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Influence of knot configuration and tying technique on the mechanical performance of sutures
The Journal of Emergency Medicine, Vol. 9, pp. 107-I 13, 1991
Printed in the USA. Copyright 0 1991 Pergamon Press plc
INFLUENCE OF KNOT CONFIGURATION AND TYING TECHNIQUE ON THE MECHANICAL PERFORMANCE OF SUTURES Christopher A. Zimmer, BA,* John G. Thacker, PhD,+ David M. Powell, BA,’ Kenneth T. Bellian, Daniel G. Becker, MD,* George T. Rodeheaver, PhD,* Richard F. Edlich, MD,POD*
BA,’
‘Department of Plastic Surgery, *Department of Otolaryngology and Head and Neck Surgery, University of Virginia School of Medicine, and the Department of +Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia Reprint address: Richard F. Edlich, MD, PhD, Department of Plastic Surgery, University of Virginia School of Medicine, Charlottesville, VA 22908
0 Abstract - The purpose of this investigation was to determine the itiuence of knot configuration and tying technique on the mechanical performance of surgical sutures. Multifilament and monofilament nylon sutures were selected for this evaluation because they are commonly used in wound closure. The mechanical performance of these sutures was judged by the following parameters: knot breakage force, configuration of secure knots, and knot run down force. During each test, tension was applied at either rapid or slow rates, which correlates with the physician’s speed of tying knots. On the basis of these mechanical performance tests, four throw square (1 = 1= l=l) knots and five throw square (l=l=l=l=l) knots are recommended for monofihunent nylon and muitifllament nylon sutures, respectively, in which the speed of application of forces to the knots is relatively slow. Because these tests can easily be replicated in any laboratory, manufacturers now have a scientific basis for recommending specific tying techniques for their surgical sutures.
mating the divided edges of tissue. Ideally, the physician should construct a knot that can be advanced (knot run down) to the wound surface, providing a preview of the apposition of the wound edges. When the advanced knot provides meticulous coaptation of the wound edges, the physician can construct additional throws to the knot that prevent slippage and ensure knot security. Such secure knots fail by breakage, rather than by slippage ( 1,2). It is essential that quantitative information be available concerning the expected performance of a surgical suture during wound closure. The purpose of this study was to develop reproducible tests that provide practical insights into the mechanical performance of suture. Utilizing these tests, the mechanical performance of two different surgical sutures, commonly used in skin closure, was measured. The results of this study provide insight into the influence of knot configuration and tying technique on the mechanical performance of these sutures. On the basis of these data, recommendations are made on the use of these sutures during wound closure.
0 Keywords - square knot; granny knot; slip knot; surgeon’s knot square
INTRODUCTION Sutures
remain
the most common
method
MATERIALS
Basic Concepts and Definitions
of approxi-
A suture is defined as a thread that either coapts adjacent cut surfaces of the wound or compresses blood vessels to stop bleeding. The mode of operation of a suture is
This research was supported by a generous gift from the Texaco Philanthropic Foundation, White Plains, New York.
= ~
AND METHODS
Original Contributions presents articles of interest to both academic and practicing physicians. This section of JEM is coordinated by John A. Marx, MD, of Denver General Hospital.
RECEIVED: 13 ACCEPTED: 23
August 1990; FINAL SUBMISSIONRECEIVED:7 September September 1990 107
1990;
0736-4679191
$3.00 + .OO
108
Doctor’s
Side
Ear
-
Patient’s
C. A. Zimmer et al.
Loop
Side
SQUARE KNOT I=I
Figure 1. Components of a tied suture with a square knot.
