutures by surgical instruments. They indicated that either the application of clamps and forceps to the suture or rough handling of sutures could damage and weaken them. Stamp et al30 incriminated the teeth in the needle holder jaws as important causal factors of sutural damage. Compression of sutures between the needle holder jaws with teeth produced morphologic changes in sutures that resulted in a marked reduction in the suture breaking strength. Similarly, the sharp edges of needle holder jaws without teeth can even crush the suture and, thereby, decrease its strength.31 Finally application of large compressive loads by pinching polypropylene sutures with DeBakey forceps decreases the strength of the suture.32
mechanical trauma
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VI. tying techniques
The surgeon may use an individual ligature (“free tie”) or a suture that is attached to a needle or ligature reel. The length of a free tie or suture attached to a needle is usually 18 inches. The longest strands of suture material are available on a reel or spool. When the suture is attached to a needle or reel, there is a free end and a fixed end; the fixed end is attached to either the needle or reel. The first throw of a knot is accomplished by wrapping the free end either once or twice (surgeon’s) around the fixed end. During practice, clamp one end of the suture with an instrument that serves to represent either the needle or the reel, which is the fixed end of the suture.
Formation of each throw of a knot is accomplished in three steps. The first step is the formation of a suture loop. In the second step, the free suture end is passed through the suture loop to create a throw. The final step is to advance the throw to the wound surface. For the first throw of a square, granny, and surgeon’s knot square, for each additional throw the direction in which tension is applied to the
suture ends is reversed. The surgeon should construct a knot by carefully snugging each throw tightly against another. The rate of applying tension to each throw should be relatively slow.
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For either the square knot or surgeon’s knot square, the direction in which the |
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free suture end is passed through the loop will be reversed for each additional |
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throw. If the free suture end is passed down through the first suture loop, it must |
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be passed up through the next suture loop. Reversal of the direction of passage of |
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the free suture end through the loops does not alter either the knot’s mechanical |
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performance or its configuration. It simply reverses the presentation of the knot |
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(Figure 8). For granny knots, the direction in which the free suture end is passed |
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through the loop is the same for each additional throw. |
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