of a loop of fixed perimeter secured in this geometry by a knot. This loop either compresses or apposes the adjacent surfaces by transfixing the tissues within the loop. A tied suture has three components (Figure 1). First, the loop, created by the knot, maintains the apposition of tissue. Second, the knot is composed of a number of throws snugged against each other. A throw is a wrapping of weaving of two strands. Finally, the “ears” are the cut ends of the suture. The ears act as insurance that the loop will not become untied because of slippage. The physician’s side of the knot is defined as the side of the knot with the ears, or the side to which tension is applied during tying. The patient’s side is the side of the knot adjacent to the loop. A single throw is formed by wrapping two strands around each other so that the angle of wrap equals 360”. In a double throw, the free end of a strand is passed twice, instead of once, around the other strand; the angle of this double weave is 720”. The process of constructing two or more throws completes the knot. The configuration of a knot can be classified into two general types by the spatial relationship between the knot ears and the loop. When the right ear and loop of a two-throw knot the creation
SURGEON’S KNOT SQUARE 2=1
SQUARE KNOT I.1
exit on the same side of the knot and are parallel to each other, the type of knot is square (Figure 2). The left ear and loop come out from the square knot in a position that is directly opposite to that of the right ear and loop. Tension is applied to the ears of the fmt and second throws of a square knot in directions that are perpendicular to the wound in horizontal planes that are parallel to the underlying tissue. The magnitude of the tension to one ear must be comparable to that exerted to the other. If the physician applies tension to only one ear that is held taut and in a perpendicular plane to the underlying tissue, the other ear will encircle the taut end. When additional throws are constructed in the same manner, a slip knot develops with a square knot configuration (Figure 2). The knot is considered a granny type if the right ear and loop cross or exit different sides of the knot. Similarly, tension is applied to the ears of the first and second throws of a granny knot in directions that are perpendicular to the wound and in horizontal planes that are parallel to the underlying tissue. A granny slip knot will develop when the physician applies tension to only one ear that is held in a direction that is perpendicular to that of the underlying tissue allowing the other ear to encircle the taut end. These relationships between the knot’s ear and loop allow knots containing two or more throws to be classified as either a granny or square type knot. A surgeon’s knot square is formed by making an initial double wrap throw, which is followed by a single throw. The relationship between the knot’s ear and loop permit the surgeon’s knot to be classified either as surgeon’s knot square or surgeon’s knot granny. A simple code has been devised to describe the configuration of the knot (4). The number of wraps involved in each throw is indicated by the appropriate arabic number. The relationship between each throw to each other, being either crossed or parallel, is signified by the symbols X or = , respectively. In accordance with this code, the knot square is designated 1 = 1, and knot granny 1 X 1. This method of describing knots
SQUARE SLIP KNOT I=I
GRANNY KNOT IXI
GRANNY SLIP KNOT 1x1
Flgura 2. The conflgumtlon of a surgeon’s knot squam, squara knot, squam slip knot, granny knot and granny slip knot.
Knot Configuration
facilitates their identification and reproduction. It is, for example, obvious what is meant by 1 X 1 X 1, without giving the knot a name, and all surgical knots can be defined unequivocally in an international language.
Sutures
All sutures were supplied by Ethicon, Inc. (Somerville, NJ). Monofilament (Lot Number C2145ABB) and multifilament (Lot Number Cl 168BA) nylon sutures were evaluated. The gauge of these sutures was 5-O. Suture diameter was measured without compression using a video microscope. The thickness of the suture was measured on the video monitor at several points along its length using calibrated cursors on the screen.
Mechanical Per$ornmnce of Knotted Suture
The mechanical performance of a knot was evaluated by measuring its knot break load, minimum number of throws required for knot security, and knot run down force using either a rapid rate (500 mm/mm) or a slow rate (50 mm/mm) of loading by an In&on@ (Instron Corp, Canton, MA) Tensile Tester (Model 1122) (1). Reproducible square, slip, granny, or surgeon’s knots were mechanically tied at a tension equivalent to 80% of the specific knot holding power. This tension is comparable to that utilized by a physician who carefully snugs each throw tightly against another. Tying the knot under known tensions eliminated the variables encountered with hand-tied knots (1). Knots were tied around a stainless steel mandrel (38 mm diameter) positioned between the clamps of the Instron@ Tensile Tester. When additional throws were required for knot security, the procedure was repeated for each additional throw. No tension was placed on the patient’s side of the knot. After completing each knot, the ears were cut so that their length was 3 mm and the loop was then divided at its midpoint. This ear length was selected because it represents the average knot ear length encountered during surgical procedures (2). The loop ends were positioned in the pneumatic jaws of the tensile tester so that the section was taut, and the knot was centered in the 50-mm gauge length. The knot was then extended at two different rates of extension until the knot either broke or came untied. The 500 mm/min strain rate represented the rate of tension application by a physician who constructs knots rapidly. A slower strain rate of 50 mm/min exemplified a rate of loading by physicians who construct knots slowly. Further throws were added to each knot until knot security was achieved.
109
A secure knot was defined as a knot with ears of 3 mm in length that went to knot break without becoming untied (slipping more than 3 mm). Failure by knot breakage, rather than by slippage, had to be encountered in all of the sutures tested for knot construction to be judged secure. The load for breakage of secure knots was recorded. In addition, the number of throws required for knot security was documented. Knot security was never encountered with slip knots consisting of 5 throws. Consequently, the load required for slippage of a 5 throw slip knot was determined. The forces applied to the ears on the physician’s side of the knot also have important clinical implications. After constructing either a two throw square (1 = l), square slip (1 = l), granny slip (1X1), or surgeon’s knot square (2 = l), the physician advances the knot to the wound surface and gains a preview of the ultimate apposition of the wound edges. If advancement of the two throw knot achieves meticulous approximation of the wound edges, the physician can construct additional throws needed for knot security. The force required to advance the knot is called knot run down force, and has been recorded for the five different knot configurations. Two throw square (I= l), square slip (1 = l), granny (1X1), granny slip (1X1), and surgeon’s knot square (2 = l), were formed around the stainless steel mandrel. A load was applied to the second throw that was equivalent to 80% of the respective knot break load. The loop was removed from the mandrel after which its ears were clamped in the pneumatic jaws of the Instrone Tensile Tester. The jaws were then extended at either the slow (50 mm/min) or rapid (500 mrn/min) strain rates. The area under the force-distance (65 mm) curve was measured by a planimeter and expressed as the mean knot run down force. Suture diameter, knot security, and knot slippage were determined separately for ten monofilament and multifilament suture samples at either the rapid or slow strain rates of extension and expressed as the mean and standard deviation for the data. Statistical significance of these data was determined by the Student’s t test.
RESULTS The mean diameter of the 5-O multifilament nylon suture (260 + 9 p,m) was 23% larger than that of the monofilament nylon suture (200 ;t 11 pm) (P < 0.05). The strain rate, suture type, and knot configuration were all important determinants of knot security (Figure 3). For both monofilament and multifilament nylon sutures, the mean load for breakage of either square knots, granny knots, or surgeon’s knot square subjected to rapid strain rates was significantly less than that for comparable
C. A. Zimmer et al.
110 t 12.4
t
I 1.6
T
I 3.
m
t
CZ9
12,
t
05
10.3
T
I 1.4
Strain Rate 50 mm/min Strain Rate SOOmm/min
0’3
0?6
I I.
T t 1.6
IO.
OT9
9. 8,
L
7,
4:7 f 0.6
6,
& ?&
5
Ofe
* 3.9 * I .2
0!6
4 3 2 Il_l!
ir
Mono Nylon
Multi. Nylon
Mono. Nylon
Multi Nylon
Granny Knot
Square Knot
Multi. Nylon
Mono. Nylon
Multi. Nylon
Mono. Nylon
Surgeon’s Square Knot
Slip Knot
Figure 3. Secure square knots, and granny knots, and surgeon’s knots square falled by breakage at loads that were nearly twofold greater than loads required for sllppage of slip knots.
Mono Nylon
Multi. Nylon
Nylon
Square
Knot
Granny
Mono. Nylon Knot
Multi. Nylon
Mono. Nylon
m
Strain Rate 50mm/min
0
Strain Rate 500mm /min
Multi. Nylon
Square Slip Knot Surgeon’s Knot Square
Figure 4. The conflguratlon of either a secure square knot, granny knot, or surgeon’s knot square had either four or five throws, while all slip knots falled by sllppage (represented here as 0 throws), rather than breakage.
Knot Configuration
111
~stram
Rate 50mm/min
0
Strain Rate 500mmhin
I-
I-
I-
:1 I
MUM Nylon
Mono Nylon
Squore
I=1
Knot
Multi Nylon
MOW0
Nylon
Gronny
III
Knot
MUIll Nylon
Mono Nylon
Squore Slip Knot I=1
Mono
Nylon
MUl?l Nylon
Granny Slip Knot 1x1
Mono Nylon
MUIll Nylon
Surgeon’s Swore Knot 2=l
Figure 5. For comparable sutures, the mean knot run down for the square knot was the greatest, followed by the granny knot, and then the slip knot. Asterisk denotes that the surgeon’s knot square dld not advance and consistently falled by breakage.
knots loaded at slower strain rates (P < 0.05). At comparable strain rates, the mean load for breakage of either secure square knot, granny knot, or surgeon’s knot square multifilament nylon knotted sutures was significantly greater than those for comparable monofilament nylon knotted sutures (P < 0.05). At similar strain rates, the mean load for breakage of secure square knots did not differ significantly from that for either secure granny knots or surgeon’s knot square constructed from comparable suture materials. It is important to point out that all 5 throw slip knots failed by slippage, rather than breakage. The mean load for breakage of secure square knots, granny knots, or surgeon’s knot square was approximately twofold greater than the mean load needed for slippage of the slip knots (P < 0.01). At a strain rate of 500 mm/mm, the configuration of secure square knots, granny knots, and surgeon’s knot square of monofilament and multifilament nylon sutures was four throws (Figure 4). In contrast, a five throw square knot, granny knot, or surgeon’s knot square was
required for security in multifilament sutures loaded at the slow strain rates. The type of suture as well as knot configuration had considerable influence on the magnitude of the mean knot run down force (Figure 5). It is important to point out that all surgeon’s knots square (2= 1) did not advance and failed by breakage, while knot run down was encountered with square knots (1 = l), square slip knots (1 = l), granny knots (1X1), and granny slip knots. The mechanical performance (knot break load) of the surgeon’s knot square loaded from the physician’s side of the knot in the knot run down test was not significantly different from that noted in the knot security evaluation in which the knot was loaded from the patient’s side of the knot. However, knot configuration (two throws) for a secure surgeon’s knot square subjected to tensions from the physician’s side of the knot was distinctly different from the secure knot configuration (four or five throws) loaded from the patient’s side of the knot. The mean knot run down force for either the square
112
C. A. Zimmer et al.
(1 = 1) or granny (1X1) knots in multifilament nylon sutures was significantly greater than for comparable knots in monofilament nylon sutures (P < 0.01). For similar sutures, the mean knot run down force for square (1 = 1) knots was greatest, followed by that for the granny knots and then the slip knots (P < 0.01). The mean knot run down forces for the granny (1X1) slip knot was significantly greater than for the square (1 = 1) slip knot tied in comparable sutures (P < 0.05). The magnitude of the strain rate did not significantly influence the mean knot run down force for the knots in different sutures, except in the case of granny slip knots. At high (500 mrn/min) strain rates, the mean knot run down force for granny (1X1) slip knots in monofilament sutures was significantly greater than that for granny (1X1) slip knots in monofilament nylon sutures subjected to lower (50 mm/min) strain rates. DISCUSSION Knot construction by a physician involves two distinct steps. The purpose of the first step is to secure precise approximation of the wound edges by advancing either a one throw or a two throw knot to the wound surface. Once the throw or throws contact the wound, the physician will have a preview of the ultimate apposition of the wound edges. When the physician forms a double wrap throw, the first throw of the surgeon’s knot square can maintain apposition of the wound edges by “locking” or temporarily securing it in place by reversing the direction of pull on its ears. The “locked” double throw is not a reliable means of maintaining wound apposition because any tension applied to the ears from the patient’s side of the knot will unlock the knot. The addition of the second throw to the surgeon’s knot square will provide additional resistance to wound disruption, but this knot will n& advance by slippage, limiting the physician’s ability to secure meticulous coaptation of the wound edges. In contrast, two throw square (1 = 1) or granny (1X1) knots can be advanced to the wound surface to secure proper wound edge apposition. These two throw square (1 = 1) or granny (1X1) knots can be easily converted into their respective siip knots by applying tension to only one ear in a direction that is in a perpendicular plane to that of tissue surface. Square (1 = 1) or granny (1X1) slip knots require lower knot run down forces for knot advancement than either the square (1 = 1) or granny (1X1) knots, but will never reach knot security even with 5 throws (1). Physicians may inadvertently tie slip knots when tying knots with the one hand technique or forming knots in deep cavities (3). The risk of tying slip knots can be obviated by applying tensions to both ears in horizontal planes parallel to the tissue surface. The second step in knot construction is the addition
of a sufficient number of throws to the knot so that it fails by breakage, rather than by slippage. The magnitude of knot breakage force is always greater than that for knot slippage force of a comparable suture, ensuring optimal protection against wound dehiscence. The magnitude of knot breakage was significantly influenced by the rate of application of forces to the knot. When constant force was applied slowly (50 mm/min) to the knot ears, the knot breakage force was significantly greater than that for knots in which the same constant force was applied rapidly to the ears. The latter loading rate is often referred to as “the jerk at the end of the tie,” especially when the knotted suture breaks. At comparable rates of loading, the knot break force for multifilament nylon sutures was significantly greater than that for monofilament nylon suture. This disparity in knot break force is attributed to the larger suture diameter of multifilament nylon sutures as compared to that of the monofilament nylon suture. After reviewing these data, the physician may be surprised that the mechanical performance (knot breakage force and knot run down force) of square knots were remarkably similar to that of granny knots. However, the mechanical performance of the granny knots and surgeon’s knots must be weighed against the physician’s ability to construct these knots. Physicians can easly construct reproducible square knots by either instrument ties or one-handed or two-handed tying techniques with an unlimited number of throws. In contrast, physicians experience considerable difficulty in constructing reproducible three or more throw granny knots using onehanded or two-handed tying techniques. Fortunately, physicians can construct reproducible granny knots using instrument ties regardless of the number of throws. On the basis of these standardized mechanical performance tests, the following recommendations are made for tying knots in 5-O monofilament and 5-O multifilament nylon sutures. The physician should construct a knot by carefully snugging each throw tightly against each other. The rate of applying tension to the knot should be relatively slow. The physician should begin knot construction by forming a two throw square knot (1 = 1) that is advanced to the wound surface, providing a preview of the ultimate coaptation of the wound edges. When meticulous approximation of the wound edges is achieved, the physician will construct two additional throws to a monofilament nylon (1 = 1 = 1) and three additional throws to the multifilament nylon knot (I= 1= 1 = 1 = l), which will then fail by breakage. When the physician applies tension to the knot ears, it is applied in directions that are in horizontal planes parallel to that of the tissue surface. The ears are then cut so that they are 3 mm long. While we realize that these recommendations apply only to a specific gauge size for two sutures,
Knot Configuration
113
the manufacturer may repeat these standardized mechanical performance tests for all other sutures, the results of
which provide a scientific basis for recommending knot tying technique for surgical sutures.
REFERENCES 1. Thacker JG, Rodeheaver GT, Kurtz L, Edgerton MT, Edlich RF. Mechanical performance of sutures in surgery. Am J Surg. 1977; 133:713-15. 2. Thacker JG, Rodeheaver GT, Moore JW, et al. Mechanical performance of surgical sutures. Am J Surg. 1975;130:374-80.
3. Taylor WF. Surgical knots. Ann Surg. 1938;107:458-68. 4. Tera H, Aberg C. Tensile strength of twelve types of knots employed in surgery, using different suture materials. Acta Chir &and. 1976;142:1-7.
Printed in the USA. Copyright 0 1991 Pergamon Press plc
INFLUENCE OF KNOT CONFIGURATION AND TYING TECHNIQUE ON THE MECHANICAL PERFORMANCE OF SUTURES Christopher A. Zimmer, BA,* John G. Thacker, PhD,+ David M. Powell, BA,’ Kenneth T. Bellian, Daniel G. Becker, MD,* George T. Rodeheaver, PhD,* Richard F. Edlich, MD,POD*
BA,’
‘Department of Plastic Surgery, *Department of Otolaryngology and Head and Neck Surgery, University of Virginia School of Medicine, and the Department of +Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia Reprint address: Richard F. Edlich, MD, PhD, Department of Plastic Surgery, University of Virginia School of Medicine, Charlottesville, VA 22908
0 Abstract - The purpose of this investigation was to determine the itiuence of knot configuration and tying technique on the mechanical performance of surgical sutures. Multifilament and monofilament nylon sutures were selected for this evaluation because they are commonly used in wound closure. The mechanical performance of these sutures was judged by the following parameters: knot breakage force, configuration of secure knots, and knot run down force. During each test, tension was applied at either rapid or slow rates, which correlates with the physician’s speed of tying knots. On the basis of these mechanical performance tests, four throw square (1 = 1= l=l) knots and five throw square (l=l=l=l=l) knots are recommended for monofihunent nylon and muitifllament nylon sutures, respectively, in which the speed of application of forces to the knots is relatively slow. Because these tests can easily be replicated in any laboratory, manufacturers now have a scientific basis for recommending specific tying techniques for their surgical sutures.
mating the divided edges of tissue. Ideally, the physician should construct a knot that can be advanced (knot run down) to the wound surface, providing a preview of the apposition of the wound edges. When the advanced knot provides meticulous coaptation of the wound edges, the physician can construct additional throws to the knot that prevent slippage and ensure knot security. Such secure knots fail by breakage, rather than by slippage ( 1,2). It is essential that quantitative information be available concerning the expected performance of a surgical suture during wound closure. The purpose of this study was to develop reproducible tests that provide practical insights into the mechanical performance of suture. Utilizing these tests, the mechanical performance of two different surgical sutures, commonly used in skin closure, was measured. The results of this study provide insight into the influence of knot configuration and tying technique on the mechanical performance of these sutures. On the basis of these data, recommendations are made on the use of these sutures during wound closure.
0 Keywords - square knot; granny knot; slip knot; surgeon’s knot square
INTRODUCTION Sutures
remain
the most common
method
MATERIALS
Basic Concepts and Definitions
of approxi-
A suture is defined as a thread that either coapts adjacent cut surfaces of the wound or compresses blood vessels to stop bleeding. The mode of operation of a suture is
This research was supported by a generous gift from the Texaco Philanthropic Foundation, White Plains, New York.
= ~
AND METHODS
Original Contributions presents articles of interest to both academic and practicing physicians. This section of JEM is coordinated by John A. Marx, MD, of Denver General Hospital.
RECEIVED: 13 ACCEPTED: 23
August 1990; FINAL SUBMISSIONRECEIVED:7 September September 1990 107
1990;
0736-4679191
$3.00 + .OO
108
Doctor’s
Side
Ear
-
Patient’s
C. A. Zimmer et al.
Loop
Side
SQUARE KNOT I=I
Figure 1. Components of a tied suture with a square knot.
of a loop of fixed perimeter secured in this geometry by a knot. This loop either compresses or apposes the adjacent surfaces by transfixing the tissues within the loop. A tied suture has three components (Figure 1). First, the loop, created by the knot, maintains the apposition of tissue. Second, the knot is composed of a number of throws snugged against each other. A throw is a wrapping of weaving of two strands. Finally, the “ears” are the cut ends of the suture. The ears act as insurance that the loop will not become untied because of slippage. The physician’s side of the knot is defined as the side of the knot with the ears, or the side to which tension is applied during tying. The patient’s side is the side of the knot adjacent to the loop. A single throw is formed by wrapping two strands around each other so that the angle of wrap equals 360”. In a double throw, the free end of a strand is passed twice, instead of once, around the other strand; the angle of this double weave is 720”. The process of constructing two or more throws completes the knot. The configuration of a knot can be classified into two general types by the spatial relationship between the knot ears and the loop. When the right ear and loop of a two-throw knot the creation
SURGEON’S KNOT SQUARE 2=1
SQUARE KNOT I.1
exit on the same side of the knot and are parallel to each other, the type of knot is square (Figure 2). The left ear and loop come out from the square knot in a position that is directly opposite to that of the right ear and loop. Tension is applied to the ears of the fmt and second throws of a square knot in directions that are perpendicular to the wound in horizontal planes that are parallel to the underlying tissue. The magnitude of the tension to one ear must be comparable to that exerted to the other. If the physician applies tension to only one ear that is held taut and in a perpendicular plane to the underlying tissue, the other ear will encircle the taut end. When additional throws are constructed in the same manner, a slip knot develops with a square knot configuration (Figure 2). The knot is considered a granny type if the right ear and loop cross or exit different sides of the knot. Similarly, tension is applied to the ears of the first and second throws of a granny knot in directions that are perpendicular to the wound and in horizontal planes that are parallel to the underlying tissue. A granny slip knot will develop when the physician applies tension to only one ear that is held in a direction that is perpendicular to that of the underlying tissue allowing the other ear to encircle the taut end. These relationships between the knot’s ear and loop allow knots containing two or more throws to be classified as either a granny or square type knot. A surgeon’s knot square is formed by making an initial double wrap throw, which is followed by a single throw. The relationship between the knot’s ear and loop permit the surgeon’s knot to be classified either as surgeon’s knot square or surgeon’s knot granny. A simple code has been devised to describe the configuration of the knot (4). The number of wraps involved in each throw is indicated by the appropriate arabic number. The relationship between each throw to each other, being either crossed or parallel, is signified by the symbols X or = , respectively. In accordance with this code, the knot square is designated 1 = 1, and knot granny 1 X 1. This method of describing knots
SQUARE SLIP KNOT I=I
GRANNY KNOT IXI
GRANNY SLIP KNOT 1x1
Flgura 2. The conflgumtlon of a surgeon’s knot squam, squara knot, squam slip knot, granny knot and granny slip knot.
Knot Configuration
facilitates their identification and reproduction. It is, for example, obvious what is meant by 1 X 1 X 1, without giving the knot a name, and all surgical knots can be defined unequivocally in an international language.
Sutures
All sutures were supplied by Ethicon, Inc. (Somerville, NJ). Monofilament (Lot Number C2145ABB) and multifilament (Lot Number Cl 168BA) nylon sutures were evaluated. The gauge of these sutures was 5-O. Suture diameter was measured without compression using a video microscope. The thickness of the suture was measured on the video monitor at several points along its length using calibrated cursors on the screen.
Mechanical Per$ornmnce of Knotted Suture
The mechanical performance of a knot was evaluated by measuring its knot break load, minimum number of throws required for knot security, and knot run down force using either a rapid rate (500 mm/mm) or a slow rate (50 mm/mm) of loading by an In&on@ (Instron Corp, Canton, MA) Tensile Tester (Model 1122) (1). Reproducible square, slip, granny, or surgeon’s knots were mechanically tied at a tension equivalent to 80% of the specific knot holding power. This tension is comparable to that utilized by a physician who carefully snugs each throw tightly against another. Tying the knot under known tensions eliminated the variables encountered with hand-tied knots (1). Knots were tied around a stainless steel mandrel (38 mm diameter) positioned between the clamps of the Instron@ Tensile Tester. When additional throws were required for knot security, the procedure was repeated for each additional throw. No tension was placed on the patient’s side of the knot. After completing each knot, the ears were cut so that their length was 3 mm and the loop was then divided at its midpoint. This ear length was selected because it represents the average knot ear length encountered during surgical procedures (2). The loop ends were positioned in the pneumatic jaws of the tensile tester so that the section was taut, and the knot was centered in the 50-mm gauge length. The knot was then extended at two different rates of extension until the knot either broke or came untied. The 500 mm/min strain rate represented the rate of tension application by a physician who constructs knots rapidly. A slower strain rate of 50 mm/min exemplified a rate of loading by physicians who construct knots slowly. Further throws were added to each knot until knot security was achieved.
109
A secure knot was defined as a knot with ears of 3 mm in length that went to knot break without becoming untied (slipping more than 3 mm). Failure by knot breakage, rather than by slippage, had to be encountered in all of the sutures tested for knot construction to be judged secure. The load for breakage of secure knots was recorded. In addition, the number of throws required for knot security was documented. Knot security was never encountered with slip knots consisting of 5 throws. Consequently, the load required for slippage of a 5 throw slip knot was determined. The forces applied to the ears on the physician’s side of the knot also have important clinical implications. After constructing either a two throw square (1 = l), square slip (1 = l), granny slip (1X1), or surgeon’s knot square (2 = l), the physician advances the knot to the wound surface and gains a preview of the ultimate apposition of the wound edges. If advancement of the two throw knot achieves meticulous approximation of the wound edges, the physician can construct additional throws needed for knot security. The force required to advance the knot is called knot run down force, and has been recorded for the five different knot configurations. Two throw square (I= l), square slip (1 = l), granny (1X1), granny slip (1X1), and surgeon’s knot square (2 = l), were formed around the stainless steel mandrel. A load was applied to the second throw that was equivalent to 80% of the respective knot break load. The loop was removed from the mandrel after which its ears were clamped in the pneumatic jaws of the Instrone Tensile Tester. The jaws were then extended at either the slow (50 mm/min) or rapid (500 mrn/min) strain rates. The area under the force-distance (65 mm) curve was measured by a planimeter and expressed as the mean knot run down force. Suture diameter, knot security, and knot slippage were determined separately for ten monofilament and multifilament suture samples at either the rapid or slow strain rates of extension and expressed as the mean and standard deviation for the data. Statistical significance of these data was determined by the Student’s t test.
RESULTS The mean diameter of the 5-O multifilament nylon suture (260 + 9 p,m) was 23% larger than that of the monofilament nylon suture (200 ;t 11 pm) (P < 0.05). The strain rate, suture type, and knot configuration were all important determinants of knot security (Figure 3). For both monofilament and multifilament nylon sutures, the mean load for breakage of either square knots, granny knots, or surgeon’s knot square subjected to rapid strain rates was significantly less than that for comparable
C. A. Zimmer et al.
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Square Knot
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Mono. Nylon
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Slip Knot
Figure 3. Secure square knots, and granny knots, and surgeon’s knots square falled by breakage at loads that were nearly twofold greater than loads required for sllppage of slip knots.
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Multi. Nylon
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Knot
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Multi. Nylon
Mono. Nylon
m
Strain Rate 50mm/min
0
Strain Rate 500mm /min
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Square Slip Knot Surgeon’s Knot Square
Figure 4. The conflguratlon of either a secure square knot, granny knot, or surgeon’s knot square had either four or five throws, while all slip knots falled by sllppage (represented here as 0 throws), rather than breakage.
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Figure 5. For comparable sutures, the mean knot run down for the square knot was the greatest, followed by the granny knot, and then the slip knot. Asterisk denotes that the surgeon’s knot square dld not advance and consistently falled by breakage.
knots loaded at slower strain rates (P < 0.05). At comparable strain rates, the mean load for breakage of either secure square knot, granny knot, or surgeon’s knot square multifilament nylon knotted sutures was significantly greater than those for comparable monofilament nylon knotted sutures (P < 0.05). At similar strain rates, the mean load for breakage of secure square knots did not differ significantly from that for either secure granny knots or surgeon’s knot square constructed from comparable suture materials. It is important to point out that all 5 throw slip knots failed by slippage, rather than breakage. The mean load for breakage of secure square knots, granny knots, or surgeon’s knot square was approximately twofold greater than the mean load needed for slippage of the slip knots (P < 0.01). At a strain rate of 500 mm/mm, the configuration of secure square knots, granny knots, and surgeon’s knot square of monofilament and multifilament nylon sutures was four throws (Figure 4). In contrast, a five throw square knot, granny knot, or surgeon’s knot square was
required for security in multifilament sutures loaded at the slow strain rates. The type of suture as well as knot configuration had considerable influence on the magnitude of the mean knot run down force (Figure 5). It is important to point out that all surgeon’s knots square (2= 1) did not advance and failed by breakage, while knot run down was encountered with square knots (1 = l), square slip knots (1 = l), granny knots (1X1), and granny slip knots. The mechanical performance (knot break load) of the surgeon’s knot square loaded from the physician’s side of the knot in the knot run down test was not significantly different from that noted in the knot security evaluation in which the knot was loaded from the patient’s side of the knot. However, knot configuration (two throws) for a secure surgeon’s knot square subjected to tensions from the physician’s side of the knot was distinctly different from the secure knot configuration (four or five throws) loaded from the patient’s side of the knot. The mean knot run down force for either the square
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(1 = 1) or granny (1X1) knots in multifilament nylon sutures was significantly greater than for comparable knots in monofilament nylon sutures (P < 0.01). For similar sutures, the mean knot run down force for square (1 = 1) knots was greatest, followed by that for the granny knots and then the slip knots (P < 0.01). The mean knot run down forces for the granny (1X1) slip knot was significantly greater than for the square (1 = 1) slip knot tied in comparable sutures (P < 0.05). The magnitude of the strain rate did not significantly influence the mean knot run down force for the knots in different sutures, except in the case of granny slip knots. At high (500 mrn/min) strain rates, the mean knot run down force for granny (1X1) slip knots in monofilament sutures was significantly greater than that for granny (1X1) slip knots in monofilament nylon sutures subjected to lower (50 mm/min) strain rates. DISCUSSION Knot construction by a physician involves two distinct steps. The purpose of the first step is to secure precise approximation of the wound edges by advancing either a one throw or a two throw knot to the wound surface. Once the throw or throws contact the wound, the physician will have a preview of the ultimate apposition of the wound edges. When the physician forms a double wrap throw, the first throw of the surgeon’s knot square can maintain apposition of the wound edges by “locking” or temporarily securing it in place by reversing the direction of pull on its ears. The “locked” double throw is not a reliable means of maintaining wound apposition because any tension applied to the ears from the patient’s side of the knot will unlock the knot. The addition of the second throw to the surgeon’s knot square will provide additional resistance to wound disruption, but this knot will n& advance by slippage, limiting the physician’s ability to secure meticulous coaptation of the wound edges. In contrast, two throw square (1 = 1) or granny (1X1) knots can be advanced to the wound surface to secure proper wound edge apposition. These two throw square (1 = 1) or granny (1X1) knots can be easily converted into their respective siip knots by applying tension to only one ear in a direction that is in a perpendicular plane to that of tissue surface. Square (1 = 1) or granny (1X1) slip knots require lower knot run down forces for knot advancement than either the square (1 = 1) or granny (1X1) knots, but will never reach knot security even with 5 throws (1). Physicians may inadvertently tie slip knots when tying knots with the one hand technique or forming knots in deep cavities (3). The risk of tying slip knots can be obviated by applying tensions to both ears in horizontal planes parallel to the tissue surface. The second step in knot construction is the addition
of a sufficient number of throws to the knot so that it fails by breakage, rather than by slippage. The magnitude of knot breakage force is always greater than that for knot slippage force of a comparable suture, ensuring optimal protection against wound dehiscence. The magnitude of knot breakage was significantly influenced by the rate of application of forces to the knot. When constant force was applied slowly (50 mm/min) to the knot ears, the knot breakage force was significantly greater than that for knots in which the same constant force was applied rapidly to the ears. The latter loading rate is often referred to as “the jerk at the end of the tie,” especially when the knotted suture breaks. At comparable rates of loading, the knot break force for multifilament nylon sutures was significantly greater than that for monofilament nylon suture. This disparity in knot break force is attributed to the larger suture diameter of multifilament nylon sutures as compared to that of the monofilament nylon suture. After reviewing these data, the physician may be surprised that the mechanical performance (knot breakage force and knot run down force) of square knots were remarkably similar to that of granny knots. However, the mechanical performance of the granny knots and surgeon’s knots must be weighed against the physician’s ability to construct these knots. Physicians can easly construct reproducible square knots by either instrument ties or one-handed or two-handed tying techniques with an unlimited number of throws. In contrast, physicians experience considerable difficulty in constructing reproducible three or more throw granny knots using onehanded or two-handed tying techniques. Fortunately, physicians can construct reproducible granny knots using instrument ties regardless of the number of throws. On the basis of these standardized mechanical performance tests, the following recommendations are made for tying knots in 5-O monofilament and 5-O multifilament nylon sutures. The physician should construct a knot by carefully snugging each throw tightly against each other. The rate of applying tension to the knot should be relatively slow. The physician should begin knot construction by forming a two throw square knot (1 = 1) that is advanced to the wound surface, providing a preview of the ultimate coaptation of the wound edges. When meticulous approximation of the wound edges is achieved, the physician will construct two additional throws to a monofilament nylon (1 = 1 = 1) and three additional throws to the multifilament nylon knot (I= 1= 1 = 1 = l), which will then fail by breakage. When the physician applies tension to the knot ears, it is applied in directions that are in horizontal planes parallel to that of the tissue surface. The ears are then cut so that they are 3 mm long. While we realize that these recommendations apply only to a specific gauge size for two sutures,
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the manufacturer may repeat these standardized mechanical performance tests for all other sutures, the results of
which provide a scientific basis for recommending knot tying technique for surgical sutures.
REFERENCES 1. Thacker JG, Rodeheaver GT, Kurtz L, Edgerton MT, Edlich RF. Mechanical performance of sutures in surgery. Am J Surg. 1977; 133:713-15. 2. Thacker JG, Rodeheaver GT, Moore JW, et al. Mechanical performance of surgical sutures. Am J Surg. 1975;130:374-80.
3. Taylor WF. Surgical knots. Ann Surg. 1938;107:458-68. 4. Tera H, Aberg C. Tensile strength of twelve types of knots employed in surgery, using different suture materials. Acta Chir &and. 1976;142:1-7.